September 2 0 0 2

Their soul and secrets

by Erwin Puts

Leica M-Lenses

Leica M Lenses [1]

I n t r o d u c t i o n

Secrets revealedIt is well known that Leica M lenses, in spite of

their compact design, deliver imaging performancewith the highest quality. But what are the reasonsbehind the fact that, over the many years that newLeica M lenses have been computed, designedand produced in an ongoing succession, additionalimprovements still continue to be achieved? Theseimprovements, in the opinions of the advertisingexperts at Leica, leave all previous advances farbehind.

In this brochure, Dutch photojournalist Erwin Putsexplains the principles on which the secrets ofLeica M lenses are based and how the extensiveknowhow and the great competence of Leicaoptical designers succeed again and again inachieving ever higher peaks in maximumperformance in their optical systems.

The author also pays special attention to thepopular concern expressed by numerous Leicausers, who wonder whether the „old“ lenses aresuperior to current Leica M lenses in terms ofcontrast range, contour sharpness and resolvingpower. With his knowhow and the experience ofnumerous test series, he juxtaposes comparablelenses and their performances. In addition to

factual explanations, he also presents charts andmeasurement curves that have never before beenpublished in this form. Guidelines for interpretingthese tables and curves make these graphics evenmore practical.

Erwin Puts has been making photographs since1960 and he explored the technical aspects ofphotography even while he was studying businessadministration. The author has been working withLeica since 1989, and in a series of more than 30articles published around the world since 1992, hediscusses the history, technology and theoperation of Leica cameras and lenses. The core ofhis work consists of lens test reports that areconducted with a meticulous thoroughness that isrecognized by his competitors. The work bringstogether optical parameters and practicalapplications, clarifying the limits of performancecapability, with proper attention to their interplaywith the different types of film materials.

For many readers, this brochure will reveal manya secret, thus clarifying the reasons for theperformance characteristics of Leica M lenses. Wewish you much enjoyment in reading thisbrochure!

Leica Camera AG

Ralph HagenauerMarketing Communication

[2] Leica M Lenses

C o n t e n t s

Leica M Lenses [3]

C o n t e n t s

Contents

Introduction 1

The Soul of Leica M-Lenses 4Core Technologies 12MTF Analysis: those shapely curves 15Color Rendition 19

21mm Lenses 20

21mm f/2,8 Elmarit-M 2421mm f/2,8 Elmarit-M ASPH 25

24mm Lenses 26

24mm f/2,8 Elmarit-M ASPH 29

28mm Lenses 30

28mm f/2,8 Elmarit-M (1979) 3228mm f/2,8 Elmarit-M (1993) 33

35mm Lenses 34

35mm f/1,4 Summilux-M 3735mm f/1,4 Summilux-M aspherical 3835mm f/1,4 Summilux-M ASPH 3935mm f/2,0 Summicron-M 4035mm f/2,0 Summicron-M ASPH 41

50mm Lenses 42

50mm f/2,8 Elmar 4650mm f/2,8 Elmar-M 4750mm f/2,0 Summicron-M 4850mm f/2,0 Summicron-M (current) 4950mm f/1,4 Summilux 5050mm f/1,4 Summilux-M 5150mm f/1,2 Noctilux 5250mm f/1,0 Noctilux-M 53

Tri Elmar-m 1:4/28-35-50 mm ASPH 54

28mm f/4,0 5635mm f/4,0 5750mm f/4,0 58

75mm Lenses 60

85mm f/1,5 Summarex-M 6275mm f/1,4 Summilux-M 63

90mm Lenses 64

90mm f/2,8 Tele Elmarit-M 6890mm f/2,8 Elmarit-M 6990mm f/2,0 Summicron-M 7090mm f/2,0 APO Summicron-M ASPH 71

135mm Lenses 72

135mm f/4,0 Elmar-M 74135mm f/2,8 Elmarit-M 75135mm f/3,4 Apo Telyt-M 76

Glossary 78

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The soul of Leica M lensesEver since Professor Max Berek de-

signed his first lens for the Leica, the 50mm f/3.5 Anastigmat/Elmax, in 1924,the optical capabilities of Leica lenseshave been intensively analyzed and dis-cussed. Some reviewers state thatLeica lenses are the standard againstwhich others are to be judged. Othersexpressed the opinion that, even thoughLeica lenses perform very well, they arebasically as good as the products ofother manufacturers. Leica lenses arealso said to have a special kind of im-age-recording quality that is often com-pared with three-dimensional renditionor with pictures that convey a three-di-mensional impression. This peculiar“optical fingerprint” is frequently dis-cussed among Leica aficionados andcollectors. Sometimes it is even claimedthat older Leica lenses have certainmythical qualities that gradually disap-peared in newer lenses that were de-signed later. Then the fact is brought upthat optical design is being performedmore and more by computers, so thatthe personal “fingerprint” of the de-signer is no longer as evident as it oncewas.

It is undoubtedly true that Leica lenseshad and have particular characteristicsand qualities that are the very reason forthe fascination and the challenge ofworking with these lenses. In my opin-ion, the question whether a photograph-er always achieves the best resultswhen he or she uses a Leica lens isquite unproductive. Every lens has alarge number of specific qualitites and itis highly unlikely that every one of thesequalities will always be of the highestdegree.

Behind every Leica lens one can sensea passionate determination to controland to eliminate the geometrical aber-rations that are present in every opticalsystem. It is true, of course that con-temporary manufacturers of opticalproducts can no longer operate withoutusing sophisticated computer installa-tions. It is a fact that modern computerprograms can produce new optical de-signs in accordance with prescribed

specifications nearly without human in-tervention or control.

The likelihood that a design generatedin that manner is an ideal solution forthe intended purpose is about one in abillion. And that is the reason why thecreativity of the designer is essential,even decisive for creating an optical sys-tem that has optimal performance.

It might seem strange that I wish todraw attention to the importance of thecreativity and the art of the lens de-signer as an important factor in opticaldesign.

The fundamentals of modern opticaldesign are rooted in mathematical andphysical theories. The widespread appli-cation of computer-assisted design me-thods by all manufacturers has fosteredthe impression that lens design nowa-days is a highly automated process.

Leitz was one of the very first opticalmanufacturers to use computers to ac-celerate the extensive and laborious cal-culations of ray tracing significantly.That was around 1955. Today the soft-ware programs used by the “OptischesRechenbüro” (optical design depart-ment) are highly refined proprietary al-gorithms. Even so, current high per-formance optical systems could not

have been achieved without a goodmeasure of intuitive creativity.

In order to understand this soul that ispresent in every Leica lens, let us take abrief look at computation techniques, de-sign procedures and optical evaluationtechniques. After this small “tour deforce” we will be able to sense and ap-preciate the “Leica spirit in the glass”.

Let us begin with a basic explanation.If we take a simple lens, for instancethe good old burning glass, to create animage of the sun on a piece of paper,the sun is rendered as a very bright spotand the paper begins to burn becausethe lens causes the sun’s energy toconcentrate in one point. In early times,a single lens element was the onlymeans of obtaining an image. For verysmall angles of view, like those of a tel-escope, for instance, one was satisfiedwith such an image. Louis JacquesMandé Daguerre, who made his veryfirst photograph in 1839, needed a con-siderably larger angle of view for his im-age plate. The image created by a singlelens element was quite sharp in thecenter, but very blurred along the peri-phery. At that time, optical aberrationswere still unknown and better solutions

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could only be found by means of experi-mentation. The phenomenon of the dis-persion of white light into various spec-tral colors had been known for a longtime, but now it became a problem inmaking Daguerreotypes. The photo-graphic plate was sensitive to blue light,but the human eye is more sensitive toyellow light. That is why it was possibleto use a simple lens element to focusan image on the ground glass with yel-low light, but the image formed by bluelight could not be focused at the sametime. It was possible to correct this lon-gitudinal chromatic aberration by usingtwo lens elements, each one of a differ-ent type of glass, so that the dispersionof one lens was compensated by theother lens.

The curved surfaces of a lens alsogenerate a curved image (just as theydid in the old box cameras). But sincethe photographic plate was flat, a com-promise had to be found. This was stillbased on knowledge obtained fromexperiments. The first opticians andlens designers disregarded theories,even though the laws of optics hadbeen known for a long time. The law ofrefraction, which is the foundation ofoptical computation, was formulated inthe 17th century. Every ray of light com-ing from an object that strikes the glasslens at a certain angle, is bent in accord-ance with a known mathematical for-mula. When this light ray passesthrough many lenses, the path of theray can be traced clearly and methodi-cally. When the object is a very distantone, such as a star in the sky, all therays coming from that point light sourcewill be parallel as they strike the lens,and they will also converge into a pointafter passing through that lens. At leastthat is what we hope. As proven byDaguerre’s lens, this is not the case. Letus consider two rays of light, one ofwhich strikes the lens near its edge, theother at its center. We can then use thelaw of refraction and knowledge of thetype of glass to calculate the pointswhere these rays will strike the imageplane. If all the rays converge into apoint on the image plane, everythingwill be in order. If they do not, we havea problem. The first person who de-signed a lens using this kind of math-ematical computation instead of using

experimental methods was JosephPetzval. And his portrait lens was clearlysuperior to lenses that had been cob-bled together experimentally. Althoughnow it was possible to use formulas totrace rays quantitatively, the knowledgewas still lacking as to why the rays werebent in this manner and why they didnot reach the ideal or the theoretical lo-cation of the image point. Around 1850,Ludwig von Seidel researched the basiclaws of image formation with lensesand he was the first person to establisha theory of imaging performance. Aber-ration (from the Latin “ab” = from, and“errare” = to stray) literally means “tostray from the right path”. He discover-ed that there are seven so-called imag-ing errors of the third order that are in-dependent of each other, and which

together cause unsharpness and distor-tions in the image.

In principle, the next step is still easy.Now that we know, at least theoretical-ly, what causes the unsharpnesses inan image, all we need to do next is tocorrect these aberrations. And this isprecisely where the creativity of the op-tical designer comes in.

There are imaging errors that are caus-ed by faulty design and there are manu-facturing errors, both of which affectthe end result (the image on the film)significantly. The seven Seidel aberra-tions are divided into three groups: 1 -sharpness errors: spherical aberration,coma, astigmatism; 2 - positioning er-rors: curvature of field and distortion; 3 -chromatic errors: longitudinal chromatic

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aberration and lateral chromatic aberra-tion.

Every lens has certain characteristics,such as the type of glass, surface curv-ature (the radii of its two surfaces).These characteristics are called “para-meters” or “degrees of freedom”. Thetheory states that each individual de-gree of freedom can be used for thecorrection of an aberration. Conversely,every degree of freedom is also involv-ed in all the aberrations. This meansthat the optical designer can assign ab-erration components to every individualsurface.

The significance of the above can beexplained by means of an example. Thisexample is very important, because itdemonstrates how an optical designergoes about his task and why creativitystill plays such a large and decisive rolein that task. The seven aberrations canbe corrected with a minimum of eightindependent system parameters (de-grees of freedom). (The focal lengthalso has to be taken into account). A tri-plet (a three-element lens) normally con-sists of two collective outside elements(crown glass) and one inside dispersiveelement (flint glass). That results in sixradii and two separating distances be-tween the three elements. At the begin-ning, the designer selects basic systemparameters, such as types of glass, ele-ment thicknesses, distances betweenelements, and curvatures (radii) of theglass surfaces. That makes six surfacesavailable to the designer, and he or shecan now calculate the amount and kindof aberrations that each surface contrib-utes. As an example, we can establish(in a very simplified manner) that in thecase of the triplet, the radius of the sec-ond surface (of the first lens element)contributes spherical aberration andchromatic aberration, and that the radius

of the third surface contributes comaand astigmatism.

The optical designer must now decidehow to correct these aberrations. Hemight try to change the curvature of thefirst lens in such a way as to reducespherical aberration. But the curvaturealso determines the focal length, whichshould not be changed. It may also hap-pen that a change in the curvature willreduce spherical aberration, but that theamount of coma will simultaneously in-crease. The designer may also chooseto distribute the correction over severalsystem parameters in order to reducethe likelihood of increasing other aberra-tions. When the task of correcting oneparticular aberration as much as possi-ble is assigned to a single system pa-rameter, there will be a problem inmanufacturing if that very parameter isnot within established tolerances. Onecould also find that, if tolerances are tootight, the manufacturing departmentmay be unable of staying within thosetolerances.

But let us return to the correction ofaberrations. The optical designer willkeep altering system parameters untilthe correction of the seven aberrationshave reached a level where residualimaging errors are very small. The de-signer will also strive to correct each ab-erration by using several degrees offreedom at the same time. The “bur-den” of correction will then be distrib-uted over several surfaces and the en-tire system will appear more balanced.The designer can select the types ofglass and the curvatures within certainlimits, but each combination will resultin a different kind of overall correction.When the triplet has been configured insuch a way that it comes close to meet-ing specifications, we may find, for in-stance, that astigmatism has nearly van-ished from along the edges of theimage, but that it is still quite evident in

the field. Here we encounter a newproblem: The seven Seidel aberrationsare not the only optical aberrations. TheSeidel aberrations are classified asimaging errors of the third order. Logi-cally, there are other imaging errors ofhigher orders. The most important onesare the errors of the fifth and seventhorder. These groups of errors are en-countered only when the apperture iswell corrected.

