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Photographic Lenses and Shutters

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Page 1: Photographic Lenses and Shutters
Page 2: Photographic Lenses and Shutters

PHOTOGRAPHICLENSES ANDSHUTTERS

.ByRICHARD W. ST. CLAIR, A.R.P.S.

Research Consultant and Lectureron Photographic Optics

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COPYRIGHT. 19~OBY ZIFF-DAVIS PUBLISHING COMPANY

AU. RIGHTS RESERVED INCLUDING THE RIGIlT TO REPRODUCETHiS BOOK OR PARTS THEREOF IN ANY FORM

• • •

ZIFF-DAVIS PUBLISHING COMPANYCHICAGO NEW YORK

PRINTED IN THE U.S.A .

CONTENTS.hapter PageIntroduction _ - . - . - . - . .. 6,

I The Camera and Its Lens. . . . . . . . . . . . . . .. 7II Simple Lenses . . . . . . . . . . . . . . . . . . . . . . . . .. 23

III Lens Measurements __ _. 50

IV The Aperture or Diaphragm. . . . . . . . . . . . .. 57V Lens Aberrations 71

VI Photographic Lenses .. ,................. 88VII Auxiliary Lenses _ 97

VIII Care of Lenses and Shutters 107IX Testing Lenses _. __ 115X Enlarger and Projector Lenses 118

XI Shutters .. _. _ __.. _.. 125

XII Useful Tables _ _ _.. - .. 149Index __ _. _. _155

5

.~...~...~~ ~..--.--

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I TRODUCTION

THIS book has been written for the purpose of supplyingthe beginner with information about his camera lens and

shutter, their principles, uses, and care. It is essentially a bookfor the everyday amateur who does not possess a technicaleducation, and is written in a simple and comprehensiblemanner.

Such few calculations as do appear and which, from experi-ence, I have found necessary in everyday work, are actuallynothing more than simple arithmetic although they may seem·algebraic at first glance. The use of letters in the equations issimply a shorthand method of stating rules that are easilyunder-stood. When these letters are replaced by the figures forwhich they stand, the equation becomes a simple problem. Ifthe reader is of a non-mathematical turn, he may omit suchcalculations altogether and stil! gain much information on thesubject.

While lenses and shutters have never received the attentionthey deserve in photographic literature for the beginner, recentdevelopments in photography have made such information anddata necessary for the aspiring amateur. The function of thelens, the principles on which it works, and a discussion of thevarious types of lenses now on the market are all matters ofthe greatest importance to those who wish to advance in thisart.

Taking good pictures involves correct exposures and theproduction of images which are as large or as small, as sharpor as soft, as desired. Both are dependent on the manipulationof the lens and shutter. The other parts of the camera aresecondary to the optical system in gaining the final results.

Richard Winthrop St. Clair, A.R.P.S.New York, N. Y.April, 1940.

6

CHAPTER I

THE CAMERA AND ITS LENS

BASICALLY, the photographic camera is a device employedfor recording images of objects which are visible to the

human eye. These images can be of momentary duration orcan be recorded permanently on specially prepared sensitizedpaper, glass plate, or celluloid for future reference.

Photographic cameras, as with many other mechanicaldevices, are more or less accurate copies of natural structures,~he principles of which have been employed by the originalInventors. The modern camera is a mechanical replica of thehuman eye. It resembles the eye in all essential respects andthis organ undoubtedly is the basis of photography as we knowit today. By making a direct comparison between the eye anda simple box camera their similarity becomes apparent.

In Fig. 1 is shown a sectional view taken through a humaneye which, at the moment, is viewing the arrow or object (0).Here we have the light-tight box or eyeball (E) with thetransparent lenses (A) and (L) through which light rays passfrom the object to the viewing screen or retina (R) at therear of the eyeball. These lenses recreate on the retina animage (M) which is exactly similar to the object (0) butinverted and much smaller. In short, the image is smaller andupside down on the retina due to the crossing of the light raysin the lenses.

Our eye, therefore, consists of a front lens (A) filled with

Fig. I. A sectional view through the human eye.

7

._.....• -~....~~~..-

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8 LITTLE TECHNICAL LIBRARY

a crystal clear fluid known as the "aqueous humor," and acrystalline lens (L) of greater consistency which serves torectify the light rays passing through the front lens. Theimage thus created falls on the nerve ends in the retina whichin turn convey the image to the brain.

Between the two lenses is the adjustable iris (I) with anopening that can be contracted or !xpanded by muscular effortin accordance with the prevailing light conditions. In weaklight, the iris is well opened while in strong sunlight it maybe closed almost to a pin-point, thus controlling the amountand intensity of the light falling upon the retina and alsomaintaining a sharp image or field of view.

In addition to the iris adjustment, the eye lenses are "fo-cused" by muscular action on the crystalline lens which isinstrumental in obtaining a sharp, well-defined image at alldistances of the object. The iris supplements the result of thefocusing action, by means that will be described later, ?o t~atthe image will be needle-sharp under all conditions of lightingand object distance. External to the eye, but acting in con-junction with it, are the eyelids (e) which, when closed, preventlight from entering the eye or its lenses.

For comparison, a simple form of camera is shown in Fig. 2,where the great similarity of the camera and the eye is strik-ing. In fact, all of the basic elements of the eye are alsoincluded in this simple camera, item for item. until we wonderwhether the camera was actually invented or simply copiedfrom nature's masterpiece of engineering.

First we have the light-tight camera box (E) which corr e-sponds'to the eyeball. In the front end of the camera box isthe glass lens or objective (L) and also the iris or aperature(I) for the control of the light and image sharpness. An image(M) is projected upon the translucent groundglass focusing

Fig. 2. Simple form of camera. Compare it with Fig. I.

PHOTOGRAPHIC LENSES· AND SHUr.rTERS 9

. creen (~) which corresponds to the retina of the eye. At (F)IS an adjustment provided for moving the lens back and forthwhile focusing or until the image attains the greatest sharpnessfor the given distance.

After Iocusing, a light-tight holder containing the sensitizedfilm !S slipped m~o place at (R) and the exposure is made byopening and closing the shutter. The shutter is really a lightvalve that controls the length of time that the image is im-pressed upon the sensitized surface of the film and occupies aplace just ahead of the iris. The camera shutter correspondsto the eyelids in admitting or cutting off the light and con-trollin.g the duration of the exposure. Thus, the modern photo-~raphlc <:amera duplicates the mechanism of the human eye,Ite.m.for rtern, with the single exception of the sensitized film.Millions of years ag:o nature designed the optical system thatwe are US111gtoday 111our most advanced cameras.

The Camera Obscura

The simple camera consisting of the box, lens, and focusingscreen IS known as a camera obscura, a very ancient type wellknown to artists years before the photographic era. It hasbeen described in detail by Leonardo Da Vinci in his sixteenth-century treatise on art and, from the tone of his writing wasnot new or novel in his day. '

T~e c~mera obscura was employed by artists to guide (heirpencils 111making sketches, the artist tracing over the imageon the focusing screen with his pencil or brush thus shorten-ing the time required for making sketches and, at the sametime, improving the accuracy of the sketch: Thus, the cameraobscura became the advance agent of fake artwork and finallythe beginning of a new art=-photography.

While lenses were used in later models of the cameraobscura, the earliest boxes had no lens but were providedwith a simple pinhole of very small diameter that served theIl';1rpose of the lens in producing the image on the screen.Fig. 3 shows a type of advanced camera obscura in which lightntering through the lens (L) is reflected upward by the mir-

ror (M) to form the image (m) on the screen (R). Thehorizontal screen is much easier to work on than the box endtype, and this camera exists even today.

The Pinhole Camera

Centuries ago, it was discovered that an image of a brightoutdoor scene was .formed when a ray of light was allowedto pass through a very small hole and fall on a screen in adarkened room. The image, thus formed, was very faint andwas inverted, but it was accurate in detail and proportion.

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Fig. 4. Showing how the pinholecamera forms an inverted image.

CAMERA OBSCURA PINHOLE CAMERA

Fig. 3. This device was the fore-runner of the present-day camera.

Later the small "pinhole" was applied t? a viewing box muchlike a' camera which is illustrated by Fig. 4. Here. light raysfrom the object (0) pass through the very small pinhole (H)in the metal plate (D) which can be removed or adjusted,and form the image (m) on the screen (R). As the pinholemust be very small in diameter to get even 3;n approximatelysharp image, very little light pass~s through It ~nd the Imageis faint as a result. Exposure time "":Ith a p inho le camerarequires minutes instead of seconds as With the modern cameraequipped with a lens. . f

When a lens is used, the admittance or conductl1~g area orlight is increased, as compared to the pinhole, wI.thout .a!1Yloss of sharpness. A lens has greater light collecting ab!lItythan the pinhole because it embraces a gre~ter bundle of lightrays, thus making it possible to m~ke rapid exposures underadverse light conditions. The admittance area of a lens maybe from 50000 to 100000 times greater than With a pinhole.However v:,ith all its faults, the pinhole has the advantage 111that the 'image it produces is. in relatively sharp focus atThlldistances, so that focusing adjustment IS n?t necessary. epinhole will be discussed at greater length 111Chapter II.

The Photographic EraThe early camera obscura became ~ true photographic cam-

era when a light-sensitive film was 1I1se~ted for permanl~n~l~recording the image formed by the .Ien,. .The use o~ ig tsensitive material for this purpose IS Var~SIY credited toDaguerre and Niepce but, as a matter °h~fact. -oClna_ny '~839)tors were working on the problem at t IS nrn J tothat the true inventor is in doubt.

Louis Jacques Mande Daguerre, commonly known as the

PHOTOGRAPHIC LENSES AND SHUTTERS 11

"father of photography," formed a partnership with Niepcein 1829 and he profited greatly by this alliance. Niepce had aworkable system before he joined Daguerre, but the latterknew little of the basic principles of optical or chemicalprocesses, and made but little progress until Niepce workedwith him. The first lenses used in this project were modifiedspectacle glasses made by Voigtlander, founder of the cameracompany bearing his name.

Present-day photographic processes differ greatly from thosepracticed in Daguerre's day in the matter of detail, but thebasic principles remain unchanged. For example, it was nec-essary to make a separate exposure for every picture by theDaguerre system, but we can now make as many copies of asingle original negative as we may desire, which is a decidedimprovemen t.

Present-day sensitized materials consist of sheets of cellu-loid or glass covered with the activated gelatin coating knownas the emulsion. The active agents in this emulsion are usu-ally salts of silver, such as silver nitrate, silver bromide, silverchloride, or a compound of these salts. The active salts aresuspended in gelatin which also forms a means of holdingthem onto the celluloid or glass base.

When light acts upon the emulsion, a change takes place inthe molecular structure of the silver salts which is not visibledirectly after exposure but which can be made visible by theaction of a liquid known as a developer. The light-affectedilver nitrate, bromide, or chloride is converted into metallic

silver by the action of the developer so that the density of thereduced silver areas is proportional to the intensity of the lightwhich strikes the film during the exposure. Thus, after develop-ment, an accurate image of the subject is reproduced in reverse,with all highlights, shadows, half-tones, and gradations oppo-site to those projected onto the emulsion by the lens.

The image produced is a negative because the highlights ofthe subject are dark in the image and the shadows of thesubject show as light or clear areas on the emulsion. Thiscauses no difficulty, however, because in making a positiveprint from the negative by contact printing or enlarging, thelights and shadows are again reversed and thus correspond tothose of the original subject.

After development, the negative emulsion is still sensitiveto light and must be treated to a further process called fixingby which the remaining active silver salts are removed by asolution of sodium thiosulfate or "hypo." This leaves only themetallic silver image in the emulsion which is not affected byfurther exposure.

In the direct contact printing process, where positives ofthe negative are produced, the negative emulsion is placed incontact with sensitized photographic paper. A strong light

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then transfers the image to the sensitized paper surface. Thisgives a positive contact print in which the highlights and shadowscorrespond to those of the subject, and consequently as manycopies can be made as desired. The paper positive IS no\~ devel-oped and fixed in much the .sa~e manner as the negative. Inmaking enlargements (or pro!ectlon prints) that are to be largerthan the negative, the negative s placed 111an enlarger whichprojects an enlarged image of the negative opto se!lSltlzed paperin the same manner that a lantern slide IS projected onto ascreen. The magnification of the .image. is produced. by a spe-cial enlarger optical system which win be described later.Developing and fixing are as before. .

Thus, photography is .partly a physical and partly a chemicalprocess in which light IS the active agent or f?rm of energythat causes the changes to take place .. A definite quantity oflight energy produces a definite change 111the, emulsion so thatproper regulation of light intensity and. duration IS n~cessary.For this reason a knowledge of the optical principles involvedwi11 aid the amateur in making better pictures.

Elements of Light Theory

The ~ssential element of all photographic.processes is the formof energy that we know. as .light. I.t is through the agency oflight that our eyes perceive impressions of f?rm and color, and'it is also through the same agency that the Image ~f an objectis traced on the sensitized surfaces of pha10gra~hlc films andplates. It is because of this basic relation of \1(Sht t~ lensesand the image that it is nec~ssary to beco.me familiar WIth someof the simple elements of light before .dlscussmg lenses.

Light energy is a Iorrn o~ wave motion that travels forwardalong a perfectly straight line unless deflected by some inter-vening object. The rate of wave :"7!bratlOn,or the frequency ofthe waves, varies through a cons idera.hle range, the ~requencyrate determining the color of the light through the visible band.At the higher frequencies, light approaches the X-rays andgamma rays, while at the lower fr equencies It approaches !henature of electromagnetic waves employed t11t~e t.r~nsmlsslonof radio signals. It should be noted that light IS visible to !heeye only through a very limited band of .frequencles, passmginto the invisible ultra-violet rays at the high end~d into theinvisible infra-red rays at the lower end of the freq,ency band.

Structure of Light

A beam of light may be considered as .made up of m~riadsof small-diameter straight lines or rays wI!h the waves .vlbrat-ing radially along the rays in all plaljles. Lighted spa.ce IS com-pletely permeated with these rays so that, under ordinary con-

13------------------------------~--------.l:'HOTOGRAPHIC LENSES AND SHUTTERS

ditions, there are no dark spaces or interruptions. However, ifpassed through a certain type of prism or filter so that only therays in one plane are allowed to pass, then the light is said to bepolarized and, when in this condition, the rays exhibit manypeculiar properties. *

Perfectly white light, such as sunlight, is a mixture of allcolors and their corresponding freq uencies. Thus, the visibleband of sunlight consists of violet, indigo, blue, green, yellow,orange, and red, any individual color of which can be removedfrom the white light by means of suitable prisms or filters.Black, on the other hand, indicates a total absence of light,hence black and white are the two extremes in the scale oflight. The above prismatic colors can be recombined to formwhite light.

Light energy manifests itself in many ways. It is detected bythe physiological sense of sight, by causing chemical and physi-cal changes in certain substances, or it may be detected bythe electric current created when it strikes a photoelectriccell. LIght ca!l be produced by chemical reactions, by electricalcurrent, by high temperature combustion, and by conversionfrom other forms of energy.

It is to be particularly noted that light travels only along astraight line unless acted upon by certain materials in the pathof the ray that cause deflection. This is a most valuable prop-erty of light in the construction of optical apparatus particularlyin the construction of lenses. '

Diffraction of Light

Light rays are bent out of their course when passing by theedge of an opaque body. This phenomenon is called diffractionand is responsible for lack of sharpness in images formed bythe pinhole camera. When very small diaphragm openingsare used with camera objectives diffraction may impair thequality of the image to some extent.

Reflection of Light

When a light ray strikes the surface of an object it is deflectedor reflected from its original path and it is largely by reflectionthat we are enabled to determine the form of objects by sight.On striking the surface of the object, the ray of light rebounds,so to speak, and enters the eye or the lens of the camera. Thusin Fig. 5, we have a source of light (S) illuminating the surfac~of thescreen (a-b-d), the rays from the light being shown bysolid lines. Then, by reflection, the original incident rays are

*This subject is discussed fully in Filter.< find Th eir Uses: (Little 'I'ech'n+ea lLibrary, No.3).

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14 LITTLE TECHNICAL LIBRARY

reflected back into the eye (E) of the observer who is thenenabled to determine the form and nature of the reflecting sur-face. The reflected rays are shown in dotted lines to distinguishthem from the original incident rays issuing from the lamp orother source of light.

It will be noted that not all of the light emitted by the source(S) falls on the screen, the greateP part of the light being d!ss~-pated uselessly in the surrounding space. More of the bgHtcould be utilized if the screen were made larger so that it couldintercept more rays or if the light source were placed nearerthe screen.

Much depends upon the nature of the reflecting surface as tothe amount and nature of the light reflected. A highly polishedsurface reflects more light than does a dull, rough s?rface ofthe same color; and, further, the polished surface will :eflectthe image of the light source to the eye. If the surface 1S dullor rough, the reflected light will be diff~sed. Iight, more evenlydistributed and softer. Dark-colored diffusing surfaces reflect.less light than lighter colors, until we reach dead. black sur-faces which reflect no light at all, the light being totallyabsorbed.

By painting or coating the face of the screen with varioussubstances we can produce what is known as selective reflec-tion so that the reflected rays will be of different color thanthe ;"'hite incident rays. Thus, by using a coating that absorbsall of the waves except those of red frequency, only red lightwill be reflected to the eye. Thus, paints are substances thatabsorb all of the colors in a beam of white light except thedesired color or color combination which they reflect.

In Fig. 6, the tree is viewed by reflected light at the ob-server's eye (E) where both the form and color of the treecan be distinguished. Light from the source (S) is incident on

E~~~----..: .••. - ....•. -- ..-, .... -.... ---...... ........,-'",-'".•.............

Fig. 5. Reflected light enablesone to see objects about him.

Fig. 6. One can also determinetheir form, size, and color.

PHOTOGRAPHIC LENSES AND SHUTTERS 15

n

R

.---------~~---------sn

n

R

n

Fig. 8. Incident ray is brokenup by a rough surface as shown.

II! foliage and trunk of the tree and is reflected from a multi-1111 (f points such as (P) and (PI) to the eye. The chlorophyll" thl' leaves absorbs all color but the greens, while the barkr th,' trunk absorbs all but the browns so that the eye can de-

1111 a faithful picture from the reflections.11'111" 7 illustrates the basic law of reflection, the angle ofn ction is equal to the angle of incidence. Thus, (S-S) is aI! h d reflecting surface and line (n-n) is a guide line drawn11"'lulicular to the surface. Both the incident ray (1) and

II II fleeted ray (R) make the equal angles (i-i) with the1\,C'lIdicular. If! Fig. 8 we have a tough or matte reflecting

III II r showing how the original incident ray (I) is brokenItI Ilito a number of reflected rays (R), illustrating the nature<111111. d light.

I lit an be controlled by reflection from suitably curved

I . 7. Diagram illustratingIh basic law of reflection.

M

R

Fig. 10. Application of mirroras it is used in a reflex ce mera.

9. Reflection from theof 0 porabolic mirror.

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surfaces as from the parabolic mirror (M) of Fig. 9 wherelight from the source (S) is reflected in the form of a bundleof parallel rays (R) from the front of the mirror. By alteringthe curve, the bundle of reflected rays can be made to divergeor converge as may be required. Such mirrors are commonlyemployed in enlargers to increase the ligh t intensity or sothat all of the light dev~oped by the source is applied usefully.

Reflection always causes a loss of light because all surfaces,no matter how highly polished, absorb some of the incidentlight. Highly polished silver reflects from 98 to 99 per centof the incident light with aluminum a close second, whilepolished cast iron may only reflect from 10 to 15 per cent ofthe light. Glass is a highly efficient reflecting surface fre-quently employed. In Fig. 10 we have a common applicationof a mirror as used in a reflex camera. A beam of light fromthe lens (L) is reflected by and turned through an angle of90 degrees by the mirror (M), so that the image falls uponthe viewing screen (R).

Some reflection takes place wherever a ray of light en-counters a change of density. Thus, as water is more densethan air, a ray from the air suffers reflection on striking thesurface of water. Even when passing from the air into aheavier gas, reflection takes place at the boundary of the twofluids. .

Refraction of Light

When a ray of light pa~~es through a transparent body, ora body that freely trans;.;v'ts light, it is subject to a deviationcalled refraction, providing that the density of the body isgreater or less than the density of air. Thus, when a ray in

n R"/

/., /

'''''//

/~

/

s

T

Fig. 1 I. Diagram illustratingthe basic law of refraction.

Fig. 12. Path of ray througha pilrallel-sided sheet of glass.

PHOTOGRAPHIC LENSES AND SHUTTERS 17

air enters a sheet of glass at an angle (see Fig. 11), the. ray isbent out of the original direction by an amount that IS gov-erned by the nature of the glass. and the frequency of thelight. As this principle is the basis of lens theory, It shouldbe carefully considered. The acute angle formed by the refractedray and the perpendicular is called the angle of r.ef~actlon ofthe transparent medium. The !lleasure of deviation of aray when passing from ope medium to another IS called theindex of refraction. In FIg. 11 the line (S-S) IS ~he bo~ndarybetween the air and the denser transparent medium WIth. th.enormal (n-n) drawn perpendicular to the surface. T~e mCI-dent ray (I), in air, strikes the surface at (0) and IS bentsharply to the left (or toward the normal). S? that t~e angleof refraction (r) is less than the angle of I~cldence (I). Theincident ray is (1-0). The refracted ray IS (O-T): If themedium below the line were less dense than. the. air .above,then the refracted ray would bend in the opposite direction.

But, as before, reflection also takes place from the surface,hence the reflected ray (O-R) makes the. same angle WIththe normal as the incident ray. On entering a tra.nsparentmedium of different density than the original medium, theincident ray is broken up into two ra:ys, one a ~efracted rayand the other a reflected ray. There IS necess~n~y a loss <;>flight in the refracted ray because part of the incident ray ISreflected while varying remaining amounts of the transmittedor refracted, ray are lost by absorption in the glass and byinternal reflections. .

When the refracted ray emerges from the dense mediumand re-enters the air, it is again refracted or bent into .theopposite direction so that if the sides of the dense mediumare parallel the final refracted ray in air is parallel to the

I TT a

Fig. 13. Light is refracted bya glass prism as shown here.

Fig. 14. A prism having spheri-cal sides refracts light rays.

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. ·T

Fig. 17. Prisms apex-to-apexdlv 'ge the refracted rays.

Fig. 18. Double spherical con.cave lens also diverges the rays.

T

I Ii is a simple explanation of the double convex spherical1••1 of Fig. 16 so frequently employed in optical work and

lilli' or more elements in a compound lens. An image ishu nu-d at the focal point or focal plane (F), but as this lensIIIIdlle'('s some distortion of the image when used alone, it is.1.111111fund in modern cameras.

III Fig. 17 the prisms (A) and (B) are placed apex-to-apex,

\lcll forming the simple equivalent of the biconcave lens ofI . 18. This lens diverges the rays and therefore has no

" II fn al point at which an image is formed, hence we haveulv III' virtual focus (F) which is simply the point at which

Ilc fll'! jected rays meet on the axis.

I I' rsion

1111passing through a glass prism, the white incident light11I1I/{('11up into a band of various colors known as the

trum, This reaction, dispersion, separates the various col-I IIf lice spectrum according to their frequencies, the prism or

I II Hiving the greatest deviation or refraction to the high-II '1111II Y colors. Violet, indigo, and blue are given moreI 11,1Iion than yellow or red and are therefore separated fromII I"wer frequencies. In Fig. 19A it will be seen that the"' Id"11I light ray (X) is broken up into the spectrum, or color

lid, at the right where (V) is violet, (I) is indigo, (B) isIII" CO) is green, (Y) is yellow, (0) is orange, and (R) is red.Ii prism is of flint optical glass so commonly used in1011.11lori s. .III "jl-(. 19B we have the same reaction except that the prism1111case is of crown optical glass. It will be seen that the

1111HI.IHS shows the greatest refraction of light rays, andI II opens up the color spacing in the spectrum so that

Fig. 15. Two prisms base-to-base converge refracted rays to point T.

incident ray. This condition is shown by Fig. 12 where theincident ray (I) enters the parallel-sided sheet of glass (0),and is refracted in the direction (o-b), and then emerges intothe air on the opposite side along (b-T') or parallel to theincident ray (1-0). . h"

In Fig 13 is a glass prism employed for refracting t e mCI-dent ray (1-0) into the final direction (b-T'). Unfor~un.atelymore than refraction takes place with the prism, for an incidentray of white light will be broken up into a band of prismaticcolors, which is not always deslrab~e. '"

A prism having spheri~al srdes, instead of straight Sides, ISshown by Fig. 14. This IS ~alf of a. biconvex lens and sh<;Jwsthat it refracts the ray of light until It mt~rsects the .optlcalaxis (a-a) at the point (T) or focal pomt-approxm:atelywhere the image is formed with a spherical lens. In Fig. 15we have two prisms, (A) and (B)., placed base-to-base forconverging the refracted beams to point (T) on the optical axis.

F

I

I

Fig. 16. Double convex spherical lens is able to form a real image.

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xx

Fig. 19. Showing dispersion of incident ray (XI by prisms offlint and crown glass. Note higher refraction of flint glass.

the colors in the band are wider and more definitely separated.Or we might say that flint glass has high refraction and highdispersion, while crown glass has a lower refraction and lowerdispersion.

Light Intensity or Illumination (The brightness or intensity of the available light is of the

greatest importance in photography. The more intense thelight, the more rapid will be the exposure because the effectof light on sensitized materials is in almost direct proportionto its intensity.

The most common unit for the measurement of light inthis country is the foot-candle, or the intensity of light on awhite surface one foot square illuminated by a standard candleplaced at a distance of one foot from the surface. This stand-ard of illumination is rather small. In bright sunlight, theintensity may reach 800 foot-candles or more at high altitudeswhere the air is free from moisture. In addition to beingaffected b)l weather conditions, the intensity of outdoor naturallight also varies with the hour of the day and the seasons, sothat the light must be accurately determined for a properexposure.

If a given amount of light of given intensity is spread overan increased area, then the intensity will decrease in pro-portion to the area. In the same way, if an equal amount oflight is concentrated on a smaller surface, then the intensityWill be increased. In cameras, the intensity of light on theconstant area of the film is varied by controlling the amountof light admitted through the iris and lens.

PHOTOGRAPHIC LENSES AND SHUTTERS 21Square Law.

IIh l co~st~nt light quantity, the intensity of the light on1111 t vanes inversely as the square of the distance between

urtnce and the source of light. Thus, if a lamp that ist II from a surface is moved out to four feet then the

II Ily will become: '(AX 2) 4

- _ - = I;' as bright.N X 4) 16

I hi is illustrated by Fig. 20 where the lamp (8) illuminatesIi III face (A) at a distance (D) from the source. TheIII quare feet of this surface are each illuminated to one

1 1.III.dle, for .example. Now, if a surface (B) is erected a;), twice the distance of (D) from the source within the same

B

Ig. 20. Diagram illustrating the inverse square law above.

L

:10 0

~L

A B

rig. 21. Principle of inverse square law applied to a camera.

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22 LITTLE TECHNICAL LIBRARY

CHAPTER II

SIMPLE LENSES

IH) M a photographic standpoint, a lens is a transparentI/Ilcly designed for creating a true image of an object by

II III iuciple of refraction. This lens may consist of a singleI IIIc'lIl or an assembly or group of simple refractive elements,I I' uding upon the purpose for which the lens is required.

II order for a structure to be called a lens, it is necessaryII t Ihe opposite sides should not be parallel. For example,

tch crystal is not necessarily a lens if the thickness isII nmc throughout, although one surface is concave and theIII I is convex. It will be noted in examining the simple formsr phcrical lenses that these sectional views indicate that the

I II i ither thicker at its periphery than at the center, orIe versa. It will also be noted that some' degree of spherical1I11',Ilme has been imparted to one or both sides of eachI mr-nt.

a rule, a practical camera lens is built up of two ortuu r simple lenses acting in combination with one another,III III order to simplify matters in this study, we will first take

U Ihe properties of the simple elements and progress fromIII point. See Fig. 22.

II I 1 Classification

light cone, this square surface will contain 16 square feetinstead of four square feet, As a result, the illuminationon each square foot of (B) will be only one-quarter as greator ~ foot-candle. ....

In Fig. 21 the same principle IS app lied to a camerahaving the constant iris opening (1) and the. lens. (L) .. If theimage is brought to focus oq.film (A), t~e light I?tenslty Willbe four times as great as on (B) at twice the distance fromthe lens.

(

I lording to their action on a ray or beam of light, allII I may be divided into two principal groups:

I. Converging type lenses which converge the light rays and concen-trate them on a point. This class is also known as positive lenses.Plverj~ing or negative lenses that diverge the light rays or spreadthem Into a cone of increasing diameter.

urfaces of these lenses are either spherical in shape orI v c lose to spherical. In short, the curvature of the lensI I an generally be defined by a radius except for the highlyflllc d types which may show some slight departure from

,Ie II' or sectors of spheres. They are as nearly transparentplIssible so that the light will pass through them with a

lnhuum of resistance and the surfaces are accurately polishedt h It the light rays will be deflected to exactly the required

111It.

I vI'ry simple lens causes certain errors and distortions, soII hnple lens is seldom used alone. A second or a third

23

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24 LITTLE TECHNICAL LIBRARY PHOTOGRAPHIC LENSES AND SHUTTERS 25

~auses a ray of incident light to diverge, spreading the lightinstead of conce~tratmg it on a focal point. Being oppositeto a convex lens, It can be used to correct or modify the actionof a convex lens in the elimination of certain aberrations ordistortions. 0

Concavo-Convex Lenses

There are some lenses which have one concave and one con-vex ~urface. In cases where the concavity predominates thelens IS known as a concave or negative meniscus. Where theconvexity predominates the lens will form an image and isknown as a convex or positive meniscus (see Fig. 23).

Figure 24A shows the well-known Wollaston meniscus lensso extensively used in box cameras. When well stopped downto about f 11 with the aperture (1), it gives quite remarkableresults for a single lens, which accounts for its extended use insimple box cameras. Since the convex face is the most deeplycurved, the convex factor rules, and the lens is therefore apositrve type that brings the lig:ht ray~ to .a focus at the point(F). Wh!l7 It ca.n be used In either direction, there is a slightadvantage In having the concave face forward as indicated, andI~ this case the lens is comparatively free from many aberra-tions and even partly overcomes astigmatism. It is too slow forall-round use, however, when stopped down to the properpoint.

In Fig, 24B is shown a special achromatic type of Wollas-ton meniscus partly corrected for chromatic aberration Twoelements, on~ of flint and one of crown glass, are ce~entedtogether. .This type of lens is known as a meniscus achromat.

There IS another type of lens structure which is used in

A B A B

D ECBA

Fig. 23. A, positive menis-cus; 8, negative meniscus.

Fig. 24. A, Wollaston meniscus used inbox cameras; 8, achromatic type of same.

POSITIVE CONVERGENT NEGATIVE DIVERGENT

Fig. 22. Simple spherical lenses: A, plano.conv~x; 8, biconvex;C, unsymmetrical biconvex; D, plano-concave; E, biconcave; F, un-symmetrical biconcave. Their characteristics are explained below.

lens is used to correct the errors in adjacent. glasses, and inthis way a very complex structure may be built up. "--.Positive Convergent Lenses

These lenses, which converge the rays of light to forrn areal image of an object, have at least one cOl;vex or bulgingsurface. They can form images on a screen without as.slstanceof other lenses, but this image is always more or less distorted.They are three in number as follows: 0

L Plano- convex lens. This lens has one sphencal convex face anda fiat face.

2. Biconvex or double-convex lens. This lens, often used as a mag-nifying glass, has two spherical convex faces and IS probably themost commonly used lens. 0

3. Unsymmetrical biconve,:, lens. There are two sphencal convexfaces with curves of different radii,

The above lenses are all thicker in the center than at theedges a fact that leads to an uneven distribution of light anduneql{al angles of refraction at various points on the lens.