Theoretically, a very small object pointis also reproduced as a very small imagepoint. This does not hold true in prac-tice, because these additional aberra-tions manifest themselves and spoil thefun. A point is not reproduced as apoint, but as a small circle with variouslevels of brightness. See illustration:Point spread function. A soon as the di-ameter of these disks becomes smallerthan a certain size, the higher order ab-errations become noticeable. This is asimplified statement, because in realitythese aberrations are always present,except that they only become notice-able when the residual errors of thethird order are small.

The example of the triplet, in whichastigmatism is still present in the field,shows the effect of the higher order ab-errations. One can use a certain verywell controlled residue of the Seidel ab-errations to compensate these errors ofthe fifth and seventh order. Naturallythis is only possible to a limited extent,and a triplet will render an acceptableimage quality only when its angle ofview and/or aperture are small.

This statement is very important. Aparticular optical system (number andconfiguration of lens elements) has alimited possibility for the correction ofaberrations. In essence this means that,when a new design is to be made, anoptical designer can only make the rightchoices if he or she has considerableexperience and knowledge.

An impossible task?

In earlier times, when there were nocomputers, optical designers only hadslide rules and logarithm tables at theirdisposal. Ray tracing was straightfor-ward, but laborious. Normally, the pathsof several rays are traced from an objectpoint as they travel through the optical

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system. These calculations are quite nu-merous and in the case of oblique rays,they are also complex. Before the ad-vent of computers, ray tracing was verylaborious. An experienced mathemati-cian required two to three months tocalculate a sufficient number of raytraces through an only mildly complexoptical system, like a triplet, for in-stance. It is understandable that ap-proximations were used and that verycomplicated calculations were simplyomitted. The resulting optical designshowed inadequate knowledge of theexact extent of optical aberrations. Still,one has to recognize that these approxi-mations helped the designers to deter-mine the characteristics of many aberra-tions exactly, and their experienceconstitutes valuable background for to-day’s optical designers at Leica.

All optical designs that are based onanalytical methods are solutions thatcan never be exact, and they only repre-sent approximations of the precise solu-tion. That is why an actual prototype ofthe lens had to be built, so that the prac-tical performance of the lens could betested. Two potential difficulties causedmany problems for the designer: thelens did not deliver the performancethat was expected, or the manufactur-ing department complained that thelens could not be built within the speci-fied tolerances. In either case the de-signer had to start all over again.

It was not easy to optimize a design.Success required much creativity and avery well developed instinct for the ef-fects of the aberrations. When onelooks at some of the older designs to-day, one is compelled to admire theachievements. An unbiased evaluationwith modern instruments shows thatthese famous designs lack refinements,but that they do have a worthy charac-ter.

As mentioned above, only proper raytracing can produce accurate results.But that causes a new array of prob-lems. First, the designer needs a largenumber of ray tracings. In the past,trigonometric formulas and logarithm ta-bles were used. At Leitz, the chief de-signer drew a diagram of the proposedoptical system and then instructed eachmember of a large group of individualmathematicians to perform a part of theray tracings and to hand the results to a

colleague. At the end of the day or theweek, the chief designer evaluated theresults and planned the next phase ofthe lens computation. For all rays thattravel in a flat plane that also containsthe optical axis, the equations for trac-ing their paths are based on plane ge-ometry and are relatively easy to use.Oblique rays require three-dimensionalor solid geometry. The respective equa-tions, however, are very complex.Therefore in those days oblique rayshad to be traced by means of approxi-mation formulas or not at all. Here too,only a partial knowledge of the perform-ance of the respective optical systemcould be gained.

With the introduction of computers,the limitations of optical calculationswere lifted, so that the (more exact) nu-merical method could now be em-ployed to full advantage. Numericalmethods can be used to achieve bettercontrol of important aberrations andthey can also be used to optimize anoptical system. This wealth of informa-tion can also entail its own problems.Did anyone ever tell you that the task ofan optical designer nowadays is easy?

The magnitude of the optical design-er’s task can be illustrated quite force-fully. There is a certain relationship be-tween the number of lens parameters(such as curvature, thickness, spacingbetween elements), i.e. the degrees offreedom, and the level of correction ofthe optical system. With more degreesof freedom, the optical designer has acorrespondingly greater number ofpossibilities of correcting a system.When a lens designer employs morelens elements, a higher level of correc-tion might be achieved. But that entailssignificant increases in costs, further-more the system may become highlyvulnerable to tight production toler-ances and/or to increases in weight.

Therefore the optical designer needsto acquire a thorough understanding ofthe basic optical potential of a given de-sign. All systems require optimizationafter an initially promising design. Whena design is not suitable for fine-tuning,the designer is only able to achieve aninferior product. A six-element 50 mmf/2 Summicron lens has 10 airglass sur-faces and radii, six thicknesses (one perlens element) and four distances be-tween elements. In addition, each type

of glass has a refractive index and a dis-persion number. The exact position ofthe iris diaphragm must also be deter-mined. With these 36 parameters (ordegrees of freedom), the designer hasto correct more than 60 (!) different ab-errations. Every parameter can have ap-proximately 10,000 distinct values andmore than 6,000 different ray pathshave to be computed for every changein a parameter.

The 36 degrees of freedom also arenot fully independent. Some need to becombined, and some are tightly con-strained by other parameters. Thus the36 degrees of freedom are in fact re-duced to only 20, making the task evenmore complicated. Given the specifiedconditions and considerations, it is notsurprising that hundreds, if not thou-sands of designs can be generated thatare very close to the desired solution. Ithas been estimated that a completeevaluation of all possible variants of thesix-element Summicron design, usinghigh speed computers that calculate raytraces at a rate of 100,000 surfaces persecond, would require 1099 years!

That is obviously impossible. In orderto select the best design from this virtu-ally infinite number of possibilities, thedesigner needs to have intimate knowl-edge of the effects of all the aberrationson the quality of the image. He or shealso has to be able to identify thosecomponents of image quality that pro-vide the necessary characteristics of thelens system. Today the design processcan keep a small staff busy for up totwo years in order to keep expenseswithin an economically viable range.There is no better way to illustrate theoverriding importance of the art of opti-cal design that is needed at the start ofa new lens system.

It appears that that the creativity of theoptical designer today is even more im-portant than it was in the past. And in-deed it is!

As stated in the description of thecomputation of the triplet, an importanttask of the lens designer is to assessthe various aberrations and then to un-dertake the corresponding alterations indesign specifications (radii, thicknesses,spacings and glass types). It is alsomost important that the starting designtype be selected judiciously, so that thedesired correction will even be possible.

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The merit function

When there are so many possibilitiesfor defining and correcting a particularoptical system, the designer has to beable to sense exactly when the desiredlevel of correction has been reached.Computers and optical design softwarecan easily produce numerical data. Theyare able to trace millions of rays within ashort time. The optical designer can usethis information to gain an insight intothe kind and the order of magnitude ofthe various aberration components. Af-ter that, two questions remain to be an-swered:

- Does the lens that has just been com-puted meet the requirements? and

- Is there an even better solution?

This is where the art of Leica lens de-signers becomes evident. It is not onlyat Leica that one is familiar with opticsand aberrations, and with the funda-mental fact that every photographic lensis a compromise between ideals and re-ality that incorporates a fine balance ofthe many aberrations that have to becompensated with one another. Thereis always a small residual aberrationcomponent in a lens. In the end, it is theweighting and the method of compen-sation of the aberration balance that de-termines how the resulting imaging per-formance is perceived and accepted byphotographers.

Leica lens designers have a verystrong ambition for developing opticalsystems with a particularly high optimi-zation of the various errors that will re-duce residual aberrations to their lowestpossible level. If one would claim that aparticular computation is not goodenough, one should have a standardwith which one could compare whatone has and what one wishes to have.The computing program is of no help inthis case. Imagine that you are in a heli-copter flying over a hilly landscape andthat you are trying to locate the deepestvalley. You will certainly be able to lo-cate a valley that is very deep in relationto its surroundings. But you don’t knowwhat is beyond the next mountain. Anoptimizing program seeks to find a deepvalley and it will certainly find a local lowpoint. But without overall knowledge of

the entire landscape, one will keep onsearching, never knowing whether onehas really found the deepest valley. Onecan only obtain this structural informa-tion if one knows the peculiarities andthe characteristics of an optical system.Leica lens designers call this the soul ofa lens. The merit function that one asso-ciates with a lens must be realistic andit should elicit the very best perform-ance from a lens. It should be pointedout, however, that every lens designerinterprets and defines “the best per-formance” differently.

We are accustomed to imagining lightrays as individual lines. That makessense for the computations. But in real-ity a flow of energy consisting of thesum of all the light rays radiated by allthe object points in the direction of thefilmplane will traverse the entire lens.The complete flow of light strikes thefront lens element and is transmittedthrough the optical system. This iscalled luminous flux. Knowledge and un-derstanding of this flow are extremelyimportant during the design stage of alens. Light energy should traverse thelens smoothly, without much deviationor resistance. That almost sounds like aconcept of Zen philosophy.

Steps in the design process

When a new lens is to be developed,the designer normally selects an exist-ing system and uses it as a start to-wards an improved design. The con-straints of dimensions and weight areparticularly important in the case of

Leica M lenses. The first attempt is con-strained by ancillary specifications suchas physical dimensions. Lenses shouldbe small and handy, and they should notobstruct the view through the view-finder. These characteristics are quitelogical from the user’s point of view, butthey constitute a constraint for the de-signer. Greater optical performance of-ten entails a greater physical volume, agood reason for applying new solutions,like aspherical lens surfaces in order toachieve the desired result. Excessiveweight must be avoided, and this limitsthe number of elements and the selec-tion of types of glass. The focal lengthand the maximum aperture already dic-tate the possibilities. The designer hasto find a creative starting point that canlead to success or that is at least prom-ising for optimization (finding the deep-est valley). Here too, there is a philo-sophical consideration: an optical designshould have a kind of beauty that onecan recognize. There are cross-sectionsof lenses that look very daring, andthere are others that possess an opticalbeauty. The latter are the best lens de-signs. Without a good starting premise,no lens will deliver the performance thatis expected. Optimization will go aroundin circles and sometimes no progresscan be made. When there is a startingpremise with which one feels comfort-able, one can proceed with the nextstep, which is the correction of theSeidel aberrations.

To correct the Seidel aberrations is ba-sically not very difficult, but we knowthat they are also used for influencingthe higher order aberrations. Therefore

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one should make a judicious choice ofpromising system parameters at thevery beginning, otherwise one can onlyachieve the desired result by usingmore complex procedures. Every addi-tional lens element can be used for cor-recting an aberration, but this also cre-ates new problems. It will quicklybecome apparent that these problemscan become utterly overwhelming. Atypical feature of Leica lenses is theirrelatively low number of lens elements.The 90 mm f/2 Apo-Summicron-MASPH. has only five elements and it de-livers outstanding performance.

The next step is the optimization ofthe system: small alterations of the lenscurvatures, the choice of glass types,spacings and thicknesses are imple-mented to achieve the desired level of

correction of the aberrations. The laststep is to balance the residual aberra-tions in relation to one another in such away as to achieve the imaging perform-ance that has been specified.

One of the quiet revolutions at Leicahas been the very close cooperation be-tween optical and mechanical engi-neers. There is no benefit in designing alens that cannot be manufactured orthat cannot be fabricated with sufficientaccuracy, or that is too expensive toproduce. The optical designer has to bevery creative in this regard. For a lensthat can only reproduce fine structures,the manufacturing tolerances that arerequired are different from those thatare needed for producing a lens that canclearly reproduce the very finest struc-tures. That is logical: when one wishes

to record tiny object details on film, onecan tolerate fewer errors than onewould tolerate in recording coarse ob-ject details. Very tight manufacturing tol-erances in the assembly stage assurethe possibility of achieving the com-puted optical performance in every indi-vidual lens. It is not an easy task to staywithin these tolerances, and it can onlybe done if these tolerances have beenworked out in close cooperation by theoptical and mechanical departments.

Leica-specific characteristics ofLeica M lenses

The progress that has been achievedin the performance of Leica M lenses inrecent times can be explained in the fol-lowing manner:

The optical design programs havebeen improved and they take into ac-count the latest findings in aberrationtheory, optimization and weighting ofimaging performance. Knowledge of thecharacteristics of the various types ofglass has also been expanded. The timehas passed when new types of glasseswere introduced in quick succession.Large suppliers of glass have cataloguesthat are quite stable. Leica lens design-ers would like to have a few additionalexotic glasses created, but it is ques-tionable whether this will ever happen.

The cooperation between the me-chanical and optical departments hasbeen intensified. The input of manufac-turing engineers to the computation of ahigh performance lens is a prerequisitefor a good result.

Leica has extensive experience withthe various aberrations, with their ef-fects on the photographic image andwith the more complex inter-relations ofthese aberrations. Current Leica M lens-es possess certain outstanding charac-teristics that can be classified as familycharacteristics. The latest Leica Mlenses feature a performance at full ap-erture that is a quantum leap better thanthe performance of their predecessors.This does not pertain so much to theperformance in the center of the image,but mostly to its field, i.e. in the imagezones. The overall contrast has alsobeen considerably and visibly height-ened. Stray light has been very wellsuppressed and this can be verified byexamining the very fine structures in theimage. Older lenses render these tinydetails blurred or they don’t record themat all, whereas the latest lenses renderthem clearly and distinctly, which be-comes especially evident in large projec-tion. The fine gradation of highlights andshadows in light and dark portionsacross virtually the entire image areaproves that the important monochro-matic aberrations, like spherical aberra-tion, coma and astigmatism have beenextremely well corrected. Brilliant anddelicately shaded colors are renderedaccurately, which is an indication of out-standing color correction. Chromatic er-rors that often become noticeable asperipheral unsharpness are well correct-ed. Another characteristic is the optimalaperture in the new lenses, which is al-ready reached by stopping down onlyone stop from full aperture. The oldaxiom that states that an aperture off/5.6 or f/8 has to be used for achievingthe best performance is no longer validso universally. The clarity of the imageis also improved because stray light, i.e.light energy that does not contribute toimage formation and that is scattered inthe optical system, is controlled effec-tively.