Negative Divergent LensesThese lenses diverge the light rays and can have only a vir-

tual image to the eye. They are most co.mmonly employed 111

combination with other lenses in assemblmg a compound lens.Three simple forms are as follows:

1. Plano-concave lens. This lens is flat on one side and concave onthe other.

2. Biconcave lens. This lens has two spherical concave surfaces,3. UnsymmetricaI bicon~ave lens ... There are two spherical concave

faces with curves of different radii.These lenses are all thicker at the edges than at the center.Every negative lens has at least one concave side which

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26 LITTLE TECHNICAL-LIBRARY

lenses for certain types of instruments but no~ to any extendin photography. These are known as cylindrical lenses, .an

derived in the same way as the sphencal lenses except~h~t they are segments of a cylinder instead of. segments. °ia sphere. Cylindrical curves are combiried with sphericacurves in spectacle lenses and have as their function the cor-rection of astigmatism in the eye. . . h f

The function of a lens is to recomb me the rays of lig t. h°!,a point source to a point in the plane of the film. A Pl1 0 ~is unable to do this in the same way as. a lens. If.a ho e 0about a quarter of an inch in diameter IS. punched m a card,the rays of light from any point in the Image diverge so asto occupy the entire diameter of the hole as they P!,ss throughthe card. The rays continue to diverge after p.assmg throug~the card and reach their maximum dlverg~nce m the plane~the film. Therefore, in the case of a pinhole apert~reimages of point sources will ?e resolved as dls~s. of l1&,ht ofgreater magnitude than the pinhole through >yhlc~ the Imageis projected, even if the point source IS at .mfimty and thesensitized film is only a few inches from the pinhole. . f

The eye is able to resolve lines separated by.a distance 011100 of an inch at a viewing distance of 10 inches. Linescloser together than this appear as a ~mgle ~tructure. In thecase of a pinhole the diameter of the pinhole IS always smallerthan the subject point resolved on .the film.. Therefore, a p.m-hole 1/100 of an inch in diameter Will have Circles of. confusion

rea tel' than 1/100 of an inch in diameter and the Image will~ppear un sharp as the overlapping of these circles of confusionwill give a fu~zy appearance to the picture. If the pinholewere 1/400 of an inch in diameter and the film were ~loseenough so that the spread of the rays were not of a magnitudeof over 4 times, the image would appear sharp when Viewedat 10 inches. It will readily be understood that the amountof light which would pass through a pinhole of .1/~00 of aninch would be extremely slight, particularly as this light mustbe spread over the entire plate.

Unlike a lens, the pinhole has no focal length but. may .beplaced at any distance from the film. On a ~lm of given sizethe nearer the pinhole the Wider the angle included, and themore distant the film the narrower the angle and the greaterthe magnification. .'

The pinhole can be used to advantage I~ extreme wide-angle work at times. However, a~ the film IS brought closerto the pinhole, the distance from pinhole to film .along the cen-tral axis becomes much shorter, while t~e distance to t~ecorners of the film becomes .muc~ gr~ater in proportIOn. Thiswill result in unevenness of Illemination, the edges of the filmbeing underexposed while the center may be overexposed,and this same condition will hold true even in the case of a

PHOTOGRAPHIC LENSES AND SHUTTERS 27

lens. In the Extreme Wide-Angle Hypergon, the lens is pro-vided with a rotating fan which is hinged below the lens.I )U!'ing about two-thirds of the exposure the fan is placednver the lens and rotated by means of a blast of air providedby a bulb. This fan prevents light from entering the center1)[ the lens but allows progressively more light to entertowards the periphery. The last third of the exposure is madewith the fan out of the way. Only in this way is it possible toK t evenly illuminated negatives in the case of very wide-anglework. Except for the unsharpness and lack of speed, the pin-1\ le would be an ideal type of lens. Because of these twoinherent faults it has, however, been relegated to the position

f an interesting curiosity.We have seen that a ray of light is bent upon its entrance

into a plate of glass and again upon its exit, the entrance andexit rays being parallel but not in the same plane. It is upon thisproperty of the refraction of light that photographic objectivesare dependent. In order that the bending be uniform and with-out distortion, it is necessary that the elements of the objectivebe developed from segments of spheres, as pointed out before.Since light is normally composed of waves of different fre-quencies, and since these various wave motions are affecteddifferently by the transparent bodies through which they mustpass, a number of anomalies arise in the image. In photographya lens may be considered to be a transparent colorless structurewhich has been polished with one plane and one sphericallyconcave or convex surface, or with both surfaces sphericallyoncave or convex.

In the early days of photography only two materials wereavailable for this purpose. These were crown and flint glass.

rown glass was composed of lime, potash, alumina (aluminumxide occurring as native corundum), and silica; whereas the

flint glass contained lead oxide in place of the alumina. Thissubstitution resulted in a product which was more highlyrefractive. Simple lenses composed of either of these materialsgave an image circular in outline which was extremely dis-torted near the margin but which became increasingly sharpertoward the center. Consequently, it was necessary to useonly the center portion of the image, and since the periphery

f the lens gave even a distorted fuzzy picture in the center,the marginal rays of the lens were eliminated by stoppingdown the diaphragm or by cutting off the thinner portion ofthe lens. See Fig. 25. By cutting down the image to the centralportion of the field and diaphrag ming off the marginal rays,it was possible to get an image of satisfactory sharpness forvisual observation.

However, this did not prove to be particularly good forphotographic purposes, since the refraction of the differentwavelengths varied considerably. The blue rays were refracted

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28 LITTLE TECHNICAL LIBRARY

most and focused closer to the lens, while the red rays wererefracted the least and focused Iarthes" from the lens.The eye focuses in the yellow-green region of ~he spectr';lmor about in the center of the refracted rays, while the activechemical rays to which the photographic film is most sensitivelie considerably in front of this visual point. Consequently t~eimage on the photographic plate had a tendency to be quitesoft or fuzzy, since the blue 2I'I1d.ultra-violet rays crossed infront of the plane focused on (Fig. 26).

One of the first problems, therefore, was to try to bring allof the light rays passing through the lens to. the sa!De focalpoint in the image plane. This could be done with a fal.r degreeof success by combining tw~ lenses made from matenals hav-ing different refractive arid disperSive propern.es. Such an objec-live is called achromatic, or without color fnnges. These firstachromatic lenses were developed about 1850 and were a greatimprovement over the simple lenses in use before that time,since they allowed a greater amount of ~orrectlOn and .at thesame time did not require as small a diaphragm openmg ashad been the case for somewhat similar results with a simplelens. fl'However. none of the lenses composed of crown and intglass could yield critically sharp images over a very wide angle,and they required so much diaphragming that they were und~lyslow. Abbe introduced the use of a natural mineral, fluorite,into microscope objectives in 1884, which allowed of grea.terrefinement in the image. About 1890, through the collaborationof Ernest Abbe and Otto Schott, a new series of optical glasseswere developed at Jena. Although glass was known. to theancients, and crown and flint glasses were common ar ticles ofcommerce and were highly developed from the standpomt offreedom from color and strain, the relatively small use of glass

r-_1 -'">-

~

__ »> I I

~~: ___ I I

--___ I I

-- l I_~ •••.• 1.•.. ,'.

Fig. 25. By using a diaphragm to cut off marginal rays, asharper image is obtained with a simple uncorrected lens.

PHOTOGRAPHIC LENSES AND SHUTTERS 29hy the optical industry had resulted in a lack of enthu~iasm onlhc part of the glass industry in attempting research on glassor this purpose. By 1870 the need for special glasses for optical

Jl,urpo~es had become so acute that through a grant from the1 russian Government and technical aid from the Zeiss Workstile problem wa.s attacked with the object of developing specialR.asses for. optical purposes. The result of this research washighly gra~lfymg arid a number of new materials were developedWith v.ery. lI1.terestmg properties.I .~lhlle It IS possible to fuse various materials which yield11g .Y transparent fluxes .. the resulting products may havephysical characte~lstJcs which are highly undesirable. For ex-ample, the material may be very soft and thus be subject to~bra~J(:)J1,or the m~tenal may be easily affected by atmosphericconditions so that in a very short time the surface may becometarnished or frosted over, due to its reaction with air and mois-ture. A good glass should be hard enough so that it will notbecome scratched by normal handling and should be inert chern-ically, At th~ same t.ime the materials must have the ability torefract and disperse Itgi: t rays in varying degrees. The researchwork at J ena resulted JJ1a number of new glasses which had!"lotonly these desirable qualities of transparency, hardness, andIl"!ertness, but also. gave us .new optical properties which werehitherto unknown in the optical industry. .

It might. be mentioned that the number of elements in ~photographic obJ~ctlv.e IS no. criterion of its excellence, sincethree elements Will yield an Image of high quality as seen intGheTriotar or the Cooke lens which is used extensively on the. raflex came.ra for high-speed work as well as color' and itIS quite possible to suppose that some of those lenses whichhave SIX, eight, or. ten elements may have been designed toovercome patent difficulties rather than optical problems. It

B G R

F!g. 26. Showing how a simple lens brings the blue and ultra-Violet rays (B) to a focus in front of the green and red rays.

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30 LITTLE TECHNICAL LIBRARY

must also be understood that it is impossible to.overcome theinherent aberrations in a lens by merely combining two ormore lenses made of the same material.

At the time when most photographic objectives were designed-and there have been very few major changes in photographicobjectives since 1900-two factors were of outstanding impor-tance. One was the fact that the photographic plate was sensi-tive only to the blue region (Ji the spectrum and fell off veryrapidly in sensitivity toward the red end, while the human eyehad its greatest sensitivity in the yellow-green region of thespectrum, and the greatest necessity was to develop objectiveswhich satisfied these requirements. However, within the lastdecade the sensitivity of the negative material has been increasedto such a degree that when used to their extremes many photo-graphic objectives are not corrected sufficiently to bring boththe ultra-violet and the infra-red to the same focus as theyellow-green.

About 1920 interest was aroused in photography using theultra-violet region of the spectrum. Very few optical glassestransmitted light below about 3500 Angstrom units. This neces-sitated the use of new materials, the most satisfactory of whichwas natural quartz. At first lenses were ground from solidquartz crystals. During recent years, manufacturing methodsfor the production of fused quartz have advanced rapidly. Thefirst fused quartz which was produced could scarcely have beencalled transparent, since the number of bubbles was so greatthat it was fit only for laboratory utensils. At the present time,however, fused quartz which is practically flawless may beobtained. Photographic objectives which are eminently satis-factory for special purposes have been made from this material.

Within the last few years, considerable publicity has beengiven to moulded lenses made of synthetic plastics. At thepresent time the plastic lens has not developed to any greatextent due to the limited number of materials available.

Focal Point and Focal Length

The principal focal point of a positive lens is commonlyunderstood to be the point at which a sharp image is formedwhen the object is at a great or infinite distance from the lensso that the incident rays from the object are parallel. In orderto locate the focal plane or focal point of the lens itself, thedimension known as the focal length or equivalent focal lengthis employed. Technically, the focal length (L) is the dis-tance from the node of emission in the lens to the focal point;but to simplify matters in this first elementary discussion, thedistance will be measured from the front face or center of thelens as indicated in the drawings referred to. The nodes ofadmission and emission will be discllssed later.

PHOTOGRAPHIC LENSES AND SHUTTERS 31..

f ~

a ~R •••..... F a

~,Fig. 27. Focal length of a plano-convex lens is shown here.

R

R

f<-- L

Fig. 28. The biconvex lens has a shorter focal length.

RIF a

R2 JL

Fig. 29. Focal length of an unsymmetrical biconvex lens.

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32 LITTLE TECHNICAL LIBRARY

As a rough estimate, the focal length (L) in the figures isgiven in terms of the radius (R) which is only approximatelytrue, as the exact focal length is also affected by the refractiveindex of the glass employed. All measurements are taken on orparallel to the optical axis (a-a) on which the light rays areassumed to converge. All parts of the lens system are sym-metrical about the optical axis which, however, is not alwaysexactly coincident with the g~metrical axis of the lens.

In the case of the plano-convex lens in Fig. 27, the focallength (L) is measured from the front face of the lens to thefocal point (F), and is approximately twice the radius (R).This is fairly close to the average case when flint optical glassis used. The focal length of the biconvex lens, Fig. 28. ismeasured from the center of the lens to (F) and is approxr-mately equal to the radius (R). Thus, the focal length of abiconvex lens is half that of a plano-convex lens having thesame radius of curvature. The biconvex lens is of more inter-est to us at present than the other lenses for the reason thatits action is similar to that of a complete camera lens, andit is therefore much used in elementary discussions of lensesin place of the more complex objective. It will be found used,from time to time, in this book for illustrative diagrams where'the complexity of the actual lens would obscure the explanation:

Figure 29 is a "crossed" or unsymmetricall?iconvex lens withthe two radii (Rl) and (R2) for the rear and front facesrespectively. This lens is frequently employed for the partialcorrection of certain aberrations or distortions that occurwith equal radii. The calculation for the focal length (L) ofthe unsymmetrical lens is more complex than the others or t -

2 X Rl X R2L =-----

Rl + R2

It should be noted that glass having a high degree of refrac-tion or a high refractive index. bends the rays more sharplythrough a greater angle than does glass having a low refrac-tive index, hence the high index glass gives a shorter focallength with a given radius than other glass. In manufacturethis factor is controlled by testing the refractive index ofevery piece used, before the lens is ground, using only theproper glass.

Object Distance and Focal Point

When the focal length of a lens is specified, the lens is as-sumed to be focused at infinity or on an object at a greatdistance from the lens. At any other object distance the actualfocal length or bellows extension is longer. Focused at infinity,

PHOTOGRAPHIC LENSES AND SHUTTERS

I~'-------~F-------+1.---

o----------------~--~~o--------------------~_o----------------------~--~0------------------~----~--1

33

F

Fig. 30A. An object at infinity is brought to fa ocus at point F.

50'--------~--

o

increases.Fig. 30B. With 50-foot object distance, image distance

Fig. 30e. Image distance increases b'more as 0 (ect approaches lens.

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84 LITTLE' TECHNICAL LIBRARY

the light rays entering the lens are parallel S? that the ben.d-ing due to refraction is less than at shorter distances. In FIg.30A the rays (0) coming from the object at infinity are paralleland come to focus at (F). Let us assume that in this case therated focal length under these standard conditions is S inches.In Fig. 30B the object distance is reduced from infinity to 50feet which means that the distance from the lens to the focalpoi~t has now increased to S.W inches. I~ Fig. 30C the objectdistance is further reduced to 6 feet while the lens to focalpoint distance is now increased to 5.37 inches. .

This shows why the changing angle of th<: light rays. atdifferent object distances also causes changes in the locationof the focal point and makes focusing a necessary operationfor the accurate camera. Some cameras are made with a "fixedfocus" and require no adjustment within certain limits, but sucharrangements are deficient in other respects and are usuallyconfined to the less expensive types.

Approximate Infinity

It is of course impossible in practice to focus upon anobject ~t an infinite distance, hence the question often arises,"how can I approximate infinity without error?". In mostcases, it will be perfectly safe to assume that any point tart~erthan one-quarter mile from the camera can be taken as infinity

3

2

c

Fig. 31. Focal length of the lens has a direct effect on theimage size as shown in this diagram and explained in the text.

with cameras having a focal length of 6 inches or less. Onemile will be safer with cameras of greater focal length. Forany lens, approximate infinity will be the focal length squaredmultiplied by the reciprocal of the desired circle of confusionas explained further in Chapter III.

PHOTOGRAPHIC LENSES AND SHUTTERS 35

Image Size

The size of the image is proportional to the focal length ofthe lens, hence a 6-inch lens will show a much larger image of.rn object than a 2-inch lens from the same viewpoint. Themall size of the image is one of ·the principal drawbacks of the

miniature camera, and explains why long focal length lensesuid telephoto lenses are supplied as auxiliary lenses for theuiiniatur es.

Figure 31 shows clearly why focal length has a direct effectupon image size, the image in this case being an arrow. Witht he shortest focal length (A) the image size is also the shortestas at (1). Increasing the focal length to (B) gives the largerimage (2), while with the extreme focal length (C) the verylarge image (3) is obtained. This is easily solved by the methodIIi" proportional triangles with the view angle (a) of constant\ .rlue. This relationship between focal length and image sizei an important consideration in the selection of a lens for theI nrnera or enlarger.

onjugate Foci

There is a fixed relation between the object-to-lens distanceund the image-to-lens distance, as suggested in the precedingparagraphs. One distance is said to be the conjugate of the

o

A _.La -J1<

Fig. 32. There is a fixed relation between object distance (AIand image distance (8), one being the conjugate of the other.

nther, and the two distances are interchangeable. This is illus-t rated by Fig. 32 where the greater distance (A) is the majorconjugate and the shorter distance (B) is the minor conjugate.l'he object size (0) bears the same relation to the image size(I) that the major conjugate bears to the minor conjugate.

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36. ~L~IT~T~L~E~T~E~C~H~N~I~CA~L~L=I=B~R~A~R_Y~

This can also be expressed by the simple formula:

size of imalle Image distance I B_-=------ or - = -size of object object distance 0 A

Thus if the minor conjugate or ~mage. distance (B~ is 100.inc~esand the major conjugate or object distance (A) IS ~O 1(0) e.s,then the ratio of the image si!e (1) to the object ?Ize . ISalso 10 to 100 or 1 to 10. The two conjugate ¥OCi are inter-changeable, it being possible to subs~ltute th: Image. for th.eobject at either end. Thus if the ratio of. object. to Image IS10, then the linear reduction will be l/~O with the image at theend of the minor conjugate. Again, If the .ratio of image toobject is 10 then the linear enlargement Will be 10 w.lth theimage at the end of the major conjugate. These ratios areexpressed as (r) and (R) respectively, .where \r~ denotes thenumber of times the linear size of the Image dl,vldes mto. thatof the object and (R) denotes the number of times the linearsize of the object divides into that of the image. These ratiosmay be expressed as follows:

size of object size of imall,er = size of imalle (linear); R = size of object (linear)

Accordingly (r) is the number of times of linear reduction and(R) the nur:"ber of times of linear enlarg;ement. h f I

Where the ratio of object to Image size (r) and t ~ ocalength of the lens (f). are known, one can .eas~ly determine themajor and minor conjugates by the following.

Major conjugate = f + (f X r)

fMinor conjullate = f +-

rf

Total distance = 2f + fr +-r

Let us suppose that with a 6-inch lens we wish to photographa Hl-irich object so that the image will be exactly 1I1~ or 1. h hi h Here the ratio (r ) is 10, and the object distanceIl~ajo:gc~njugate), bellows draw (minor conjugate), and totaldistance from object to ground glass would be calculated asfollows:

Major conjugate = 6 + (6 X 10) = 66 inches

6Minor conjullate = 6 + - = 6.6 inches

106

Total distance = 12 + 60 + - = 72.6 Inches10

PHOTOGRAPHIC LENSES AND SHUTTERS 37

The conjugate foci and focal length of the lens are dependentone on the other as we have just seen. This relationship canbe expressed by another basic and simple formula from whichcan be derived a number of other formulas which are oftenuseful to the camera owner for calculating an unknown factor.We will not attempt here to delve into the mathematics bywhich these formulas are evolved. Some of the equivalentvalues are obtained by constructing a diagram showing theprinciples of image formation and considering the proportionalparts of similar triangles. Among the various factors broughtinto use are the values (v-f) and (u-f) which represent the dif-ference between each of the conjugate foci and the focal lengthof the lens. They are known as the extra-focal distances andserve to simplify calculations. It can be demonstrated that theextra-focal object distance is equal to the focal length dividedby the ratio of image to object, and the extra-focal image dis-tance is equal to the focal length of the lens multiplied by theratio of image to object. Here, then, are a number of workingformulas which will be found useful to the amateur:

Let: f = Focal lengrh of the lens.u =Distance of object from the lens.v = Distance of Image from the lens (bellows);D =Total distance from object to Image. The nodal space Is

disregarded as In most cases It Is very small comparedwtrh D.

R =Number of times of enlargement (linear size of Imagedivided by linear size of object).

r =Number of times of reduction (linear size of objectdivided by linear size of Image),

For extveme accuracy the distances (u) and (v) are calculatedfrom the nodes of admission and emission respectively. How-rver, for practical purposes we can consider them as measuredfrom the surfaces of a single lens or the diaphragm of a com-pound lens. The basic formula mentioned above is:

I I I-= -+-f u v

(I' m which are derived other formulas for calculating objectcli tance, image distance, scale of enlargement or reduction,(t cal length, etc., when certain of the factors are known:

u xvf---;

u+v

fXvU=--;

v+-f

fXuv =--

u-f

fR- --;

u -·f

v-fR= --

f

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38 LITTLE TECHNICAL LIBRARY

u u-fr= -; r= r=

v v-f

uXR v u vXrf=--; f=--; f~ f~

R+I R+I r+1 r+1

f=DXR

(R+1)'

DXr

(r+1)'f=

vU=-;

R

fu=-+f: u=vXr; u=(r+1)Xf

R

v=uXRu f

v~(f XR)+f ; v= -; v= - +f ;r

RXD Dv= --; v=--

R+I r+1

1D=fX (R+ -+2);

R

1D=fX(r+-+2)

r

D=vX(R+1)

RD~vX(r+1)

uX(r+l)D=uX(R+l): D= ---

r

uFrom the formula (r =-) it will be seen that when the

v .object distance and image distance a~e equal, the image Sizeis the same as the object size. According t? the basic formulaboth distances will be equal to 2 X f or .twlce the focal. length.

. Thus if one desires to photograph an object the same size witha 6-i~ch lens the object will be 12 inches from the lens and t~eimage will b'e located the same distance behind the lens. T~l1Sexplains the popularity of cameras With groundglass focusingbacks and double-extension bellows.

PHOTOGRAPHIC LENSES AND SHUTTERS 39

Back Focus

This is the distance measured from the rear face of the lensto the film or focusing screen surface and is a unit that is seldomused. The original reasons for the employment of the backfocal length were to afford a definite point on the lens fromwhich to measure the focal length, and to give an idea of theminimum distance required between the camera lens andgroundglass screen.

Optical Center

For the measurement of the focal length and other distancesfrom the lens, it is frequently the practice to make measure-ments from the optical center of the lens, assuming that thecenter is the point of light concentration. This is only approxi-mate at best and subject to some error, but at least it gives\15 a definite point from which we can repeat measurementswhen required.

In the case of the biconvex lens of Fig. 33, the optical center(0) is also the geometrical center, being halfway between thetwo surfaces on the optical axis. The focal length (L) is meas-ured from (0) to the focal point (F). The point (0) is deter-mined by drawing two parallel lines from the centers of curva-ture (C) until they intersect the surfaces at (rn-rn) , and con-necting these intersections by the line (rn-rn). The intersectionof the latter line with the principal axis at (0) will give the( ptical cen ter.

In the case of a plano-convex lens, Fig. 34, the optical center(0) lies at the intersection of the convex surface with theoptical centerline, and measurements (L) to the focal point (F)are made from this point as shown.

The center of an unsymmetrical lens, Fig. 35, is determinedhy the same method as shown in Fig. 33, using the centers ofcurvature (C) and drawing the diagonal (m-rn) so that it inter-ccts the optical center at (0). This shows that the optical

(' nter always lies closest to the most deeply curved surface ofthe positive lenses-even on the curve in the case of the plano-ronvex lens.

Lens Nodes or Gauss Points

A more accurate basis for lens measurement is provided bythe nodes or Gauss points determined by estimation or actualIt'Rt. These are imaginary points that are invariable under allrunditions and are therefore fixed points of reference that havebeen adopted by opticians.

Except in cases where the points may be coincident, there

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40 LITTLE TECHNICAL LIBRARYPHOTOGRAPHIC LENSES AND SHUTTERS 41

lire two nodes. One is the node of admission concerned withthe rays entering the lens. The second node is the node ofmission concerned with the light rays leaving the lens and

flflSsing to the picture plane or focal point. By definition, theocal length of a lens is the distance from the node of emission

t the focal point. The object distance is the distance from thebject to the node of admission. With certain lens designs,

the two nodes may occur at the same point or be coincidentwhile in other lenses one or both nodes may lie entirely outsideof the body of the lens.

In the plano-convex lens of Fig. 36, the node of admission isat (a) while the node of emission is (e), both of which arenear the deeply curved surface, the node of admission lyingdirectly on the curved surface as in the case of the opticalcenter, As shown, the actual focal length (L) is the distancefrom the node of emission (e) to the focal point (F). The node

f emission is determined by projecting or continuing the line ofthe emitted rays until they intersect the centerline at (e) asindicated by the short dotted lines. In Fig. 37 the nodes areshown for a symmetrical biconvex lens where they are closeto the respective lens surfaces. It will be seen that a con sid-rable error in calculations would be involved if the central

optical center were used for the measurement of the focalI ngth instead of the node of emission. The optical center,it will be remembered, was at the geometrical center of thelens for a biconvex type so that the error would amount toabout 20 per cent of the lens thickness.

The nodal planes, which run vertically through the nodes,are shown at (m) and (n). In this case, where the node ofadmission is ahead of the node of emission, the interveningdistance (d) called the nodal space is a considerable dimen-

c

F

Fig.: 33. Finding the op+ical center of a biconvex lens.

c F

Fig. 34. Optical center of plano-convex lens shown here.

Fig. 36. Nodes of II plano-convex lens: a, node of admission,e, node of emission. The nodal planes are shown by m and n.

lC Fc

Fig. 35. Optical center of an unsymmetrical biconvex lens.

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LITTLE TECHNICAL LIBRARY

Fig. 37. Nodes of admission and emission of a. symmetricalbiconvex lens are located near the respective surfaces.

sion and must be added to the object-to-image ?is.tance.Where the nodes are crossed, with the node of adrnis aion atthe rear of the lens, the noda.l space must be deducte~. Thematter of nodes has been discussed in order to expl.a111.whatthey are, as they are an important factor in the des igriing oflenses. The amateur photographer, however, need not be con-cerned with them.

Distribution of Light on Lens

Since a single illuminated point sends o.ut.an infinite or num- .berless series of rays in every dlrectlOn, It IS evident that raysfrom such a point will completely cover. the surface of ~ lensfacing such a point-not only at a few P01l1ts but ev.ery P01l1tonthe lens surface. Such a condition is shown by Fig. 38 wherethe illuminated point (S) radiates rays al.l over the surface of thelens as indicated by the radi;;tl1ines. W ith 'a lens dl~m.eter (D),the cone angle of the contacting rays .IS (a). This IS a ve.ryimportant point to remember because It has much to do Withlens performance. . ldi '11Cutting off a portion of the rays by shie mg WI notentirely suppress the image of the p01l1t on the screen. Thus,if a sheet of cardboard (~). is u.sed ~o shield the lens. fromthe upper rays, the image point Will still al?pe~r on the pictureplane but with less intensity, as the shleld111g reduces thequantity of light passing through the lens .. Placing a patchon the surface of the lens Will not block the image, hence thebubbles frequently appearing in lens glass Will not interferewith the projection of the image unless they are so numerousthat they completely block the opening. .

This condition, fortunately, permits us to paint out lens

PHOTOGRAPHIC LENSES AND SHUTTERS 43

fD

Fig. 38. Light from a given object point. is distributedevenly over the entire surface of a lens as shown here.

scratches with black paint, fill in bubbles that have brokenthrough the s.urface of the lens, or repair lens cracks withoutinterfering With ~he projection of the complete image. Theonly drawback Will be a slight loss in the lens speed due tothe amount of light blocked off.

Path of Light Through Simple Positive Lenses

Up to th~ pr.esent time yre have adopted the most elementaryClf. schematic light path diagrams for simplicity in explanationwithout attempting to indicate how the rays actually passthrough the .Iens. This was done to avoid unnecessary com-phcation until such time that the basic principles were under-lood.But, befor.e we begin. with the more advanced diagrams, it

hould be said that the light entering the camera lens is seldom11 single r~y fr~m. a concentrated source of light, but ratherI'-flected light Issu~ng from many reflecting surfaces such astl~e leaves of trees 111a land~cap~. These reflected rays, coming[10m every unobstructed direction, require further discussionwhen constructing ?- lens diagram.

A ~Iagram sh.ow1l1.gthe primary conditions encountered inIocusing an o.bJect instead of a concentrated point of lightI shown b~ FI~. 39, wh~re the. object is the arrow (A-B). TheI orr esponding inverted Image IS formed at (A' -B') by the lens(L).

A point .(A) . selected a.t the arrowhead radiates light raysIII every direction of which the band falling on the lens islncluded 111the angle (a). Passing through the lens (L)tit ,se ra;ys a!,e refracted and reproduce the point of light at(A). Likewise, a P01l1t (B) selected on the tail of the arrow

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44 LITTLE TECHNICAL LIBRARY

A~~----~-- ~ B'

,,

,/,/,/,,/,/,/~~ ~~ ='-- --"A

B

Fig. 39. Sketch showing the primary conditions enc;ountere.d infocusing an obiect instead of a concentrated point of hght.

creates a bundle of radial light rays in the angle (b) that alsocover the lens (L). The latter rays are brought to a focus at(B') on the image. The rays from the arrowhead are shownas solid lines while those from the tall are sh.own as dot~edlines for ease in distinguishing the rays. This same actiontakes place with every light-emitting point on the arrow sothat the complete image is formed along (A'-B'): The raysystem for each point is complete within itself WIthout CON-

flict with any adjacent systems. .'It is evident from the diagram that the quantity of lIght

from each point increases with an increase in the angles (a)and (b), hence the quantity of light entermg the .lens (and,therefore, the lens speed), will increase WIth a~ mcrease.111the diameter of the lens (L), since the angles mcrease WIththe diameter. .In Fig. 40, is shown a complete diagram, of a simple smgle-lens optical system employing a biconvex lens. By me~ns offour light rays from the object we a~e able to lo~ate the Image.To simplify the explanatIOn. we WIll not copslder the nodalpoints of the lens. The optical center .(C) IS on t)"le opticalaxis (X-X). The point (F) is the principal focal point, or thepoint where rays whi~h ar_e parallel to the op tical axis of thelens converge to a point, . .

Light rays from points (A) and (B) on the object stnke thelens surface at (b) and (C;), respective.ly .. They are refractedand intersect the axis (X-X) at the pnnclpal focus (F). Theprincipal focal length (f) is the distance ~ro.m (C) to (F)(actually the distance from the node of emISSIOn to F). Therays cross the axis at (F) and are indefinitely prolonged untilthey intersect the dotted lines at (N) and (B') where theyform the image on the picture plane. The dotted lines from (A)

PHOTOGRAPHIC LENSES AND SHUTTERS 45

A~~------------------~-- r--o

------B 1.-

u

Fig. 40. This sin:'ple single-lens op+lcel system shows how, bymeans of four hght rays, the image plane can be located.

add .(B) subtend the object and enter- the lens (at the node ofa mISSIOn) where they cros~ ~t the optical axis. They leavet~e lefs (at the node of eITIlSSlOn)and their intersection with~ e re racted rays at (A') and (B') determines the focal orImage plane.hT~e angle (D) varies with the distance of the object from

~ e ens, becoming smaller as the object is removed farther\om the lens. Decreasing the angle by increasing the object

dl~ta~ce (u) cause~ the intersection to take place closer to thep~~nclp.al"focal I?omt (F) so that, at a sufficient distance~ mfimty.> the Ima.ge becomes coincident with the principaiocu~. W!th the object close to the lens, there is a very per-

cep tible ddleren~e between (f) and the image distance (v).When the object. distance (u) is 100 times the focal length

squared, the.n the plc~ure plane position can be substituted forthe focal. point (F) wl~h less ~ha!1 1 per cent error, and (F) can~A'B,n)sldT~ed as. being coincident with the picture plane

.- . . us, WIth a. focal length of 6 inches, the minimumobject distance at .whlch (v) can be substituted for (f) is:6 X 6 X 100 =3600 inches = 300 feet

Since !1egative lenses have no actual focal point, an imaginaryfocal pomt known as the virtual focus is employed for makingmeasurements or calculations with them. When combined withf convex lens having a focal length equal to the virtual focalcngth of the companion negative lens, the light ray passes

through .the combination without deviation as through aparallel-SIded sheet of glass.

A plano-concave lens is shown in Fig. 41, flat on one sidell~ld concave on the other. The incident rays (i) s read ordiverge .by refraction as at (t). This is frequently k~own asa reducing glass as objects viewed through it appear smaller

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46 LITTLE TECHNICAL LIBRARY

VF .,,/

Fig. 42. Combination, each lenswith same radius of curvature.Fig. 41. The plano-concave lens

has only a virtual focal point.

than actual size. While seldom used ~lone in a camera lensit is frequently used in combmatlOn wl~h other convex lensesand particularly in telephoto assemblies where the plano-concave is employed to increase t~e focal lengt~ of. theassembly. The biconcave lens,. which IS also a negative diver-gent' type, is a lens unit that IS very frequently employe? l.ncompound lens assemblies. Having two concave faces, It IStwice as effective as the plano-concave lens.

The effective focal length of both the plano-concave and thebiconcave lenses is the virtual focal length measured to thevirtual focal point (VF). This focal point .ISobtained by ~ro-jecting the diverging rays back where the~ intersect the opticalaxis. This point locates the plane 10 which the imagmary orvirtual image is assumed.

Simple Combinations of Lenses

Various combinations of concave and convex lenses are inuse. In Fig. 42 we have a concave and a convex lens assemblyseparated by an air space. Since both lenses have an equalradius of curvature, the rays of light .are unaffected. and passstraight through as indicated. Fig. 43 IS an achromatic doubletfrequently encountered in the cheaper b!?x earner as. It con-sists of a biconvex lens cemented with Canada balsamto a plano-concave lens. By its use color aberratIOns are larg~lysuppressed. Two kinds of glass are employed, .generally flintglass for the biconvex and crown glass of equivalent for ~heplano-concave. Fig. 44 is another achromatic. doublet employmga biconcave lens element, cemented to a biconvex lens madeof dissimilar glass. This is very frequently used as the frontelement of a compound lens.

PHOTOGRAPHIC LENSES AND SHUTTERS 47

F

Fig.43_ Doublet with biconvexand plano-concave elements.

Fig. 44. Similar doublet withbiconvex and biconcave lenses.

Admittance Factor

The amount of light admitted to the camera depends uponthe free or net area of the lens through which the light passes.For high-speed exposures it is evident that a large opening isnecessary at. the lens. The greater the amount of light fallingon the sensitized surface of the film the more rapidly willthe change be made in the emulsion by the light.

Admittance IS proportional to the free area of the lens andfor the round openings commonly employed, this varies as th~square of the diameter and not directly as the diameter. In.om.e cases, ~he admittance factor is based upon the quantityof light passing through the opening. In some specificationsthe admittance factor is the net admittance factor taken at th~rear end of the lens. This takes into account the manylosses ~hat. occur within the lens such as those caused byabsorpt ion 1!1 t~e glass, by reflection, etc. This is the logicalfactor as It indica tes the net available light.

Lens Area and Image Brightness

Lenses differ in the amount of light they admit. Exposure isdepel?-dent upon. the amount of light falling on a given area offilm 10. the camera. As was stated in Chapter I, the principlef the inverse square law applies in the formation of an ima~e.

The illumination of an image depends on the area of the Aefu-J.~lnd ItS Iocal length. If a lens of given aperture for s~an 6/

Image 2 inches square at a distance of 3 inches from th lenswhen ~ocused on infinity, it will form an image 4 inches q~are:l.t a distance of 6 inches from the lens when copying a ralize. This second image will cover 4 times the are ov-

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48 LITTLE TECHNICAL LIBRARY

ered by the first image although the film may use only apart of the image formed. The same light will be spreadover 4 times as great an area and consequently the image willbe y,; as bright as the image formed at a distance of 3 inches.Thus, the brightness of the image is inversely proportional tothe square of the focal length of the lens. Since the amount oflight admitted by the lens depends on the aperture of the lens,and area of the aperture depend4J on the square of its diameter,image brightness will vary directly with the square of thediameter of the aperture.

The f Function

Simply specifying the lens opening gives no direct indica-tion of the lens speed for the reason that the area illuminatedis also concerned in the matter. If a given lens opening ininches or millimeters is to be used in two objectives of differentfocal lengths, one covering a 4x5 film and the second coveringan 8xlO film, it is evident that four times the exposure time willbe required to cover the larger film because it has four timesas much area as the 4xs film and the objective will be twice asfar from the film as is the case with the 4xs film.

Of course, this relative speed can be expressed by a com-parison between the lens and film areas, but a much simplersystem has been devised in which computations are basedentirely upon the lens factors-the free lens opening and itsfocal length. Thus we have lens speed factors such as f8,f4.s, f2.s, etc., all measurable from the lens itself. Insteadof employing the film area, the proportional focal length ofthe lens is employed, as the focal length is proportional to thefilm area. It is evident that if the focal length is doubled theimage size and film area will also be doubled. The lens-opening diameter (light admittance) is compared to the focallength to obtain the speed function (f). '

The f-number of a lens is equal to the focal length dividedby the diameter of the free lens opening. Thus, if the freeopening diameter in the lens is 1.5 inches and the focal lengthis 7.5 inches, then:

7.51= - = 151.5

When the camera is provided with an adjustable or iris'dia-phragm, a great variety of lens speeds can be had to fitvarious lighting and distance conditions. The lower thef-number, the faster the lens, hence an f2 lens is much fasterthan an f8 lens. ..

The relative speeds of a lens are inversely proportional tothe square of the f-numbers. Thus, in comparing an f2.0 lens

PHOTOGRAPHIC LENSES AND SHUTTERS 49

with an f8, we compare the square of 8 to the square of 2, or:(8 X 8) 64-- =- = 16.0(2 X 2) 4

In .other wo.r~s,. an f2 lens is 16 times faster than an f8 Ior'llf ~he f2 Ir~s IS stopped down to f8, the speed of the car::rs~WI e one-sixteenth the speed when wide open at f2 Th

texpohsures required at different stops are directly propo~tionalo t e square of .the f-numbers.

!hi free ?pemng. or effective aperture used in the above~~ cu a~lOhs IS

fthe diameter of a beam of light after passing

roug t e orward part of the lens and ahead of the ..Ii I:

hus rally \he diameter of the aperture plate placed just bl:~k

o e. ~ont ens element. In a simple lens with the sto infront, It. IS.t~e actual diameter of the stop opening. The mefhodof determining effective aperture is explained in Chapter III.

Resolving Power of a Lens

The. ability of a lens to show fine detail is affected bits'resolving power. In practice a circle 1/100' h j di y. d at a di ' mc 111 iametervl~we at a dlsta~ce of 10 inches, will appear to the eye as ~POll1t~a!1d·11100 inch has therefore been taken as the lar estpermissible circle of confusion in photographic lenses for ;on-~a~t .PIr.mtll1&,. Every commercial lens is expected to reproduce

e ai 111Ul1lt~no .larger than 11100 of an inch.hMany spe11<:1high-grade lenses designed for aerial cameras

s ow a reso vmg power as. high a~ 1I500-inch, but it is neces-fiary JOt ~le. these lenses with special film capable of recording

ne e ai 111order to obtain the maximum efficiency of thele~s. Tht ~esofilvlll1gpower is somewhat dependent upon the

tnha.ure b~ t Ie m and the developer. More will be said on

IS su ject ater.

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CHAPTER III

LENS MEASUREMENTS

Focusing Methods

FOCUSING on a given object distance consists of adjustingthe obj~ctive of the camera .in res?ect t~ the focal plane

(or film emulsion) until a sharp ~ma~e IS obtamed. A~ there ~5a distinct position for every unit dIstance o~ the object, It .ISevident that reiocu~ing is necessary every time that the dIS-tance of the object IS changed. f

The exact method of focusing depends UpOIo th~ type 0camera and its construction. but basically the le~s IS movedin respect to the picture plane in all systems. Class!fied accord-ing to the focusing methods, we have the following types ofcameras:

View cameras. The image is shown on a groundglass screen ,at the1. rear of the camera where it can be observed for. sharpness while the

lensboard (or back) is racked back and forth during focusmg.

2. ~{e~s~toif~;e~Si~~dh~~~e~aili:r~C~le :t,11:i~~gt~~li~~aSt:d ;~cf~~f ~!::t~~:to agree with the various object distances. Distance IS estimated.

3. Universal or fixed focus cameras in whicb the lens is rigidly heldin one position, focused on the hyperfocal distance .• ThIS l2a'ferta ;~good only for a limited range of dIstances, say irorn eeinfinity at a relatively small aperture.

Coupled range finder cameras are provided with a distance.me~surihg4. device coupled with the focusing meehan.lsm In such a way t at t e

lens is moved automatically with the adjustment of the rangefinder.a. Rangefinder WIth a pivoted rmrror system.b. Rangefinder with a rotating pnsm.

ReRex cameras in which the full size image is reflected to a viewing5. r en b a mirror, the Image being ro~used on the sc.reen.

sc: Sini;.le.lens reflex with tilting mirror that IS lifted above ~e• 0 tical axis and out of the way when the e~po5ure 15 ma e.1single lens is used both for focusing and making the exposur"d

b Twin-lens reflex. One lens for the Iocusirig finder screen ~an. a second lens for the exposure, both lenses being moved at one

time during focusing. Fixed mirror WIth finder lens.

All of these methods attain the same result .by dii'fer.ent ways.The groundglass screen view camera permits ~he view to lbdcomposed on the screen with greater ease while the coup e

50

,PHOTOGRAPHIC LENSES AND SHUTTERS 51

rangefinder and reflex cameras are easier to focus and can befocused more rapidly than the view camera. .

In the majority of cameras, the entire lens and its mount aremoved back and forth during focusing. In the view and handcameras, the lens is mounted in the front movable lens boardwhich in turn is moved back and forth along the camera bedby means of a rack and pinion. In most miniature cameras,the lens is mounted in a focusing mount and is focused byrotating the mount. A screw thread in the mount determinesthe movement along the axis. A third method is the frontelement focusing system in which the rear lens element is rig-idly attached to the camera while the front lens element ismoved b~ck and forth by turning a screw thread (helix).

The distances en~raved on the focusing scales are usuallyquite accurate, particularly those on the focusing-mount typeof lens, but occasionally it may be necessary to remark orreadjust the scales because of an accident to the camera orchanges made in the camera. It is then that we require aknowledge of focusing measurements and adjustments.

Focal Length of Lenses

The focal length of most lenses is marked on the front endof the lens: this is quite accurate a:,-d.not subject to change. Ifa sufficient change takes place WIthin the lens to materiallychange its focal length, then you may be assured that otherdifficulties will also be discovered that make repairs necessary.In the following table are given the equivalent lens focal lengthsin inches for corresponding dimensions in metric me asur e itbeing common practice to quote focal lengths in both syste~s.

M!111- Centl- Milli- Centi-meters meters Inches meters meters(mm) (ern) (in) (mm) (em)

15 1.5 0.5906 135 13.525 2.5 0.9843 150 15.035 3.5 1.3780 155 15.545 4.5 1.7717 165 16.0

180 18.0195 19.550

60758090

5.06.07.58.09.0

1.96852.36222.95283.14963.5433

Inches(in)

5.31505.90566.10246.29927.08677.6772

210240250270300360500

21.024.025.027.030.036.050.0

8.26789.44899.8426

10.630011.811114.173419.6850

105120125

10.512.012.5

4.13394.72454.9214

Even when we know the exact focal length or a lens. accu-rate measurements are usually difficult or impossible as theoptical center is seldom marked on the mounting and we.

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therefore, have no basic point from which to make measu.re-ments. However, rough calculations can be made by makingmeasurements from the center of the lens mount.

Focal Length Measurements

The principal focus (also called infinity focus or the focallength) of a normal lens is the distance from the node of .e~I1I~-sion to the picture plane or surface of the film when the. Ins ISwide open and when the lens is focused upon an object atinfinity. This is the standard focal length. Telepho~o lenses donot follow this law. It should also be noted that with the fullyopened lens focused at infini~y, the len~ is in its closest prox-imity to the focal plane while the pomter. IS ~t the extremeinside of the camera focusing scale at the infinity mark (00)It is impossible to bring an image into focus with the lensmoved closer to the focusing screen. To focus the lens on anearer object makes it necessary to extend the bellows andmove the lens away from the picture plane. Usually the camerais provided with a stop so that the lens cannot be moved closerto the film than at the infinity position. .

In the infinity position, with the lens closest to the film, Itis evident that the illumination on the film is most in tense atthis point and that this is the position for the highest lensspeed. Focused on a nearby object, the lens is farther fro~ thefilm and both the illumination and speed are correspondinglyreduced.

In cases where the focal length is marked on the lens, wecan determine the location of the node of emission very easilyand mark it on the lens barrel for future reference. Focus thecamera on infinity with the iris wide open and then measurethe exact focal length as marked on the lens from the pictureplane, or face of the film, !o .a point .on. the barrel. Mark thispoint; it is the node of errussron. This IS true only for normallenses and does not hold true for telephotos.

In cases where the focal length is not marked on the lensand must be determined by measurement, we have severalmethods open to us which can be employed without much diffi-culty. After the focal length has been measured, it should beemployed for marking the node of emission on the lens barrelby the method just described.

METHOD 1. Focus a ruler or yardstick sharply, using a magnifyi~gglass to examine the image on the focusing screen. There need be '.'0 definitesize of image except that the image should be large enough t~ be easily VISIble.N ext measure the distance between the ruler and the focusing screen. Nextdeter;"'ine the ratio of reduction which will- be 3.0 if a 4·inch image of the

PHOTOGRAPHIC LENSES AND SHUTTERS 53

12-inch rule is shown on the screen. Multiply the distance by the ratio and thendivide the product by the ratio plus 1, squared.

Let: R =Ratio of full-size rule to image of rule on focusing screenD =Distance of rule from focusing screen in inches •

.D XRThen: Focal Length = ---

(R + 1)'

EXAMPLE. Let the distance be 41 inches and the ratio 3.0.Then:

41 X 3 123 123= --- = -- = ?? inches.

(3.0 + 1)' (4 X 4) 16

METHOD 2. This is a very inaccurate method but one that will give anapproximate idea of the focal length, just so long as the shutter lies near theoptical center of the lens. Measure the distance between the focal plane andthe lens shutter when the camera is focused sharply on an object at infinity toobtain the focal length.

METHOD 3. With the iris full open, focus on some far off object, at leastone-half mile away. Mark the position of the lens at infinity. Next, focus aruler or yardstick until the image of the inches on the screen are full size, andmake a second mark on the scale. You will now find that the distance betweenthe first and second marks is equal to the focal length of the lens because thedistance of the lens from the screen is twice the focal length when the imageis the full size of the object.

Determining Approximate Infinity

Elsewhere in this book, it was explained that the pictureplane became coincident with the focal point when the objectwas distant from the lens by the focal length squared multipliedby the reciprocal of the desired circle of confusion. This beingthe case, an approximation to infinity may be determined bymultiplying the square of the focal length of a lens by thereciprocal of the desired circle of confusion. Dividing by 12gives the distance in feet. Thus, if the focal length of the lensis 6 inches and the desired circle of confusion is 1/100 inch,the nearest approach to infinity will be:

(6 X 6) X 100 = 3,600 inches = 300 feet.

However, if a smaller circle of confusion is desired, as isusually the case with lenses of shorter focal length, then ap-proximate infinity will be found at a greater distance. Forpractical purposes, with most cameras, focusing will be satis-factory if an object at 500 tp 600 feet is taken as infinity.

Making Focusing Scales

Sometimes the addition of a new lens to the camera or otherchange makes it necessary to recalibrate the focusing- scale sothat it will meet the new conditions. The most accurate method

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54 LITTLE TECHNICAL LIBRARY

of making the new scale is to first determine the infinity posi-tion and then measure off the other distances with a tapemeasure focusing sharply at each distance. Another method isto compute the intermediate distances according to the principleof the conjugate foci. Actual measurement with the tapemeasure is to be preferred in any case.

Angle of View

The field or image-filled space covered by the lens is circularin form with the optical axis as the center of the circularilluminated disc. The circle is, in reality, the base of a cone inwhich the objective lens is at the apex of the cone and theangle formed by the sides is the angle of view. In practice, the

A

Fig. 45. The angle of view must enclose the negative's diagonal.

actual circle of illumination is somewhat larger than the effec-tive angle of illumination as some allowance is made for thedistortions along the outer edge of the circle.

In Fig. 45, we have the lens at (L) with circle of illumina-tion (A-B-C-D), the angle of view being (a). The inscribednegative or film is (A-B-C-D) having the diagonal (A-C).The distance (m) is the focal length. It will be seen that thisforms a triangle (A-L-C), which when laid out to scale, givesthe view angle (a) which in turn determines the coveringpower of the lens. In the following table will be found thediagonal lengths for a number of popular negative sizes:

Film Size Dia~onal Film Size DiagonalInches Inches Inches Inches

1 xl% 1.8 31,4x 41,4 5.41~X21,4 2.8 4 x 5 6.42 .x2% 3.2 5 x 7 8.62%x3% 4.0 ' 6%x8% 10.72%x41,4 5,0 8 x10 12.8

PHOTOGRAPHIC LENSES AND SHUTTERS 55

Determining Effective Aperture

In the case of ~ single lens, with the iris placed in front ofthe lens, the effective aperture is the diameter of this stop open-mg. All of th~ light that enters must enter through thisaperture. But, m the case of a multi-element lens the lightbeam entering the front lens is slightly converged or condensedby: the front lens element before it reaches the iris which inthis type of objective, is placed between the front and rear ele-ments .. The diameter of the beam is now less at the iris than atthe POlpt ~f entry and the whole matter is rather uncertain. Inshort, It Will be safer to measure the beam of light in this casethan to attempt measuring the aperture.

A small beam of light is admitted through the center of therear lens element. from. a position normally occupied by thegroundglass, and IS projected on a screen held just in frontof the front element. Obviously, the diameter of this beam isthe effective dla,meter of the aperture when measured at thescre~n. This diameter is divided into the focal length toobtain the maximum f-number.

Angular Aperture

The angular aperture of a diaphragm is associated withdepth of field calculations and is seldo"'m employed except inlens design. ~s a rule it is slightly larger than the effective~pert1}re, and IS obtained by projecting back the two converg-ing ,lmes from t,he pr incipal focus until they intersect the:vertical plane wh.lch. passes through the node of emission andIS .called .the pnr:clpa.l plane, The distance between thesepomts of intersection IS the angular aperture.

Measuring the f-Number

Afte~ the focal length and the effective aperture have beendetermined, the value of the f-number can be computed. Thus:

Lf =-

d

where (d) is the effective aperture and (L) is the focal lengthboth .m terms of inches, For example, if the focal length of ~lc,:!s IS 6 inches and the effective diameter of the aperture is2 inches, then the f-num ber becomes:

6f = - = f3

2

The camera should be focused at infinity when determining

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the focal length of the lens. At any other distance, the valueof the f-number varies because the focal length varies. Thus,if the bellows of the camera is extended for focusing a nearobject, the focal length is increased and the f-number becomeslarger.

Assisting Screen Focusing

In making lens measurements involving the use of a ground-glass screen (for a view or reflex camera) it is not alwayspossible to see the image distinctly on the screen. This isparticularly the case with lenses slower than f6.3, as the smallerapertures admit very little visible light that can be used withcertainty for focusing.

In most cases where a critically sharp image must be had,a magnifying glass may be used. This glass not only. enlargesthe image but it also collects light and concentrates It so thatthe image is both sharper and brighter. The special two-element focusing glasses are best for this purpose but a simplereading glass can also be used in an emergency. The spe~ialfocusing magnifier is generally provided with a rubber suctioncup that holds it in place on the groundglass screen and makesthe job easier to handle.

But these focusing glasses have their drawbacks. Unfortu-nately, they magnify the grain of the glass as well as the imag;,so that the image is confused by glass crystal shadows. Thiscan be cured, however, by means of an oil stain or a drop ofCanada balsam on the grainy side which extinguishes thegrain at that point but which leaves a remarkably smoothfocusing surface. This oil stain or the Canada balsam spotcan be placed permanently in the middle of the glass andmarked by a small penciled cross. The magnifying glass is nowfocused on the pencil mark and can then be moved to anypoint where it is desired to see the image.

In using the magnify!ng, glass in conj.upction wit~ the l?en-ciled cross or "cross hairs,' the most Critical focus IS obtainedby moving the eye from one side of the magnifier to the otherso as to change the relative position of the cross hairs in thefield of view. If the cross hairs do not change their positionrelative to the image, lying in the same plane, the focus maybe considered to be critically sharp. This method is known asparallax focusing.

CHAPTER IV

THE APERTURE OR DIAPHRAGM

Purpose of the Diaphragm

'r HE. aperture, "stop", iris, or diaphragm is a simple devicebut very effe~tlve and necessary to the proper operation

of a cal?era. It. IS a simple, accurately-cut circular hole in aplate, either variable or fixed in size, that is combined withthe lens assembly.

The location of the diaphragm in regard to the lens elementsdcpe~ds. Upon the type of lens, and it may be placed in frontof, within, or to tile rear of the lens mounting. In the case ofu s~mp.le meniscus lens it may be placed in front of the lenswhile 111 a symmetrical doublet it is usually placed betweenthe front and real:' elements within the mounting. With the~xceptl?n of the inexp ensrvs box cameras and certain typesof studio cameras, th~. ~r~at majori~y of diaphragms are nowI)f the adjustable or. iris type which permits a great rangeof openings by the Simple movement of a lever

In general, the aperture or ?iaphragm serves three pur-poses, from. an optical standpoint, making perfect exposurecontrol possible.

I.Light volume control. Acts as a valve in the control of the lightentenng. the camera, slowong down the exposure in bright lightor speeding the exposure in dull light. By this means, the film isbrought within the range of the camera's shutter speeds.

2. Reduces or removes residual aberrations, By .proper manipulation,the diaphragm Will subdue or eliminate aberrations remaining in thelens and, In partIcular, residual spherical aberrations.

3. Improves the. depth of fielq or makes it possible to control the zoneof sharpness in near and distant objects in a view.

The f-Number or Ratio

. The quantity of light admitted .to the film through the objec-tive and the speed of the objective depend upon the effectiveopening of the aperture as previously explained. It is evidenttlit'ref.ore, that by.combining these two factors we can arriv~III n SImple expression of lens speed. This is expressed in prac-

57

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tice by a ratio known as the f-number and written as follows:

Focal length of lens

j-Number = Diameter of aperture

Thus if the effective diameter of the lens is 2 inches and thefocal'length of the lens is 4 inches, then the f-number or lensspeed can be expressed by:

4I ~ - = 2

2

If the aperture remains constant and the focal length i.slibn-creased to 8 inches, then the speed of the O~Jectlve wII ereduced to f4 by the same method of c.a1c~latlOn. The largerthe f-number, the slower will be the obiecuve- ber I t

But we must note that the value o~ a grven f-~um er IS n~always constant with a given objective under dIfferent condi-tions. The amount of light reaching the ~lm plane decreaseSwhen the camera is focused upon a near object, for under thdseconditions the distance from lens to film increases, thus ~-creasing the f-value even though the aperture may remainconstant. When the fully opened aper tur e IS.given ItS standardrating; the camera must be focused at infinity.

Effective I-Number for Close-ups

The f-rating of a lens as marked on the diaphragm setting iscalculated for the lens when focused on infinity .. For example,if a lens has a focal lengt~ of .4 .inches and Its maximumaperture has a diameter of I JJ1ch,It IS rated as an f4. lens. Fodrdistances less than infinity the speed of the lens IS reducebecause the bellows extension is increased, for the shorterobject distances; the lens. is farther from the film and th.eillumination is correspondll1gl:l; reduced .. Because of the Ida~l-tude of modern black-and-white film, this factor can be IS-regarded up to a certain point with most ?-mateur cameras.However beyond a certain bellows draw this factor becomesimportant, particul~r1y. when close-ups ~re made or wherenatural color film WIth ItS narrow latitude I.Sused. b

Required increase in exposure and effective f-number can edetermined as follows:

Let: L =Focal length of the lens. ! hv =Image distance, which wil.!be th~ tocal lengtb pus t e

extension past the infilllty posttton-I= Aperture or indicated I-number.

L XLThen: increased exposure = --

vX v

PHOTOGRAPHIC LENSES AND SHUTTERS 59T?e effective f-number at any object distance less than infinity,With the bellows extended past infinity, is

v XIEffective I -number = --

L

EXf\MPLE: If the bellows extension or image distance shouldbe ll~creased to double the rated focal length with a 6-inchlens in order to take 'a close-up giving an image the same sizeas the object, and the aperture set at the indicated f8, then theexposure increase over normal exposure required without theadditional extension would be

6 X 6 36 1--- = - = - or an increase of 4 times12 X 12 144 4

and the effective f-number is

12 X 8

6

96= - or 116

6

In this case the effective f-number becomes the actual f-numberund~r the new conditions and the rated f-number no longerapplies, .

So. far we have assumed. that no loss of light takes place afterpassmg through the objective, and that our lens is abso-lutely trapsparent with no light loss due to internal absorptionor reflection. Unfortunately, there is a considerable loss of lightby absorption, even with the best and most transparent gradesof optical glass. It is estimated that each surface of a glass lensor element reflects from 5 to 6 per cent of. the incident light,hence when we have SIX air-spaced reflecting surfaces in thel~ns, the loss will amount to 6 X 0.06 = 36 per cent of the totallight. This loss, however, does not take place to such a greatextent where the lenses. ar~ cemented together, and from thisstandpoint cemented objectives are more efficient

I tis .very apparent that the .light actually passing through thelens .W.lt? a given f-number IS much less than that estimatedby dividing the focal length by the aperture for in this calcu-lation, we take no losses into account. For e~ample, a lensfully open might show as an f3 by direct measurement butactually It~ effect upon the film might be only equivalent to anf4.5, the difference being absorbed by the various losses.

The rapid mer ease in the use of exposure meters has broughtabout a great deal of confusion because of differences in theperformance of various makes of lenses bearing the same t-rating. It would seem probable in the future that the value of fw:i1l be determined by photometric methods rather than bydirect measurement of the aperture and focal length.

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Comparisons of Lens Speeds

The relative speeds of various lenses can be compared by acomparison of the "squares" of the several f-numbers. Forexample, let us say that a 2-second exposure is necessary in acertain case with an 14 lens wide open, and that we wish to findthe exposure required when the iris of the lens is stopped downto f8.

(8 X 8) 64Exposure at 18 = --- X 2 = - X 2 = 4 X 2 = 8 seconds.

(4 X 4) 16

Thus we see that the f8 opening requires four times the ex-posure necessary with the f4 opening. With an opening of f6.3,the square of which is (6.3 X 6.3) = 49.99, the exposure willbe approximately 2.5 times greater than at f4.0 with an ex-posure time of (2.5 X 2) 5 seconds. .

Most diaphragm scales are marked in such a way that eachsucceeding number on the scale either halves or doubles theeffective exposure. This arrangement is a great convenienceand saves much 'calculation. The following is an example ofsuch a marking system, with the square of the number placedbelow each division.

15.6 f 8

(31.36) (64.0)

HI f16 f22

(121.0) (256.0) (484.0)

12.9

(8.41)

f4(16)

Thus if an exposure is made at stop f 11 and we wish to in-crease or double the speed, we simply move back to the nextstop f8 which is approximately twice the speed of f11.

In general, two different systems of f-number arrangementsare used, the English and the Continental system. They areshown below where it will be seen that the difference is notgreat.

En~lIsh 2.8 4 5.6Continental 2.3 3.2 ,~.5

11 16 229 12.5 18

3225

86.3

In the case of miniature cameras, the smallest aperture nowcommonly used is f22, the 132 now being discarded with thistype of camera. However, the lenses for large studio and com-mercial cameras, view cameras and the like are provided withmuch smaller stops, including 132, 145, f64 and sometimes fl28in addition to some of the larger stops listed above.

The U. S. or Uniform System

This is an old system of diaphragm notation rarely found inmodern cameras but often used on old-rime rectilinear lenses

PHOTOGRAPHIC LENSES AND SHUTTERS 61and early anastigmats. This system starts with 1 as the equiv-,~lent of f4 an~ proceeds in whole numbers to U. S. 64. Theligures used with the U. S. scale represent relative exposures so~~at each succeeding number doubles the preceding number.1he table below gives the comparison between the two systems.

f-NumberU. S. Number

4 5.6 8 11.3 16 22.61 2 4 8 16 32

3264

The only point of agreement in the two scales is at 16.

Circle of Confusion

When the lens is focused on a certain predetermined distancethe light rays from various other distances are not all broughtto a focus exactly on the, focal plane. Some are focused in theplane, some ahead of the plane, and some behind the planeEach ~ay that IS ~rought. to focus in front of or behind the focalplan~ IS not a POI1'~tof lI~ht but shows as a small circle calledthe Circle of confUSion which affects the definition adversely andreduces. the sharpness of the image., In fIll. 46,. the film. is indicated by (F) while the sensitizedrnulsiori facing the light from the lens is at (e). It so hap-

pel!s that the lens. brings the pencil of light rays from anobject at a grven distance (a). to a sharp focus on the surfaceof the emulsion as a small POInt of light. The light pencil (b)from an object farther away, however, is brought to a focusat a distance In front of the emulsion or picture plane and theprolonged rays pa:,smg the focal point (dotted) form a smallCircle on t?e emulslOp so that the light strikes as a blurred circlef confusion, A third pencil of light rays (c) from a closer

object is brought to a focusbehind the emulsion whereagain a smal! circle is formedon the emulsion. If the lenswere able to bring objects atvarying distances to a sharpfocus in one plane, we wouldobtain infinitely sharp defini-tion all over the film for alldistances, but since this iscontrary to optical principlesthe image will be composedof many sharp points fromthe plane focused on plusmany overlapping circles ofconfusion from objects eithernearer or farther than the

F-->-If<--

--~ ------:---~

-::: --- c

Fig. 46. The circle of confusionis one cause of blurred images.

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plane focused on, which may blur the image and destroy thedefinition.

A tiny circle appears as a point to the naked eye if it is 1/100of an inch or smaller. This size, then, is the largest permissiblecircle of confusion where a sharp image is desired. When thediameter of the circles of confusion is 1/100 of an inch or lessand the point is viewed at a distance of 10 inches, the image willappear sharp. However, if sush a negative is enlarged ~o twi~eits size, the relative diameter of the circles of confusion willalso be doubled to 1/50 of an inch and the picture will losesharpness proportionately. It is for. this reason that t~e l!lr~estcircle of confusion encountered in high-grade cameras IS limitedto about 1/200 inch, and much smaller in the better miniaturecameras where the small negatives are always given a consid-erable amount of enlargement. It will be seen that, when en-larged 10 times a negative with l/1000-inch circle of confusionwill show as IhOO-inch in the finished enlargement.

Theoretically, there can be only a single plane of criticalfocus for objects at any fixed distance but, practically, .manyof the rays strike the film at such an acute angle that the filmcan receive rays from a number of points at different dis-tances and thus render them sharp, since the contacting raysyield points not greater than 11100 inch. The distance of thecamera from points in different object planes, and the size ofthe diaphragm opening, determine whether the image will besharp in the plane of the film.

Hyperfocal Distance

If we take a view camera, with the lens fully opened, andrack the lens forward we shall reach a point where a distant'object will come into ~ritically sharp fo.cus. .If the entirefield is examined, we will note that certain objects near theinfinity point are also in focus. Looking ~gain, we find. t.hatother points nearer the camera are beginning to lose criticalsharpness until objects immediately in front of the lens arepronouncedly soft and fuzzy. .

The sharpness or unsharpriess of objects at various distanceswill depend much upon the focal length of the lens. The longerthe focal length, the sooner the foreground objec.ts will becomesoft. With a short focal length lens there will be a muchgreater zone of sharpness. This test will show how much"depth" our lens will give at maximum aperture when focusedon infinity or, better yet, will indicate the closest distance atwhich objects will.be sufficiently sharp. ...

Now we will again slowly focus forward; the infinity pointstill remains sharp but the depth of field or region of sharp-ness is increased in the foreground. Racking farther we find

PHOTOGRAPHIC LENSES AND SHUTTERS 63that, sudd~nly, the infinity point has started to lose its sharp-ness and, If we stop Just before this point is reached we willh~ve th.e. camera focused on the hyperfocal distance whichgives critical sharpness at infinity as well as the greatest depthof field.

The hyperfo~al distance is the nearest point at which objectsare ~n appr~xunate~y sharp focus with the lens focused oninfinity. This relation IS shown by Fig. 47, where the lensIS focused on .1l1finlty.(00) with the hyperfocal point at (h). Thehypel:focal distance IS (H) and the focal length is (F) withthe diaphragm !it (d). Everything between (h), the hyperfocalP?l11t,and mfimty (00) IS In sharp focus. This is the hyperfocaldl.stance for the lens at its largest opening. As shown inPig. 48! when the lens !S focused critically on this distanceeverythmg will be sufficiently sharp from half this distance./\

0:> h I~I I \VI II II II I

I Y II Ir-SHARP Iii' H I' F ----+iI I

Fig. 47. With lens focused at infinity, the hyperfocal point is h.

~------SHARP----------~~ I---+~-- F-..lI

h

Fig. 48. Focused at h, the area in sharp focus greatly increases.