These general characteristics of thelatest Leica M lenses are clearly dis-cernible in the photographic image. Thefull performance potential of Leicalenses can only be exploited when thephotographer has a thorough grasp ofhis technique. The correction level ofthese lenses is of a very high level, andit can only be fully appreciated when thedemands are high. A good 20 x 25 cm

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(8” x 10”) black-and-white print cannotshow all the details that a lens is capa-ble of reproducing and systemic un-sharpness in the small details cannot beseen. But when a greater enlargementis made, for instance 30 x 40 cm (12” x16”), the results are much more dra-matic. Now it is essential that every linkin the performance chain is utilized tobest advantage. In this case the lens be-comes the most important link in thatchain and the photographer can makegood use of its characteristics.

Unsharpness and sharpnesstransition

There is only one plane of sharpness,and that is the film plane. That meansthat a bundle of light rays that comesfrom an object point traverses the filmplane like a cone of light. In the idealcase, the tip of the cone will be locatedexactly at the film plane, and the pointthat is being reproduced will be ren-dered as small as possible. The crosssection of the cone on both sides of thispoint is larger and the point is repro-

duced as a small disk. This is normallycalled the circle of confusion. If thecone of light is generally narrow, thenthe difference in the diameters of thepoint and the circles in front of and be-hind the tip is also small. In that casethe transition from sharpness tounsharpness is smooth. New Leica Mlenses are corrected in such a way thatthey are capable of reproducing the fin-est structures and details of the object.That also means that the tips of the lightcones have to be very small and thatthey have to subtend a larger angle (seeillustration). The circle of confusion willbe relatively larger (also absolutelylarger) than it was in the previous exam-ple. A characteristic of current Leica Mlenses is a visually faster transition fromsharpness to unsharpness. This is help-ful in composing the picture at full aper-ture, because pictorially important por-tions of the picture will stand outdistinctly from the background. The cir-cles of confusion will often appearsomewhat more disturbing, and thisshould be taken into account as the pic-ture is created.

The latest Leica M lenses are not onlysuperior to their predecessors optically,they also have a different kind of imagerendition that must be taken into ac-count when one changes over from anolder to a newer version of a lens. Butthat is precisely the fascination and thebeauty of Leica M lenses: one shouldbecome familiar with them and oneshould study their “personalities”.

Core technologiesCurrent Leica M lenses embody a

thorough understanding of, and a sensi-tivity to issues of geometrical and physi-cal optics, of mechanical engineering, ofoptical fabrication, of glass selection, ofthe relationship between residual aber-rations and image quality. Leica lens de-signs result from ingenuity, creativityand from a solid scientific knowledge ofall relevant aspects of an optical sys-tem. Most important are, of course, theguiding principles of the great designersof the Wetzlar era, notably Max Berek,and the accumulated experience andinsights of his successors. Part of thisknowledge has been incorporated into

current computer programs. There isone aspect of overriding importance,however, that cannot be codified intorules or algorithms: the culture of study-ing the true image potential of a newdesign and the know-how for transform-ing such a design into a real master-piece of photographic optics.

Leica lenses are not only highlycorrected, they are also meticulouslycrafted works of optical art. They arehoned to deliver a fidelity of reproduc-tion that reflects the combination ofphilosophy and state-of-the-art opticaldesign that is so unique to Leica de-signers.

If we were to identify the most impor-tant tools of Leica designers we wouldproduce this list:

•aspherical surfaces•apochromatic correction•glass selection•thin film coatings•engineering of lens mounts.None of these areas is an exclusive

domain of Leica. In fact, many manufac-turers around the globe use aspherics,apochromatic correction and have ac-cess to the same glass catalogs thatLeica designers use.

When I discussed these topics withLeica designers, I cited the example

Leica M Lenses [11]

T h e S o u l o f L e i c a

that aspherics technology has been inuse since the thirties and is now inwidespread use by many optical manu-facturers. They responded with charac-teristic modesty that they themselvesmight know a few things about asphe-rics that helps them to design lenseswith improved imagery. Let us look atthese tools, some of which are surpris-ingly old.

Aspherical surfaces.

Most lenses used in photographic op-tics have spherical surfaces, whichmeans that the curvature of the sur-faces has the form of a sphere. The lim-iting case is a plane or flat surface, i.e. asphere with an infinite radius. Sphericalsurfaces are relatively easy to make andray tracing is also simple (at least con-ceptually). An asphere (a-sphere) is de-fined negatively: any surface with ashape that departs from a sphere iscalled an asphere. A spherical surfacehas a radius R with the center of thecurvature somewhere on the opticalaxis. The radius will define all pointsabove and below the optical axis. For anaspherical surface we need more infor-mation. We define the difference be-tween the reference sphere and the ac-tual asphere at several heights aboveand below the optical axis and enterthese figures into an equation. Theequation can be very complex, but in itssimpler forms it defines a parabola, orellipsoid or hyperbola. An asphere mayhave a surface that has several zones ofasphericity, one paraboloid, and anotherellipsoid. The complexity of the surfaceshould be weighted against the cost ofmanufacture and the function within theoverall optical system.

There is a tendency to interpret theuse of aspherical lenses in an opticalsystem as a sure sign of superior opticalperformance. It is not. Some opticaldesigners can create fabulous designsby using a particular computer program,while another person employing thesame program will get moderateresults.

An aspherical surface introduces somecarefully controlled aberrations on top ofthe aberrations that result from spheri-cal surfaces. If you do not have a verythorough understanding of the basic ab-

errations in the system, the addition ofan aspherical surface may not be suc-cessful.

A prototypical case would be a lenswith spherical aberration, among severalother aberrations. The designer coulduse the spherical system to correct allaberrations, except the spherical aberra-tion. Then, by adding an asphere, he orshe can correct the spherical aberration.

The use of aspherical surfaces on mir-rors and telescopes is very old. Asphe-rics were already produced in the 18thcentury, using trial and error methods.Therefore they are not new tools for ab-erration correction.

Aspherics are used when systemsusing only spherical surfaces becomevery complex, or when systems be-come too large, and for many more rea-sons.

Using an aspherical surface gives thedesigner an additional degree of free-dom in the correction of optical sys-tems, which enables the designer tobuild high quality optics quite com-pactly. The advantages from a correc-tional perspective are the elimination ofspherical aberration and the correctionof the spherical aberration of the pupil(distortion). Aspherics can be used toachieve wider apertures, wider field an-gles, reduction of weight and volume(one aspherical surface replaces twospherical ones). In fact, many opticaland mechanical challenges can be metwith aspherics.

The manufacture of aspherical sur-faces requires extreme precision. Theaspherical deformations are calculatedto a very small fraction of the wave-length, but this level of precision is notattainable in manufacture. The requiredprecision for high quality optics is 1/4wavelength of light (that is 1/3000 ofthe width of a human hair!). In opticalshop testing, this level of precision canbe approached by using interferograms.It is very difficult to test aspherics in thismanner and in order to guarantee a pre-cision of 0.5 micrometer, one needs touse CNC grinding and polishing equip-ment.

Leica, however, does use an interfer-ometer to check the sphericity oflenses. They employ a compensationsystem to adapt the spherical wavefrontfrom the interferometer to the asphe-ricity of the lens surface. These com-

pensation systems can consist of aspherical lens. The most recent methodfor a compensation system is the use ofCGH’s (computer generated holo-grams). Leica is now using this tech-nique.

The required aspherical form can bepressed in plastic, or it can be a hybridform consisting of a glass lens with amolded plastic form attached to it or, asused by Kodak on the disc camera, itcan be a glass lens with pressure-mold-ed aspherical surfaces.

The 50 mm f/1.2 Noctilux, introducedin 1966, was the first lens produced byLeica that had two aspherical surfaces.In those days, aspherical lens surfaceswere polished to an approximate shapeand then hand-corrected afterwards. Asthe level of deformation is very small onmost aspherical surfaces, there is a riskthat the polishing at the end may re-store the spherical form! Only a veryfew workers at Leitz were able tomanually correct the aspherical form,and even they produced surfaces be-yond the required shape. This costly andlaborious manufacturing process wassoon abandoned. The potential of theaspherical surface for vast improve-ments of the image quality, however,was too attractive not to pursue. Huy-gens described its theoretical potentialas early as 1678. Three hundred yearslater the manufacture of precision as-pherics became reality. At first Leicaused a new technique, jointly developedby Leica, Schott and Hoya. Leica con-tributed the technology of the moldingtool. This methodology was first em-ployed in the 35 mm f/1.4 Summilux-Maspherical (the 21 mm, 24 mm and 35mm f/2 lenses are all of the Summiluxtype). It generates high precision sur-faces, but the technique is restricted tolenses with a small radius (about20mm). Furthermore only a few glasstypes can be used that can withstandthe heating, pressing and cooling with-out adverse effects. This restricts thechoice of glasses and many designerscomplain that the more than 100 glasstypes now in glass manufacturers’ cata-logs of the are not sufficient.

The next step is the use of computer-controlled grinding and polishing ma-chines that allow the designer freedomto choose glass types and radius asneeded. In the line of Leica M lenses,

[12] Leica M Lenses

C o r e T e c h n o l o g i e s

the 90 mm f/2 Apo-Summicron-M ASPHis the first lens to have an asphericalsurface produced by this promisingtechnique.

The Leica designer always needs morepossibilities for the correction of aberra-tions. As soon as a certain level of imagequality has been reached and the under-standing of a lens system has improved,a higher level of aberrations has to bedealt with. Therefore the designer needsmore parameters to change and influ-ence. The demand for ever more exoticglass types, more lens elements neverends. Aspherics are a very effective andelegant technique for the design and con-struction of complex optical systems.

The theory and technology of asphe-rics is in its infancy, however, and it iscertainly not as well understood asspherical technology and correction

theory. Leica designers employ as-pherics whenever there are clear ad-vantages for improving image quality,reduce volume or number of lens ele-ments or when designs can be createdthat would not be possible without theuse of aspherics.

The mere use of aspherics does notautomatically signify a high performancelens. Sometimes the designer can cre-ate a lens with spherical surfaces thattheoretically has the same performanceas the aspherical one. It might, how-ever, be impossible to build that lenswith the required precision and toler-ances. So when Leica designers employaspherical surfaces, it is a well consid-ered component of the complete lensdesign.

Apochromatic correction.

Ernst Abbe computed the firstapochromatic lenses around 1895. In

those days, the field of microscopy wasexpanding rapidly and the very highresolution required for microscopiclenses demanded that all aberrations bevery small, that is, close to the diffrac-tion limit. There is always an apochro-matic error in the photographic imageand this error extends over the wholeimage field. Generally it has a lowermagnitude than other aberrations andso will not be identified separately. Thevisible result of the apochromatic erroris a degradation of contrast and afuzziness of small image details. Oneshould look for the apochromatic errorin the center of the image (on axis). Onaxis the most disturbing aberrationshave been corrected quite effectivelyand so one is left with the more difficultaberrations, like spherical aberration,

chromatic error of the spherical aberra-tion and apochromatic error.

What is this apochromatic error? Ifpolychromatic light enters a glass, it willbe refracted into a number of rays, eachof a different wavelength. Each ray willfollow a slightly different path. The bluecolor will be focused closer to the lensthan the red color. The difference inlength between the two locations iscalled the longitudinal chromatic aberra-tion. Because the blue light convergesto a focus closer to the lens, the result-ing patch on the image plane will alsobe larger. This is called lateral chromaticaberration. One can see this defect as aseries of color fringes around a spot.

The magnitude of the chromatic aber-ration depends on the Abbe-number,the refractive index, the focal length andthe field angle. Note that the focallength is important, which explains whylenses with long focal lengths need tobe corrected for chromatic aberrations

in particular. The change in focal lengththat results from the fact that the refrac-tive index of a glass is different for dif-ferent colors is called dispersive power.As an indication of the very smallmagnitudes that are involved, we maynote that the distance between the redand the blue focal points corresponds to1/60 to 1/30 of the focal length. Thischromatic variation of index is called dis-persion. If one would plot the curves forrefractive index versus wavelength fortwo different glasses, for exampleSchott BK7 and SF2, the curves wouldbe non-linear and different. Every glasshas its own and unique graph. Crownglasses have relative low dispersionsand flint glasses relatively high disper-sions. Overall dispersion defines thegeneral dispersion characteristic. But if

we are interested in the blue part of thespectrum we need to study the disper-sion of the blue part. Two glasses havedifferent amounts of dispersion and theshape of the dispersion curve is differ-ent. So, in addition to the overall disper-sion, we also need figures for partial dis-persion, ‘partial’ referring to a part of thespectrum. If a glass produces a longblue spectrum it is called a long glass. Aglass with a short blue spectrum is, notsurprisingly, called a short glass. Mostcrown glasses are short and most flintglasses are long. Some glasses do notconform to this general rule. We have afew long crowns and short flints. These‘out of line’ glasses are called glass withanomalous dispersion.

Most glasses on the glass chart liealong a straight or slightly curved line,the so-called normal glass line. The out-of-line glasses, the ones with anoma-lous dispersion, are also referred to asglasses outside the normal line.

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C o r e T e c h n o l o g i e s

It is quite easy, at least in principle, tomatch two glasses with opposite dis-persions in order to make sure that atleast two colors (red and blue) will cometo a focus at the same point on the opti-cal axis. Then we have an achromat,which is quite often a flint/crown pair.Other colors of the spectrum (apartfrom the blue and red) such as greenand purple will still be out of focus.These residual errors are called the sec-ondary spectrum or secondary color.

The apochromatic error is the result ofdifferent partial dispersions or differentproportions of partial aberrations of theglass types.

Theoretically it should be possible toget an apochromatic correction by usingthree glasses with different dispersions.But the non-linearity of the dispersioncurve and the partial match of the partialdispersions will make life hard for thedesigner. For the correction of theapochromatic error, the use of specialglasses outside the normal line may beadvantageous. But these glasses haveproperties that make them hard to em-ploy. They are soft and very difficult topolish, they may be not available in therequired diameters, and they may alsobe very expensive. As a result, design-ers can attempt to achieve an apochro-matic correction with normal glasses(the three-glass-solution). In order touse such glasses, however, one musttake into account monochromatic er-rors. Sometimes the designer also en-counters difficulties in the correctionbalance and he may run out of usefulparameters so that he would need asystem that is too complex.

I mentioned earlier that the designer,when he corrects a system for twocolors, is left with residual chromatic ab-errations. There is no rule that stateshow large this residue should be. Nor isthere any rule that specifies how smallthe apochromatic error must be in orderfor the lens to be called a trueapochromat.

Pragmatically, all one can say is thatthere is a long bandwidth between anachromat, a semi-apochromat and a trueapochromat. Therefore, any lens withvery small chromatic aberrations can becalled an apochromat, even when thecorrection has been accomplished byusing glasses from the normal glassline.

Leica uses glasses for its apochro-matic corrections that lie outside thenormal glass line. These glasses arealso known incorrectly as APO-glasses,which is a misnomer. In reality they areglasses with anomalous dispersion. Thedispersion curves are non-linear, makingit difficult to compute corrections withthese glasses. The curves will nevermatch completely, so that some re-sidual aberrations will be left in the sys-tem. The residuals left after achromaticcorrection are called the secondaryspectrum, and it is no surprise that theresiduals left after apochromatic correc-tion are called the tertiary spectrum.

Leica designers know the non-linearityof these glasses very well. The art is toknow what glasses to employ where inthe design. As noted before, glasseswith anomalous dispersion can be foundin the catalogs of Schott, Hoya, Corningand others, which also list all the glasscharacteristics. Using such glasses maynot be unique to Leica. But the knowl-edge and expertise needed to extractthe most out of these types of glass, incombination with the creativity and ex-perience that the designer draws uponto balance the conflicting characteristicsof the lens system, are part of the coretechnology of Leica. The result is a lenswith a very small apochromatic errorthat has been balanced with all theother aberrations and that has been cor-rected over the entire image area, forexcellent results at full aperture orstopped down.

Thin-film coating.

Uncoated glass reflects a small part(4%) of the incident light per surface.The resulting problem is not so much areduction of the transmitted light, butthe increase of stray light. This straylight is scattered over the image plane,causing a dull and flat image with lowercontrast.

There are two possible solutions. Onis the application of thin film anti-reflec-tion lens coatings, invented in 1935 byDr. Smakula of Zeiss. The other is thecareful prevention of internal light reflec-tions by interior mechanical surfaces ofthe lens mounts.

The coating technique is basically asimple process. A very thin interference

layer of a material of lower refractive in-dex is applied to a glass surface with ahigher refractive index. The actual math-ematical computations are very com-plex. The thickness and the refractive in-dex of a layer must be computed sothat a destructive interference will re-sult. A single layer coating can be opti-mized for only one wavelength, usuallygreen, which is why the surface lookspurple by reflected light. This type ofcoating has an optical thickness of 1/4of the wavelength that is targeted. It isalso called quarter-wave coating. Thecoating material is often MagnesiumFluoride with a refractive index of 1.38.

For glass with a low refractive index asingle coating often suffices. It is not ef-fective on glasses with higher refractiveindices. With three or more layers, amore effective broadband low reflectioncoating can be achieved. A three-layercoating produces an anti-reflectioncurve with three minima that corre-spond to the selected wavelengths. Thenumber of layers can become quitelarge (6 to 11 stacks of layers), and theymay be multi-purpose, used for reduc-tion of reflections in order to improvetransmission and for balancing spectraltransmission.

It can be shown that a four-layer coat-ing with two different refractive indicesis very effective. The most frequentlyused technique of coating a lens is ther-mal evaporation coating. The coatingmaterial is heated in a vacuum chamberthat contains the lenses to be coated.The coating vapor is then deposited onthe glass surfaces. The correct thick-ness of the layer is monitored by a pho-tometer, but irregularities can occur.Not all coating materials can be depos-ited in this manner and sometimesmuch higher temperatures are required.That is when the technique of electronbeam coating is employed. Also in ahigh vacuum, the glass is bombarded bya beam of high-energy electrons, whichforms a layer on the heated glass.

This heating and cooling must be per-formed very carefully as the glass isvery sensitive to this treatment. Thelayer must be deposited on a smoothand clean surface, as any irregularity willcause unwanted local reflections. Thecleaning process is very important.Leica requires that some lenses mustbe coated within a few hours after

[14] Leica M Lenses

T h e S o u l o f L e i c a

MTF diagrams:those seductive curves!

cleaning to ensure that the air does notaffect the surface. After the applicationof the aforementioned coating tech-niques, the lens surfaces are coveredwith a layer of a microscopically smallpillar-like structure. The structure of thesurface of this layer is not amorphous,it consists of rows of very small pointedpillars, somewhat like rows of nailswith the tips pointed upwards. The re-sulting coated surface still has a micro-scopically small roughness.

The complicated and time-consum-ing processes of cleaning, heating, va-

porization, cooling for many layers inevi-tably generates errors.

Leica now uses a new technique, de-veloped in cooperation with Leybold:the plasma ion-assisted deposition.(IAD: ion-assisted deposition). With thistechnique the heating and coolingstages are no longer necessary and thegrowth of the coating layer is not pillar-like but amorphous, producing asmoother surface. The technique basi-cally consists of bombarding the target,which consists of the coating material,with argon ions, setting free atoms that

are deposited on the substrate to formthe coating.

The employment of this technique isanother example of Leica core tech-nology.

Every aspect of the optical system, beit glass selection, cleaning of glass sur-faces, coating, mounting, computation,or quality control, is scrutinized to findthe best solution to achieve the goal ofhigh performance optics.

The best and most convincing proof ofperformance is, of course, a picture. Itcan be in the form of a black-and-whiteprint, a color print, or a projected image,which is still the most impressive pres-entation of Leica photography. In prac-tice however, there are too many vari-ables that have to be taken into accountwhen we compare pictures. We don’tjust compare lenses, we compare theentire chain of performance. Thereforewe need a standard for the imaging per-formance of an optical system. In thepast, it was thought that a simple solu-tion was to use resolving power in linesper millimeter for these evaluations. Butvarious problems surfaced, some of avisual kind, others of a theoretical na-ture, which made the use of such astandard unreliable. We are not just in-terested in reproducing separated linesbut, more important, whether theselines can actually be seen as clearlyseparated lines. That is why an indica-tion of contrast is needed. The differ-

ence between a bright and a dark bandcan be discerned far better when thecontrast between the two bands is sig-nificant. When we look at a light gray /dark gray pair of bands instead of awhite and black pair of bands, the differ-ence is certainly less pronounced.

Residual aberrations that remain in thelens basically only cause unsharpness ina picture. By unsharpness we mean thatlight rays do not converge into aminiscule spot, but that they come to-gether into a slightly larger spot calledcircle of confusion, which only meansthat contrast is reduced.

Let us imagine a grid that consists ofblack and white stripes of equal width.When such a grid is reproduced by alens, diffraction, aberrations and straylight cause part of the light from thewhite stripes to reach the black stripes.This redistribution of light results in thereduction of contrast. The narrower thestripes, the more light from the whitestripes will spill into the black stripes,

further reducing the contrast. A lenswith inadequate correction of aberra-tions has larger circles of confusion,which means that contrast will be evenlower, whereas a lens with excellentcorrection of aberrations will also haveexcellent contrast rendition. Unfortu-nately the reverse does not hold true, inthat a good contrast rendition does notnecessarily mean a well-corrected lens.Design begins with a concerted effort tocorrect the many aberrations and whenthat has been achieved, high contrast isone of the results. The reverse is not al-ways true.

The fineness of the grid can be ad-justed by changing the width of thestripes. Let us consider stripes with awidth of 1/10 of a millimeter. Therefore10 such stripes make up 1 millimeter.This is defined as spatial frequency inlines per millimeter. In the aforemen-tioned example, that spatial frequencyamounts to 10 lines/mm. Because onecannot see black without white, it was

Leica M Lenses [15]

M T F G r a p h s

agreed to use line pairs as a structuralperiod. Therefore the figures stated inMTF diagrams, such as 5 or 10 l/mm,must be interpreted as periods or linepairs per mm. In other words, 5 lp/mmmeans 10 stripes that are alternatelylight and dark.

Leica furnishes MTF diagrams for 5,10, 20 and 40 line pairs or periods (10,20, 40, 80 light/dark stripes). The morestripes per millimeter, the finer the de-tails that can be reproduced.

One often wonders why the fineststructures are limited to these 40 lp/mm. There have been articles in thepress citing lenses that can record 200or more lines per millimeter. But now ithas become clear that the number oflines per mm is interesting only whenstated in conjunction with the respec-tive contrast. At 200 lines per mm thecontrast is so low that it is virtually im-possible to distinguish anything at all.The 40 periods used by Leica as a sen-sible lower limit result in a dot size of1/80 mm or 0.0125 mm. It is difficult toimagine how small that dot is on a 35mm negative! Take a negative and aruler calibrated in millimeters. Then sub-divide the width of one millimeter into80 tiny individual units. That gives youan idea of the performance capability oftoday’s lenses. Once we have estab-lished an understanding of how narrowsuch a unit is, then it becomes obvious

that the smallest vibration can spoil theentire picture. And a small amount ofunsharpness from inaccurate focusinghas a disturbingly large effect whensuch small image details are important.

How to read MTF diagrams?

The vertical axis is calibrated in per-cent of contrast, always based on anoriginal contrast of 100%. The subjectthat is being reproduced is that grid oflight/dark stripes in ever smaller widthsor periods. Every light/dark pair of linesin the original subject, even if it is veryfine, has an ideal contrast of 100%. Thismeans that all the light energy comesfrom the light stripe, and none at allfrom the dark stripe. The lens distrib-utes this energy over both stripes, thusreducing the absolute contrast. Thefiner the structure, the more that con-trast is lowered. With 5 lp/mm one canstill achieve a contrast of nearly 100%.At 40 lp/mm however, one would bepleased if contrast were as high as50%.

Thus the reduction of contrast is afunction of spatial frequency. Since thecontrast in an image is an interpretation,or a transfer (modulation, change) of theoriginal contrast, the relation betweenthe original contrast and the reproducedcontrast is called the Modulation Trans-fer Function, or MTF.

The effect of aberrations is less pro-nounced in the center of the image thanin the outer parts of that image (i.e. inthe “field”). A standard 35 mm negativehas a diagonal of 43.2 mm. Thereforethe maximum distance from the centerof an image to its outermost corners is21.6 mm. The performance of a lensvaries between the center of the imageand its field, because there are aberra-tions that disturb the field more than thecenter. That is why the horizontal axis ofan MTF diagram is calibrated in dis-tances from the center of the image, i.e.‘0’ represents the center of the imageand ‘21’ (mm) its corner. Therefore ‘12’indicates the height of a standard hori-zontal 35 mm frame, and ‘18’ its maxi-mum width. When one wishes to exam-ine an MTF diagram more closely, oneshould concentrate on the region be-tween 6 and 15 mm from the center ofthe image, because that region containsthe pictorially important part of an im-age. The central portion of the image,from 0 to 6 mm from its center, is satis-factory in most cases.

An MTF diagram thus provides a greatdeal of information because it showsthe contrast reduction for different kindsof image details for the entire 35 mmformat. The 5 lp/mm curve describesthe reproduction of very coarse imagedetails, the 10 lp/mm curve the repro-duction of clearly visible details, the 20lp/mm the reproduction of very finestructures and the 40 lp/mm curve thereproduction of the finest details of thesubject.

Low contrast values for 5 and 10lp/mm indicate a flat image, and highvalues for 20 and 40 lp/mm indicate thatfine details of the subject are repro-duced cleanly and clearly separated.

It should be kept in mind that each dia-gram is valid for a specific aperture.When diagrams are available for variousapertures, one can observe the behaviorof performance as the lens is stoppeddown. All the MTF diagrams shownhere display the curves for full apertureand for optimal aperture. The directionof the stripes is also taken into account.They can be orientated horizontally orvertically, and this has an effect onimaging performance. When the twocurves (tangential = vertical and sagittal= horizontal) are widely separated, thisoften means a blurry reproduction of

figure 1

[16] Leica M Lenses

M T F G r a p h s

subject details. In such cases, a repro-duction with good contrast is achievedonly when the details are orientated inthe “good” direction.