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\.

to infinity. To calculate the hyp.erfocal distance for a lens:

LeT: F =Focal ien~th of lens in inches.H =Hypertocai distance in feet.f -!)iaphra~m openfng or i-number. h

C ;;;Reciprocal of the diameter of circle of confusion in inc es.

F' x cThen: H = -'---

f X 12

For example to find the hyperfocal distance for a lens havinga focal length of 10 inches and an aperture of f 4, the permissiblecircle of confusion being 11500 of an inch:

(10 X 10) X 500. 50,000 fH = = --- = 1,041 eet.

. 4 X 12 48

F this lens 1041 feet is the hyperfocal distance whi~h willi~~ th~ greatest depth of field at maximum aperture., WI~h the

Fe focused at 1 041 feet this field will extend from infinity toapnpsroximately h;lf the hyperfocal distance or 520 feet from thecamera but this is a considerable distance from the camerawith a blurred foreground extending for 500 f~et,

The reason for this result is that we have, 111the first place,used a very large aperture (f4) and, second, have demanded

ery highly corrected lens with a. small Circle of co~fuslOn.Ei~her of these factors will increase the hyp erfocal dl~tan~e,hence we must make a suitable correction ~o bring the, dis-tance down within reason. One very effective method IS toreduce the focal length of the lens, effective because thedistance varies as the squar<: of the focal length. J ';1st as anillustration, let us use a 6-1I1ch instead of the l U-inch lensin the first problem:

(6 X 6) X 500 36 X 500 18,000H = = -- = 375 teet.

4 X 12 4 X 12 48

This makes a noticeable difference in ,the depth o~ field,reducing (H) from 1,041 feet to 375 fe,et, w ith ou t disturbing thevery desirable small circle of confusion or slowmg down thelens by the use of a smaller aperture, But, by ordinarymeasure the depth of field is sti11 too short so we will, forexample: cut the focal length to 3 inches and stop down thelens to flO by means of the iris, We do not Wish to disturbthe circle of confusion as yet,

(3 X 3) X 500

10 X 12

9 X 500

10 X 12

4500= -- = 37,5 feet.120

PHOTOGRAPHIC LENSES AND SHUTTERS 65

This decrease in the hyperfocal distance is well worth whileeven though we did lose speed by stopping down the lens. Thezone of sharp definition now extends from 18.6 feet to infinitywith the lens focused at 37,S feet. The experiment also showshow the hyperfocal distance is reduced by the use of a shorterfocal length and, therefore, why' a miniature' camera showsa greater depth of field than a large camera with a long focallength lens,

N ow, cutting the hyperfocal length down to a very lowfigure will mean that we will have to sacrifice the advantagesof a very smal! circle of confusion and use a camera with a2-inch focal length lens, Let the new circle of confusion be11200 of an inch and the iris be set to f16. Then:

(2 X 2) X 200 4 X 200 800~ --- = - = 4.2 feet.

16 X 12 16 X 12 192

Here, we have about. the practical rmrumum in hyperfocaldistance by the use of a miniature camera and using an averagelens with a small value of circle diameter, Everything will besatisfactorily sharp from half the hyperfocal distance to infinityby focusing on the hyperfocal point instead of on infinity.

Focusing on the Hyperfocal Point

When the camera is focused on infinity, everything will besharp between the hyperfocal point and infinity. But, as statedbefore, if we refocus the camera so that it is now focused onthe hyperfocal point, the zone of sharpness will be increasedfrom half the hyperfocal distance to infinity (Fig. 48). Thus,if the hyper focal distance is 20 feet when focused on infinity,the zone of sharpness can be increased by focusing the cameraon the hyperfocal point and, under this condition, the zoneof sharpness will begin at half the hyperfocal distance or10 feet.

The fixed lenses in box cameras are always focused on thehyperfocal point instead of on infinity, in order to gain thegreatest possible range of sharpness. Several camera manu-facturers have specially marked focusing scales so that advan-tage can be taken of short focusing, There are two red dots,one on the focusing scale at about the 20-foot mark and theother dot will be found at about f 8 on the iris scale, Bysetting the focus 'at 20 feet and the iris at f8, everything willbe in focus from 10 feet to infinity, and it is then only necessaryto adjust the shutter speed,

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Determination of Stop

As the iris or stop is the most convenient method of con-trolling the depth of field, we will transpose the originalequation so that we can solve for the necessary I-number.

Let: F = Focal length in inches.H = Desired byperfoca] distance in feet.f =.Aperture opantng ,

C = Reciprocal of the diameter of circle of confusion in inches.

F' XCThen: f= ---

HX12

For example, let 115 say that we wish to have everything from10 feet to infinity in the range of critical sharpness, hence thehyperfocal distance will be twice this figure or 20 feet. The.circle of confusion is 11200 of an inch and the focal length is6 inches. We wish to find the stop necessary:

(6 X 6) X 200

(20 X 12)

7200=--= (30

240

Depth of Field

We have seen that when a lens is focused on a certainobject, other objects closer to and farther from the camerathan this principal object (which is in critically sharp' focus)will be in approximately sharp focus. The distance between theplanes in which lie the nearest and the farthest objects in suf-ficiently sharp focus is called depth of field. This depth willvary with the distance of the object in critically sharp focus,the focal length of the lens, and the aperture used.

Figure 49 illustrates the manner in which various points of anobject are recorded on the image plane. Here it will be seenthat object point (A) is critically focused on the image planeat (A'). The more distant point (B) is brought to sharp focusin front of the focal plane at (B') and the diverging rays forma circle or disc of confusion on the focal plane. The neare .•object point (C) is brought to sharp focus in back of the focalplane and the converging rays form a disc of confusion on thefocal plane. If the diameter of discs (B") and (cn) is not greaterthan the largest permissible circle of confusion in order toproduce a satisfactorily sharp image, then the distance betweenobject points (B) and (C) is by definition the depth of field.Likewise, all object points lying between the near depth planeand the far depth plane are said to be in focus.

In order to take advantage of selective focusing, as well asto be sure that all objects in the selected field of view will be

PHOTOGRAPHIC. LENSES AND SHUTTERS 67

;::»o'"1)oZ-i»

-0Z[Ilz'-<

1)"''''

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in sharp focus, the amateur often wants to know the depth offield of his lens. This is a simple matter which can be deter-mined by means of two formulas, As was observed before, thehyperfocal distance of the lens used is a determining factor.

Let: D = Distance of object focused on.H =Hyperfocal distance of lens.N =Near depth plane.F =Far depth plane.

HXDThen: N= ---

H +D

H XDF~--

H-D

Suppose we are using a lens with a 6-inch focal lengthstopped down to 111, and desire a circle of confusion of 11500inch. According to formula, the hyperfocal distance would be136 feet. If we focus on an object 40 feet from the camera,

136 X 40N =----

136 + 40

136 X 40

5440= -- = 30.9 feet

176

5440= -- - 56.6 feet

96F=---

136 - 40

Depth of field, then, extends from 30.9 feet to 56.6 feet, andobjects falling within these limits will be sufficiently sharp.Now suppose we focus the camera on the hyperfocal distance.Then:

136 X 136N=---

136 + 136

136 X 136

18496= -- = 68 feet272

18496= -- ~ 18496 feet or 00

oF=

136 - 136

proving that objects from half the hyperfocal distance to in-finity are approximately sharp when the camera is focused atthe hyper focal distance, as stated previously.

It is often possible to obtain a depth of field table for yourlens from the manufacturer. If not, it's a good idea to makeone, as such a table will prove very useful. It not only shows -'the limits of sharpness under different conditions, but also thezones of softness outside the limits where a soft backgroundmay be desired. Thus a portrait can be kept within the area ofsharpness while the background can be in the soft zone whereit is not prominent.

Depth of Focus

Depth of focus must not be confused with depth of field, forthey are decidedl:v different. Whereas depth of field refers to

PHOTOGRAPHIC LENSES AND SHUTTERS 69an area in front of and behind the main object plane, depth offocus defines the small range of positions which the focal orimage plane may occupy without noticeably affecting thesharpness of the image. It is particularly important withgreatly magnified views, say two or three times the size of thesubject; we will find that the shorter the depth of field, thegreater will be the depth of focus.

For example, if we are enlarging two or three times thesubject size, the lens focusing adjustment is very critical, afraction of a millimeter in some cases, but the back focus can bechanged through quite a distance without affecting the sharp-ness when the groundglass screen is moved for this purpose.View cameras and studio cameras are provided with methodsof back-focusing by which the groundglass can be moved backand forth on the bed as well as the front lens board.

The back focusing system is not as sensitive as the front lensfocusing. This is of assistance in many cases where the frontfocusing operation requires a small fraction of a millimeter ofadjustment as it often does in microscopic photography.

Resolving Power and (I)

The greatest resolving power of a theoretically perfect lensis at maximum aperture because the effects of diffraction are ata minimum under this condition. This assumes, of course, thatall aberrations are reduced to zero and that depth of field doesnot enter the problem.

The fact that there is some improvement with commerciallenses when the iris is moved down one or two points hasnothing to do with the resolving power of the lens but is con-cerned with other factors such as imperfect or incompletecorrection. Another effect that acts adversely upon the resolv-ing power of the lens is lack of sufficient resolving power in theordinary film now being produced so that the resolving powerof the lens is secondary to the many other factors entering II1tothe problem.

Where there are a number of objects at varying distancesfrom the lens, stopping down the lens will make a considerableimprovement by increasing the depth of field, but this is in-dependent of the basic resolving power of the lens. Again,where the full correction has not been made for sphericalaberration, the stopping down may cut off offending rays fromthe outer circumference of the lens where the greatest aberra-tion exists.

Thus, we must carefully distinguish between the resolvingpower of the lens itself and its ability to preserve detail at wideopen aperture and definition, which may be controlled by meansentirely outside of the lens.

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Aperture and Definition

Always within limits, the adjustment of the iris diaphragmcan be made to improve definition under certain conditions.Certain aberrations are minimized, notably spherical aberration,with the iris down to f 16, principally for the reason that dis-torted rays from the outer circumference of the lens are sub-dued while the less distorted rays from the center of the lensnow predominate. The total effect's to reduce the general hazethat obscures detail when spherical aberration is present andthe lack of accurate focusing when chromatic aberration ispresent.

However, when we get an opening smaller than f 16, there isa sudden increase in the effects of diffraction and, when thislimit is passed, the image once more grows hazy. At one timeit was believed that the improvement at small aperture was dueto the narrowing of the pencil of rays by the aperture, but asall of the other elements are narrowed and reduced as well asthe circle of confusion. this seems hardly to be the answer.

CHAPTER V

LENS ABERRATIONS

NOLENS design c'an be theoretically perfect .• no mat.terhow highly developed it may be, because this perfection

would be contrary to the basic nature of light. The diffractio~of light or that property that causes the ray to bend when Itpasses through an aperture, prohibits perfection in lens design.We can only hope to produce a lens that will give satisfactoryresults in practice or one that closely approximates the desiredperformance.

The best that can be expected is a compromise between sixor seven aberrations, even with. a complicated objective, thatwill most nearly satisfy the conditions under which the lens is tooperate. Some conditio.n.s call for a sharp-cutting lens cap~bleof producing fine definition and detail, while other c~ndlhonsplace a premium on softness or r~undness. for pictor ial w?rkand portraiture. Still other operating ~ondltlons call for highlens speed in defiance to all other qual;tles-and so on, througha long list of conflicting factors of a like nature.

The popularity of color photography has placed anotherburden on lens design, as this requires complete elimination ofcolor distortions that exist with many lenses that are perfectlysatisfactory on black-and-white photography, In general, thefollowing are the principal requirements of a satisfactoryobjective:

1. Color-correction so that rays of all colors are brought to a focus inthe same plane.

2. Perfect coverage so that a uniform degree of definition is maintainedall over the negative, in the corners as well as in the center of theimage; a flat field.

3. Freedom from astigmatism with equal sharpness on vertical andhorizontal lines, even in the corners.

4. Freedom from coma or local distortions.

5. Rectilinear truth by which straight lines on the object are shown asequivalent straight lines in the image.

6. Freedom Irom flare or fog patches due to interior difficulties withinthe lens.

7. Uniform illumination or equal lighting all over the picture-at theouter edges as well as in the center of the negative.

8. Sharp definition or a high degree of sharpness, distinctness, andclarity in the small details,

71

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Most desirable, of course, is a lens of high speed with freedomfrom distortion and which produces good definition. Consider-ing the natural aberrations in the materials used and whichmust be overcome, some of the modern products of the lens-maker's art are indeed scientific wonders. Let us look into thematter of these aberrations before discussing the differenttypes of photographic lenses.

Chromatic Aberration

The image produced by a simple lens shows a number offaults which detract from it, In the first place, due to thedifference in refraction of the component light rays, as we havepointed out before, the blue rays witl focus in front of the lessactinic, but more conspicuous, visual rays. This defect canbe compensated for to a certain extent in two ways. First, bythe use of monochrome filters which limit the light to a verynarrow region of the spectrum. and secondly by focusing bythe chromatic difference-that is, by recognizing how far infront of the critical visual focus the actinic rays focus, and bymoving the groundglass forward this amount after the imagehas been critically focused to the eye.

In using some of the older semi-achromatic portrait lenseswhich give rather soft images it has been suggested that thegroundglass be racked forward beyond the focal point, and thenracked away from the lens until the image is in focus. Bydoing this, and by stopping as soon as the image becomes criti-cally sharp, we shall get a sharper image on our film thanwe would if we extended the bellows beyond the focal pointand then racked toward the lens, since there is a fairly widezone in which the image will appear visually sharp. Due tothe fact that the blue rays are refracted the most, by startingwith the bellows fairly well collapsed we rack back to reachthis point of greatest sharpness and stop as soon as the imageis satisfactorily sharp. See Fig. 50,

The usual difference in focus for the sharp visual image andthe sharp chemical focus is about 2% of the focal length.Therefore, if the camera is focused critically to the eye and-then racked toward the lens by about this amount the image onthe negative should be quite sharp-certainly much sharperthan it would be if the picture were taken at the point of bestvisual focus. The effect of chromatic aberration is much morepronounced in negatives made on panchromatic film than onthose made with process or color-blind film, since these filmsare not affected to any extent by the yellow or red regions of·the spectrum; if we adjust for the blue rays the out-of-focusred rays will not have any material effect on the image.

To show up chromatic aberration in a simple lens or a lensof poor quality, point the camera toward the sun and receive

PHOTOGRAPHIC LENSES AND SHUTTERS 73

its image, on the groundglass. If the image is out 'of focus sothat the Image of the sun is about 0 inch in diameter it willbe seen that the margin of the image is surrounded by a bluehalo. By changing the focus the color of the halo will changefrom blue to red.

In correcting lens systems for color, the purpose for whichthe system is to be used determines the type of correctionattempted. Instruments designed for visual use only are cor-rected to give their most critical definition in the region of theD line of t~e spectrum-a-that ,is, in the yellow or yellow-greenregion-e-while those instruments designed for photographicwork ~ould be corrected in the blue and violet regions. Photo-graphic instruments which require visual focusing would there-fore necessrtate the yellow and blue regions focusing at thesame p0111t.

An understanding of the above will indicate why the resultsobtained by using a binocular in front of a photographic lensusually give poor definition, since the binocular or telescope iscorrected only for the visual rays and is not intended for photo-graphic purposes.

Achromatic lenses are those in which the chromatic aberra-tion .has been removed. This is accomplished by combiningP?slt!ve and neg:atlve elements having the same dispersion butdifferent refraction. In this way the separation of the com-ponent wavelengths is prevented while the entire bundle ofrays is bent as a unit. Practically, this is not possible for allcolors at the same time and is generally limited to the twospectral bands principally concerned. In photography, thiswould be the regrons around 6000 and 4000 Angstrom units(the yellow-green and the blue-violet regions). For processwork another zone must be included up around 7000 Angstrom

R

Fig. 50. This shows in exaggerated form the different distancesat which various colors are brought to a focus by a simple lens.

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LITTLE TECHNICAL LIBRARY 75PHOTOGRAPHIC LENSES AND SHUTTERS74units in the red region. Lenses corrected for three colors arecalled process or apochromatic lenses.

This property of light rays of different wavelengths to focusat different distances behind an uncorrected lens produces whatis known as chromatic aberration.

Curvature of Field

Another defect which is commonly encountered in simplelenses is produced by the fact that the distance from the lensto various points on the plane occupied by the film varies.In other words, suppose in our biconvex lens the distancefrom a perpendicular drawn through the center of the lensto the center of the plate is 7 inches. If the plate is 8xlOinches in size, the distance to the corner of the plate fromthe center of the lens will be about 90 inches (Fig. 51). Thisresults in the marginal image being blurred due to the fact thatthe critical focal point lies in front of the plate. This defectcould be remedied if the plate, instead of lying in a plane,were curved, the radius of the curve being equivalent to aline drawn from the center of the lens to the nearest point ofthe plate, in this case 7 inches. This is done in certain typesof astronomical work.

This condition is known as curvature of field. It is quitepronounced in simple lenses and is recognized by the fact thatthere is a rapid falling off in sharpness from the center- aswe proceed toward the margins of the picture. The usualmethod for overcoming this defect is to use only the centerof the projected image. This results, of course, in limitingthe angle included in the picture. Diaphragming will not mate-rially help this condition, since it is caused by the varyingdistances at which the film lies from the lens. It is normallycorrected by combining lenses of different curvatures.

In the case of positive lenses the curvature is concave towardthe lens as shown in Fig. 52 and is sometimes called positivecurvature of field, or undercorrection for curvature of field.In the case of negative or concave lenses the curvature is thereverse of that seen in the convex lens.

Fig. 51. Distances from lens to emulsion vary.

Fig. 52. Curvature of field makes soft margins.

Spherical Aberration

Curvature of field should not be confused with sphericalaberration. Notice that in Fig. 51 the rays depicted passthrough the same region of the lens, the difficulty being dueto the varying distances to different parts of the film fromthe center of the lens. Spherical aberration is concernedwith the difference in focus existing between the edge and Fig. 53 ", Spherical aberration aives general blur:

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central rays (Fig. 53). The convex lens brings the marginalrays to a shorter focus than the axial rays. This conditionis called spherical undercorrection. In the case of the concavelens the reverse condition exists and the condition is known asspherical overcorrection. This condition is corrected in ma~u-facture by combining positive and negative elements whichtend to neutralize the fault. For experimental purposes Itmay be corrected by using a ediaphragm which cuts off. themarginal rays. This, however, reduces the speed .mat~nally.Spherical aberration can be demonstrated by placing a circulardisc before the lens so that about two-thirds of the centralportion is covered. If the camera is. n?w focused c~refullyand then the diaphragm cut down until It meets the disc andthen the disc removed, any blurriness in the image wouldsuggest spherical aberration. A lens corrected for sphericalaberration is called an aplanat.

Astigmatism

Astigmatism is concerned with the phenomenon wherebypoint sources are reproduced in the image as minute,. crossedlines one of which is out of focus when the other IS sharp.Astigmatism is an aberration of the oblique rays and is not afault which affects the axial rays. It is caused by the factthat rays passing through the horizontal meridian do notfocus in the same plane as the rays passing through ..thevertical meridian. The discrepancy between these two POintsof focus results in a point being reproduced as a line at thepoint of focus of either the vertical or horizontal meridians.These two lines will be at right angles to each other andboth will not be sharp at the same time. The point of bestFocus will be in the region between the focus of the crossedlines indicated by circle (C) Fig. 54. In an image not focusedin this central region the image will be composed of crossedlines (one of which will be fuzzy due to lack of focus andwill lie at right angles to the sharp line) instead of points asis the case in anastigmatic lenses. Lines in different meridiansof the object will be reproduced with varying degrees ofdensity and sharpness depending on whether or not they lieparallel or at right angles to the plane of greatest sharpnessin the astigmatic image.

To show up astigmatism in a lens, cut a cross in a sheet ofcardboard, the bars of which measure 8 inches in length byabout y,; inch in width. Place the camera on a firm supportfacing a window. If the card is now placed before the Windowin such a position that the cross appea~s In a corner of ~hegroundglass one line may be sharp while the other remainsindistinct-in fact in some very poorly corrected lensesit is possible to focus one line critically sharp and have the

PHOTOGRAPHIC LENSES AND SHUTTERS 77

"",m ",JIN FOCUSHORIZONTAL LINE

IN FOCUS

Fig. 54. In a lens subject to astigmatism, the point of best focus liesat C, or midway between the focal planes where the vertical andhorizontal components of the cross are brought to a sharp focus.

GROUNDGLASS

CROSS AT MARGIN OF FIELDCROSS AT CENTER OF FIELD

Fig. 55. Astigmatism, if present, is found only at the marginsof a picture. As the lens is focused, the image of a cross assumessuccessively the shapes shown in the lower right corner above.

other line so far out of focus as to be indistinguishable (Fig.55). This test would be of value only in single lenses of verypoor quality. In lenses of better quality the astigmatism couldonly be shown up by picking up the image with a microscopein the focal plane and magnifying the image several times.Lenses corrected for astigmatism are called anastigmats.

Coma

A form of spherical aberration concerned with oblique raysinstead of symmetrical rays is called coma. In this case therays below the axis are refracted more sharply than thoseabove the axis, therefore the focal points do not meet at asingle point but in a series of noints (Fig. 56). Point sources

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Fig. 56. Coma is an aberration in a lens which. when present.produces a pear-shaped image of a circular shape or of a point.

are therefore resolved as pear-shaped discs. In the case ofthe meniscus lens. if the concave surface faces the film thepoints of the pear-shaped discs, wil! face toward the centerof the film; if the lens is reversed the condition wi!1 be reversed,and the points will face toward the periphery. These condi-:tions are sometimes referred to as inward or outward coma.In the case of lenses having this fault it may be overcome toa marked degree by cutting down the iris.

To demonstrate coma with a lens of poor quality, proceedas described under Astigmatism. Place the camera on a tablefacing a window. Put a card having a circular apertureabout y, inch in diameter in such a position that the imageof the hole appears in one of the corners of the groundglassand focus criticalJy. If the hole appears pear-shaped, comais present. Coma is also known as zonal aberration.

Distortion

Distortion (curvilinear distortion) is recognized by verticalor horizontal lines appearing curved vin the picture, and isdue to the fact that the lens causes progressively greaterrefraction as we pass from the periphery toward the center,or vice versa depending on whether the stop is placed infront of or behind the lens. Lines passing through the cen-ter of the lens are not affected by this condition nor are con-centric rings if their center is located at the optical axis ofthe lens. However, verticals or horizontals near the edgeof the film will appear to bulge toward the margin if thestop is placed before the lens and this is called barrel distor-tion. If, on the other hand, the stop is placed behind the lens,the center of horizontal or vertical lines will bulge toward

PHOTOGRAPHIC LENSES AND SHUTTERS 79

DIAPHRAGM1 LENS

BARREL DISTORTION

PINCUSH ION DISTORTION

Fig. 57. The two types of curvilinear distortion are given the names "bar-rel" and "pincushion" from the obvious shapes of the images produced.

the center of the picture, causing- pincushion distortion. SeeFig. 57. ~

Curvilinear distortion should not be confused with the dis-tortion in perspective resulting from lilting the camera as issometimes done in the photography of high buildings. Herethe distortion is in perspective, the extremities of paralJel linesconverging toward a common point. The lines, however, inthis case do not show any curvature as i the case in that formof abberation referred to above.

If the lens is carefully made and the stop placed at theoptimum position, the curvilinear distortion can be reducedto a negligible degree for ordinary work and may be removedentirely by using two lenses of opposite curvature which areeparated by a diaphragm. A lens of this type is called

rectilinear.

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Lateral Chromatic Aberration

Under Chromatic Aberration we have noted that whenwhite light strikes a simple lens the exit ray is both refractedand dispersed into its spectral components, the blue focusingnearest the lens, and the red at a greater distance. In dis-cussing spherical aberration we suggested that the rays pass-ing through the lens near its ~enter were not refracted as muchas were the rays passing through the lens near its periphery.

In lenses composed of crown and flint glass it is not possibleto correct the spherical aberration for more than one color.When the spherical aberration has been removed as far aspossible for the center of the spectrum, there remains a spher-ical under correction for the red, and an overcorrection forthe blue and violet rays. This defect has been noted in prac-tice as a more or less marked inequality between the chro-matic corrections for the central and the peripheral zones ofthe objective. The introduction of the phosphate and borateglasses by Abbe and Schott made it possible to correct thechromatic spherical aberration for two different colors atonce (and therefore practically so for all colors).

In all objectives of large aperture, composed of crown andflint, in which the front element cannot be made achromaticby itself, there remains, even when the color deviation alongthe axis has been corrected as completely as possible, a notinconsiderable difference in the magnifying power for differ-en t colors (difference of the focal length of the objective fordifferent colors when the position of the anterior focal pointis the same). This gives rise to marked color deviation out-side the center of the field which makes itself apparent inconspicuous borders of colors at the margin. (The imageformed by the blue and violet rays is larger than that of thered and yellow. It coincides with the latter at the center of thefield but extends over it more and more towards the margin.)

Flare

Two types of flare are met with at "times in examining pho-tographic objectives. The first and simplest type is mechanical,flare which is caused by bright spots of metal in the lens mountand can be eliminated by a careful examination of the mountand painting over with a dead black lacquer any spots whichare found and which might reflect light onto the glass sur-faces. Optical flare cannot be avoided completely in any lensas it is caused by reflections from the lens surfaces them-selves. However, in a lens of good quality this will be reducedto a minimum. It becomes more pronounced, however, as thenumber of glass-to-air surfaces increases.

Tolerance Limits

While the aim is toward perfection in all lenses, it shouldbe understood that a definite limit is placed upon the degreeof correction by the price of the lens. I t cannot be expectedthat the lens for a $2 box camera can be corrected to thesame degree as the objective for a $200 camera.

Where the price for a lens is so low that it must be placedinto the equivalent of mass production, it cannot be of thesame high grade as an expensive objective that represents manyhours of hand labor and intensive inspection. When com-menting upon the qualities of a lens, the price should be takeninto consideration.

PHOTOGRAPHIC LENSES AND SHUTTERS 81

Uneven Illumination

Since the distance from the lens to the film varies from thecenter of the film to its edges, it is quite apparent that theamount of light reaching the edge oi the film will be c~:m-siderably less than that striking t~e center. Other m~chal1lcalfeatures of the lens mount and diaphragm may also influencethe illumination of the film, but these are quite variable anddepend upon the obliquity of the rays. and th~ effect of thediaphragm on the marginal rays, par ticularly 111the case.ofthe older portrait lenses. In the modern compact anastig-mats the first-mentioned difficulty is the most common. It IScorrected to a great extent by the negative element whichtends to diverge the light passing through the lens; hO"Yever,it is not possible to do this completely I~ all cases .wlthoutintroducing other more serious errors. S1l1C~tl~e latitude ofpresent-day films is so great the effect of this inherent faultis not as noticeable as formerly. In order to check a lens forthis condition, photograph the unobstructed sky on process film,exposing so as to yield a negative which is ,not too dense. Ifthe negative is of even density all over (medium gray) WIthoutany falling off in silver deposit near the corners, the lens maybe considered of high quality in this respect.

Limits of Definition

While errors in the definition or sharpness of a lens are notordinarily considered in the light of ~berrations, yet poo,r defini-tion might be considered an aberration and should be includedas one of the seven errors more commonly known as aberra-tions. Definition is the degree of sharpnes.s, distinctness, ~ndclarity with which a lens shows small details on the ne!SatIve.In the majority of cases, definition is one of the !!lost .1I?por-tant factors concerned with a lens, particularly m miniature

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camera lenses which cal1 for relatively great enlargement,news photography, architectural, and mechanical work. Itis not of so much importance in portraiture or pictorial pho-tography.

Lens definition is largely controlled by the accuracy of theworkmanship and the effects of diffraction, that property whichcauses a ray of light to spread out after passing through anaperture. Diffraction converts the fine point of light into ablur that destroys sharpness .and detail. The smaller theaperture, the larger will be the blur and the poorer will bethe definition. It is therefore desirable to limit the size ofthe iris or f-number to f22 with the average hand camerasto avoid diffraction, and with very fast lenses this aperturemay reach a minimum of f 11. This difficulty largely disap-pears with lenses that are highly corrected for spherical aber-ration, and with such lenses sharp images and contrast canbe obtained at almost any stop opening. The matter ofspherical aberration was discussed earlier. Objectives havinga high degree of definition usually contain at least three simpleunit lenses and sometimes 12 or 14. Single lenses have poordefinition, except the Wollaston meniscus which shows fairlywell when stopped down to a very small aperture.

Resolving Power

This factor, which also has to do with sharp definition, isnot usual1y included in the list of aberrations, but as it is afactor of design and workmanship it is mentioned here. Theobjectives of high-grade miniature cameras have a high resolvingpower, and aerial cameras that must distinguish fine details inthe terrain several thousands of feet below the airplane, mustalso show high resolution.

Resolving power is the ability of the lens to form dis-tinguishable images of objects separated by very small angulardistances so that they show as two distinctly separate pointsThe greater the separation, the better will be the detailResolving power is improved by a small field and narrow angle ofview, and also by the elimination of n!Sidual spherical aber-ration. The measure of resolving power is the ability of thelens to separate bundles of small parallel lines ruled witha known spacing. The lowest limit is the separation of linesspaced 11100 of an inch apart which will show as separatelines when viewed from a distance of 10 inches. If these linesare bunched so that they do not show separately, then theresolving power is less than 1/100 of an inch. At a distanceof 10 inches this is equivalent to about 3 minutes of arc. Theexamples on the next page show much greater resolving powerthan this minimum.

PHOTOGRAPHIC LENSES AND SHUTTERS 83

Resolving Power Specifications _U. S. Army Air Corps low limit. ..•...•.•.•......•....... 1/200"Plaubel Anticomar f 2 ...................•..•.....•••.... 1/400"Leica f 2 Summar •....................•.•.............. 1/1500"Cine Kodak 1/1000"Miniature Kodaks ....•.......•.....•..•................ 1/500"Folding Kodaks ••.•...................••...•........... 1/200"Zeiss Contax (declared) 1/750"

The resolving power of al1 theoretically correct lenses ofthe same effective diameter, regardless of the f-number, IS thesame. Also, the linear separation of images just resolved inthe focal plane by a given lens is proportional to the f-numberand independent of the effective diameter.

Monocular Vision

The ordinary camera has only one eye or lens while thenormal human being possesses two eyes or viewing lensesseparated by a perceptible distance. It is perfectly natural,therefore, that the photograph produced by the single-eyedcamera and the view observed by two lenses from two P01l1tsof view do not agree in many particulars.

The single-lens camera sees along one straight line, hencethere is no suggestion of depth or solidity except that a distantobject shows smaller than one close at hand. The depth ofthe subject or its "third dimension" is entirely lacking, andit is largely for this reason that an animal does not recognizeobjects in single-plane representations.

But viewing an object from two points of view, as with theeyes, gives us the third dimension and the impression of thick-ness or depth with a solid object. The two eyes see alongtwo independent converging lines with the object at the inter-

Fig. 58. Where the single lens of the camera sees objects as inone plane, the eyes-through the effort of focusing and the sepa-rated viewpoints-see depth through an added third dimension.

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section of the view lines and it is the distance of the inter-section from the eyes that gives the impression of depth.Further, in the case of small objects, the two eyes may seeon both sides of the object, once again adding to the depthimpression.

Thus, in Fig. 58A the single lens (L) of the camera can makeno distinction between the same object when placed succes-sively in the three positions (A-B-C), except in the point ofsize. It only sees one face of tlte object at any position withno impression of depth. In short, the lens sees only one fiatpresented plane.

In the case of the two eyes (E-E) in Fig. 58B the threeincluded view angles (a-b-e) vary with the distance at thepositions (A-B-C). Thus the angular difference gives. animpression of depth that is lacking with the single cameralens. Note also, that the two eyes "see around" the surfaceof the object so that a part of the sides is seen as well as apoint-blank front view. This is known as a stereoscopic visionwhich yields natural, plastic views, a phenomenon of binocularvision.

To obtain these views photographically, two-lens camerasknown as stereoscopic cameras are built, with two carefullymatched lenses and irises that take two pictures simultaneously.Viewed separately, these negatives reveal nothing more than isobtained with an ordinary lens, but when prints are viewed simul-taneously through a special two-lens viewer, or by means of twomirrors; the depth of objects in the picture immediately becomeapparent. Stereoscopic lenses are remarkable only in the factthat they must be very carefully matched for focal length, speed,and field, so as to make the two negatives match perfectly.