MTF data therefore provide an accu-rate and comprehensive description ofthe imaging performance of a lens. Nev-ertheless, it has to be used with cau-tion. In practice, small differences be-tween the curves are of negligibleconsequence. One should examine theentire image. One should also keep inmind that, although these diagrams pro-vide a very good description of all theaberrations, they do not cover all theconsiderations. Stray light, vignetting,color correction, distortion, performancein the close-up range, for instance, can-not be evaluated by means of MTF dia-grams. Because the method of creatingthese diagrams is not standardized, oneshould only compare the data providedby different manufacturers with greatcaution or better yet, not at all if it is notknown whether they were created bythe same method. An important factoris the quality of the light that is used forobtaining the measurements. Whitelight contains a great number of wave-lengths. Measured values are differentwhen only three wavelengths are used,compared to readings obtained withseven wavelengths. The weighting ofthe wavelengths is another influentialfactor.

One often wonders whether MTF datacan really describe optical performance

in a way that can be used in practice.We know that the objects being photo-graphed are three-dimensional, thatthey have depth. Even a wall has a sur-face texture that has depth. We assumethat the test grids (with the light anddark stripes) that are used for obtainingMTF measurements are only two-di-mensional (i.e. they have height andwidth, but no depth), so that they arenot representative of real photographicsubjects.

One should disregard these thoughts.The transfer of the contrast of the struc-tural periods (the grid) is a measure ofthe optical efficiency and the optical per-formance of a lens in general. What isactually measured is how much light en-ergy from a subject point reaches thecorresponding image point and how theenergy distribution is shaped in the im-age point (actually the image disc). Thisactually shows the effects of the aberra-tions. These image points can be lo-cated in the plane of sharpness but alsoin front of, or behind that plane, i.e. inthe range of unsharpness. Although thereproduction of the subject point will bedifferent, but the principle remains thesame. The aberrations determine the lo-cation and the shape of the image point,as well as its energy distribution. Be-cause points in the range ofunsharpness create the impression ofthree-dimensionality, MTF data is alsovalid, in principle, for depth perception.

How are MTF measurementsactually obtained?

There are two methods: one methodcomputes MTF data, the other methodmeasures MTF values. Basically, thereare no differences, and Leica uses ei-ther method, whichever is most appro-priate: the optical design departmentcomputes the MTF values, and themanufacturing department uses anMTF-measuring instrument to obtainMTF data (see the diagrams). Bothmethods are based on the same theo-retical principles, so that their resultsshould not be different from one an-other. A variance between the two val-ues only occurs when the lens assem-bly department can not conform tocalculated tolerance values.

Let us return to the image point as arepresentation of the ideal subject point.This point source is reproduced as a tinydisc in which light rays are distributedwithin a circle. Most of the rays will con-verge in the center of this circle, whilesome of them are scattered towardsthe perimeter of that circle. Light distri-bution can be described in the form of apoint spread function or spot diagramsby means of a 3D-diagram (with x-, y-and z-axes). The x- and y-axes (depthand width) show the shape of the discand the z-axis (height) shows the inten-sity of the light distribution (see the il-lustration of a point spread function).Figure 1 (point spread function) showsan ideal image point. The width of itsimage point spread is small and thepeak is very high. This means thatnearly all the light energy is concen-trated in a very small circle (like a point).The actual height of this point reproduc-tion is 20 µm (twenty thousandths of amillimeter) and the “point” is 5 µm wideat its base (five thousandths of amillimeter)! That means 200 pointswithin one millimeter, or 100 line pairsper mm (lp/mm)! Figure 2 shows thispoint again, but this time as a cross-sec-tion. It contains the same information,shown in a different manner. Figure 3shows a point spread function with adifferent shape. Its base is now approxi-mately 8 µm wide, and its height issmaller. This point can be described ashaving 62.5 line pairs. This is logical: thelight energy coming from a given sub-ject point remains the same, but this en-

figure 2

Leica M Lenses [17]

M T F G r a p h s

ergy can be distributed over small circlewith a high peak, or a large circle with alower peak. These computer-generatedvalues and images are based on classicray-tracing principles. The computed po-sition and distribution of the light raysfrom the subject point can be used forgenerating the point spread functionwith mathematical methods.

For actual MTF measurements, an in-strument is used that is capable ofanalyzing the distribution of light acrossa narrow slit. This narrow slit is illumi-nated from behind and a lens images iton a detector. A synchronous motordrives a scanner with another slit (whichis much narrower than the rear-illumi-nated slit) across the width of the illumi-nated slit. Measurements at the edge ofthe slit show the transition from dark tolight. This measurement is shown inFigure 4. The distribution of light energyat the edge of the slit has the sameshape as the cross-section of the pointspread function. There is no mathemati-cal difference between a point and aline, which is defined as an infinitenumber of points in a row.

Using mathematical formulas onceagain, one can use the measured lightdistribution of the slit (which is a lineimage) to arrive at a point image. Con-versely, it is also possible to derive aline image from the point spread func-tion.

One can occasionally read or hearclaims that an MTF diagram derivedfrom measured values is superior to acomputed MTF diagram, but this shouldnot be taken very seriously. One shouldalso consider the fact that at Leica, themeasured version and the computedversion are nearly identical, which isalso a sign that the manufacturing de-partment is capable of producing whatthe optical designers have devised.

In conclusion

MTF diagrams are not intended to re-place your own practical evaluation of alens. Nevertheless it has been proventhat an MTF evaluation comes veryclose to a visual and subjective evalua-tion of a lens. While MTF diagrams canonly be evaluated by someone with agood fundamental knowledge of optics,they nevertheless also represent a good

general indication of lens performance.If personal photographs do not corre-spond to expectations that are based onMTF diagrams, one should re-evaluateone’s own photographic technique. Theentire chain of reproduction especially,should be analyzed. MTF values repre-sent pure lens performance, which doesnot take into account exposure circum-stances and material characteristics.The MTF is an excellent means for com-paring and evaluating different lenses.In practice, it is very difficult to evaluatecomparison photographs that have realexpressive power. Small variations inexposure circumstances have a signifi-cant effect on the resulting photo-graphs.

After all, in practice there are a greatmany variables in exposure techniquesand also in personal conditions andevaluations. An individual who only cre-ates photographs using small apertures,certainly has different evaluation criteriathan one who frequently uses full aper-tures. When comparing MTF diagrams,one should also observe another funda-mental rule: it is not customary to com-pare lenses that have different focallengths. First of all, the correction of ab-errations is different. A wide-angle lenshas a greater angle of view, and oblique

light rays have a different weightingthan those in a lens with a longer focallength, in which chromatic aberrationshave a greater influence. One shouldalso be aware of the fact that, while alllenses are described in terms of con-trast values for 5, 10, 20 and 40 lp/mm,this data must be correlated with the re-production ratio. A 28 mm lens repro-duces an object much smaller than a135 mm lens, and since the lp/mm val-ues are valid for the reproduction on thefilm, they should be interpreted accord-ingly.

Personal experience and expectations,not purely MTF-based considerations,are the basis for the acceptance of anewly purchased lens. Even so, at a cer-tain moment in the decision process,MTF diagrams provide a very neutraland objective evaluation of the perform-ance capability of a lens. MTF is neces-sary when an objective evaluation of alens is wanted, but it is not the only cri-terion for purchasing a lens: methodsare only as good as the way a photogra-pher uses them, and information is onlyuseful when one knows how to inter-pret the data correctly.

figure 3

[18] Leica M Lenses

C o l o r R e n d i t i o n

Color renditionThe color rendition of a lens is obvi-

ously a very important characteristicwhich may take precedence over othercharacteristics. The photographer is of-ten very sensitive to changes in colortransmission when changing lenses, es-pecially when photographing the samescene.

Lenses with a yellowish or reddishcolor cast are called warm and thosewith a bluish cast are called cool.

Some glasses (for example extradense flints) contain a large amount oflead oxide and appear to the eye as paleyellow. If such a glass is used in a lens,the resulting color cast will be warm.The cast can be offset by suitable coat-ing, but there are limits here.

The notion of color rendition and colorfidelity is a very tricky one. The eye ishardly an accurate detector of color dif-ferences as the eye is easily fooled,partly by its great adaptation powers.

But the existing light changes also. At12.00 clock on a summer day the sun ina blue sky may have 10.000 degreesKelvin and a few hours later 6.000 de-grees. The eye may not detect this dif-ference, but the film does. Films alsodiffer in their spectral responses.

The imaging chain of existing light,scene to be photographed, color rendi-tion of the lens, color response of thefilm and the eye, introduces so manyvariables that a meaningful discussionand assessment is difficult.

The International Standards Organiza-tion has introduced standard #6728,which is called: “Determination of ISOcolor contribution index” (ISO/CCI) in1983. To start somewhere, the ISOanalyzed 57 typical camera lenses ofhigh quality in 1979 and used the aver-age relative spectral transmittance val-ues as a base figure: the ISO standardcamera lens.

As the typical photographic daylightthe ISO used 5.500 Kelvin, which is alsoused by the emulsion industry. This isthe situation when the sun is 40 de-grees above the horizon in a cloudlessatmosphere. Flashlight is also desig-nated 5.500 Kelvin, but the spectral dis-tribution may be different.

For the spectral sensitivity of films,data from the manufactures for the sen-sitivity of the blue, green and red sensi-tive layers were weighted so that a nor-malized sensitivity per layer could beestablished.

The spectral characteristics of a lenscan be evaluated in terms of its total ef-fect on the several layers of an averagecolor film. The effect on the blue sensi-tive layers is called the blue photo-graphic response of the lens. These re-sponses can be calculated.

The CCI value is computed by multi-plying the relative transmittance valuesof a lens by the weighted spectral sensi-tivity values for the blue, green and redsensitive layers. The total response isobtained by summation. and will give anumber for blue, one for green, one forred. To simplify, make the smallest ele-ment of the three number designationsequal to zero. If a lens has a Color Con-tribution Index of 0/5/4, this means thatthe average color film in standard illumi-nation sees the lens as providing moregreen (5) and more red layer (4) re-sponse relative to the blue (0) than thatobtained with no lens in the system.This lens would give a yellow cast.

The ISO however has established thatwith the sensitivity data of emulsionmanufacturers, the standard lens shouldconform to the 0/5/4 index.

The tolerances are established asfollows.

Blue is 0 with +3 and -4.Green is 5 with +0 and -2Red is 4 with +1 and -2.

These tolerances must be derivedfrom a trilinear graph and not by addi-tion. Leica lenses are aimed at the CCIof 0/5/4 and can be considered as neu-tral in transmission when they arewithin the tolerance band.

The following is a list of representativeLeica M lenses and their CCI values.Values lower than -/5/4 produce a bluishcast, values above -/5/4 a reddish cast

Elmarit-M 2,8/21 ASPH 0 6 5Emarit M 2,8/21 0 5 2Emarit M 24 mm f/2.8 ASPH 0 5 5Elmarit-M 2,8/28 current 0 5 4Elmarit-M 2,8/28 previous 0 4 2Summilux-M 1.4/35 aspherical 0 6 6Summilux-M 1.4/35 ASPH 0 5 5Summilux-M 1.4/35 0 4 1Summicron-M 2/35 ASPH 0 4 3Noctilux -M 1/50 0 8 6Sumilux-M 1,4/50 0 5 2Summicron-M 2/50 0 5 4Elmar-M 2.8/50 0 6 5Summilux-M 1.4/75 0 6 3APO-Summicron-M 2/90 ASPH 0 6 4Apo-Telyt-M 3.4/135 0 5 4Elmarit-M 2.8/135 0 6 4Tri-Elmar 4/28 0 5 3Tri-Elmar 4/35 0 5 3Tri-Elmar 4/50 0 5 3

From this list you can get a very goodimpression how of close to neutral mostLeica lenses are.

The Summicron-M 2/50 or the Apo-Telyt 3.4/135 might be considered asreference lenses. Take pictures with onrof these lenses and your favorite film.Then take pictures with other lenses un-der identical circumstances and on thesame film, preferably slide film, so thatthe lab can not introduce additional colorcorrections. Then compare the picturescarefully and try to find differences incolor rendition. This will give you a goodidea of what shifts to expect and whatfilters to use, if any, to correct a smallcast.

The difference of 1 point may not benoticeable.

Leica M Lenses [19]

C o l o r R e n d i t i o n

[20] Leica M Lenses

2 1 m m l e n s e s

21 mm lensesThe optical system with a 90º angle of

view is a comparatively late addition tothe line of Leica lenses. The design ofsuch an extreme wide-angle lens for the35 mm format had to contend withthree serious problems. Distortion is theobvious one. Optical vignetting is thereal villain here. For a lens with a 90º an-gle of view, peripheral illumination isonly about 25% of axial illumination, as-suming distortion-free imagery. To beatthis so-called Cosine-Fourth vignetting,the designer should accept distortion,but this is not acceptable. An optical‘trick’ can be used, however, to in-crease the level of light energy towardsthe periphery without increasing distor-tion. Basically one uses the propertiesand relations between the entrance andexit pupils to accomplish this feat. Awide aperture is the third villain. Earlierdesigns had to be stopped down to f/11or even smaller apertures to get accept-able or even usable image quality.These earlier designs were used onlarger format cameras where the needfor enlargements is less pronouncedthan it is in the ‘small negative-big en-largement’ philosophy of 35 mm pho-tography. Zeiss gets the credit for beingthe first in 1954 to cross this barrierwith its seminal rangefinder-coupled 21mm f/4 Biogon lens for the Contax cam-era, that perennial and friendly chal-lenger of the rangefinder Leica. Leitz in-troduced its version of a 21mm lens in1958 in the form of the 21 mm f/4 LeitzSuper-Angulon lens.