Stereo-Aberration

When the diameter of a single lens exceeds approximately30 to 4 inches we are subject to a peculiar aberration that isdue to the large single lens acting as a stereo lens. This erroris not in evidence with- small-diameter lenses but when thelens diameter becomes large enough to ~pproach the spacingof the eyes, then we will have a left and right hand imagesimultaneously.

Perspective and Focal Length

The fact that images of objects become smaller as theirdistance is increased from the eye or camera, gives us thephenomenon known as perspective. Thus, when we viewobjects having long straight lines, the lines seem to convergetoward the more distant points until they finally unite on

PHOTOGRAPHIC LENSES AND SHUTTERS· 85

VI V2

H ~=====~t=:Lt±=======~ H

Fig. 59. This shows what may happen when the camera isheld in a tilted position to photograph tall buildings.

the horizon. Perspective, while not always apparent withirregular or disconnected bodies, still exists.. In Fig, 59 we have a perspective layout of a tall building111 which (H-H) is the horizon with the two vanishing points(Vi) and (V2). It will be noted that all normally horizontallines intersect at (Vi) and (V2) on the horizon so that thereare no parallel lines. All normal vertical lines intersect at thethird vanishing point (V3) located on the vertical' hence thebuilding contracts toward the top when the camera is pointedupwards. This distortion, which is frequently seen in pho-tographs, is simply due to the natural effects of perspective.

There has been much discussion on the effect of focal lengthon perspective and it is unfortunate that it has become theaccepted but erroneous belief that a long-focus lens is requiredfor properly rendering perspective values. which is not thecase. It can be proved that, under. equal conditions, the per-spective effect with a short focal length lens is identical withthat of a long-focus type providing that the camera positionis relatively the same, and that the objects are more than6 feet from the lens.

For example, if a view of a building is taken with a lO-inchlens so that the image completely fills the picture space, the

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perspective will be identical with the same view taken witha 2-inch lens from the same camera position if the imageformed by the 2-inch lens is enlarged to the same size as thepicture made with the la-inch lens.

Depth Distortion

When the lens is placed very near the subject in taking aclose-up view, say at a distan~ less than 6 feet, there willbe a considerable amount of distortion. The parts of the objectnearest the lens will appear unduly magnified while portionslying only a short distance behind the foreground will appearmuch smaller in proportion. Thus, if a portrait is taken witha very short focal length lens, it will be necessary to' placethe lens very close to the subject in order to fill the picturespace and the subject's nose will be so greatly magnified thatit is all out of proportion to the remaining features of his face.

This is not due to incorrect or distorted perspective, butto exaggerated magnification which occurs when a certain min-imum object distance is reached. To avoid this distortion, thefocal length of the lens should be great enough to give thedesired size of image at a distance of at least 6 feet from thesubject. Commercial portrait studio lenses are of compara-tively great focal length, say 12 to 24 inches or even more,and with such lengths it is possible to obtain a large image ata considerable distance from the object. The use of 2-inchand 3-inch focal lengths with amateur miniature camerasusually accounts for the unnatural and distorted portraits takenwith such cameras. In such cases, it is better to take asmaller image at a comparatively great distance and increasethe enlargemen t.

Where close-ups are taken frequently, as in a studio, along-focus lens should form a part of the equipment. Roughly,the focal length should be about 50 per cent greater than thelong side of the negative. If the camera takes 4x5 picturesfor example, the focal length should be 50 per cent greaterthan 5 inches or 7y,; inches.

Roundness and Gradation

In portraiture and pictorial photography, sharp, hard defini-tion is not ordinarily desirable, 'hence some of the aberrationsare permitted to remain in lenses designed for these uses.In many studios, the old achromatic single lenses and rapidrectilinears are still in service because they retain a sufficientdegree of spherical aberration to conceal sharp changes incontour and therefore give the characteristic of "roundness"to the image.

PHOTOGRAPHIC LENSES AND SHUTTERS 87

Soft gradation is directly associated with roundness. It isthe almost imperceptible blending of one surface into anotherwithout noticeable demarcation. To obtain a soft, pleasingnegative does not mean that the lens is thrown out of focusto break the sharp lines. A truly soft lens preserves its defini-tion all over the plate but in a subdued degree. A lens canproduce a soft picture and yet be in focus.

In some cases this is attained by residual aberrations, suchas residual chromatic aberration, while in other lenses thecircle of confusion is increased in size to 1150 or 1/75 inch,the overlap of the circles being sufficient to break brilliantsharp contrast between adjacent areas. Certain special studiolenses are made adjustable so that the hardness or softnesscan be varied from a sharp critical focus to a soft diffusedpicture bordering on fuzziness.

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CHAPTER VI

PHOTOGRAPHIC LENSES

THE first lens to meet wita universal acclaim was themeniscus of Wollaston (1812), since it worked at an aper-

ture of about f 11 and covered an angle of about 45 degreeswith satisfactory sharpness. This lens was mounted with theconcave side facing the subject and the diaphragm was placedin front of the lens. Since a single lens cannot be chromaticallycorrected it was necessary to focus sharply for the visual raysand then to correct for the chemically active blue and violetrays by racking forward about 2% of the focal length in order

Fig. 60. The simplest lens is the meniscus, developed in 1812.

to obtain a sharp picture. This lens is stil! used universally- ininexpensive box cameras, and since it is not possible to focusthese instruments visually, the manufacturers have placed thelens at the point of optimum focus (Fig. 60).

Chevalier introduced an achromatic doublet in 1821 whichbrought the chemical and visual rays !o a comm<;)!1focus <l:ndalso improved somewhat on the spherical aberration to which

Fig. 61. The achromatic doublet was introduced about 1821.

Wollaston's meniscus was subject. This objective consisted ofa biconvex lens cemented to a biconcave, both lenses beingsymmetrical (Fig. 61).

In 1857 Grubb patented an achromatic doublet composed ofa meniscus similar to Wollaston's, combined with a negative

88

PHOTOGRAPHIC LENSES AND SHUTTERS 89

element of flint (Fig. 62). This combination departed fromthe symmetrical type which had been introduced by Chevalier.

In 1869 Goddard reversed the Chevalier landscape lens andadded a meniscus to the negative element, separated from it byan air space (Fig. 63).

Many modifications have been built up around these basic

Fig. 62. Unsymetrical achromatic doublet was made in 1857.

lenses and many of them are still being n:anufactured for por-trait and landscape work. Semi-achromatic lenses have certainqualities which are not obtainable with the anastigmats. Aerialperspective, roundness, and diff~lsion which blends one .surfaceinto another are their outstanding features. In por trait work

I-- ....

IFig. 63. In 1869 Goddard made this variety of +heChevaller lens.

they give a pleasing softness which eliminates the need for agreat deal of retouching, since blemishes are not so apparent.The softness of fabrics and furs is accentuated by these ob-jectives and consequently they have been used successfully inadvertising work.

The Double Achromatic Lens

The first practical achromatic doublet was the rapid recti-

IFig. 64. Steinheil's rapid rectilinear or aplanat was made in 1866.

linear or aplanat which was introduced by Steinheil in 1866.From Fig. 64 it will be seen that this combination consisted es-

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sentially of two combinations similar to the Grubb landscapelens mounted with their concave surfaces facing each other andseparated by an air space. The diaphragm was placed half-waybetween the doublets. The resulting complete lens was practi-cally free from the aberrations to which the previous lenseswere heir. The entire combination worked at an aperture ofabout f 8 and since the objective was symmetrical, either thefront. <?rthe rear element could be used separately, under whichcondition the focal length was aboat twice that of the com-bination; however, the speed was materially reduced.

Petzval made the first real step forward with his portrait lenswhich he developed in 1840. This lens had the remarkablespeed of f 3.4 and shortened the time of exposure to the extentthat portraits could be taken by Daguerre's method without

I

Fig. 65. Petzval's portrait lens gave the then fast speed of f 3.4.

undue discomfort. Petzval's lens was not symmetrical. Thefront doublet consisted of a biconvex lens of crown glasscemented to a biconcave flint, while the rear element was com-posed of a concavo-convex flint separated from a biconvexcrown by. an air space. The axial pencil of rays is very wellcorrected in this lens, but It covers a very narrow angle criti-cally and except for portrait work has been replaced by theanastigrnats (Fig. 65).

The Anastigmats

Although it was possible to overcome most of the aberrationsby combining cro:wn and flint glasses, astigmatism remained a-

Fig. 66. The Gauss lens of Jena glass first correded astigmatism.

stumbling block until the Jena glasses were· introduced byAbbe and Schott in 1886. Following the announcement of these

PHOTOGRAPHIC LENSES AND SHUTTERS 91new glasses, opticians set to work to develop lenses. free fromastigmatism as well as the other aberrations.

The Gauss Objective

This is probably the simplest of the lenses which were de-signed to correct astigmatism, since it is composed of only twoglasses. The front element consists of a concavo-convex nega-tive lens with the concave side facing the subject, and the rearelement, which is separated from the front lens by a very nar-row air space is biconvex (Fig. 66).

The Cooke Triplet

As has been mentioned before, the number of elements is notnecessarily the criterion upon which to postulate the excellenceof a photographic objective. In the Cooke triplet we have a

Fig. 67. The Cooke triplet is simply constructed, yet very effective.

3-lens objective covering a relatively wide angle at a very highaperture (550 at f 3.5). None of the elements are cemented;all are separated by air spaces. In construction, the Cookeconsists of two identical positive elements separated by a nega-tive. By placing the diaphragm between the last two elements,flare has been eliminated. The usual aberrations have been

I

Fig. 68. The Protar type of lens gave exceptionally fine definition.

reduced to a minimum; this objective has been used extensivelyfor color work with excellent results. (Fig. 67).

The ProtarThis objective, developed by Dr. Rudolph, is a 4-lens cornbina-

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tion. The front cemented doublet consists of a crown and a flintachromat similar to the combination used in the rapid recti-linear. The rear cemented doublet is composed of the newer] ena glasses. to correct for astigmatism which could not beovercome with the old crown and flints. Although this objec-tive was relatively slow it had exceptionally fine definition.(Fig. 68.)

The Tessar

This objective represents the most popular of the 4-lens ob-jectives. Many modifications of it have been made during the

Fig. 69. The Tessar type le~s has been in wide use for thirty years.

past 30 years, and some improvements have been made in thisway for special purposes. It is completely unsymmetrical. Thefront element is a plano-convex separated from a biconcavenegative element by an air space, while the rear element is a-cemented doublet composed of a biconcave or plano-concavenegative element combined with a biconvex positive. The dia-phragm is situated between the middle and rear combinations.This type of construction has been used on the Kinamo cinecamera at an aperture of IZ.7, although its standard aperture isusually considered as 14.5. Depending upon the aperture, theangle has been varied from 45· to 75· (Fig. 69).

The Ektar

Within the past few years a number of n{w objectives havebeen developed by the Eastman Kodak Company, based uponthe Tessar type and known as the Ektar. The Ektar 1 2, how-ever, deviates from the Tessar type and approaches more theunsymmetrical type which was developed from Rudolph's sym-metrical Planar. In general construction it resembles the Optic1 Z of Taylor-Hobson. Reports on these new objectives indicatethat a number of refinements have been made which have dis-tinctly improved these lenses for special purposes over theirolder counterparts.

Ektar f 6.3-36 em. This objective was designed for making

PHOTOGRAPHIC LENSES AND SHUTTERS 93

direct color transparencies or separations. It has- been cor-rected for all types of work where the image size does not

Fig. 70. The newly designed Ektar series is based on the Tessar.

exceed one-third of the object size. The angle of view is about53·, which will amply cover a 10xlZ" plate (Fig. 70).

Ektar 1 3.7-10.7 cm. This objective is similar to the Tessar

Fig. 71. This shorter Ektar is designed for 21/4x31j4 size cameras.

except that the rear combination has been reversed. It hasbeen designed to cover a plate size Zy,\x3y,\" (Fig. 71).

Ektar 1 3.5-5 ern. This lens is typical of the Tessar type of

Fig: 72. Still in the Ektar series, this design differs widely.

construction and is designed to cover the regular double frame35 m111film.

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Ektar f 2-4.5 ern, This objective departs' from the Tessar·type and resembles more the Taylor-Hobson Optic f 2 or theZeiss Biotar f 1.4 (Fig. 72).

The Radiar (Gundlach)

This is a modification of the Tessar type in which the rear

Fig. 73. The Radiar is another derivative of the Tessar type.

element consists of a cemented triplet. the added lens being anegative concavo-convex (Fig. 73).

The Goerz Dagor

Ever since its inception in 1893 the Dagor has been highlyregarded and is still extensively used. In construction it con-

Fig. 74. The Dagor lens has been in very wide use since 18'93:

sists of two symmetrical cemented triplets" separated by "an airspace. Either the front or rear elements Play be used independ-ently, thus doubling the focal length for getting a larger imagefrom the same point of view (Fig. 74).

Zeiss Sonnar f 1.5

In the Sonnar f 1.5 we have an example of a 7-lens objective.The Sonnar construction is related to the Triotar or triplet.since these lenses consist of three combinations with the dia-phragm located between the two back combinations. Due to the

\

PHOTOGRAPHIC LENSES AND SHUTTERS 95

angle and aperture which have been maintained it has beennecessary to substitute cemented triplets for the simple lenses'found in the Triotar or triplet (Fig, 75).

Fig. 75. This example of the Sonnar series is made with 7 lenses.

Convertible Pro tar

For versatility and perfection of correction, the ConvertibleProtar is among the best. Its only possible drawback lies inits relatively slow speed, working as it does at from f 6.3 tof 12,5. In this objective Rudolph modified his Pro tar and com-

Fig. 76. The Convertible Protar is derived from the first Protar.

bined all four lenses into a single cemented element. Thecomplete objective consists oftwo of these 4-lens combinationsseparated by a centrally located diaphragm. Since Protar ele-ments may be obtained in a number of focal lengths and usedeither separately or in combination, a very versatile lens systemmay be built up (Fig. 76).

Turner-Reich Convertible

The most 'complex of the objectives which we shall consideris the Turner-Reich Convertible, which consists of two ele-ments composed of five cemented lenses each. This objectivehas similar features to those listed under the ConvertiblePro tar (Fig, 77).

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Fig. 77. The Turner-Reich Convertible has two S-Iens elements.

Apochromats

These lenses are of only passing interest to the amateur astheir principal use is .in the field of process and color work.They represent the highest type of correction for all aberra-tions, and in addition are corrected for three colors instead oftwo as is the case with the Anastigmats.

f

CHAPTER VII

AUXILIARY LENSES

THE regular objective furnished with the camera takes care ofaverage conditions only; when extreme perfor:mance of

any sort is demanded, additional or auxiliary lenses must beobtained.

In general, auxiliary lenses can be divided into two classesaccording to the method of mounting them:

1. Special mounted lenses, complete within themselves, used as sub-stitutes lor the normal lens and attached to the lensboard 01 thecamera in the same manner as the normal objective.

2. Slip-on lenses. Simple lenses used in combination with the normallens by slipping them on the barrel 01 the normal lens, modifying itscharacteristics.

The special mounted lenses of the above classification arecomplete lens assemblies and are substituted for the normal lens.The slip-on lenses are usually simple single lenses placed infront of the normal lens to create the desired effect. The latterlenses are the cheapest and least satisfactory of the auxiliaries,but can be used where complete rectification is not necessary.

There are many auxiliary lenses for various purposes, butthe most commonly used lens is probably the telephotowhich is employed for photographing distant objects to alarger size or scale than is possible with the normal lens. Thenwe have the portrait lenses especially adapted for close-ups,and so on, according to the itemization in the following list.In most cases, both mounted lenses and slip-ons can be hadfor e.ach purpose indicated.

1. Telephoto lenses used lor enlarging distant scenes or lor "bringing updistant objects,"

2. Portrait lenses for close-up views in portraiture or for photographingsmall objects in close-up views.

3. Wide-angle lenses having an angle of view greater than the normallens to include more of the view at a short distance.

4. Copying lenses for photographing line drawings, copying printedmatter, or other flat field close-up photography.

5. Salt-focus lenses for pictorial work of a general nature. Also knownas diffusion lenses.

5. Salt-sharp lenses which can be adjusted for any degree of softness orhardness required.

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Telephoto and Long-Focus Lenses

These are special objectives for magnifying the images ofdistant objects, or apparently "bringing the objects up to thecamera." They have been used for many years but have gainednew life with the advent of the miniature cameras that haveappeared in the last few years.

Actually, a special telephoto len~ is not required for the pro-duction of large images if the belJows extension is long enough,for all that is really necessary is a lens having a longer focallength than the normal lens. Thus, if the normal lens has afocal length of 2 inches, then a regular 6-inch objective lenswill give an image three times as large as that of the normallens from the same camera position. This is the simplest andcheapest way out of the difficulty if the construction of thecamera will permit the use of a long-focus lens, but very fre-quently the bellows extension is not sufficient to handle aregular lens of this size.

The true telephoto lens differs from the regular objectivein the arrangement of the elements, and requires less bellowsextension than a regular lens for the same effective focal length.

In Fig. 78 we have a typical simple objective with a long focallength (L), focusing at (F). The angle 0.£ the light cone is(a), and the length of the lens tube is (T). In general, thisis the same as any other objective lens of normal focal length.In Fig. 79 is shown a typical telephoto lens with the positivebiconvex objective lens (A) and a negative biconcave diverg-ing lens (B) at the rear of the objective. The positive frontelement (A) would normally bring the rays to a focus at (Fl)but, by the diverging effect of the biconcave lens (B), the focalpoint is prolonged to (F2), thus effectively increasing the focallength and the size of the image. Note that the angle (b)of the light cone is very much smaller than with the singlelong-focus objective, In many telephoto objectives the focallength of the lens and, therefore, the size of the image, cas, bevaried by changing the distance between the positive and nega-tive images by means of a rack and pinion.

The magnification of the image is the rrfbsure of the effec-tiveness of the telephoto lens. The magnification factor (M)is the number of times that the image of the positive lens isincreased by the addition of the negative lens. Thus, if theimage given by the positive objective (A) alone is 1 inch andthe size of the telephoto combined image at (F2) is 3 inches,then the magnification factor is 3 X (three times). We canfind the total effective length of the combination by multi-plying the focal length (Fl) of the positive lens (A) aloneby the magnification factor, Thus:

Total focal Iengtb (F2) - M XL

PHOTOGRAPHIC LENSES AND SHUTTERS 99

The separation of the two lens elements (S) is measured fromthe node of emission in the positive lens to the node of admis-sion. on the negative lens. This separation (S) of the twolenses must be greater' than the difference between the focallength of the two lenses but less than the focal length of thepositive lens (A). The best focal length for the negative lensis from one-third to one-half the focal length of the positive lens.

Let: S =Separatioll between the positive and negattve lensesin the combination.

L = Focal lengt h of positive lens.L' = Focal len~th of negative lens.M =Ma~nification or number of times Image ~iven by

complete lens is larger than that given by positivealone.

L'Then: S=L - L' +-

M

Fig. 78. This shows the path of light rays in a typical objective.

Fig. 79. In telephotos, focal length is increased by negative element.

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Camera extension (E) which is necessary for a given mag-nification is found by multiplying the focal length of the nega-tive lens by the magnification less 1.

E = L' (M - 1)

and when working at a given camera extension the magnifica-tion (M) is found by dividing the extension by the focal lengthof the negative lens and adding ••.

EM=-+L'

The effective f-number of the complete telephoto lens isequal to the f-number of the positive lens multiplied by themagnification. Thus, if the f-number of the positive lens. isf 4.5, then the speed of the combination with a magnificationfactor of 4 is: 4.5 X 4=f 18. The telephoto lens is slower be-cause the same amount of light is distributed over a greaterarea. It is usually advisable to work at even a smaller stopthan indicated here because the magnification of the Imagereduces the definition and, to recover normal definition, thelens should be well stopped down.

All this naturally increases the exposure time so that wemust arrive at-some means of computing the time if provisionhas not already been provided by markings on the lens barrel.Fortunately, many modern telephotos have equivalent f-nuin-bers marked on the barrel so that adjustment is as simple aswith any lens; but in some cases computation must be made,particularly with lenses of the adjustable magnification type.

The exposure time varies with the square of the magnifica-tion hence the exposure with a magnification' of 4 requires:4 X '4= 16 times the exposure necessary with the front positivelens alone. With a magnification factor of 6, the expqspr e willbe: 6 X 6 = 36 times as long. The exposure tim~ with anordinary long-focus objective lens will be the same as with anyother normal lens having the same f-numper.

Plate coverage or the amount of the view on the plate dependsupon the magnification of the telephoto. With high magnifica-tion and a large image we naturally do not have as much of theimage on the plate as with a normal lens. For- example, anentire building with room to spare at top and bottom may beshown with a normal lens while only a few windows or detailswill be accommodated on the plate with a high-power tele-photo. This means that we must have a special finder for thetelephoto if the camera is of the finder type. Screen focusingcameras and reflex types, of course, show matters as theyactually are.

PHOTOGRAPHIC LENSES AND SHUTTERS 101The Schneider Te1e-Xenar f 4.5 is a 5-1ens- unsymmetrical

combination partly cemented. The rear element, as is the casewith all true telephotos, is a negative (Fig. 80). Dallmeyer hasdeveloped a telephoto combination composed of two cemented

Fig. 80. The Tele-Xenar telephoto is a 5-lens combination.

elements, the front doublet being a positive combination whilethe rear element is a cemented negative doublet. Telephotocombinations have been developed by this firm having aperturesas high as f3.3 (Fig. 81).

Since the front lenses of telephoto objectives are usually

Fig. 81. The Dallmeyer telephoto has two cemented elements.

quite large, special care should be given to the selection offilters for telephotography. Optical glass filters are recom-mended, but if gelatin filters are used they should be mountedin "A" glass only.

Slip-On Telephotos

Slip-on lenses used by fitting them over the mounting of thenormal camera lens are frequently very useful and can be had ata low price. These lenses are usually of the biconcave type,either plain or achromatic, but must be used with a camerahaving a double or triple bellows extension. They cannot beused with single-extension folding cameras or miniatures sincethey increase the focal length of the normal lens (and conse-quently narrow the angle of view). The corrected slip-ons arecommonly known as Distars from the trade name of the Zeisslenses that have' been in use for so many years.

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Portrait Lenses

Supplementary portrait lenses can be had either as mountedlenses or as slip-oris, and are very useful auxiliaries if muchportraiture or close-up work is done, Their principal purposeis to avoid the exaggerated perspective shown by normal lensesand to introduce a sense of roundness and softness into the por-trait. Some of these lenses are pr01'lded with adjustments bywhich any desired degree of softness can be obtained by vary-ing the degree of diffusion in the lens.

Slip-on portrait lenses are positive lenses which, when placedover the normal lens, shorten its focal length and permit amuch larger. image of a small object at close range. .

Wide-Angle Lenses

Wide-angle lenses ar~ very useful accessories. The view anglecan be increased from tne average 50° of the normal lens to 80°or 90° so that a much greater portion of the subject can be takenin at a short distance. They are particularly useful for archi-tectural work or interior views where the distances are short

and where a considerablebreadth of view must be cov-ered at short range.

In Fig. 82, we show thedistance (a-b) taken in by a50° angle at the distance (L)and the much greater length(A-B) taken at the same dis-tance with a 90° wide-angle \lens. This is a very imporf-ant matter in situationswhere the camera distance(L) is limited.

Unfortunately, the speedof a lens mast decrease whenthe width of its field in-creases, hence wide-angleIe n s e s a r e essentially slow

lenses. On the average, they range from f 9 to f 18 at full aper-ture and are usually stopped down more than this where sharpdefinition is desired all over the plate. Wide-angle lenses forminiature cameras will average f 6.8 full aperture with an occa-sional f 4.5.

Wide-angle slip-ens can be applied to the normal lens of thecamera with a decided improvement in the angle but with aconsiderable loss in definition and speed. In the majority of,

A

B

Fig. 82. A wide-angle lens givesa field wide than the normal.

PHOTOGRAPHIC LENSES AND SHUTTERS 103cases, the normal lens must be stopped down as far as possibleto retain defin ition when a wide-angle supplementary is used.This type of auxiliary lens is referred to as a Proxar from theZeiss trade name.

Wide-Angle Objectives

The Goerz Dagor has already been referred to under NormalObjectives. When used at the full aperture of f 6.8 the Dagorhas a useful angle of 70° but when stopped down this may beincreased to about 90°, thus bringing it into the class of wide-angle objectives. The definition of the Dagor is extremelysharp up to the corners of the plate and it is fully corrected forastigmatism as well as spherical and chromatic aberrations andthe internal reflections are practically non-existent. '

A Wide-Angle Dagor has been developed which works at amaximum aperture of f 9. The curves of this objective havebeen modified somewhat so that the angle has been increasedto 100°, making it an ideal objective for banquet work becauseof its wide angle and relatively high speed.

The Hypergon which was developed by Goerz some yearsago embraces an angle of 140°. This is a symmetrical com-bination cons,isting ~f two spherical elements. An interestingfeature of this lens IS the fact that the lens covers an imageabout 5 times the focal length of the objective (a 4~-inchHypergon covers 12xI6"). Focusing is done at f 22 and aber-ration is eliminated by using an aperture of f 32 for taking thepicture. To obtain even illumination over the whole plate a

Fig. 83. The Hypergon is an extreme wide-angle lens.

small hinged star diaphragm is required to shield the center ofthe objective, During 2/3 of the exposure it is placed in frontof the lens and rotated by means of air pressure furnished bya bulb, and the remainder of the exposure is made with thediaphragm removed (Fig. 83).

One of the most interesting lenses designed for wide-anglework was the Robin Hill lens made by Beck of London. Thisobjective had the remarkable angle of 180° and was designedto photograph the whole sky on a single plate when the camerawas faced vertically upward. A number of interesting pictureshave been reproduced which were taken with this objective. Aslong as the camera is pointed vertically up or down, rather

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interesting patterns may be obtained; if, however, the camerais pointed horizontally the 'distortion of verticals (barrel dis-tortion) is very noticeable and not particularly pleasing, Its

Fig. 84. This Robbin Hill lens covers a field of 180 degrees.

application except for trick effects is very limited. OriginallyIt was designed for meteorological record work (Fig. 84).

Copying Lenses

These lenses, including the slip-on type, are essentially flat-field lenses used for photographing drawings, paintings, printedmatter, small objects at close range, etc. This class also in-cludes the highly corrected process lenses employed by engrav- ,ers. All of the copying lenses must show exceptionally finedefinition in reproducing all possible detail in the original copy.

Slip-on copying lenses are very convenient for the amateurand many of them give very good results in combination withthe regular objective. This lens is usually a biconvex typewhich shortens the focal length of the normal lens but whichin effect increases the bellows extension so t1J.at small objectscan be taken full size without distortion. They make possiblewith a single extension bellows what would require a doubleextension bellows without the supplementary lens, while inthe case of cameras having double extension bellows, theywill permit copying at considerable magnification,

Soft-Focus Slip-Oris

These are really diffusion discs placed in front of the normallens for softening the outlines of pictorial views and resemblethe portrait diffusion type, It is believed by many that better

PHOTOGRAPHIC LENSES AND SHUTTERS 105results are obtained by diffusing or sof~ening the view with theenlarger or printer rather than by this lens.

Two types of supplementary attachments have been devel-oped to increase diffusion in the lens. One of these is a flatglass plate which has concentric rings and radiating spokespressed in relief on the surface. The other is in the form of asupplementary lens which introduces a certain amount ofchromatic aberration into the image, without altering the focusof the camera lens to any extent. While these attachmentstend to soften the image the aerial perspective is not renderedas effectively as is the case with the semi-achromatic lenses.

Notes on Slip-On Lenses

For'the best results, the achromatic type of slip-on lenseswith two cemented elements should be used .: These removeat least one aberration and are superior to the plain singlelenses that sell at lower prices. Slip-ons slow up the mainlens by some 20 to 30 per cent because of the added absorp-tion and reflections. This applies to all of the lenses evenwhere the apparent f-number is increased by a decrease inthe focal length of the lens. To insure proper working, themain lens should be well stopped down.

For proper operation, the supplementary lens should beplaced as close as possible to the normal lens. This is par-ticularly necessary when the supplementary is a biconvex lens.Care should also be taken that the slip-on ring fits the barrelsnugly and that the lens is not cocked nor tilted.

Calculations for Slip-On Lenses

In Fig. 85 we have the camera objective (A) and thebiconvex slip-on lens (B). We wish to determine the com-bined focal length when the camera lens focal length is(Ll ) and the focal length of the slip-on is (L2).

Ll X L2= --- (with biconvex slip-on)

Ll + L2

Ll X L2= --- (with biconcave s\lp-on)

Ll - L2

Total focal lerrg rh

The lenses are assumed to be in contact in the above formulasbut where there is a slight separation (S), we have:

Ll X L2LT =----

Ll + L2 - S

This is illustrated by Fig. 85, where (S) is seen to be the

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F

~----- v ----.1Fig. 85. It is comparatively easy to calculate theeffects of slip-on lenses with the accompanying formulas.

separation measured from the nodes of emission and admissionfor biconvex lens.

Let: u = Dtstance [rom supplementary lens to subject.y =Dlstance from center of camera lens to film.

M =Magntflcatton.D = Distance as marked on Iocustng scale.

LT = Combined focal tengrh.

Then:LT X v

v - LT u u- LT

LT Xuv=---

u -LT

u X L2D=---

L2 - u

The above formulas are for biconvex lenses, but when J"icon-\cave lenses are used as. supplementary lenses, the focal length(L2) IS used as a negative quantity.

Obviously the i-numbers of a lens will be changed when thefocal length is altered by a slip-on lens. To find the effectivei-number for the combination, the new focal length is multi-plied by the original i-number as marked on the rezular lensand the product then divided by the focal length of the regula;lens. Thus if

v LTu= M

F = focal lengrb of camera lensLT = Combined focal Ieng rh

!=!-number marked on lens

Then: effective I-number =LTXf

F

CHA.PTER VIII

CARE OF LENSES AND SHUTTERS

As A general rule, the less that the amateur has to do withhis lens, except for its protection, the better will be the

final results. Too much tinkering, rubbing, and fussing withthe lens assembly may, sooner or later, result in serious damageto a delicate and expensive piece of equipment.

Remember that no watch is as accurately adjusted as is thelens elements and the shutter mechanism of a good lens. Thismeans, in short, that the lens and shutter should be treatedvery gently and most certainly' should not be tossed about.Even a short fall may bump the lens elements out of align-ment so that the lens loses its sharpness or develops othertroubles.

Again, any camera deserving of the name should be worthyof a good carrying case for its protection against dampness,dust, or bumps. The eveready cases, while very convenient,are seldom tight enough to be a great deal of protectionagainst moisture, and moisture is a camera's worst enemy.Further, a miniature camera should always be provided withshoulder straps, and the straps should always be over theshoulders while the camera is being handled to prevent damageshould the camera accidentally slip out of your hands.

Lens Protection

Every lens should be provided with a tight lens cap that willexclude all dust and moisture. This may, under conditions,be left off with certain folding cameras that act as their owncaps when the bed is folded up; but other cameras should,under all conditions, be provided with caps. They may bevelvet lined or plain, but they must fit the lens mount tightly.

Lens glass is much softer than window or bottle glass, andis much more easily damaged. Further, the barium glass isof such a nature that it is acted on chemically by salt water,perspiration, fingerprint marks, or even salty air. This glassshould not be exposed to the air any longer than necessary, andspecial protection against moisture should be provided whenthe camera is carried in the pockets or otherwise exposed toperspiration. "

Even when well protected, certain lenses will "bloom" or

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become covered with a grayish haze, but in the majority ofcases this is harmless if care is used in wiping it from the lens.This should not be allowed to occur often as frequent rubbingmay destroy the polish on the lens, making it necessary toreturn the camera to the manufacturer for an' expensiverepolish job-if the lens justifies this expense. When thesurface of the glass becomes covered with a dark-bluish coatingthat indicates chemical action on the barium glass, the repolishjob is usually the only way out. This hard coating, whichextends down below the surface of the glass, is also the resultof the oil and salty perspiration contained in fingerprints.

Fingerprints should be removed immediately, for they areamong the worst disasters that can happen to a high-gradelens. They imbed themselves in a remarkably short time sothat they cannot be removed by any ordinary means and,besides, soften the glass surface so that it is more easily injuredduring cleaning.