At full aperture, this lens featuresgood performance in the center and, ifthe clear rendition of fine details is notthe prime objective, that aperture isquite usable. The 21mm f/3.4 Leitz Su-per-Angulon followed in 1963 and it re-mained in production until 1980. Quite along working life. But in those days opti-cal progress could almost be measuredin generations.

Generally speaking the 21 mm f/3.4Super Angulon lens is a very good per-

former. It fine qualities are restrictedonly by the presence of astigmatismthat reduces overall contrast and thecrisp rendition of fine details at full andmedium apertures.

21 mm f/2.8 Elmarit-M

The first fully Leitz-designed lens forthe demanding 90° angle was intro-duced in 1980. This lens had to have along back focal length (distance fromthe rear lens surface to the film plane)to make room for the M5 exposure me-tering mechanism. This retrofocus de-sign requires a totally different type ofcorrection. Coma, distortion and trans-verse chromatic aberrations are difficultto correct. In this case, a bit of distortionhad to be allowed in order to achieve ahigher correction of the other aberra-tions.

At f/2.8 the light fall-off is slightly lessthan that of its predecessor. Flatness offield is less well corrected than distor-tion. At full aperture, the on-axis per-formance (a circle with a diameter of6mm) produces an image that is slightlycleaner than that of the Super-Angulonlens. This generally somewhat higher

contrast results in a clearer delineationof fine details. Details are quite soft inthe field and in the corners the image.

The general performance of theElmarit-M lens is better than that of theSuper-Angulon at f/3.4. This is partlydue to the reduction of astigmatism,which results in improved rendition offine details.

At f/4.0 extremely fine details are vis-ible with good contrast in the center andwithin a 12 mm diameter circle aroundthe center. From there to the cornersthe image details become progressivelysofter, but fine details remain within adetectable range.

Optimum performance is reached at f/5.6 with extremely fine details now vis-ible over the entire image area into theoutermost corners. Subject outlines, es-pecially in the outer zones, have softedges, giving an overall impression of asmooth, somewhat subdued image.Stopping down to f/11 and smaller aper-tures diminishes image quality.Decentering was not measurable.

Generally speaking, this lens is a com-mendable performer and an improve-ment over the Super-Angulon lens. Inthe field at the wider apertures, imagequality is a bit modest.

Close-up performance

A 21mm lens by nature shows a greatdeal of empty foreground, which couldbe filled with some objects close to thelens. The essence of the 21mm focallength is the surrounding of an interest-ing object in the foreground by a large

Leica M Lenses [21]

2 1 m m l e n s e s

environmental background. It is of primeimportance to know how the lens be-haves at distances around 1 meter.

At f/2.8 the corners are noticeablydarker, but vignetting is gone at f/4.

At full aperture the overall contrast ismedium. The Elmarit, however, repro-duces very fine details just a shademore crisply than its predecessors.From f/5.6 the performance becomesuniform across the entire frame and wehave an excellent image with very finecrisp details into the corners.

Flare suppression:

The Elmarit-M handles light sourcesquite well at full aperture. In strongbacklight the silhouettes of treebranches are dark (no leaking of lightaround the edges) and sharply outlined.

21 mm f/2.8 Elmarit-M ASPH

At full aperture, contrast is high andextremely fine details are rendered verycrisply from the center to an imageheight of 11 mm. That is the coverageof an image circle of 22mm. From thereto the edges, the contrast decreases alittle but it is still vastly superior to thatof all its predecessors. Astigmatism andcurvature of field are almost fully cor-rected. Subject outlines and fine detailshave very high edge contrast and arerendered clearly, almost lucidly. In theouter zones and in the corners this su-perior performance decreases slightly.

The overall image quality is a quantumleap forward in relation to all previous21mm lenses in the Leica stable.

To place it in perspective: the perform-ance at f/2.8 is better in all respectsthan that of the f/3.4 Super-Angulon at f/5.6.

At f/4 contrast and the clear renditionof very fine details improve, with thecorners still lagging a bit behind. Overallcontrast is now at its optimum, with ex-ceptional performance over a large partof the image field. At f/5.6 overall con-trast drops a little, but very fine detailsare still crisp into the far outer zones.

From f/8 the performance drops everso slightly and at f/16 it is noticeably be-low optimum.

Decentering is not measurable. Overallassessment: this lens produces out-standing image quality at full aperture,which continues to improve as it isstopped down as far as f/8.

It is by far the best 21mm lens in Leicahistory and the only recommendedchoice for the person who needs supe-rior performance from a 21 mm lensstarting at f/2.8.

Close-up performance

Close-up performance at full apertureis better than that of the Elmarit-M f/2.8,producing higher overall contrast. Moreimportant, the high level of micro-con-trast gives a clear rendering of very fineimage details over the entire field, in-cluding the extreme corners. Even theperformance from the center to the cor-ners for the complete range of aper-tures is quite impressive.

Flare suppression:

The Elmarit-M ASPH lens improves onthe performance of its predecessor. Thesuppression of halos, especially at fullaperture, can be described as quite ef-fective.

Distortion

Look at the distortion graphs for theElmarit-M and Elmarit-M ASPH lenses.The curve of the Elmarit looks better asthe distortion at the fringe of the image

area becomes zero, where the ASPHversion shows more distortion. Theabrupt inward curve of the Elmarit, how-ever, increases the visibility of the dis-tortion, while the smooth curve of theASPH is easier on the eye and it re-duces the noticeable presence of distor-tion.

Design considerations

I wish to draw attention to the veryhigh level of finely tuned aberration cor-rections in modern Leica lenses. In thefirst part of this brochure I mentionedthat any optical system can only be cor-rected to a certain level and that someaberrations will always be present as re-sidual aberrations.

A careful design will not neglect theseaberrations because they will generate‘noise’ in the system. White light con-sists of all wavelengths, but photo-graphically important wavelengthsshould be weighted more heavily for ul-timate image quality. In the precedingElmarit lens, slightly lower importancewas assigned to certain wavelengths.Nowadays a Leica designer will strive toconcentrate all these wavelengths into aspot that is as small as possible. This isthe case with the 21 mm f/2.8 Elmarit-M ASPH. One can only see a faint traceof color residuals of the marginal rays.

Performance at infinity

This is an intriguing topic.The performance of extremely wide-

angle lenses at infinity is sometimesdiscussed as if it were a bit below ex-pectation when compared to lenseswith smaller angles of view. As I alwaysuse the same scene for a comparison, Iwas able to compare the image qualityof a distant scene as recorded with a28mm Elmarit-M lens (latest generationand a superb representative of its kind).The 28mm produces a high-contrast im-age, with extremely fine details, ren-dered very crisply. The overall imagealso has a clarity and lucidity that is diffi-cult to quantify.

In direct comparison, 21mm lensesare softer, they lack the overall clarityand crisp reproduction of very fine de-tails. The same amount of details as ob-

[22] Leica M Lenses

2 1 m m l e n s e s

tained with 28mm lenses can be no-ticed without difficulty, but image de-tails are slightly ‘fuzzier’.

Optical progress can easily be fol-lowed in the discussion that follows,with the 21 mm f/2.8 Elmarit-M ASPHrepresenting a very high level ofprogress. It is the only 21 mm lens thatcompares favorably with the 28mm f/2.8 Elmarit-M.

The Elmarit-M at full aperture showsvery fine details, but it renders themwith slight fuzzy edge contrast and ex-tremely fine details are lost. One needsto stop down to f/5.6 in order to renderthese extremely fine details with goodseparation.

The Elmarit ASPH produces better im-ages at full aperture. Overall contrast ishigher, and extremely fine details arejust discernible. The ASPH gives a no-ticeably crisper image at all aperturesand the level of just discernible details ishigher (more details that is). At f/4 per-formance is as good as the predeces-sor’s at f/5.6 to f/8.

Is a direct comparison between lensesof different focal length acceptable?

Not really! The reproduction factor ofthe extremely fine details is higher witha 21 mm than it is with a 28 mm lens.Even if the grain and the film would al-low it, the 21 mm image needs a highermagnification to show the same detailsat the same size and therefore it needsa higher degree of optical corrections ata higher spatial frequency.

The prerequisites for such an infinitytest are simple: extreme care must betaken if one wants best quality at infin-ity: rigid tripod, correct exposure, dis-tance setting at infinity, low speed film.

The test shows clearly that perform-ance at infinity with the latest genera-tions of 21mm lenses is at a very highlevel. When comparing results betweendifferent focal lengths one should take

into account the reproduction factor andall other image-degrading components.

I also conducted a test to ascertainwhether the performance level changesif one sets the focusing ring just a frac-tion before the infinity mark, thus usingdepth of field to secure good renditionof details. The drop in performance be-tween these two settings is clearly vis-ible. So, if you need the best imagequality at infinity, focus the lens at infin-ity and forget about depth of field con-cerns in such cases. We also noted thatimage quality will be reduced substan-tially if the objects at infinity are overex-posed, which is quite often the casewith small objects (like trees) againstthe sky. This behavior is inherent in 35mm format photography where overex-posure is one of the most seriouscauses of image degradation.

Leica M Lenses [23]

2 1 m m l e n s e sM T F m e a s u r e m e n t

[24] Leica M Lenses

2 1 m m l e n s e s

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21mm f/2,8 Elmarit-M

This lens is the firstretrofocus design byLeica for the M body.Stopped down to mediumapertures it delivers ex-cellent image quality overmost of the picture area. .At full aperture the onaxis performance is verygood, but in the field isonly acceptable.

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21mm f/2,8 Elmarit-M ASPH

A truly outstanding lensand a fine example ofLeica’s drive to designstate-of-the-art lenses. Atfull aperture the lens al-ready provides a high con-trast image with a crisprendition of very fine de-tails. This performanceimproves when stoppingdown and extends to thefull picture area.

[2,8] [4,0]

[26] Leica M Lenses

2 4 m m l e n s e s

24 mm lenses

24 mm f/2.8 Elmarit-M ASPH

The gap between 21 mm and 28 mmfocal lengths has long existed in therange of Leica lenses for Leica cameraswith coupled rangefinders. In the reflexworld the 24 or 25mm focal length hasa longer history. It maybe the renewedconfidence of many users who chosethe Leica M as the top reportage cam-era that inspired the Leica designers tocompute this focal length for the M se-ries. One may also note that the M bodyfavors the design of high quality shortfocal length lenses. As a historical re-mark we may note that Nikon andCanon rangefinders from the gloriousfifties offered the 25 mm focal length,thus closing the gap between the 21and 28 mm focal lengths long ago.

I used an example of the 24 mm f/2.8Elmarit-M ASPH lens and compared it tothe 25 mm f/4 Voigtländer SnapshotSkopar lens. The 84º angle of view isvery interesting, but also quite demand-ing creatively. Close range photographyof single persons or small groups con-veys an intimacy of close contact. Atthe same time you can make a strong

statement about the wider surroundingswhere these people are located. Encap-sulated intimacy might be the approach.When taking pictures you are naturallyinclined to tilt the camera downwards abit in order to include more foregroundin the image. At eye level the 84 degreeangle of view often encompasses toomuch horizon. In this position, perspec-tive distortion of background objects atthe sides of the picture area tends to bepronounced. This perspective distortionshould be carefully separated from theoptical one. Optically this lens showshardly any distortion.

After some use I acquired the habit oftaking pictures from a lower, but levelperspective. Here the unusually goodimage quality really shines.

In use its angle of view is quite fasci-nating. I used it in reportage-style pic-ture-taking situations and I was able toget very interesting pictures at closerange. The trick in using this lens is theselection of subject matter at about 1and 2 meters (approximately 3 to 6feet).

If the subject in the foregroundcatches the eye because of its nature orbecause of its composition, the wholeimage will be interesting. In many in-stances you might be tempted to go forthe grand view. Make sure the fore-ground space is dominant and full of in-teresting details.

At full aperture the lens exhibits a veryhigh contrast image from the centeracross the entire field. Only the far cor-ners drop in contrast and produce softdetails. Over an image circle with a di-ameter of 12 mm the outlines of subjectshapes and details are delineated withsuperb edge contrast and extremelyfine details are rendered crisply andclearly. In the rest of the field, very finedetails are etched crisply in the emul-sion with extremely fine details ren-dered visibly but with softer edges. Ex-ceedingly fine details are rendered just

above the threshold of visibility, butwith slightly lower contrast.

Going from center to corner the con-trast of extremely fine details dropssomewhat. While a bit soft at the edgesand of lower contrast, these details arestill clearly visible.

Stopping down to f/4 the contrast invery fine details improves and the ex-ceedingly fine details now are clearlyvisible. Corners still lag a bit but centerand zonal performance (12 mm imagecircle) is at its optimum. This aperturecan be called the optimal aperture. Stop-ping down to f/5.6 we see that the fin-est possible details become a bit morecrisp, but the outlines of shapes and de-tails start to soften faintly. Thus overallcontrast is a bit lower. It is a matter ofpriorities which aperture is ideal. I wouldsay that this lens is at its best at f/4.

At f/8 corners continue to improvewhere the center now drops in contrast.At f/16 the overall image contrast islower and very fine details suffer as dif-fraction sets in.