Special auxiliary lenses, such as wide-angle and telephotolenses, should be stored carefully in the special cases designedfor them by the makers. 'they are usually the most expensiveof lenses and thus deserve special care in handling and storage.If the original maker's package has become damaged or is notavailable, then a chamois-skin bag with zippers or draw-strings should be made for holding the lens, and the bag shouldbe placed in a box as a protection against mechanical injury.The safest place for any lens is when it is mounted in thecamera; accidents are doubly apt to happen when it is allowedto lie around.

Dust and Dirt

In the course of time all lenses accumulate dust and dirtwhich, if allowed to increase, will blanket the lens and reduceits speed. While it is eventually necessary to clean the lens,the intervals should be as long as possible and the number ofcleanings reduced to a minimum, for every cleaning removessome of the vital lens polish. A new !J.ighly polished lensglass is almost invisible except for the reflections that takeplace on its surface. If it is light colored or of solid appear-ance, it is entirely possible that this is due to a multitude offine scratches on the surface that have destroyed the polishand will soon cause trouble with the lens.

Soft, fibrous dust does little harm except that it calls foranother cleaning, but there may be some hard, sharp gritincluded that will scratch the glass as soon as it is rubbed.Therefore, it is always safest to dust off possible grit by meansof a soft camel hair brush to dislodge the grit before attemptingto rub the surface. Bad scratches have occurred by the neglect

PHOTOGRAPHIC LENSES AND SHUTTERS 109of this advice. And, while you are at it, clean the rear end ofthe lens as well as the front, first brushing off any film oremulsion dust that may have been deposited on the rearelement.

For rubbing off the dust and grit, which should be donevery gently and with little pressure, we come to the conclusionthat there is no material that is as soft and free from scratchingtendencies as the special lens tissues now on sale at all stores.Never use a linen or cotton handkerchief for this purpose,even though it may be soft from repeated washings. The fibersare hard and much troublesome lint is deposited. Silk hand-kerchiefs are to be preferred to cotton or linen, but the silkfibers are also hard and have little real cleaning value. Spe-cially prepared "wash-leather" or chamois-skin wipers are usedin England but the ordinary drug store chamois skins arelikely to contain dangerous grit. Chamois, when properlyprepared, leaves no lint and is quite soft.

Avoid the use of chemicals for cleaning lenses, for many ofthese preparations contain alcohol, ether, or acids that willaffect the glass or the metal mountings. Ether will removegreasy deposits from the glass but it will also remove thelacquer from the mountings and deposit this varnish on theglass. Soap of any kind will leave greasy rings and deposits,and the alkali in the soap may attack the barium in the lensglass. Water, if applied in any quantity, may run back into theinside of the mounting where it will cause unlimited damage.

Breathe gently on the glass, not forcibly, and then with agentle circular motion rub off the fog with a lens tissue.Breathing hard on the lens may drive vapor back into themount where it will cool off and be deposited in the form ofdroplets on the inside surface of the glass which is inaccessible.Now, view the lens by reflected light which will reveal anydust or spots that may be left after the first cleaning. Donot rub the glass a second time unless it is necessary, andunder no condition scrub hard on the glass. If this treatmentdoes not suffice, better spend a little money with a lens repair-man and let him clean the lens-inside and out.

Bubbles and Specks in the Glass

Do not worry if you discover small bubbles in the glass.I t is almost impossible to avoid bubbles in the manufactureof certain high-grade optical glasses and they do no harmunless they break through the surface. Some of the best lensescontain whole chains and colonies of bubbles.

On looking through a lens, one will frequently see blackspecks on the inside of the lens caused by small flakes ofblack lacquer being detached from the mounting. Unless these

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specks are large or are located directly in the center of thelens, they need be no cause for worry. If it is necessary to takethe objective apart it should be sent to the maker or repairmanfor disassembly, internal cleaning, and new lacquer. Do notattempt to remove the specks .yourself.

Lens Scratches

Very often a deep scratch on the front lens element willcause no trouble, and even when the edges of the fron t elementare chipped there is no interference. If this occurs on therear element it is another matter altogether, because thescratches are likely to cause shadows on the rear of the lens.In the first place, the scratches on the front element are soclose that they are out of focus, and second, they do littleexcept to reduce the lens area very slightly. Certain scratchesmay cause internal reflections and flare, and such scratchesmust be carefully filled up with black paint to prevent lightfrom entering the scratch. The same is true when a bubblebreaks through the surface; it should be filled with a dotof black paint.

Cracked Lenses

About the least damage that a hard fall can cause is acracked lens element, surprising as it may seem. Lenses withcracks clear across the diameter of the lens have been used,and they functioned perfectly with no evidence of thecrack. However, the lenses did not open up along the crack,and the story might have been different if the halves hadpulled open with a second blow, So long as the halves fittogether tightly along the crack, it is seldom that evidence ofthe flaw will show on the picture.

Out of Alignment

The greatest damage that can be caused shy a hard fall is tothrow the lens elements out of their proper alignment, This isa job for an experienced lens repairman who must reassemblethe lenses in their proper relation, I-Ie has the equipment fordoing this job-you have not.

Rubbed Rear Element

Very often the owner of a miniature camera will find a seriesof scratches or a dul! white rough area on the rear elementof the lens. So far as this camera is concerned, it is throughas a camera unless the lens is replaced. A few words may help

PHOTOGRAPHIC LENSES AND SHUTTERS 111to prevent this accident from happening to you. In someminiatures of the folding type, the lens mount is set so farto the rear that it comes into actual contact with the film.If the film is moved up fOF the next picture while the camerais closed and the film is contacting the lens, then the film willscrape along the glass and cause scratches. In all cases, afolding-type miniature camera should be ·opened before thefilm is advanced,

Out of Focus at "Infinity"

In some cases, a camera will focus accurately with thefocusing scale at every point except the "infinity" position.Again, it may only do this trick occasionally. It is very annoy-ing at any time or for any reason, but the remedy is usuallyvery simple. This annoyance may be caused by a loose lensmount in the camera. If loose, tighten the mount in thecamera and try it again. Second, the pressure plate springmay be so weak that it does not properly force the film downon the film aperture plate. This is, 0.£ course, easily fixed.

Out of Focus at All Points

This may be the result of a nome-made lens repair jobwhich frequently shows up on second-hand cameras. Second,the pointer at the focusing scale may be bent or may haveslipped. Third, the focusing scale may have moved. The onlyremedy is to focus the camera at "infinity," using a piece ofgr oun dglass in place of the film, and find whether this"infinity" agrees with that marked on the scale. If it does not,move the scale, move the pointer, or remark the scale, which-ever may be the easiest.

Another cause, although infrequent, is the adjustment of thelens mount which may not be far enough back in the frontboard. It also occurs with focusing mounts 'of the frontelement focusing type where imperfections in the helix orrough spots on the barrel may prevent the lens from movingproperly in the mount.

Fern-Like Discoloration

Occasionally, the Canada balsam used for cementing the lensunits dries out or decomposes, leaving a fern-like tracery inthe lens. This means that the lens must be recemented by ;>lens man at a considerable cost. Likewise, the cement mayturn brown, slowing up the lens and killing its resolvingpower. This also necessitates recementing,

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Lens Foggy and Speckled

A lens showing a foggy 'inside surface or a fine mist ofdroplets on the inside surfaces needs i~mediate attentionThis .mist may be due to moisture that has gained access to th~mtenor of the lens .mo~nt or it may be oil vapor that has beencaused by the lubrication of the camera. Never oil any partof a camera, particularly with the ordinary oils found aroundthe home. The. oil vapori.zes .and eventually may condenseand form deposits on the mtenor of the lens. This means acomplete lens disassembly, cleaning, and reassembly by anexpert.

Cold Weather Hints

N eve.r u~co:ver the lens of a cold camera immediately afteryou brmg It into the h0l!se. Allow it to warm up graduallywith the cap on to aVOId condensation on and within thelens. Make the temperature changes gradually. With certaintypes of focusing lens mounts, the cold may congeal the greasearound the helix and cause the mount to move so hard that itmay be strained or broken.

Fi!m base material is always brittle in cold weather an~ care isreq~lred In wmdmg film after the exposure to prevent itsteann&, across, or along the edges. It is easy to tear out-sprockethole.s in 35 mm film 111 cold weather by winding it rapidly orIor cib ly.

Circular Spot on Film

Occasionally a large, black, circular spot will appear in thecenter. of the film when a folding camera is used. In somecases It may occupy as IToIuC;has half the film area. It is usually~ very dense black. This IS due to lack of proper air ventingin the back of the <;amera, the air being compressed to rathera high pressure inside the box when th.e bellows is pushed insuddenly on closing the camera. The h~h air pressure undersuc~ cond~tions, may momentarily force the shutte: open,leaving a Circular s.hadow on the film. This happens frequentlywith cameras haVing the old-style pneumatic bulb shuttersand the only remedy is to close the bellows slowly if th~camera cannot be vented properly.

Haze Due to Lighting

Fog, haze, or thin bl~ck streaks may be caused by facing thecamera into a strong light or by reflections from windows orother glossy surfaces. This can be avoided by the use of the

PHOTOGRAPHIC LENSES AND SHUTTERS 113popular lens shades which can be slipped over the lens forprotection against reflections and direct light. Strong reflec-tions from windows may be subdued by the use of the anti-glare polarizing filters.

Blurred Subject

When a moving object passes rapidly across the field of thelens, too rapidly for the shutter speed being used, the imageof the object will appear blurred while surrounding stationaryobjects will be as clearly defined and sharp as usual. Theremedy is to increase the shutter speed so that .it is fast enoughto "stop" the object. It is not safe to use a speed lower than11100 second where motion is involved even though the motion

·may seem slow.*

Camera Movement

When all of the objects on the film are blurred, both movingand stationary, the trouble is due to camera movement duringthe exposure. When a camera is held in the hands, or whenthe camera is on a tripod in a strong wind, the shutter speedshould never be less than 1/25 second and preferably 1150second where the lighting conditions will permit. Time andbulb exposures require the support of the camera by a rigidtripod. Any slight vibration during a time exposure will causeblurring that is greatest on distant objects. With the modernminiature camera, shutter speeds should be at least 11100 of asecond to avoid the possibility of movement during the exposure.

Care of the Shutter

Modern shutters are very well built and reliable, but theywill not stand abuse. The parts are as delicate as those of awatch and are easily injured by falls, hard blows, or by excessmoisture. The following hints will be of service to the amateur.

1. DO NOT OIL THE SHUTTER OR IRIS. Oil, even in very smallamounts, will cause the leaves to stick together and cause other difficultiesthat call for the services of an expert repairman.

2. Do not leave the shutter "cocked" for any length of time. This grad-ually weakens the springs and changes the timing. Rele as e the shutter whenthrough for the day or when the camera is to be laid aside ior a time.

3. The focal-plane shutters oi such cameras as the Grafiex and SpeedGraphic will last longer without repairs if the curtain tension is released atthe end of the day.

4. The use of the cable release instead oi the trigger will avoid thetendency toward camera movement, and the cable should thereiore be used atspeeds of 1/50 second or lower.

.This subject is discussed £uIly in Photographing A.ction. (Little Tech ..nical Library, No. 14).

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5. Do not attempt to force or "atron " "not act normally, investigate it for th ~ arm a shckmg shutter. If it doesstn~ a focal-plane shutter by attempti;'g e~~o~·rc/lt.1S a very easy matter to

. An old, Worn shutter IS mdlcated b h dpurchase a second-hand shutter if th b{ dscratc e , scuffed blades. Never

7. Shutters of all ty es and ~ a es are marred or rubbed.likely to change timing ~r 'stick i~artlIdlarlY ~ocal-plane shutters, are more

8.. In the course of time, natural \~~a weat er. .ter wIll cause small pinholes in the fab r. o~htht c11tam of ~ focal-plane shut-film. Usually these pinholes can be touched a t WI I cause Iighf to strike theor black paint on the end of a small b h au WIt 1 a drop of rubber cement. 9. Do not use the top shutter speedu~ fIt places an unnecessary strain on th a h 0 tener tha? nece~sary becauseabove 1/200 second. e mec amsm, This applIes to speeds

10. When purchasillg a camera or I tThe speed most likely to stick is (T)e~~ teft each shutter s!?eed individually.not remain open when released Th d t ciequently acts lIke (B) and willbe tested before accepting the ~ame:a e lye relerse mechanism should alsoshutter c,an be cleaned and washed out' with casb 0 an emer~ency, a stickingwater, OIl, or gasoline under any circumsta~~~s~.n tetrachloride. Do not use

Iris Diaphragm

This mechanism consists of b f - .of very thin metal that mai a .num er 0 interlacing leavesopening of varying diameter I~ta~h an approximately circularThese leaves fit very c1osel/~oge~h~~abtu~entIJ of t~e lens.trouble, even after long service TI se om gIVe anyoccasionally to see that the f . . rey should ~e exam1hedworking. Because of the c1~seug~t~od 1hOP~!y WIth all leavesthe leaves, they should never be ~led e 6i11nwllta~. ukedhfortogether so that they cannot be moved. . I S IC t em

CHAPTER IX

TESTING LENSES

EVER Y lens should be carefully inspected and tested be-fore final acceptance. It makes no difference whether the

lens is expensive 0 inexpensive, it should fulfill its purpose,else why add it to your collection? Of course, we cannotexpect a $10 lens to equal a $100 lens in performance, but itshould be sufficient in its price class, Not a bargain, perhaps,but good value for the price. However, we venture to statethat any defects which show up in or on a lens made by areputable manufacturer can generally be traced to lack ofcare in handling. . .

Lenses should be carefully examined for scratches on thesurface or for dull spots where the polish has been rubbedoff the glass, leaving dull gray patches. The surface of awell-polished lens is nearly invisible except where patches oflight are reflected, but a worn lens surface is strongly visibleand does not give such brilliant reflections. If the surface ofthe' lens shows a reflected bluish cast, it is likely that thepolish has been destroyed by exposure to moisture and thusit will not be long until a repolish job will be needed.

Dented mounts, bent flanges, and deep scratches in themount are evidence of rough treatment and such lenses shouldbe regarded with suspicion. Dropping lenses or giving themheavy blows sufficient to dent the mount will be very likelycauses for alignment troubles. It the elements are out ofalignment, it is usually an expensive job to get them backinto place, requiring a highly skilled optician for the job.When the mount is in bad condition, it is likely that theiris or the shutter has been injured and careful tests shouldbe made of these parts, proceeding step by step at each t-number or speed. Deeply scratched or rubbed shutter bladesindicate that the lens has had long, hard service; likely as not,the shutter and iris are worn out. The iris should open andclose very smoothly without catching at various points, andthe shutter should be free from rattles and clicks whenoperated. .

Focal-plane shutters of the cloth curtain type are subject tomany disorders that are not always noted in a casual inspec-tion. The cloth curtain may be so worn that the sizing hasdropped out, leaving pinholes which will cause light-struckfilm, fog, and streaks on the film. The replacement or repair

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of a focal-plane shutter is usually an expensive job and everyprecaution should be taken against getting a defective shutter.

All lenses should be supplied with tight-fitting lens capsfor the exclusion of dust, grit, and moisture. Such a capshould be supplied with the lens and the absence of the capshould be regarded with suspicion when purchasing a usedlens. Lack of the cap shows indifference to handling a lens,and this is a bad sign.

Very often, small specks oto flakes of black varnish orlacquer will become detached from the interior surface of thebarrel and adhere to the interior of the lens. If these flakesare small and well away from the center of the lens, theywill do no harm, but they are not desirable. Bubbles in theglass, as we have remarked before, are not objectionable ifthey have not broken through the surface of the glass.

Slight yellow discoloration of the glass, due to aging of thelens cement, need not be objectionable but it is not an assetbecause the stain will slow up the lens. Only an exposuretest made with ortho film and timed by an exposure meter,will tell whether it is too deeply stained. If uniformly stainedto a yellow color all over the surface, the lens becomes a self-contained lens and filter. ".

Photographic Tests

The only true lens test is to take test negatives with thelens under uniform if not standard conditions. An ounce- oftest is worth a ton of conjecture. Photographs made of testcharts immediately reveal any defects that" the lens may havewhen carefully analyzed. A flat white sheet of cardboard,ruled with heavy black lines into various geometrical figures, isthe best type of chart for making lens tests, because anyaberrations or distortions are made immediately evident bytheir effects upon the lines. Large printed letters and figurespasted on the chart are also of service, particularly for obtain-ing critical focus and sharp definition.

Charts for this purpose can now be had from your cameradealer with full instructions as to their tt!>eand interpretation.Such charts should contain horizontal lines, vertical lines, at-least one pair of lines at an angle of 45 degrees, and a fewgroups of circles. Checkerboard designs in the corners arealso useful for determining resolving power. Where possible,the chart should be of the same proportions as the negativeso that the image of the chart will nearly fill the negativespace. The negatives should be made on finegrain film.

In general, the charts will reveal the various aberrationsas follows, but after some experience with the charts, manydefects can be determined easily.

PHOTOGRAPHIC LENSES AND SHUTTERS 117

1. Spherical aberration causes un sharpness all over the chart withoutsharp focus at any point except, in some cases, near the center ofthe chart.

2. Curvilinear distortion is indicated by the straight lines curving nearthe edges of the chart. Objects near the edges change 10 size fromthe same size figures near the center of the chart.

3. Sharpness of the letters and figures indicates resolving power as dothe small squares in the checkers. Loss of sharpness at the comershows that the lens has not sufficient covering power.

4. Astigmatism is indicated by.unequal sharpness in. the horizontal andvertical lines. Certain portions of the CIrcle WIll also be blurredwhile the remainder of the arcs will be sharp. .

5. Chromatic aberration may be indicated by unsharpness all over thechart at normal visual focus which becomes sharper on a secondnegative when the lens is racked slightly farther forward. . .

9. Lack of flatness in the field will generally show as good definitionnear the center of the chart which gradually loses sharpness towardthe edges.

7. Poor illumination shows the light material brighter near the centralportions than near the edges.

8. Flare spots will show as bright discs of dlight, gefnerhallybneart' thecenter of the chart. These are the least efinite 0 tea. erra Ionsbecause they depend upon certain definite lighting conditions thatdo not always exist.

9. Coma shows as comet-shaped or oval patches of light, variously dis-tributed.

Optical Alignment

It may. be necessary at times .to check the optical alignment ofa lens system. When an objective leaves the factory th~ vano~selements are centered in their mounts so that the. optical axisis central to all of the components. However, If It IS sub-jected to hard usage or is dropped, the lenses may becomedecentered in their mounts. In order to check an objective forcentration it is necessary to mount it in a horizontal position a~dplace a candle or small light source a foot or so in front of It.If one now looks through the lens obliquely the candle canbe seen reflected on the various surfaces of the lens. elernen ts.The candle should be adjusted until the images he directlyin the center of each lens element. If the lens IS now slowlyrotated the relative positions of the images should _not change.On the other hand, if one of the images rises an.d !alls I~ ~~la-tion to the other images as the lens is rotated. It IS an mdica-tion that the lens is out of alignment. In this. case the lensshould be returned to the manufacturer for readjustment.

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CHAPTER X

ENLARGER AND PROJECTOR LENSES

THE lenses used for enlargers .are very similar to cameralenses; in fact camera lenses are frequently used inter-

changeably in the camera and enlarger. However, speciallydesigned enlarger lenses are the most satisfactory for thispurpose and give the best all-round results. There is alwaysdanger of breaking an expensive camera lens in making thetransfer from the camera to the enlarger and the heat emittedby some enlargers does not improve the balsam-cemented ele-ments in the camera lens. .

The first and primary requirement of an enlarger lens is aperfectly flat field, and these lenses are designed with this inmind. They should .have good resolving power and fairly highspeed, as enlarging is a somewhat slow process under certainconditions with dense negatives and great magnification. ~hesensitized enlarger papers' are much slower than film emulsionsand we wish to have as much light as commensurate with sharpdefinition and detail.

Again, where the enlarger is employed for making colorseparation negatives, the lens should be entirely free from-lateral color aberration so that all of the negatives will be ofexactly the same size.

Good depth of field is essential when working with a tiltedeasel, hence the enlarger lens should carry an iris diaphragmwhich is also employed for the control of the light and formaintaining sharp definition under all ordinary conditions. Thequantity of light passing to the paper from the lamp is moreeasily -controlled by the iris than by controlling the lamp.Shutters are not necessary and are not provided with standardenlarger lenses.

By far the greater number of· enlarger lenses are anastig-mats, but occasionally we find a rapid rectilinear lens in a por-trait studio darkroom where the control of softness is duringthe enlarging rather than with the camera lens during exposure.Soft-sharp adjustable diffusing lenses are sometimes used forthis purpose and where such lenses are not available, softnessis attained by one of several types of diffusing devices. .

Lens Size and Speed

Because of the great variety of enlarger sizes, enlarger

118

PHOTOGRAPHIC LENSES AND SHUTTERS 119

lenses are supplied in a number of focal lengths· that ra.ngefrom Z inches to 10 inches or more. The focal length requireddepends upon the maximum !legative size for which the enlargeris designed, and negative sizes Will range from. 35 mrn film,lx1.5", to the 8xlO" employed in commercial studios. T~e lensused should have a focal length slightly greater than the diagonalof the negative to be enlarged. Having a much greater. focallength will require elevating th elamphouse and gate to a~ mcon-venient height for a given size enlargement an~ Will thusincrease the tendency toward vibration and blurring. If thefocal length is too short, then the lens Will not entirely coverthe negative and will cut the corner~ of the enlargement. Itis weIJ to use the focal lengths specified, even though two ormore lenses may be required for covering the range of nega-tive sizes handled. Three sizes of lenses WIIJ suffice forminiature negatives ranging from 35 ml1!- to Zy.(x3y.("-aZ-inch lens, a 3-inch lens and a 4-II1'Chlens Will cover the rangeadequately. .

For the large commercial enlargers, the lens speeds Will rangefrom about 16.8 to 14.5. The lenses for fast miniature enlargerswill range from 16.3 to 13.5, and in all. but the .cheapest .en-largers will be 14.5. The 13.5 lens, now in ~xtenslve use, grvesvery fast enlarging and short expo~~res with dense negativesand slow chloride papers. The additional cost of the (3.5 !ensis well justified in view of the additional speed. A diffusion-type of enlarger gives a uniform distribution of ligh.t ?ver theenlarging paper and has many other adva~tage~, but I~ IS ratherslow because of the loss of light. The light IS distributed bydiffusing it through a groundglass plate and by reflection f;omthe lamphouse walls and reflector so that ,:ery uniform lightfalls on the negative. The shadow of. the Image 01). th~ filmpasses to the objective lens which projects an enlarged Ima~eof the film on the sensitized paper. The focu.s of the I~ns IScontrolled by mo,:ement of the .bellows. The Size of the Im.ag~on the paper is adjusted by moving th~ lamphou~e and negative,the greater the distance the larger will be. the image. .

To increase the speed of the enlarger Without mcreasll1~ thesize of the lamp, the light must be collected more efficientlythan with the diffusion system, and all the ligh.t must be con-centrated on the negative area s~ as to obtain the grea.testpossible intensity. At the same time, the concentrated lightmust be spread out evenly w:ithout shadows or bright spots.The collection and concentration of light IS.usually performedby condenser lenses in an assembly. This consists of twoplano-convex lenses, curved faces together, placed bet.ween t~elamp and film so that the Jigh.t beam covers tl:e negative. Thisspot of light is very much brighter than the light at al}-yotherpoint in the larnphouse because it represents all of the hg~t col-lected over the area of the lenses, concentrated down II1tO a

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small spot. The intensity may be increased from ten to twelvetimes by this method with a correspondingly greater amountof light passing through the film.

The light then passes through the objective lens to the paperin the usual way. This, while an old method, is very effectivebut has the disadvantage of showing up all of the defects,scratches, or rub marks on the film. It will show filmscratches or cracks that are not r.vealed by the diffusion sys-tem, because the condensers direct the rays along definite lines.To avoid this trouble, Some condenser-type lamphouses aredesigned so. that diffusion takes place in addition to the con-denser concentration.

Some makes of, condenser-type enlargers are' provided withseveral sets of condenser lenses. This is not absolutely neces-sary, but it does assist in maintaining a higher efficiency thancan be obtained with one pair of condensers when handli~ anumber of negative sizes. Thus, a condenser that will snowthe greatest concentration OVer a 35 rnrn negative will notthrow a spot large enough to cover a 2Y<!x3y<!" negative. Acondenser for the latter size will be too dim for the highestspeed with the 35 mrn film, and so on.

Degree of Enlargement

The magnification or enlargement depends Upon the focallength of the lens and the distance between the negative and.the paper. With a short focal-length lens, this projectiondistance is at a minimum for a given enlargement, and this isan advantage in getting critical focus.

If sufficient space is available, then the matter of focal lengthis not of such great importance but, unfortunately, there is adecided limit to the permissible movement of the enlargerhead imposed by the height of the enlarger guide posts, etc.The use of wide-angle lenses will assist enlargement in limitedspace, but the wide-angle lens is not desirable if it can beavoided. Turning the enlarger in a horizontal position and pro-jecting against a wall gives the greatest amount of lens distance;this is inconvenient, however, with a nurnbej" of enlargers, butothers are designed to facilitate use in this manner.

Projector Optical System

The projector is a device for throwing an enlarged image of apositive slide on a screen or wall for direct viewing. Duringthe past few years, small projectors have been in great demandfor showing 35 mm color transparencies and for the projectionof home movies. The optical system of the projector is verysimilar to the enlarger, except that the optical axis of the pro-jector is usually horizontal instead of vertical.

PHOTOGRAPHIC LENSES AND SHUTTERS 121

5

k----- T ---.,K

5

F 86 This diagram shows the optical system usedi~g~an; fine projectors of the double-condenser type.

. f a tvni cal projector withFigure 86 shows the optical system 0 ad YPI 1 ses (CI-C2)fil t I (L) con enser en ,the concentrated amen .amp I ' (G) The image is pro-

film aperture (F), lind obJeglvS) !,nhile th~ positive film (K-K)jected upon the wa h s~ee~ f ~he lamp by the transparent heatis protected from t e ea 0 ay be a glass plate slide (filmfilter (e-e). The transparency m es) or a roll of film as?an~wichebd b(eKtwJ{e)ng¥h: ~l~~~rsl~~~~ used with 35 mm posi-indica ted y - . .tives are usually 2x2 inches, . (L) close to the

It will be noted that, by placing thef l~hePlight is embracedcondenser lenses, a la.rg\y.erc(tf~~g;rgjection. The concen.tra-by the angle (a) an IS u IIZefvely great for all of the lighttion of the. light Is(~o)m~a~t~ condense;s is concentrated onover the diameter o.the smaller aperture operung (d). d the focal

. h . t d image depen s uponThe size of t. e proJec e anld the "throw" or distance (T).

length of the objective (G(~) or the shorter the focal length,The greater .the dIstance. irna e. The design of the ob-the .larger w.llI be theIP~o~~fft:~~t t1~an that used with the en-jective (G) IS somew ia I

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larger for the reason that a mu h .IS required with the ro iecto b c greater focusing movementin the length of the ~hr~w rThec~use of the greater variationanastigmat but it is not nece e ens mayor may not be annition for direct viewing as for s:~i:igi~O ha;e. sh'ch sharp d~fi-where not much room is available f g. lor orne projectionshort focal-length lenses are used A rr· i lng throw, ratherhad at a distance of a few fee' al: y arge. Image can belens, but it is likely that lensest ~~en u4ng a6?-mch or 3-.inchcommonly used. rom to mches are more

Enlarger Calculations

The calculations employed with enlarge I d .r enses 0 not differfrom those used with ~rdi-nary. camera lenses with theDoss~ble. exception that thea~phcatlOn is changed in de-tail. The direction of the~ight i~ reversed and the ob-ject distance is exchangedfor th~ bellows extension.The principal elements con-cerned are illustrated by Fig87. .

The basic quantity is theenlargement factor (R), .or~he number of times that theImage is increased over thenegative size. Thus, the en-!argement is 2 or (R) is 2,In the case where a 4x5 nega-tive is enlarged to 8xlO. Thisis linear enlargement and isoften quoted in diameters'h~nce, in the example jusfgiven, the enlargement is 2diameters.

NEG'~rnu

~vlEASEL\

Fig. 87. The degree of enlarge-ment is very readily calculated.

Let

o

R =Enlargement factor or number of diameters of enlaraemenr, eo •.

f = Focal length of the lens.u : Distance from the lens to the negative.v -Distance from lens to paper or easel.

D =Total distance from the negattve to paper or easel. Thesum of u and v (plus the nodal distance for extremeaccuracy).

PHOTOGRAPHIC LENSES AND SHUTTERS 123

D - (2 X f)Then: R = (approx.);

fvR=-u

v - f fR =--; R =--;

u - f

Any of the above formulas can be employed for determiningthe enlargement, depending upon the known quantities. Forexample, the lens-to-easel distance is 24 inches and the lens-to-negative distance is 12 inches. What is the enlargement?

v 24R = - = - = 2 times.

u 12

Again, let us say that the negative-to-easel distance (D)inches and that the focal length of the lens is 6 inches.the enlargement factor:

D - (2 X f) 24 - (2 X 6)R =-----

6

is 24Find

24 - 12= = 2 times.6

N egative-to-Easel Calculations

In this series of calculations, the height of the negative car-rier above the easel forms the method controlling the size ofthe enlargement.

D = f X (R + 2); D = u X (R + 1)

EXAMPLE. The focal length of the lens is 3 inches and wewish to make a 6-times enlargement. What is the distancebetween the negative and easel?

D = f X (R + 2) = 3 X (6 + 2) = 3 X 8 = 24 inches

Required Focal Length of Lens

In the following formulas will be found methods of deter-mining the correct focal length of a lens for a given enlargingcondition. The results should be checked with diagonal ofnegative used to insure that coverage is obtained.

u X R v Df=--- f=---; f=---

R+1 R+1 R+2

EXAMPLE. The enlarging factor is 6 and the negative-to-easel distance (D) is 32 inches. What will be the fOJ:al lengthof the lens required?

D 32f=---

R +2 6 + 2

32= - = 4 inches.

8

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124 LI'I'TLE TECHNICAL LIBRARY

If th f·be sufta~f;fol;~h~lzeurdoes not exceed 2}4x~y,;, this lens willthe focal length w~lf bi~~~at~~ ~~di:eSIV\lS Ibrger than this,increase the negatlve-to-easel dimens· (Dt)en e necessary to

IOn .

Conjugate Foci

Where the enlarging factor (RJ d f(f) are known it is a si I tre an ocal length of the lensdistance (v) a~d bello~~~;t~~~o~ ~~/~~r~~nfO~r;~~~~ction

v = (R + 1) X f; then, u =.:R

6EXAMPLE. A 2-diameter enlargement is-inch lens. Then: desired, usi~g a

v = (2 + 1) X 6 = 18 inches; and u 18= 2" = 9 inches.

CHAPTER XI

SHUTTERS

THE shutter is a mechanical device attached to the camera,either permanently or temporarily, in different positions,

between the sensitized film or plate and the subject to bephotographed, and is used as a means by which the film may beexposed to light for varying lengths of time. The shutter,therefore, is the mechanism which regulates the amount of ex-posure that a film receives.

In the early days of photography when film speeds wereextremely slow, a simple lens cap or merely an obstructionover the pinhole was sufficient and worked satisfactorily, forseconds one way or the other in length of exposure made littledifference then. As film emulsions more sensitive to light weredeveloped, some arrangement had to be made on the camerawhich would allow the plate to be exposed to light for shorterand more exact periods of time. Consequently the shutterdeveloped in complexity until today we have highly developedmechanisms such as the Supermatic and Compur shutterswhich have little in common with their predecessors. Thereare a great number of different types of shutters, constructedon diverse principles but most of them may be classified underfour headings:

1. Before-the-Iens2. Between-the-Iens (diaphragmatic)3. Behind-the-Iens4. Focal plane.

First let us talk about the ideal or perfect shutter and thensee how well the different types now in use measure up to thatstandard. The ideal shutter would expose every portion ofthe entire picture area simultaneously, taking no time to openor. close. Of course that ideal is impossible to attain because ofmechanical reasons. The next best thing is to open the shutteras quickly as possible and then close it just as rapidly, so thatthe shutter is wide open for the great majority of the timeof exposure.

Numerous other characteristics are desirable, but all are notfound in anyone shutter. For example, the shutter should notlag at the moment when it is released, particularly on actionshots. It should operate without vibration which might dis-tort the image. It also should not rebound after 'closing, for

125

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that action might give a secondary image on the film, particu-larly in the brightest portions of .the subject. In portrait workthe shutter should operate silently so as not to distract thesubject. The diameter of the shutter opening should belarge enough so as not to vignette at the full working apertureof the lens to which it is fitted. Even if the lens might be outof center, the shutter should not limit the useful field. Too; itshould work equally well in any pasition in which the cameramight be placed. Naturally, it should be light in weight, com-pact, and strong. When used on a camera where the film isleft uncovered, and that means most of them, the shutter shouldnot open when it is set or "cocked," until released by the opera-tor. It should be as efficient as possible. By efficiency we meanthe relation between the light passed and the total period -efoperation-from the time the shutter begins to open until itis again closed.

Most important of all, the shutter should be accurate as totime relationships. It is not so bad if the exact time is notcorrect (such as 1/25 second being actually 1130 second) aslong as the other times are proportional to it. On somecameras tested, the I/2S-second mark is exactly the samespeed as that marked 1150, which would throw any photog-rapher off unless he knew that such a condition existed. Agreat many faulty negatives are due to this cause. Over aperiod of years no shutter will perform with uniform accuracy.Even if the shutter has not been used, the steel in the springs.will undergo a change. Temperature differences will affect thesmooth operation of the leaves of the shutter, and also offriction brakes where they are used to control the time.

Next to the importance of accurate time relationship in tshutter is the matter of uniform field density and even illumina-tion over the entire area of the negative being exposed. If theshutter opens partially at a rapid rate of speed and then com-pletes its opening more slowly, the negative will be uneven indensity.

On some shutters the Time (T) is omitted and there isprovision only for Bulb (B) exposures for fong intervals. Inthis instance the trigger must be depressed during the entireexposure. Others have this arrangement as well as Time, inwhich instance the trigger is pressed once to open and againto close. Usually there will be less opportunity to jar thecamera if the Bulb exposure is used, provided it is done bymeans of a flexible cable release. Some shutters must be'adjusted to "I" which stands for Instantaneous, before thesecond or fractional second exposure can be made. -On German-made shutters not for export the letter "0" (Offen) is the sameas the English "B" or Bulb; "Z" (Zeit) means Time, and "M"(Moment) means Instantaneous.

PHOTOGRAPHIC LENSES AND SHUTTERS

Before-the-Lens Shutters

be arate and detachable fromThis type of. shuttebr mba~lt ~ntS;~he body of the camera. It

the camera or It may e ur I d th subjectis ~ocatehdin fron~ ~f ~!¥hl:::\~~t:e~ee~~lh~t~l~~ ~fthis

etype of

bemg p otograp e . shutter: I-the slotted dropshutters 2-the rotary shut-ters 3~the studio shutters,4-the roller-blind shutters,and 5-the blade shutters.There is also one cal.led .theGuillotine shutter which IS avariation of the drop. slotshutter. Flap shutters, eithersingle or built like a yene-tian blind, are s om et im e splaced before the lens.

The slotted drop shutter,an early type, consisted ofan opaque screen having arectangular cut-out portronat least equal to the srze ofthe lens used. This opaquescreen was allowed to fall byits own weight in front ofthe lens thereby making theexposure, although some-times it was accelerated bymeans of a spring w~lchpulled it down more .rapldl~.Historically speaking, this

Fig. 88. ~he fi~st shutter ,!,as was the first type of actualsimply a slide With an opening. shutter other than the cap

which was placed over the apertl!rel'84FSouAblt t~~5ri~~~:~~~this type to photograph the sun in . • ou '. I dwet collodion plate had appeared, it was more extensive y usewith a modification in the form ,?f achangeable slope th¥hwoluldslow down the passing of the slide III front of the len\ e ess

er endicular the slope, the slower the fall of the s ot acrossfhe Plens. Jarnin, in 1862, put this type of b~uttbr ~et':i-~enil~lens and assisted its fall by means of a ru er an. e h dor plate had to be inserted in the camera after the shutter a

beAb~~t (~~~ ~~~e time, 18S8, a' circular shuttej apPfaredtlthe first rotary-type mechanism. A flat, circulr p at~ 0 n:;t~aor a segment of a circle revolved, by mea~s 0

1a sr:ng, :~ c~~

completely or in a half turn. In the flat, crrcu ar p a e w

I ! I, II I,

IIII

I II

BIIIIII,II, -,-,_-.,;/'IIII

III,

I II \

L '-

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128 LITTLE TECHNICAL L IBRARY

a rectangular hole it two sid . .the same width ;./ tSheW~p~1/s curDng. Wlt~ the circular disc,aperture passed in front or ~~e. I unng Its movement thisThis shutter was, of Course m~ch eys to mabke the exposure.drop type. . , ess cum ersome than the. This rotary sh utter is the t th . f .Ine-:,pensive box cameras. Th~P~ ~t IS ound today on theto ItS starting position except 0 t~VIng part does not returnspring pulls it around' ~ e ne~t exposure when thetion. A few shutters cfta~~i!hte aperture In the opposite dir ec-exposure, thereby maki ype ddo return after making oneThe duratio f ing a secon exposure On the return1125 of a se~o~d.ex~~seu:e :t the rfot~ry shutter is usually about~o do with the variability nfro~ef~e ~t:ndn~g have a great dsalIn the spring will sometimes slo it ar exposure, FatIgueond. Dust and dirt '1 . w J up to ~s much as 1110 see-normally it is unprotec~:JI y M~cs~umulate In this shutter, forslide which may be ulled' 0 t t cameras are provided with amovement and thusP allow~' 0 stop the rotary shutter in itsexposure lever when moved irne exposures to be made; thereturn to its o~iginal position aa~~c~lnd time, allows the disc tobox cameras three diaphragm 1.UScover the lens, In thefor by means of a metal slide .ope~l.n~s hare usually provideddifferent size ar I~ w IC tree holes, each of away in th l' e cut. When thiS metal slide is pushed all theI, e argest openrng or at" ..of the lens. This is th per ure IS In positron in frontThis open' . b e proper size for normal snapshot workand the s~:11~~t ~~t fll. The ~ext small~r one would be fl6slide is pulled oft 'M The ope7111gs click into position as thevariety have the ~ota/ny smg e-lens cameras of the foldipgstop than has the box YcaS~~;;er Thd onded.a.dditional smaller

. at a rtiorial stop would

A-SHUTTER BLADE OPENINGB-APERTURE'"C-PIVOTD-PIN LIMITING SWING OF

DISC BETWEEN SHOULDERSE ANDF

Fig. 89. This rotary shutter is in use in many simple cameras.

PHOTOGRAPHIC LENSES AND SHUTTERS 129be approximately f 32. The diaphragms on these folding cam-eras are usually made from a single circular disc with the"holes" or aperture openings evenly spaced around the cir-cumference. The disc,is rotated to its various openings by atoothed edge either at the side or bottom of the shutter case.A groove near each opening is provided so that the openingchosen will be centered in front of the lens (Fig. 89).

The roller-blind shutter is a variation of the drop shutter.It was first suggested by Relandin in 1855 and later perfectedby Kershaw, It works either before or just behind the lens,but preferably behind the lens, since not only can lenses bechanged without removing the shutter, but additional compact-ness is attained in the entire camera. A long strip of opaquematerial, usually a thin black fabric impregnated with a blackrubber solution, is wound on a roller at the top of a small,shallow box and attached to another roller at the bottom. Theblind is about three times the length of the box and nearlythe same width as the box throughout its length. This shallowbox in which the rollers are mounted forms the body of theshutter, which is not as clumsy or cumbersome as it sounds.The bottom roller is hollow and contains a powerful springsimilar to that of a window shade. The center third of theentire strip is cut away, leaving a rectangular hole in the blindor curtain. Narrow ribbons on the sides connect the top thirdwith the bottom third. The shallow box has a circular holecut in the front and back slightly larger than the lens, theback hole fitting over the lens hood or mount. A knurled knobon the side at the top of the box allows the shutter curtainto be wound up, increasing the tension on the coiled spring inthe lower roller. The shutter blind is held up in this positionby a ratchet and pawl mechanism. When this ratchet is re-leased the lower spring will pull the shutter curtain down veryrapidly, drawing the rectangular opening in front of the lensand exposing the film. Shutters of this type have speedsranging from 1115 to 1190 second, or the curtain may bestopped with the opening in front of the lens to give "time"exposures. The older type had a string leading from the topof the box to facilitate the rapid winding up of the curtain.Release of the ratchet to set the shutter in motion is accom-plished either by a trigger, which when pressed "kicks" thepawl out, or by a "pneumatic release," an India rubber ball andtube which when squeezed inflates a rubber bladder under thetrigger.

This type of roller-blind shutter must have a dark slide inthe shutter box, for the lens is uncovered while the curtainis being wound up. The slide is, of course, withdrawn beforethe shutter is released. In the roUer-blind shutter, the lengthof the rectangular opening in the curtain is usually one

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the plate was greater in the center by 40 per cent: In using oneof these double-flap shutters it had to be examined closely /0see that both flaps were parallel and wo~kmg togetherJ ~ron some forms one of the flaps could be dls~onnected an t eother used merely as a single-flap shutter (Fig. 90). .

Tauveron later combined the single-flap shutter with Itlheroller-blind type. The ro ~rblind was adjusted so that Itwould release when the flapreached its top point. Thiswas not a bad idea, becausethe noise that the roller-blindshutter made occurred in theclosing of the aperture andany movemen~ on the. partof the subject in portraiture,resulting therefrom, made nodifference. Also, the shuttercould be set, i.e., the flaplowered without re-exposingthe film', as the roller blindterminated the exposure bydropping, remaining in theclosed position.

There were several othertypes of shutters which werevariations of the flap shutter.One having the appearanceof a' Venetian blind, consist-ed of a number of light flapsor plates pivoted around

Fig. 90. The flap shutter is still horizontal axes and overlap-being used by some portrait men. ping one another at the be-

ginning and end of each ex-posure. This type of shutter had an efficiency of not more than33% because the flaps, being always parallel to one another,allowed the central beams of light to pass fully w~en the flapswere parallel to the optical axis, but when completing the shut-ter movement they cut off a great part of the oblique rays.Despite the fact that a shutter of this type was ma~e that wouldgive an exposure as sho-rt as 11400. of a second, It soon wentout of existence due to the low efficiency. .

Another variation was the bellows shutte~. This type wasextensively used for some time by portrait. ph~tog:aphers,principally because the greatest amount of illumination wafpassed through the center of the lens and on !o the center athe plate in which position the image of the subject was usuallyplaced. This shutter was made of two bellows of black opaque

- •..•.•,<, -,

\\\II

\ /X

\\II

//

131and one-half times the width. The tension On the spring maybe changed by a knob attached to the lower roller, prewindingthe spring so that when the curtain is wound up it will havegreater tension than normal. On this type of shutter or anyother one having spring tension the spring should never beleft under tension. The efficiency of this shutter is dependenton the tension of the spring, and is generally considered to behigher than between-the-lens and blade shutters.

In efficiency, the roller-blind tyr1e is equal to the straight-edged drop shutter. The length of the rectangular openingin the drop, rotary, and roller shutters should nearly alwaysbe one and one-half times the width for greatest efficiency. TherolIer-blind shutter was the forerunner of Our modern focal .•••plane shutter in principle. (At times a secondary roller blindwas employed on this type of shutter in order that the actualshutter could be set again while the camera was loaded with-out exposing the film.)

We shall return again to the slot type of shutter later.Historically, the flap shutter in many variations follows next.This type had been in use some time before 1878 when J. W. T.Cadett made a model that was operated by a pneumatic bellows.In 1880 C. Guerry refined the flap shutter and advocated itsuse inside the belIows, behind the lens. This type of shutteris still being used, particularly by portrait photographers. Theflap, which is extremely light in weight, is usually fastened atthe top and swings upward to open and downward to close ..The flap is covered with black velvet and the inside of theshutter case is lined with the same material so that when theflap is closed the entire arrangement is light-tight. A pneu-matic bulb when squeezed opens the flap which remains in thliOpen position until the pressure on the bulb is released. Forlong exposures a clamp is placed in the tube near the bulb,this clamp holding the pressure until released.

The size of the flap opening is always greater than thediameter of the lens, and depends on the distance of the flapfrom the lens. As the flap swings upward more light wilLenter through the lower part of the lens lhan through theupper. In some landscape work this is desirable, becauseusually less light is reflected from the foreground than fromthe sky. This is helpful only when exposures are longer than113 of a second, as this is the shortest exposure possible withthis type of shutter. Guerry's placement of the flap shutterbehind the lens was an advantage in portraiture, for the sittercould not see when the exposure was being made.

In 1880 Jouber suggested using a second flap synchronizedwith the first so that it always remained parallel to it. Thisdouble-flap shutter permitted shorter exposure times. How-ever, employing this double-flap shutter, the density of field on

././

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Fig. 91. The Guillotine shutter was a furtherdevelopment of the early falling-slide shutter.

I

II

m

PHOTOGRAPHIC LENSES AND SHUTTERS 133

cloth mounted on a metal frame. When the two bellows wereclosed they formed a half moon. Operated by a bulb pistonthey opened along a median line in the region along the verticalaxis of the lens. Short exposures were not possible, but thatfact is not important in a portrait studio.

A "noiseless" shutter is the Luc, a diaphragm type many-bladed shutter mentioned here because it is usually placedbefore the lens; but its mechanism will be described later,being similar to the Cornpur type.

Reverting to the drop or slotted type of shutter, a descriptionof the modern form of the Guillotine shutter should be includedamong the before-the-lens type shutters. This one has arectilinear movement and is used on several foreign handcameras. It is made of two thin steel plates, one used to coverthe lens before, and the other after the exposure. These platesslide one on top of the other, the inner one having a squareopening larger than the diameter of the lens, the outer onehaving no opening. On instantaneous exposures the inner plateis automatically released by the outer one on its arrival at theend of its travel. The two plates are worked independently forlong exposures.

Fig. 91 illustrates this type of shutter and shows diagram-matically how such a shutter works (the driving springs,ratchets, and the shutter release not being shown). In theposition of rest (position 1.), the plate (A) rests against thestops (cc) and engages, by means of the pins (dd), the plate(B), the solid part of which covers the aperture. To set theshutter, plate (A) is drawn towards the left, and in so doingcovers first of all the opening in (B) and then drags (B) withit until it is stopped from going farther by the studs (e)(position II.). The release sets (A) free, so that under theforce of the driving spring it moves to the right hand and inso doing uncovers the aperture (position III.). After it haspassed completely over the aperture and the opening in plate(B), it engages the latter and pulls it over so as to cover theaperture again (position IV.). The greater the length of thenotches in (A) in which the stops (dd) slide, relative to thediameter of the aperture, the greater is the efficiency of theshutter.

Bladed shutters are of many types, among which we mustinclude the rotary shutter already described. What we com-monly think of as the blade shutter is usually composed offrom two to thirty individual, thin, wafer-like segments oper-ating from pivot points equally spaced around the shutteropening in a base plate. Most of the many-bladed types havesimilar mechanical and operating features. As has been stated,this type of shutter may be used either before the lens, betweenthe lens, or behind the lens. Generally speaking, the greater

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the number of blades ther h .IS reached. This is becau:e a~~ch ebld

re. quickly full apertureshorter distance to travel H a e ffis.small~r and has amore than seven blades a;e us~wever, e Cle~cy IS lost whenof parts and the increased fricti~n~lef tOtt~e Sncre~sed numberblades be made that a shutter ·11fit ~c 01. 0 thin can thesethan 1/32 of an inch in thickn~1 F.mt92a space of not morea two-bladed shutter appear ~. Ig: shows clearly howThe clear space inside the d~te~ p.ar\lal!y opened and closed.the blades are partially opened B1cI~ce is the aper~ure whenthe pornts (P) and (P) The· d tta des)" and II prvot aboutfigur~ shows the positio~ of one ~f ~h bJedon thhe right-handture IS fully opened e a es w en the aper-

Fig. 93 shows o~ly 0 bl d f -illustrate the action of thne ~ e ~ a three-blade shutter tospring (E) in actuating ~h~e~I~~ttnng (D) and the operating<,E) is attached to the lug (K) e:i blade (C). T~,is springring. This spring is operated or an .moves. the entire sectorrelease on the outside of the sh St~t 111 ThtlO~ by the triggerto the shutter blade (C) move .u erd e pin (F) attachedsector ring at a speed de e .s in an out of the slots in thethe spring (E). The pOinf (~t~W:on .the tensIOn placed onprn (H) limits the moveme IS e Pivot of the blade. Theseen from the illustration T~t ~~ tthe sechtor nng as can beto move is ver shor· e IS ance .t at the pin (F) hascapable of high ~peeds:' consequently this type of shutter is

There are several meth d .shutters, all of w'iich mayo ~ Id use.btod regulate tge speed of

e escn e as retarding devices.

P

1&II- SHUTTER BLADESA-APERTUREP- PIVOT

B-POSITION OF SHUTTER BLADE WHENAPERTURE IS OPENED

Fig. 92. The two-blade sh tt • '11 tu er IS I us rated in diagram.

PHOTOGRAPHIC LENSES AND SHUTTERS 135The first method involves the use of a piston which movesinside of a cylinder, the piston during its motion operating alever bar connected to the sector ring to which the leaves ofthe shutter are connected or directly to the shutter blade itselfin the case of one- or two-blade shutters. As shown in Fig. 94,when the piston (A) is set in motion by the shutter the airinside of the cylinder (B) is compressed, causing a brakingaction. A cam acting on the pin (C) regulates the stroke ofthe piston, thereby regulating the exposure time. There areseveral disadvantages to this piston type of exposure regulator,although generally speaking this form of shutter control worksquite successfully. One of the disadvantages is that irregularaction may result from the lubricating oil between the pistonand the cylinder; and secondly, the viscosity of the oil changeswith the temperature so that the speed of the shutter is notalways accurate although the relationship between the variousspeeds will remain unaltered, which is the most important con-sideration. This form of retarding device is found on com-pound shutters which usually have an additional piston orair pump for bulb release.

The bladed shutters which have been described are oftenplaced before the lens and called Studio shutters. More com-monly, the bladed shutters are used between the front and rearelements of the lens in modern cameras. In this form theshutter casing forms the basic support or holder into whichthe front and rear elements of the objective are screwed, thisbeing particularly true of convertible lenses.

Between-the-lens Shutters

The well-known Cornpur, the Supermatic, the W.ollensak,

I~c_'-:.· _

Fig. 93. One blade in shutter. Fig. 94. An exposure regulator

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the Hex, etc., are of the between-the-lens type of shutters.They ar~ relatively inexpensive and extremely compact,mounted 111the barrel of the lens tube or forming the supportfor the lens. Generally .speaking, all of them are operated onthe se<;tor rrng p:1l1clple, regulated either by clockworkmechanism or by air prstons. The clockwork mechanism asu.se<;lIn these shutters is highly complicated and precise, quitesimilar to a fine watch. In one t~e of clockwork mechanisma small wheel with vanes like a turbine is caused to rotatewhen the shutter release is actuated and the friction ~f the airon the vanes causes a braking actio~. The faster the whee} is 'caused to rotate the slower the time of exposure for the brakingaction is greater, holding the leaves of the shut'ter open longer.. The .other type of clockwork mechanism has gearing whichIS applied to a crown wheel WIth an escapement. The familiarrotating disc on the front of the shutter, marked off in T, B,1, 112, 1/10, 1/25, 1/50, 11100, etc., sets a cam which regulatesthe length. -of time a ro~k~ng lever is engaged by a toothsegment WIth a toothed prrnon. This cam acts on an escape-m~nt le,:,er. The cam must first be "set" by a cocking leverw~th wh.lch all of the. shutters of the Compur type are equipped.FIg. 95 Illustrates this principle. In some models the between-

A-SETTING LEVERB- CAM ACTING ONC- LEVER TO RELEASED- ESCAPEMENT FROME- CROWN WHEEL FOR SHORT

EXPOSURESF - ROCKING ARM ENGAGESG - TO DETERMINE LENGTH OF EXPOSURE

Fig. 95. This is the mechanism controlling the Compur.type shutter.

PHOTOGRAPHIC LENSES AND SHUTTERS 137the-lens type of shutter is made automatic, usually' those withslower speeds, 1125 to 1150, so that the shutter sets itselfafter each exposure is made.

Historically speaking, this type of shutter which is oftencalled a diaphragm shutter was made in 1887 by Beauchampand Dallmeyer. Their shutters had several pivoted plates oper-ated simultaneously by an internal ring concentric with thediaphragm. These pivoted plates opened like the leaves of aniris diaphragm.

The Volute and the X-Excello shutters with ten plates haveiris diaphragm leaves which function as shutters, so that thereis only one set of leaves for the entire arrangement. When theshutter is released the leaves will open, not completely, but tothe pre-selected diaphragm opening, and upon completion ofthe time of exposure will then close again. This type ofshutter is highly complicated and fragile, rather high inprice, and has an efficiency of not more than 50 per cent.

A new Press type of Com pur shutter is now being furnishedwith the 13.5 ern Zeiss Tessar f4.5 lenses. In the regular typeof Cornpur shutter the knurled collar has to be turned to themark "1''' in order to open the shutter for groundglass focusing,and after focusing the collar is set at the instantaneous exposuredesired. On the 'Press type Cornpur shutter a knob on the topof the shutter can be pressed back even while the collar is seton any of the instantaneous exposures and the shutter openedby the shutter release for groundglass focusing .. After focusing,the winding lever can be set and the shutter IS ready for .theinstantaneous exposure. On the Compur shutter, the timeexposures or bulb exposures are made automatically, as theshutter does not require winding at these two positions. Theshutter of course, must be wound for instantaneous exposuresfrom o~e second to the maximum speed. The shutter is woundby moving the winding lever until it locks into position. Thiscan be done either before or after setting the collar to speedsbetween one second and 11100 second, but for higher speedsthe setting collar should be set before winding.

While the letters "T" and "B" are set to the index thewinding lever is locked and when the collar is set for instan-taneous exposures the time and bulb settings are put out. ofaction so that no accidental exposure may occur even WIthcareless handling as long as the shutter is not wound. up. Anyintermediate speed may be secured between the settings from1/25 second to 11100 second-that is, between 1150 and 1/100the shutter can be set to an approximate value of 1175 second;however, it cannot be set to any definite value between 11100and the maximum speed of the shutter (11200, 11250, 11300,11400, 11500), neither can this be done between bulb and 1125second.

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In the automatic shutter, i.e., the type which does not haveto be cocked, the trigger action of the release lever consists ofan arrangement of two springs on the main lever of the shutterso that the shutter blades are closed by one spring aofter beingforced by hand against the pressure of another spring. Its onlydisadvantage is that the spring which opens the blades againstthe spring which releases them ;must be stronger than therelease spring. Therefore, an auj,pmatic shutter never closesas fast as it opens, and because orthis reason the between-the-lens shutters of the fastest types are not automatic but mustbe set by hand. -,

The double-drop type 0.£ shutter might best be designed foruse between the lens, but it has also been used behind andbefore the objective. However, L. R. Decaux, in 1893, designeda very fine shutter for use behind the objective, i.e., betweenthe lens and the film, located quite close to the rear elementof the lens. It is best used with a lens of large aperture dueto the fact that the period of time during which full light isallowed to enter usually is more than half the full time ofoperation of the shutter.

It takes the double-drop shutter about 1/400 second to closeand about the same time to open. As the shortest time at fullaperture is also about the same, the fastest exposure runsabout 1/120 second. The shutter then has an efficiency ofabout 60 per cent, which efficiency increases to nearly 80 percent when the exposure is 1150 second and continually in-creases as the exposure lengthens. This is true of all shuttersother than focal plane, for as has been stated, the quantity oflight transmitted during the operation of the shutter in com-parison to the quantity of light if the shutter opened all at anteand closed all at once, is the measure of its efficiency, so thatthe longer the time of exposure the greater the quantity ofIigh tat full aperture.

Focal-Plane Shutters

It has been found from experience that between-the-Iensshutters have certain disadvantages. At high speeds theirefficiency is low. Large size shutters are usually slower thansmall ones of the same design because of the inertia of theparts when setting them in motion and because of the increasedfriction. Also, the greatest speed obtainable with any accuracyin a between-the-Iens shutter is 1/500 second. For high-speedphotography the focal-plane shutter is almost universally usedand certainly in the case of large aperture lenses even wherehigh speed is not the important consideration. The focal-planeshutter is actually a drop shutter consisting of a blind or curtainsimilar to that described as a roller-blind shutter. The differ-

PHOTOGRAPHIC LENSES AND SHUTTERS 139

ence is that the width of the slit, which in part defermines theduration of exposure, can be vaned.

There are several forms of focal-plane shutters, some con-structed with only one roller at the top and bottom, other:!employing as many as four rollers. Some types are constructeof metal slats while others are C~)l1structed of black opaquecloth or of cloth impregnated with rubber .. These. shuttersshould never be left on tension, or. spring fatigue W!1l follow,resulting in inaccurate exposure time:;. The curtain .shouldnever be exposed directly to strong sunlight for fear of pinholesin the material. For that matter, no camera should be sub-

Fig. 96. The roll-curtain or focal plane shutter i~ the Graflex ~~vesvertically with the slits horizontal when held In normal position.

jected long to abnormal weather or temperature conditi~ns. .The most important fact about focal-plane shutters IS their

placement next to the film. They are called focal-plane shut-ters because they operate in the. focal plane .of the camera.Their efficiency is directly proportional to the distance betweenthe shutter and the photographic plate. The formula fordetermining such efficiency will be given l~ter. . .

To describe simply a focal-plane shutter IS to say that It IS ablack curtain similar to a window shade, ~aste?ed to tworollers with slits of varying widths ~ut ~cross It. Fig. 96.showsthe largest opening at the top, which IS used for focusing onthe ground glass and for time exposures, and the four smallerslits, each usually one-half the width. 0'£ the ne;xt larger one,although in some cases the smallest slit IS one-third of the nextlarger, particularly on Graflex cameras. A sprrng tensl~:>n.onthis curtain is variable and controlled by a separate W1I1~1I1gkey It is understood that this opaque curtain moves In aplane parallel. to the film but that the speed with which .thecurtain moves in front of the film is not cc;mstant for a!1 pointson the film area due to inertia in starting, The slits haveparallel sides or 'edges, reaching entirely acr,?ss the ~lm, lI:ndare cut in the curtain perpendicular to the direction m whichthe curtain moves.

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As can be seen from the foregoing facts, the ~xposure isa progressive process c,!mpleted by this moving band of lightwhich contains progressively the optical image, the same widthas the aperture opening, moving across the film at a determinedspeed. The exposure event can then be divided into twoparts-fi:st, the t~me necessary for the aperture or slit to passany particular POInt on the film, and secondly the time neces-sary for the apert~re to pass all ••he points on the film. We~an say, then, that In photographing any still object the follow-Ing factor's must be considered. First, the width of the curtain-opening, secondly, the speed of this opening, and thirdly, thelen&'th ~r space the curtain must travel to pass all points of thesubject 5 Image on the film. From this it can be seen thatt~e exposure time will vary directly with the width of theslit, ,:,-ssu:nIng that ~he speed remains the same. If the curtainopening IS doubled It. will then take the opening twice the time~o pass any given POInt. on the film and if this curtain openingIS halved the time required will be Just half. The variation inexposure wit~ the change in the speed of travel of the curtaininvo lves a different law, There is a point beyond which thevelocity ?f the curtain cannot be increased; therefore greatchang~s In exposure n;ust be accomplished by variations inthe Width of the curtain aperture. The highest speed in thefocal-plane shutter, which is about 1/1250 second, is accorn-plished by usmg the narro:'1est aperture opening with thegreatest terision on the curtain roller spring.

The focal-plane. shutter does not operate at the speed thatthe blades of a diaphragm shutter do, but achieves a higherexposure speed due to the fact that it exposes only a smallportlO,n of the film at one time. To illustrate this, assume tha]the slit or cur tain aperture of a certain size moves across thefilm at a given speed and that the exposure for anyone partof the film area will be 1/50 second. Now let us assumethat the curtain aperture has been narrowed down to 1/20 ofits former width; then each part of the image on the film will-re~elve light for only 1/20 of the time, and the resulting exposureWill be 1/1000 of a second. It must be dnderstood that asecondary blind is used in some focal-plane shutters to coverthe film when the curtain is wound up again after each exposure.The ~W? blinds are usually coupled together so that when thecurtain IS wound the protective blind is in front of the film.In other cases a dark slide must be inserted in front of the film.

There is o~e disadvantage of the focal-plane type of shutterand that IS distortion, caused by the fact that an object photo-graphed while :uovin&, is nC?t completely stopped. In otherwords, t.he movll~g object being photographed alters its posi-tion during t~e tlI1?-ethat the slit is moving across the face ofthe film. It IS qurte common to see fast-moving automobile

PHOTOGRAPHIC LENSES AND SHUTTERS 141wheels elliptical in a photograph taken by a focal-plane shutter.This distorting tendency makes the focal-plane shutter uselessfor types of aerial photography where freedom from distortIOnis essential. A few years ago a picture of a baseball batterwas taken as he was striking at the ball. In the picture, theshadow of the ball was some distance from the shadow. of thebat while the actual ball was shown to .be in contact ~Ith thebat In this case the rapidly moving slit had been I~ Just theright position in front of the film to record the ball In con tactwith the bat but upon moving down across the face of the fil~the slit did ~ot record the shadow of the ball and the bat untilan infinitesimal part of a second later when the ball had alreadyleft the surface of the bat.

The focal-plane shutter was first suggested by H, Farmerin 1882 but was not used commercially until af.ter 1888, whenO. Anschutz used this type of shutter In studying movementsof animals, Those early focal-plane s.hutters wer~ on the sameorder as the roller-blind shutter previously descr ibed .. In 1900,R Huttig recommended a series of permanent slits in a longc~rtain, placed at a distance from each other o.n the curtainequal at least to the length of the film. C~rtaIn. fecal-planeshutters have a' variable Width 111 the curtain, this var iationbeing controlled by cords through srnallIoops placed at eachend of the aperture opening; The cur tam IS wound more orless, according to the size slit desired for any grven exposure,and then is automatically stopped after that slit has traversedthe area of the film so that the cur tam cannot unroll any ofthe other slits even though they might have been wound upon the roller above one chosen for that exposure. :Usua~lyan indicator on the outside of the ca:nera shows which sizeof slit is in position. Generally, the slits are arranged In sucha manner that merely by changing them the ~hotographerautomatically changes the tension at the same time, therebycausing the total and local exposure times to vary In thesame manner.

The additional curtain which cove:s the film w.hen the aper-ture curtain is being wound up IS replaced I~ the reflext pe of cameras by a mirror flap which drops 111tO posrtionJiverting the light from the I~ns to the ground.glas.s on thetop of the camera at the same time as the roller blind IS wound,

Figure 97 shows the arrangeI?ent of a focal-plane ~hutteras well as the auxiliary blind which covers the film dunn~ thesetting of the shutter. (A) is the driving roller and (B) IS ~hesetting roller, usually located at the top or above the fiirn.The auxiliary blind is represented by (C) and .(D), this blIhdending at (X) with two narrow ribbons winding' around t eupper roller (D) so that dur!ng the actual exposure the fullpart of the auxiliary curtain IS wound around the roller (C),

I

I

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A gear (E) operates settingrollers (B) and (D) simul-taneously and they are bothrel~ased at the same time.This roller (D) is held on itsaxle by friction so that whenthe edge of this auxiliaryblind (X) is down beyondthe field of view the settingroller (B) can still be con-trolled by a key on the out-side of the camera to changethe choice' of sli ts to be usedin the exposure.

I t is true that the velocityof the curtain aperture variesfrom the beginning of itstravel to the end o.f its traveldu~ to its inertia in starting.I t IS possible, then, to havea situation in which the lastportion of the film overwhich the aperture passeswill be less dense than thefirst part which will receivemore light. This difficulty iscomparable to that of bladeshutters in which the centerof the film receives morelight than the outer edges onshort exposures. As a resultof tests it has been found insome cases that the velocityof the aperture at the end ofthheexposure may be twice as much as at the start in focal-plane -s utters. .

.In most focal-plan~ shutters the distance (D) between the cur-tam and the film IS such that there is an overiappina ofexposure .on anyone g}ven point on the film surface. Ideoallvthe curtam .should be rn actual contact with the film so thatfro~ any given part of the film to the next part would be aEass.mg from cornp leta iIl1!mination to complete darkness. Fig.8 Illustrates this undeSIrable condition in the majority of

f~)Cal-plane. shutters. It can be seen that the band of illumina-tion (A) I.S gr;ater than the width of the curtain aperture(~F). ThIS wl.dt~ of the illumination band rather than theWIdth ~f the slit IS the factor to be considered in makin aproportion between the times of exposure. It will follow ttat

FILM ,IIIII

k~

Fig. 97. A sectional view of avariety of rell-cur+aln shutter.