Close-up performance

At close range (± 70cm, approximately±28 inches), this excellent performanceis preserved. A wide angle lens like a24mm is not recommended if its close-up performance is not the same as itsperformance at infinity. As most lensesare optimized for longer distances, weneed to stop down to f/5,6 to get thebest of performance in the close-uprange.

Flare suppressionand other topics

Flare suppression is perfect. Night pic-tures with Kodachrome 64 show excel-lent gradation in strong highlights anddistance point sources are renderedclearly, without any trace of halo. This

Leica M Lenses [27]

2 4 m m l e n s e s

suppression of halo and flare gives pic-tures taken with the 24 mm f/2,8Elmarit-M ASPH a very realistic, almosttactile rendition.

Of course some light fall-off is visibleat full aperture, but it is negligible inmost picture-taking situations.

On the optical bench a faint trace ofdecentering was observed. On theother hand flatness of field and astigma-tism are very well controlled. No comawas observed.

Distortion is also hardly noticeable. Ofcourse when you intentionally take pic-tures in oblique positions, perspective isout of line. Used in a level position thislens is virtually distortion-free.

In common with most recently de-signed Leica lenses, the out-of focusblur at the wider apertures is quitefuzzy. While the outlines of bigger sub-jects are preserved, finer details fadeinto the background blur quite rapidly.

Conclusion

The Elmarit-M is without a doubt amasterpiece of optical engineering, alandmark design within the Leica Mrange.

The M version of the 24 mm focallength is in a different performanceleague than the 25mm f/4 VoigtländerSkopar. It gives the discerning Leicaphotographer so great an imaging qual-ity potential that it becomes a challengeto exploit. While the Skopar is quite acapable performer, the M version issimply outstanding.

At full aperture the M version is al-ready nearly at its optimum, achieving along-standing goal of Leica optical de-signers: to provide the best quality atfull aperture over the entire image area.This lens is quite demanding on the ca-pabilities of film emulsions. Pictures onISO 400 transparency film howeverproved that this lens shows its qualitieseven when relatively grainy films are be-ing used. The very high contrast of sub-ject outlines of the 24 mm f/2.8 ASPHgives additional power to the crisp grainof modern high speed film emulsions.

Within the Leica lens range this lenshas a premium position. Its angle ofview will give fresh views of interestingobjects in our world at close range andits optical capabilities add a novel im-

pact to pictures taken with fairly wideangle lenses.

No Leica M user should be withoutthis lens. The M style of photographydemands intimate close range photogra-phy and the 24 mm lens is one of thebest lenses to explore this area. At thiswriting it delivers unsurpassed quality inthe 24 mm focal length.

Voigländer 25mm f/4Snapshot Skopar

For some time a number of lenses forLeica rangefinder cameras have beenavailable from other manufacturers. It ismost interesting to compare the per-formance of these lenses to the Leicadesigns.

This lens does not couple to the range-finder and it has a several fixed distancetabs on the distance scale. It shows afair amount of decentering. In the outerzonal areas and at the edges small lightsources produce halo-like degradation.Distortion is very low, but light fall-off isnoticeable, but not disturbingly so. Atfull aperture the overall contrast is highand very fine details are rendered crisplyover an image area with a diameter ofabout 6 mm. Fine details stay visibleover most of the image area and be-come barely detectable in the corners.This excellent performance continuesup to f/8, but with reduced contrast onaxis. Close-up performance is as goodas performance at the infinity setting

In comparison, the 24 mm f/2.8Elmarit shows a high contrast imagewith exceedingly fine details renderedvery crisply over an image area with adiameter of 12 mm. Corners are softand slightly prone to flare. No improve-ments can be detected after f/4.0.

I shot comparison images on the samefilm (lens by lens) (my bayonet is thefirst item to exchange because of us-age) and noted that the Skopar lens ex-hibits some characteristics of Leicalenses of earlier generations. Overall theimages are duller and a bit muddier thanthose made with current Leica lenses.

A most interesting phenomenon be-came evident with these side-by-side-shots. The Skopar gives images with agrainier pattern and with grain clumpsthat are rougher than those in imagesmade with Leica lenses. This is caused

by the lower aberration content of theSkopar lens. When aberrations areabundant the light rays emanating froma point source of light do not convergeto a point in the image but have a morerandom pattern around the central core.These more widely spread rays energizemore silver grains around the centerspot and they do so randomly. The re-sult is a rough clumping.

Modern Leica lenses produce asmooth pattern of very tightly containedclumps of grains, which helps to pre-serve the rendition of very fine detailsand the smooth gradation of fine lightmodulations.

Leica lenses exhibit a crisp clarity ofthe finest possible details that thirdparty designs cannot match. On its ownthe Skopar lens is an excellent valueand the Skopar at f/4 is a capable per-former on axis. But the outer zones areno match for the Leica (at f/2,8!)though.

The fine performance of the Skopar ispartly the result of the modest aperture.The higher aberration content will notbe detectable in many picture-takingsituations, hiding as it were behind thedepth of field, among other things.

The generally weaker performance inthe field is another characteristic thatdistinguishes these lenses from Leicalenses.

The riddle of the wider apertures

The increase of the maximum apertureof f/4 to f/2.8 seems a modest one. Justas the step from f/2.8 to f/2 looks easywith current computational power. Butphysics is not that simple.

What then is the optical problem? Anylens produces a circular image areawithin which the 24x36mm format hasto fit. This circular area can be divided inthree parts, the center or axis, the zonalarea or field and the outermost zones oredges. The center (or the paraxial zoneor Gaussian zone) is quite easy to com-pute. The zonal areas are more difficultto correct. Optical aberrations tend togrow disproportionately as the apertureand/or the field-angle becomes larger.Many aberrations grow at the rate ofthe square root or the cubic root in rela-tion to the aperture diameter, or evenmore.

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OK one might say, lets settle for a bitless image quality in the corners. Thereis a disadvantage, however: zonal aber-rations have a strong influence on theperformance in the center. Moreover,when stopping down, the effect onsome aberrations is not reduced. Thecombined result of all aberrations is al-ways a reduction in contrast: a soften-ing of small details and a low overallcontrast. The burden of the lens-de-signer however, is not lessened if hesucceeds in reconciling all these con-

flicting demands. Aberrations can beclassified as third order, fifth order andseventh order aberrations and so on, un-til the nth order. Third order aberrationsare large and suppress all other aberra-tions in the series.

If a designer can tame these third or-der errors, he will be unpleasantly con-fronted with the next aberration in line.Balancing third order aberrations oftenrequires a change in focus position. Thewell-known statement that you cancompute a lens for high contrast or high

resolution ultimately boils down to thiskind of balancing. Fifth order aberrationsare mathematically quite challenging.The high quality of Leica lenses is basedupon a thorough understanding of thisgroup of aberrations. As usual, balanc-ing of conflicting demands and fine-tun-ing of parameters is needed to computea lens to this very high level of correc-tion.

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24mm f/2,8 Elmarit-MASPH

A landmark design in theM series, the lens deliv-ers astounding perform-ance at full aperture, be-coming outstanding whenstopped down a bit. Theangle of field encouragesthe style of close rangereportage photographythat made the M famous.

[2,8] [4,0]

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28 mm lensesThe 28 mm lens for the Leica range-

finder cameras was introduced in 1935.The Leitz Hektor 28 mm (1935) had avery modest full aperture of f/8. At thisaperture, image quality was quite goodand this lens remained in production for20 years. Leitz produced three different28 mm designs and for versions of thelast design. The 28 mm lens is a strongcandidate for being the most rede-signed lens in Leica history.

The photographer who employs a 28mm lens with its 74° angle of view hassome strong demands. A high-contrastimage with very fine details over thewhole image field, evenly illuminatedand distortion-free would be high on thelist. Pictorially the 28mm excels with im-ages where the main object is relativelyclose to the camera and is surroundedby the background. The 28 mm is a lensfor story-telling. With an aperture of f/2,8, this lens will be used quite often inspaces or buildings with strong lightsources that will be in the picture. Ex-cellent suppression of flare andbacklighting is a clear advantage in suchcircumstances. To provide an impres-sion of optical progress, we will com-pare the Hektor at f/6.3 with the Elmarit-M (third version of 1979) at f/2,8.

A demanding subject would be part ofa dark alley with some illumination fromstreet lamps. Recording this night scenethe Hektor gives a low contrast image,the highlights are washed out and pointsources show strong halos. The blackparts are darkish gray as flare and un-wanted stray light ‘illuminate’ the shad-ows, which show very few details.

The Elmarit-M gives a high contrastimage with finely graded details in thehighlights, flare is nonexistent, allowingclean black areas. The shadows have arich tonal scale showing small detailswith good clarity and clean colors.

The first versions of the 28 mmElmarit-M (from 1962 and 1972) wereimprovements of the 28 mm f/5.6

Summaron. A two-stop increase in aper-ture and a retrofocus design were quitedemanding. The narrow diameter of thebayonet mount did not make life anyeasier for the designer. The first 28 mmElmarit showed the familiar pattern ofmany older designs: low to mediumoverall contrast, good image quality onaxis, rapidly dropping in the field, softedges of the larger object contours, anda high level of image noise that reducesthe clear rendition of fine details. Thisoptical ‘noise’ is, of course, the effect ofresidual aberrations that could not becorrected to a high level. Images with

older Leica lenses at wider aperturesexhibit a veil of dullness and an absenceof really fine textural details. This per-formance has been identified some-times as smoothness of rendition, be-cause the gradient from sharp tounsharp and from details to outlines isnot very abrupt.

Stopped down to medium apertures inbright daylight the differences betweenmodern and older lenses diminish to asurprisingly small level. The higher cor-rected lenses are able to exploit the cur-rent emulsions to a fuller extent thanthe older ones.

28 mm f/2.8 Elmarit-M

The current version of the Elmarit-Mhas an unusual design with a plano frontlens element. Primary design objectivewas to achieve improved imagery in asmaller package. The M user wants su-perior optical performance and lensessmall enough not to obstruct the clearview of the viewfinder. These two de-mands (optical performance and smallsize) are difficult to achieve together.

The latest (1993) version of the 28 mmElmarit-M shows many characteristicsof the new era of Leica M lens design.Between 1980 and 1990 no new de-signs for the M were computed. Thelast of the previous generation were the21 mm f/2.8 Elmarit-M, the 75 mm f/1.4Summilux-M and the first of the newgeneration was the 35 mm f/1.4Summilux-M (with two aspherical sur-faces).

The 28 mm f/2.8 Elmarit-M at full aper-ture gives a very high contrast imagewith extremely fine details crisply ren-dered over much of the picture field (im-age circle of 16 mm). Only the far cor-ners and outer edges (left and right) arenoticeably softer.

This clarity of very fine textural detailsis a hallmark of many modern Leica Mlenses. In addition to high overall con-trast and a virtually free of flare trans-mission of light energy, this lens at fullaperture should provide the user withfirst order recording capabilities. Stop-ping down one stop brings already theoptimum aperture and at f/5.6 macrocontrast drops a little, while of coursethe corners still improve somewhat. Asmarginal rays are cut off from reachingthe emulsion (when stopping down) theedges of fine details become somewhatmore crisp. Presumably these small dif-ferences are lost in the emulsion and inother small image-degrading effects.

Compared to its predecessor, centerperformance at full aperture is almostthe same. In the field and when lookingat very fine details, the current lens hasclearly improved optical capabilities.Modulation transfer function (MTF)graphs also show a more rigid correc-tion of the nasty sagittal rays, which blurthe rendition of finely graded color hueson small object areas. Clearly, the cur-rent lens is corrected to a higher degreeand it is at least one stop ahead of the1979 Elmarit. The fourth version of the28 mm focal length lens has only onedrawback: its modest aperture of f/2.8.

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The 74° angle of view is very useful incramped spaces and, spoiled as weLeica M users are with very high speedmoderate wide-angle lenses, we wouldoccasionally wish we had a handful ofphotons more to activate our silverhalide-based emulsions.

Summicron-M 1:2/28mm Asph.

The optical prescription of the lens isquite fascinating. It fits in the genealogyof the seminal Summilux ASPH, a de-sign that decisively departs from theclassical Double-Gauss formula. This de-sign-type, now more than a 100 yearsold, has been stretched to the limits anda performance plateau has beenreached. The new Summilux design, in-corporates the negative front and backsurfaces and the aspherical surface. It isprobably the first lens that has been de-signed specifically around the use ofaspherics. Retrofocus designs are a sec-ond approach to step out of the shad-ows of the Double-Gauss formula. Morelens elements can potentially improveperformance, as more parameters canbe controlled. The new Summicron-M1:2/28mm ASPH picks up design ele-ments of both: the lens group in front ofthe aperture is an enhancement of theSummilux (front group) design and thelens group behind the aperture fits intothe retrofocus family and is a derivativeof the 2.8/28 formula. We should notpress the point, however, as a lens de-sign is a creative whole and not a mix ofready made components. The messageshould be that the new Summicron isbased on the best design principles cur-rently available in Solms thinking. Thelocation of the aspherical surface is dif-ferent and probably decisive for this de-sign.

The ergonomics

The new 2/28 is indeed a very com-pact lens, comparable to the current2.8/28 version.

Measurements are (2.8 version in pa-rentheses): length from flange: 41mm(41.4mm), overall diameter: 53mm(53mm), front diameter: 49mm (48mm).Both lenses use filtersize E46.