PHOTOGRAPHIC LENSES AND SHUTTERS 143this slight added exposure will be proportionately greater fornarrow slits than for wide ones.

There appears to be no reason why a negative produced bya focal-plane shutter is in any way different from one recordedby a between-the-lens shutter. Generally speaking, it is truethat the focal-plane shutter allows about one-third more lightto pass than does the best between-the-lens shutter. Thereason "for this is that no shutter parts hinder the light frompassing through the lens while the curtain is being operate.d.It is for this reason that focal-plane shutters are needed 111high-speed photography. In extreme high-speed photograp.hywith a focal-plane shutter there results a blurring effect whichis a distortion giving a fuzzy image due to the mov~ment ofthe object being faster than the movement of the curtain acrossthe film when the camera is stationary. Ideally, the 'camerashould move in the same direction as the object being photo-graphed which would of course blur the background but wouldhelp to produce as sharp an image of the object as though bothcamera and object were motionless.

In photographing a horizontally moving object with a focal-plane shutter the best results are obtained by moving the slitvertically, that is, moving either from the top down or fromthe bottom up, rather than having the slit mov~ horizontallyeither in the same direction as that of the Image, or 111

TA

-L.,....-,-_

FI LM->r'

Fig. 98. For clarity, the space between the film and the rell-curtain shutter has been exaggerated to show change in width.

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144 LITTLE TECHNICAL LIBRARY

th~ oppo.site direction. First, the proportionate 'size of theobject will be truer to the normal size, using a verticallym?vIng shutter, and secondly, the blurring effect will be mini-mized, However, to· produce an elongated image of a moving~bJect the shutter should n:ove horizontally in the same direc-tron .that the Image IS mov mg, By this method a short auto-mobile ca~ be made to look long, or if the shutter moves in theopposite direction from the moviig image, the short car wouldlook even shorter. When the focal-plane shutter moves ver-tically, the vertical lines of the object become slightly tiltedfrom the normal.

Perhaps it is no~ s~ch ;; bad thing that the wheels of a racingcar ~re shown ellipticaj in the finished print, for that conveysthe rmpressron of speed much better than a picture with allmot1On. perfectly stopped would do. We can appreciate betterthe action and the power of such a subject in a picture whichdoes not show action completely "frozen."

Mention should be made of the focal-plane shutter used inson;te of the bet.ter miniature cameras, which provides a slit ofvana?le. WIdth Instead of a number of slits. Both operationsof winding the shutter and transporting the film have beengeared together. The shutter is made up of two curtains andthe separation of the ends allows for the variation in the widthof the slit. The proper width of the slit is secured in settingthe shutter for exposure by winding up the curtains on oneroller. When the exposure is made two separate rollers on theothe: SIde of the film area draw the two curtains by springtension across the film as one unit. The width of the curtainaperture in most cameras can be checked by holdin z the hindon the se.tting button while making the release. This permitsthe curtain to move very slowly and it can then be stoppedhalf way. After the curtain aperture has been measured theshutter may then be released completely and then rewound.

It is importa.nt in .using a focal-plane shutter to keep thecamera quite still during the total exposure, which will alwaysbe muc~ longer than the local exposure-16cal exposure refersto. the time of exposure of anyone portion of the film, deter-mirred by the speed of the curtain, while the total exposuremeans the time necessary for the slit to move from one endof the film to the other. For that reason it is always best to~se the greatest possible speed so that the total exposure timeIS as short as possible. Another important rule to rememberis the fact which has already been stated-that when the slitm?ves along the same direction that the image does, the imageWIll ~ppe~r to be lengthened and that if it moves opposite tothe dlrect10n of the Image, the image will be shortened.

PHOTOGRAPHIC LENSES AND SHUTTERS

Shutter Testing \

There are manv methods of testing a shutter, some simple,others more complex, depending on whether the full moveme~tof the shutter blades is to be observed or merely an approxi-mate check is to be made on the speed of the shutter. .

P. G. Nutting, of the Eastman Kodak Laboratories, hasdevised a complicated system using a mirror-wheel assemblyon which twenty small, flat mirrors are mounted. Between·this horizontal wheel and a drum on which a film stnp ISplaced, is set up a shutter holder,. about midway between thetwo. An arc lamp IS used as a light s?urce and a constant-speed motor drives the wheel ?n which are mounte1 themirrors. A small lens forms an Image of the blade which ISphotographed on the film strip mounted on the horizontal drumwheel. Separate images are photographed giving an accuratedetermination of efficiency. Each record on the film shows theopening of the lens blades .at time .intervals of 111000 secondeach. In this system the mirror cylinder turns. at exac.t1y 3000revolutions per minute so that the twenty mirror s give 1000flashes per second on the shutter blades. Results obtained bythis method give a very accurate check on the speeds of theshutter at various settings.

A complete analysis of shutter action can be made by photo-metric methods, but that is extremely .comphcated for theamateur. If one does not desire to test hIS own shutter It canbe sent to the National Physical Laboratory where a completeanalysis will be carried out in retu~n for a fee. However, thereare several simple methods of testing the speed of the shutterwith a fair degree of accuracy. .

One method is to use a phonograph turntable, Sll1C~ thespeed is fairly constant at about 78 to 80 revolutions a minute,This method is good for testing shutter speeds 112.5 secondand slower. However, many turntables c.an .be adjusted toabout 120 revolutions per minute by an adJustIng. screw. Thecamera should be mounted or held in some way directly abovethe turntable. A small flashlight lamp is placed on the extremeedge of the turntable, th}s connected to a small dry-cell orflashlight battery placed In the middle of the ~urntable. Thelighted flashlight bulb provides a moving object at a pre-determined soeed suitable for calculating the length of ex-posure. Focu's on the groundglass of the camera or measureaccuratelv the distance between the turntable and the lens,settinz the distance properly on the lens mount. Set the turn-table "in motion, allowing it to rotate for several ~econdsbefore making the exposure on the film. After a prmt hasbeen made measure the angle through which the bulb travelsduring the'exposure, the duration of which shows on the prmt

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as an arc of light from the flashlight bulb over some portionof the field, using the formula:

60 AngleR.P.M. X 360" equals shutter speed in seconds. _,

. One way of determining the revolutions per minute whichIS the only rea~:hng neces~ary during testing, is by ho'lding astoI? watch. on It for a period of a'T least two minutes. This isan mterestmg test and on~ which is quite accurate (Fig. 99).

Another method of testmg the shutter speed is to photo-graph Neon tub~s,. which usually operate on a 60-cycle alter-natmg current, grving 120 flashes per second. This is good forthe ?low shutter speeds .. Of Course the pictures must be takenat night .. Stand about SIXfeet away from the sign and chooseone vertical tube which is fairly well separated from the rest~f the sl~n. Be sure that you are in such a position that otherlights will not fog the film. During the exposure swing thecamera steadily so t~at the image of the Neon tube progressesevenly across the middle of the film field while the shutter is

~FLASHLIGHTBULB

f

Fig. 99. A phonograph turntable can be used to test shutters.

PHOTOGRAPHIC LENSES AND SHUTTERS 147

open. Therefore, for every 1/120 second that the shutter wasopen there will be one flash of light on the film. This is assum-ing that the light has a 60-cycle source. Now count the numbercU lines showing on the film and the length of exposure canbe calculated from that. For example, suppose that you foundsix flashes of light on the film, the actual shutter speed wouldbe six times 1/120 or 1/20 second.

A method more recently devised for testing shutters is bymeans of an oscillator with a cathode-ray oscillograph. Theoscillograph is set to a frequency which is a multiple of theshutter speed to be tested-SOO cycles, when testing a shutterspeed of 1/S0 second. The screen of the oscillograph is thenphotographed and in this example there should be ten cyclesshowing. The light traces are easily defined and usually morecycles than necessary are thrown on the screen in order toallow for an error in the shutter. In our example, if twelvecycles showed instead of ten, which is two more than shouldhave shown, the error in the shutter speed would be 20 % onthe slow side of the shutter. The light traces of the oscillo-graph contain their own timing element and the great numberof cycles adds to the accuracy of the test. Today a greatmany radio shops have an oscillograph and oscillator amongtheir testing equipment.

Synchronization

This is the process of firing a flashbulb at the exact momentthe camera shutter is opened. This can be done either byelectro-magnetic or mechanical devices attached to the cameraat the shutter or somewhere on the body of the camera. Flashsynchronizing units are usually adjustable for individual camerasand also for special firing times of different flashbulbs. All ofthe synchronizers with the exception of the focal-plane typeconsist of a mechanism between the camera shutter and therelease.

In speaking of flash synchronization, the time lag refersto the interval of time it takes the flashbulb to reach its flashpeak intensity after electrical contact has been made. Ideally,the shutter should be most fully open at the flash peak. Withthe majority of the flashbulbs on the market this time lag isabout 20/1000 second. The flash duration must also be takeninto account, particularly with focal-plane shutters forshutter slit or aperture opening takes longer to travel a .tflthe film area than the operating time of the between-t .• •type of shutter. Most manufacturers make special flas bibswith longer flash durations designed for focal-plane shu e .

In testing for synchronization, the simplest way is takeseveral exposures, and from the results either increase r e-

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crease the tension on the cable release attachment which usuallyscrews right into the cable release socket on the shutter.A more elaborate method of testing is that of the GeneralElectric Synchrograph. This is a revolving drum placed infront of the camera lens with a strong light behind the ground-glass in the camera, and a flashbulb connected to the batterycase and synchronizer. On this revolving drum is fastened thetest negative. The drum is revol~d at the moment of testingand the camera trigger released. The test negative withinthe revolving drum is developed, showing the time the flash-bulb was fired and the time of opening of the shutter, indicatingwhether the shutter opens too soon or too late. These twolight traces on the film are given by two slits in the revolvingdrum, one for the flash and the other for the strong lightbehind the groundglass which shines through the lens whenthe shutter is opened.

The Kalart Synchroscope, a small testing device, can beplaced. in front of the lens; it shows two parallel slits as youlook in to it when the flash test bulb is fired. If one slit is aboveor below the other the flash is being fired too early or toolate, and adjustment of the tension screw can be made untilthey are in line. This can be done with a strong light behindthe camera and no flashbulbs need be wasted.

To depend upon the human ear and eye for judgment as tosynchronization is a mistake, even despite the speed with which.sensory nerve impulses travel. Light travels much faster thansound so that to say "I heard the shutter trip at the same timethat I saw the light go on," using a test bulb, is not a depend-able method. Synchronization should be tested at frequenti:ntervals to insure dependable results.

------------------------~~---------. -~.-------.

CHAPTER XII

USEFUL TABLES

Equivalent j-Numbers for Fractions of Speed ShownU) (1)'

1/2 1/3 1/4 1/5 1/10 1/20

1.00 0.72 0.52 0.50 0.45 0.33 .........1.00.68 0.60 0.54 0.38 .........1.2 1.44 0.84

0.64 0.58 0.41 · . . . .. . . .1.69 0.90 0.761.30.86 0.77 0.67 0.48 ...1.5 2.25 1.11

3.24 1.30 1.02 0.90 0.81 0.56 ... , .....1.81.12 0.92 0.85 0.60 .........1.9 3.61 1-.35

4.00 1.41 1.18 1.00 0.89 0.63 .........2.01.27 1.21 0.92 0.67 · . .. . . . . .2.2 4.84 1.49

1.28 1.12 0.79 · . . . . . . . .6.25 1.77 1.442.51.61 1.37 1.25 0.88 .........7.84 1.972.81.70 1.48 1.30 0.92 .........2.9 8.81 2.09

9.00 2.12 1.73 1.42 1.35 0.94 ..... , '"3.01.41 1.023.2 10.24 2.26 1.84 1.58 ...... ' ..

12.25 2.47 2.00 1.73 1.55 1.11 , ........3.51.70 1.123.8 14.44 2.68 2.19 1.89 .........

16.00 2.83 2.24 2.00 1.79 1.26 0.914.02.59 2.24 2.00 1.42 1.024.5 20.25 3.212.88 2.49 2.23 1.58 1.095.0 25.00 3.46

2.50 1.75 1.225.6 31.36 3.87 3.16 2.791.78 1.373.61 3.16 2.826.3 39.69 4.35

3.32 3.08 2.15 1.4546.24 4.79 3.876.84.36 3.87 3.44 .2.43 1.707.7 59.29 5.38

4.00 3.59 2.53 1.794.588.0 64.00 5.654.36 3.87 2.77 1.958.8 77.44 6.16 5.39

4.90 3.32 2.47121.60 7.81 6.32 5.4811.05.39 3.46 2.68144.00 8.48 6.93 . 6.0012.07.14 5.10 3.7416.0 256.00 10.95 9.22 8.00

4.789.00 8.06 5.6518.0 324.00 12.65 10.39

484.00 15.55 12.68 11.00 9.79 6.92 6.4422.014.17 10.90 7.141024.00 22.52 18.44 15.8132.020.25 14.17 10.0945.0 2025.00 33.16 25.88 22.36

EXAMP.LE. An f 4.5 lens requires 1/100 seco!,d for a certain exposure.What f·number will be required at 1/200 s.econd, In the ••y," column andopposite f4.5, will be found f3.21, the required aperture.

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TABLE OF HYPERFOCAL DISTANCES (in feet)

Focal StopsI'gthIns. 11 11.4 12 /2.8 13.2 f4 14.5 15.6 18

,III fl6 122-- -- -- -- -- -- -- ---- -- --

I 84 60 42 30 27 21 19 15 11 8 6 41% 125 89 63 45 40 32 28 23 16 11 8 62 168 119 84 60 53

~~!137 30 21 15 11 8

2'12 209 149 105 75 66 47 38 27 19 14 103 .... , 178 126 89 79 63 56 45 32 23 16 123% ..... 208 147 104 92 74 65 53 37 26 19 134 ..... ..... 168 119 105 84 75 60 42 30 21 154% ..... ..... 189 134 118 95 84 68 48 34 24 175 ..... .... . 209 149 131 105 93 75 53 38 27 195'12 ..... ..... , .... 163 145 116 103 82 58 41 29 216 ..... ..... . .... 178 158 126 112 89 63 45 32 236% ..... ..... ..... 193 171 137 121 97 69 49 35 257 ..... ..... ..... 208 184 147 130 104 74 52 37 26

Figures in this table are based on a circle of confusion of 1/1000 of the foe Ilength o£ the lens, or £/1000. The distances are doubled when a two.pow:rtelePhhoto lens IS used, and increased similarly for other powers in proportionto t e power.

FOCAL LENGTHS AND NEGATIVE SIZES

Focal Lengths Max. Focal Lengths Max.

InchesNegative Negative

mm Size, In Ins. mny' Inches Size, In Ins.

15 % %x % 145 53A 31Ax 41A20 7fs 16mm 150 6 4 x 532 llA 35mm 155 6% 4 x 535 13/8 llAxl 160 6;1 4 x 540 1% 35mm 165 6~ 4 x 547 17/8 1 :<1% 170 6~ 3%x 5%

50 2175 7 5 I< 7

1 xI% 180 7'1. 5 x 755 21A Ilhx13A 190 7'12 5 x 760 2% 2 x2 205 .- 8 5 x 775 3 2%x21A 210 81A 5 I<785 3% 2 x3 215 8'12 5 x 890 3 '12 2 x3 225 9 5 x 890 3 '12 21Ax3% 230 9% 5 I< 8

100 4240 9% 6%x 8112

2 1<3 250 10 6%x 8%105 4% 21Ax31~ 260 lOti 7 x 9110 4~6 2~x31. 270 7 I< 9115

10 •4~ 2 ~x23A 280 11 ,7 x 9

120 43• 2%x3% 300 12 8 dO125 5 3 x4 350 14 10 xl2130 5% 3 IAx4IA135

390 15 '12 11 x1451A 3%x4% 475 19 14 x17

140 5% ·31Ax41A

PHOTOGRAPHIC LENSES AND SHUTTERS 151Commercial Values of Circle of Cori'fusion

The following commercial circle of cOl~fusion limits are ofinterest and in most cases, have been derived from the manu-facturer's literature, or from specifications.

=Aplanat rapid rectilinear lens.=Wollaston meniscus in box camera.=B & L Rapid rectilinear lens (1902).e-Lower- limit allowed In the speCifications of the U. S.

Army Air Corps.=Folding Kodaks in low price range.=Plaubel Antlcomar I 2.9.=Minlature Kodaks. Small negatives requiring extensive

enlargement.1/750" =Dec1ared value for Zeiss Contax.f/l000" =Declared value for other Zeiss lenses.1/1000" =Eastman Cine' Kodaks.1/1000" =Taylor-Hobson-Cooke Cine' lens.1/1500" =Standard specifications for the Lelca Summar lenses.1/1500" =Turner-Reich 14.5 lens. Commercial.

1/80"1/60"1/100"1/200"

1/200"1/400"1/500"

METRIC FILM SIZE CONVERSIONS INTO U. S. STANDARDS

Metric Metric Equivalent Nearest

(mm) (ern) in Inches U.S.A. Remarks

Sizes Sizes StandardSize

20x 25 2.0x 2.5 0.78741<0.9843 35mm Single frame 35 mm,

25x 38 2.5x 3.8 0.98431<1.4961 35mm Double frame 35 mm,

30x 40 3 x 4 1.181h:1.5748 1/2 V.P. No. 127 Roll film.

40" 40 4 x4 1.57481<1.5748 Full V.P. No. 127 Roll film.

40" 60 4 x 6 1.57481<2.3622 Full V.P. No. 127 Roll film.

60" 60 6 x 6 2.36221<2.3622 2%"x21AH No. 120 Roll film.

60x 90 6 ,,9 2.3622,,3.5433 2%"x3IA" No. 120 Roll film.

65x 90 6.5x 9 2.3819x3.5433 ............ Film packs.

90x120 9 x12 3.5433x4.7245 3~"x43A.IJ' Film packs.

120x150 12 xI5 4.7245x5. 9056 4 ~'x6" Film packs.

Auxiliary Lenses for Mini.atur~s .This table shows the amount of magnificatIOn or reductton

obtainable with lenses of various focal lengths as comparedwith the normal 2-inch lens.

Magnification or reductionFocal Length compared with 2" lens.1 'Is" ( 2.8 cm) .. . .. .. . . .. .. .. . . .. . . . . .. . . .. .. . . . 0.54l::tt (3.5 em) ... .. .. .. .. .... . . .. . . .. .. . . .. .. . . . 0.67

~~6" ~(t ~:L:::::::::::::::::::::::::.:::::: t~733/." 8.5 ern). .. . . .. .. •. .. . . . . .. .• •. . . .. ....•. . 1.65 '~" 13.5 cm). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6

~i:'"~i!~:L:::::::::::::.::::::::::::::::::: U

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152 LITTLE TECHNICAL LIBRARY

. Exposure Increase for Close-Up WorkIn copying and .close-up work with any camera where t~e

bellows extension IS extended beyond normal limits, increasedexposure time must be given to compensate for the increaseddlstanc.e fro~ lens t? plate. The following table gives theap~roxlmate. increase In exposure needed, based on the rnagnifi.cations obtained,

Reproduction Scale:25 Xnatural size. . . . . . .15Xnatural size .10 Xnatural size " .4Xnatural size " .2 Xnarurat size .

1/~~t'::lu~~fsl~:I) : : : : : : :: : : : : :

~j~ ~:~~~:l~l~~::::::::::::::::::::::::::::::::1/4 natural size : : : : : : : : :: : : : : : : .: : : .: :1/5 natural size. . . .. .. . . . ..•........b~ natural size .1/8 ~:~~~:: ~:;:: : : .: .

~jro ~:~~~::~:~:: .... : : :: :: : : : : : : : : :: :: : : :: : : : : :.. """ , .

Exposure to bemultiplied by:

676256 .12125

942.82.31.81.61.51.41.31.251.21.2

Diopter Conversion. In optical literature the term diopter is often seen. The

diopter IS the unit of measure of the converging or divergingpower of a lens, and is equal to the reciprocal of the foca(length expressed in meters. Thus, 1 diopter refers to a focallength of 1 meter; 2 diopters, 0 ';TIeter; 3 diopters, % meter; etc.A SImple method of convertmg diop ters into inches is as follows:

since 1 meter =39.37 Inchesand D =focal length In dlopters

then39.37D = focal length In inches

.Amateurs often desire to make improvised supplementaryslip-on lenses for their cameras and WIsh to pdrchase a spectaclelens from .their optician for the purpose. As these lenses arerated in diopters rather than inches it is necessary to knowwhat lens to. ask for. First determine the focal length of thelens desired III Inches (plus or minus), then

39.37-----------=focallength in diopters

focal length in inches

PHOTOGRAPHIC LENSES AND SHUTTERS 153Distance and Altitude from Photos

To determine the altitude by photographs taken from air-craft, or to determine horizontal distances from photographs,we must have the following information:

1. Focal length (f) of the camera lens used.2. Horizontal distance between any two points on the ground that

can be easily identified on the photograph.

Distances may be estimated with fair accuracy in manycases from well known standards: Curb to curb street width:Size of a block or square, etc. (railroad track gauge is4' -8;/,").

Let: s = Scale of photograph as a fraction.h = Height or altitude above ground in inches.f =Focal length of lens in inches.

W = Width of ground covered by photo.w=Width of film.L = Length of ground area covered by photo.I = Length of film.

f fThen: s= -; W= -Xw;

h h

hL=-XI;

f

f.h e= -

s

EXAMPLE. Suppose that the distance b~tween two points on the photographis 2.5 inches and tltat the corresponding distance on the ground IS 1,500 feet or18,000 inches. Then, the scale as a fraction is:

2.5s'=

1,500 X12 7,200

If the focal Iength of the lens is 20 inches, then:Focal length 20

Altitude = --- = 144,000 inches or 12,000 feet.Scale 1

7,200

The focal length of aerial cameras used in civilian work willaverage about 12 inches, with a view angle of approximately 42°.These lenses are used at altitudes of from 10,000 to 15,000 feetmaximum. A shorter focal length gives a greater angle of viewbut less detail. A lO-inch lens will cover twice as much groundas a 20·inch lens, but the scale of the 20-inch lens will be twicethat of the lO-inch lens.

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CONVERSION TABLES

Fractions of Inches Into m1111- Millimeters Into Ins. Inches tometers and decimal fractions and decimals Centimeters

MUli- Inches Milll- Inches Inches Centl-Inches meters Decimal meters meters

fractions

1 25.4 1 1 • 0.04 1 2.5415/16 23.8 0.9375 ~I 0.08 2 5.089/10 23.0 0.9000 0.12 3 7.627/8 22.2 0.8750 4 0.16 4 10.16

13/16 20.6 0.8125 5 0.20 5 12.703/4 19.1 0.7500 6 0.24 6 15.24

11/16 17.5 0.6875 7 0.28 7 17.785/8 16.9 0.6250 8 0.31 8 20.329/16 14.3 0.5625 9 0.35 9 22.861/2 12.7 0.5000 10 0.39 10 25.407/16 11.1 0.4375 11 0.43 11 27.943/8 9.5 0.3750 12 0.47 12 30.48

11/32 8.7 0.3432 13 0.51 13 33025/16 7.9 0.3125 14 0.55 14 35.569/32 7.1 0.2808 15 0.59 15 38.101/4 6.4 0.2500 16 0.63 16 40.647/32 5.6 0.2184 17 0.67 17 43.183/16 4.8 0.1875 18 0.71 18 45.721/8 3.2 0.1250 19 0.75 19 48.263/32 2.4 0.0936 20 0.79 20 50.801/16 1.6 0.0625 21 0.831/32 0.8 0.0312 22 0.871/64 0.4 0.0156 23 0.901/100 0.25 0.0100 24 0.941/200 0.13 0.0050 25 0.981/320 0.003 0.0031 25.4 1.0

ABBE, Ernest, 28Aberrations of lenses, 25, 71Absorption of light, 14, 17Achromatic lens, 25, 28, 73--, double, 89-- doublet, 88Admission, node of, 30, 41Admittance factor, 47Alignment of lenses, 110, 117Anastigmat lens, 90Angle of incidence, 15-- of refraction, 17-- of view, 54Angular aperture, 55Aperture, 57--, angular, 55--, effective, 49,' 55-- of telephoto lens, 100Aplanat lens, 74Apochromatic lenses, 74, 96Astigmatism, 76

--, test for, 116Attachments, telephoto, 101Auxiliary lenses, 97Axis of lens, 18, 32

BACK focus, 39Barrel distortion, 78Berore-tbe-Iens shutter, 127Between-the-lens shutter, 136Biconcave lenses, 24Biconvex lenses, 24Blackening internal metal-work,

80Broken lenses, 110Brown stains, removing, 111Bubbles in lenses, 109

CAMERA, comparison with eye, 7-- obscura, 9--, pinhole, 9Cap, lens, 107Care of lenses, 107-- -- shutters, 113

INDEXCen ter of lens, finding, 39Chart, test, making, 116Chemical focus, 72Chromatic aberration, 72-- --, lateral, 80Circle of confusion, 61, 124-- -- illumination, 54Classification of lenses, 24Cleaning lenses, 109Colors, primary, 19Coma, 77Combined focal length, finding,

105Combining lenses, 46Com pur shutter, 136Concave lens, 19, 24, 45-- --, finding focus of, 46-- --, virtual focus, 46Concavo-convex, 25Conj ugate foci, 35Continental stop markings, 60Conversion, metric film sizes, 151--, diopter, 152-- tables, 154Convertible lens, 95Copying lenses, 104-- --, supplementary, 104Correcting distortion, 74Correction for chromatic aberra-

tion, 73Covering power, 54Cracked lenses, 110Crossed lens, 32Curvature of field, 74Curvilinear distortion, 78

DAGOR lens, 103Daguerre, 10Dallmeyer telephoto lens, 101Damaged lenses, 110Defects in lenses, 71Definition, 70--, limits of, 81--, testing, 115

155

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156

Depth of field, 66-- of focus, 68Determining distance and alti-

tude from photos, 153Diagonals of plates and films, ,54Diaphragm, 57Diffraction, 13, 71Diffusion, 14Diopters, converting to inches, 152Discoloration of lenses, 111Dispersion, 19Distance, hyperfocal, 62-- scales, 53Distances, extra-focal, 37Distal', 101Distortion, barrel, 78--, correcting, 79--, curvilinear, 78--, depth, 86--, pincushion, 79-- with focal-plane shutter, 143Diverging beam of light, 19Doublet, achromatic, 88Double achromatic, 89Dulled lenses, 110Dust inside lenses, 108Dusting lenses, 109

EFFECTIVE aperture, 49, 55Effective '-number, for closeups,

58-- --, for slip-on lenses, 106Ektar lens, 92Emission, node of, 30, 41Enlarger calculations, 122Enlarging, distances for, 122-- lenses, 118Equivalent {-numbers (table), 149-- focal point, 30Exposure increase for closeup, 152-- with different stops, 60, 149Extra-focal distances, 37

{-FUNCTION, 48, 57{-Numbers, effective, for closeups,

58--, measuring, 55-- with telephoto lenses, 100-- system of marking stops, fiO

Faults in lenses, 71FerJi:Iike markings, 111Field, curvature of, 74Fixed focus, 65-- -- cameras, 50Flare, 80Flatness of field, 71Focal length, combined, finding,

!t5-- -- of concave lens, finding,

46------ convex lens, find-

ing, 32, 52-- --, effect on perspective, 84

effect on size of image,35

-- --, finding, 32, 52lengthening, 101, 105

-- -- negative sizes (table),150

-- -- of lenses, 51-- -- of telephoto lens, 99-- --, shortening, 102, 105Focal-plane shutter, 1381!'oci, conjugate, 35, 124Focus, 24, 30, 44--, chemical, 72--, depth of, 68--, virtual, 19, 45Focusing methods, 50-- mount, 51-- scales, 53-- screen, 56Foot-candle, 20Form ulas, useful, 36, 5R, 64, 99.

105, 123

GAUSS points, 39-, - objective, 91Glass, optical, 19, 28Goerz Dager lens, 103Gradation, 86Gundlach Radial' lens, 94

HYPERFOCAL distance, 62---- (table), 150Hypergon lens, 103

ILEX shutter, 136Illumination. uneven. 81

PHOTOGRAPHIC LENSES AND SHUTTERS 157

Image, 7, 22, 44--, brightness, 47--, real, 24-- size, 35--, virtual, 24, 46Incidence, angle of, 15Index of refraction, 17Infinity, 32--, approximate. 34--, finding, 53Infra-red rays, 12Inverse square law, 21Invisible rays, 12Iris diaphragm, 57, 114

JENA glass, 29, 90

LATERAL chromatic aberration,80

Lengthening focus, 98Lens speeds, 60Ligh t, absorptto n of, 14, 17--, theory of, 12-- intensity, 20-- waves, 21Luc shutter, 133

MAGNIFICATION, telephoto, 98Meniscus lenses, 25, 8MMetric film size conversion, 151Miniature, auxiliary lenses for,

151Monocular vision, 83Moulded lenses, 30

NEGATIVE lenses, 24Nodal planes, 41-- points, 39-- space, 41Nolle of admission. 41-- -- emission, 41Normal, 17

OPTICAL center, 39-..I glass, 19, 28

PARALLEL rays, 16, 34-- surfaces, refraction at, 18Perspective, focal length and, 84Petzval portrait lens, 90

Pincushion distortion, 79Plano-concave lens, 24Plano-convex lens, 24Portrait lenses, 102Positive lens, 24Primary colors, 19Principal axis, 39-- focus, 30Prism, dispersion by, 19--, refraction by, 18Projection lenses, 118Projector optical system, 120Pro tar lens, 91-- --, convertible, 95Proxar, 103

QUALITIES desirable in lens. 71---- in shutter, 125Quartz lenses, 30

RADIAR lens, 94Rapid rectilinear lens, 79, 81JRays, Infra-red, 12--, invisible, 12-- of light, 12--, ultra-violet, 12Real image, 24Rectilinear lens, 79Reduction, distances for, 37Reflected light, 13Refraction, 16--, angle of, 17Refractive index, 17Resolving power, 49, 69, 82Reversi ble action of lens, 35Robin Hill lens, 103Roller-blind shutters, 129Rotary shutters, 128Ru dolp h, Dr" 91, 95

SCALE of image, 35Scales, focusing. 53Schneider ,Tel-Xenar lens, 101Schott, Otto, 28Scratches, 110Screen focusing, 50. 56Separation of supplementary

lenses, 105-- -- telephoto lenses, 99Shortening focus, 102

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158 LITTLE TECHNICAL LIBRARY

Shutter, bladed, 133--, Compur, 136--, flap, 130--, focal-plane, 138--, Ilex, 136--, rotary, 128-- speeds, testing, 145-- Supermatic, 136Shutters, care and repair, 107,

113--, classification of, 125--, desirable qualities in, 125--, rOller-blind, 129--, testing, 145Simple lens, aberrations of, 71-- --, properties of, 24Simple lenses used in photog-

raphy, 24, 88Single achromatic lens, 25Slip-on lenses, 97, 105----, effective {-number for,

106Soft-focus lens, 104Sonnar lens, 94Spectacle glass supplementary

lenses, 152Speed of lens, 60Speeds, shutter, testing, 145Spherical aberration, 74Stereo-Aberration, 84Stereoscopic work, lenses for, 84Stop, effect of, 57--, determination of, 66Stops, 60Supermatic shutter, 136Supplementary lenses, 97

-- --, concave, 101-- --, copying with, 104

\ -- --, {-numbers with, 106-- --, scale of image, 106Symmetrical lens, 90Synchronization of shutter, 147

T~EPHOTO attachments, 101-- lenses. 98-- --, adjustable, 98-- --, Dallmeyer, 101-- --, field of view, 98Telephotography, exposure, 100Tessar lens, 92Test chart, making, 116Testing lenses, 115Tests of shutter speeds, 145Triplet lens, Cooke, 91Turner-Reich convertible lens, 95'l'ypes of lenses, 24, 88 \

ULTRA- VIOLET rays, 12Uniform System of marking

stops, 60"U. S." system of marking stops,

60

VIEW, angle of, 54Virtual focus, 19, 45-- image, 46

WIDE-ANGLE lenses, 102Wollaston lens, 24, 88

ZEISS Protar lens, 91-- Sonnar lens, 94-- Tessar lens, 92Zonal aberration, 78

,.;