For a lens with twice the speed this isa remarkable feat. This design indicatesthe direction of future Leica M designs:compact and high speed and high per-formance. The somewhat weak per-formance of the old Summilux 1.4/35mm could be excused with referenceto its compactness, which forced thedesigners in those days to find a com-promise between size and performance.Now the circle has been squared.

The lens operates very smoothly, andthe aperture ring clicks with just theright amount of resistance and fluidity.When taking pictures with the newSummicron 28, I was amazed howquickly I could focus with the focusingtab and I have to confess that I hardlymissed a shot, when focusing movingobjects. The depth of field with a 28mmlens, even at an aperture of 1:2 exceedsof course the DoF of the Summicron 50by a factor of 2, which brings real advan-tages in street shooting.

The performance

At full aperture this lens exhibits ahigh contrast with crisp definition of ex-ceedingly fine detail over most of theimage field, softening in the field fromimage height of 9mm. A faint trace ofastigmatism and field curvature can bedetected. Stopping down to 2.8 im-proves the center area (diameter 12mm)and also brings in a higher microcontrastin the outer zones. Corners however laga bit and stay soft with a limited defini-tion of coarse detail. Stopped down to4, contrast becomes very high and theoptimum is reached with a very even

performance over the whole imagearea, excepting the extreme corners. At5.6 we se a small drop in microcontrastof the fine textures and from 8, theoverall contrast drops a bit. We have toput this in perspective, of course as werelate it to the optimum aperture. At 5.6and smaller, the Elmarit-M 2.8/28 is a bitbehind the new Summicron 28.

Distortion is about the same as withthe Elmarit 28mm and vignetting is justvisible with 2 stops in the corners at fullaperture, about the same as the Elmaritat 1:2.8. In general use, this falloff canbe neglected: even on slide film one hassome difficulty noticing the darkening ofthe extreme corners.

Close up performance at 0.7 metersand full aperture shows excellent per-formance with high contrast rendition ofvery fine detail.

Night pictures retain high contrast inthe shadow areas, and (when exposureis right) finer gradations in the highlightsare recorded as well. At least with slidefilm and Black and White. Bright lightsources have cleanly delineated out-lines, indicating effective elimination ofhalo effects. Coma cannot be detectedin these situations (light points in theimage field).

Flare is very well suppressed in day-light shooting too, in contre-jour situa-tions and when the sun strikes the frontlens obliquely. Of course: you can con-struct situations where secondary im-ages and veiling glare is quite visible,but even here the images retain con-trast and some saturation. A lens shadeis needed, when the light sources mayshine in or close to the front lens. I willgive this topic a separate treatment.Leica has redesigned the front part ofthe lens where the shade is attached foreasier handling. All wide angle lensessuffer the same problems here. It is atribute to the design team that theyhave given this topic additional atten-tion.

The transition from the sharpnessplane to the unsharp areas is relativelysmooth, but really out-of-focus areasshow the tendency to break up detailsin coarse and fuzzypatches. There is acertain harshness in the out of focusrendition that is typical of modern Leicalenses. It is related to the level of aber-ration correction.

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28mm f/2,8 Elmarit-M (1993)

This redesign aimed atreduction of volume andimproved full apertureperformance. At f/2.8 thislens now has better im-age quality than the pred-ecessor at f/4 does. It is afirst-class performer, butone would not expectless given its modest ap-erture and angle of field.

[2,8] [4,0]

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The new Summicron-M1:2/28mm Asph. marksthe top level of perform-ance from wideanglelenses and is one of thebest Leica M lenses.

It was introduced atphotokina 2000.

SUMMICRON-M 1:2/28mm ASPH

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35 mm lensesThe direct comparison between the

first 35 mm f/1.4 Summilux-M and themost recent one with one asphericalsurface (1994) shows progress on twodifferent levels. The immediately vis-ible progress is a very marked improve-ment of image quality. The more subtleand far-reaching innovation is a radicaldeparture from a classical Gaussian de-sign. The original Summicron andSummilux designs are variants of theDouble Gauss concept, pioneered along time ago. During the design stageit became clear that the desired imagequality could not be achieved with theuse of aspherical surfaces alone. Thedesign goal was a much-improved cor-rection of the marginal zonal areas. As Imentioned in the introduction, weshould approach image quality in amore integral way. All light energy thatstreams through the optical system isaffected by a variety of aberrations. Thelens designer, of course, will analyzethe contribution of every single aberra-tion to the overall performance in orderto adjust and balance the correctionsthat are needed. Aberrations that affectmarginal areas the most will invariablyalso play havoc with on-axis perform-ance. Any successful optical design is avery carefully balanced trade-off be-tween the many aberrations that stub-bornly tend to degrade image quality.

It is not easy to correct an optical sys-tem with a wide aperture and a largefield of view. The total energy flow (theluminous flux) through a high-aperturelens is much greater than it is through alens with a smaller aperture and asmaller field of view. The effect of ab-errations is also many times larger andmore difficult to correct. Many aberra-tions increase at the rate of the squareroot or even the cube root when thefield of view is widened.

The revolutionary idea behind theSummilux aspherical lens is the radicaldeparture from the Double-Gauss de-sign. The optical system consists of

five groups with the first surface of thefirst element and the last surface of thelast element having a concave shape.In the future, highly corrected, high-speed lenses for small format photog-raphy could be based on this concept.Production technology however hasnot yet advanced to a level that everyconceivable optical design can bemanufactured within required engi-neering tolerances and necessary com-mercial parameters.

35 mm f/1.4 Summilux

At full aperture, the 35 mm f/1.4Summilux produces a low contrast im-age with fine details clearly visible inthe center and rapidly softening in thefield and corners.

Very fine details in the field are fuzzybut just discernible. At this aperture thelens shows a veiling glare and stronghalos and double images around pointlight sources

At f/2.0 the overall performance im-proves and at one stop smaller (f/2,8)contrast now is quite high and very finedetails become considerably morecrisp, showing quite a high edge con-trast. In the center, extremely fine de-tails emerge, but in the field very finedetails remain soft. Optimum apertureis reached at f/8. This pattern is typicalof older generation lenses where stop-ping down improves contrast and therendition of fine details, the latter onlyto a fair level. However a number of ab-errations are not affected by the reduc-tion of the aperture and continue to de-grade the image quality.

35 mm f/2 Summicron-M

The 35 mm f/2 Summicron-M is inher-ently corrected to a higher level. At fullaperture, contrast in the center is highand very fine details are rendered

crisply. Extremely fine details areslightly soft at the edges with a visiblecontrast reduction in small adjacent ar-eas of different illumination. This fineperformance is not sustained in thefield where rendition of very fine de-tails is progressively lost in aberrationnoise. At f/2.8 the excellent center per-formance now extends over most ofthe field, only the corners still lag be-hind. At f/8 the optimal aperture hasbeen reached and here we encounter avery high level of image quality. Highoverall contrast and high micro-contrastof extremely fine details over most ofthe image field bring a performancelevel that is difficult to exploit withoutappropriate types of emulsions.

From f/2.8 both the Summilux and theSummicron have a very similar optical‘fingerprint’. Indeed the performance isso similar at medium apertures thatone is inclined to assume that theSummilux is a Summicron designopened up just one stop too far. Thehigher inherent flare level of theSummilux reduces the rendition of verysmall details in the field more than theSummicron does.

The performance of the Summicron-M is excellent and would be called out-standing if the aspherical version didnot exist. In the course of these re-views I have to address a somewhatsensitive topic. Many older and recentLeica lenses are held in high esteemby users all over the world. Fortunatelythe standards of performance have ad-vanced and Leica designers are willingand able to push these standards a fewnotches higher. Many Leica lenses thathave been described by users, by thepress and in factory brochures as ‘out-standing’, get lower marks in my re-views. Any evaluation is relative to thestate-of-the-art available at the time ofthat evaluation.

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35 mm f/1.4 Summilux-M ASPH

At full aperture the 35 mm SummiluxASPH delivers a high contrast imagewith excellent micro-contrast and acrisp rendition of very fine details in thecenter and over a large part of the field.Extremely fine details are clearly visiblebut they show soft edges, which re-duces this lens’ ability to recordsmoothly graded illumination differ-ences. Light fall-off is noticeable, butrestricted to a very small zone. Flatnessof field is outstanding and distortionjust noticeable. Centering proved to beperfect.

At f/2 the overall image quality im-proves with slightly higher contrast forthe fine details over most of the imagearea. The finest details are recordedvery cleanly with crisp edges. At f/2.8there is a small improvement over thewhole image field and at f/4 the per-formance peaks at a very high level.

All the virtues of the design are nowclearly visible. Extremely fine detailsfrom center to corner, very smoothcolor hues in small object areas, crisprendition of object outlines, both largeand small, very good suppression offlare around strong points of light (sunreflections in water droplets) are thefingerprint of this lens.

In addition to these characteristics thenew generation of Leica lenses, ofwhich this Summilux ASPH is a primeexample, has the almost unique prop-erty of image clarity that is the result ofa very highly corrected optical system.Remember that in the introduction Idiscussed the effect of residual aberra-tions. These image-degrading aberra-

tions can be compared to dust in an air-tight room. This “dust” produces a thinfog that reduces the clarity of the view.This “dust” cannot be removed butonly redistributed in such a way that itwill no longer fog our vision. Leica de-signers are able, by studying the ‘soul’of a design, to minimize this amount of“dust”. Their designs ease the light en-ergy through the many glass elementswith minimal bending and disruption ofthe light rays. Small patches of lightwith a very high concentration of en-ergy in the core are the result.

After f/5,6 performance drops a dif-fraction begins to affect the straightpassage of light rays. This drop is rela-tive, of course and up to f/11 imagequality is of a very high order indeed.When using these apertures or smallerones be prepared to accept an imagethat is slightly softer overall. To providesome comparative remarks: the 35 mmf/1.4 Summilux ASPH reaches its opti-mum at f/4.0, with the f/2.8 aperturejust behind. One will note that theaspherical version reaches its optimumtwo to three aperture stops earlier thanthe predecessor does. And at f/4,0 theaspherical version has higher imagequality than the original Summilux atf/8.

35mm f/2 Summicron-M ASPH

The Summicron-M ASPH 35 mm atfull aperture gives quite comparableperformance to the Summilux ASPH atf/2.0, with a very high contrast imageover a large part of the picture field.The finest details are rendered a frac-tion softer at the edges and with some-

what lower micro contrast. TheSummilux-M ASPH at f/2.0 is slightlyahead of the Summicron according tothe MTF graphs in the outer zones. Thebetter flare suppression of theSummicron produces a slightly tighteroverall image. I would prefer to call it adifference in fingerprint or characteris-tic of image rendering. TheSummicron-M ASPH shows a pattern ofextremely high quality on axis, becom-ing less so when going outwards to thecorners. The difference between theavailable image quality on axis and inthe field is quite gradual. TheSummilux-M ASPH at its full aperture off/1.4 has the same pattern, but stoppeddown to f/2.0 shows very even cover-age over most of the field. That is re-markable after only one stop. The non-aspherical Summicron/Summilux 35mm versions follow the classical muchmore pronounced fall-off in quality be-tween on-axis and the field or zonal ar-eas. Evidently the Summicron-M ASPHat f/2.0 is better than the Summilux-MASPH 35 mm at f/1.4. The differencesbetween the ASPH versions ofSummilux and Summicron are muchsmaller that those between theSummilux and Summicron predeces-sors.

Close-up performanceof both lenses

Close up performance (±1 meter) isgood for both ASPH lenses. At full aper-ture the Summilux-M ASPH 35 mmhowever shows curvature of field andvignetting. Here I would not go into adetailed comparison. Both are verycompetent in this area, with theSummicron-M ASPH 35 mm slightlyahead.

Flare of both lenses.

At f/2,0 flare and stray light are ex-tremely well repressed in theSummicron-M ASPH 35 mm, it deliversexcellent separation of highlight de-tails, virtually flarefree images and finedetails are clearly defined with verygood gradation and color nuances. Italso exhibits a very crisp image of bril-liant clarity. The image of Summilux-M

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ASPH 35 mm at this aperture is moreflare prone at least in this situation. Thelarger front lens element element justcaptures more oblique rays of light thatcan soften the image a bit.

Interestingly at f/1.4 Summilux-MASPH 35 mm is better than at f/2,0, asthe diaphragm blades do not reflectsome of the light that pass through thelens at this wide opening. At f/2,0 thediaphragm is closed a little more andnow present an obstacle to the lightrays and some of them are reflectedinto the lens.

In another situation (portraits takenwith very strong backlighting, but no in-ternal reflections or flare) both versionsof the ASPH could take advantage ofthe better micro contrast and producedimages with slightly better retention offine details in the highlights. At thislevel he differences are however small.

Performance at infinity

Some Leica users question the abilityof modern wide-angle lenses to becritically ‘sharp’ at infinity.

‘Sharpness’ is not a measurable con-cept. We do have a visual sharpnessimpression that is based on the edgecontrast of the larger object outlines(also referred to as acutance). A Japa-nese drawing looks very sharp becauseits black contour lines that define theshapes of larger object details standout with high contrast from the back-ground. If you take a picture of trees sil-houetted against the sky the sharpnessimpression is quite high. Take anotherpicture at the same distance from abuilding with very fine architectural de-tails and we are led to assume thatsharpness is less because the overallcontrast is lower. Careful tests showthat both ASPH lenses at infinity record

very fine details with high contrast.Here however the expression that theweakest link is responsible for thestrength of the chain is true. The tripodmust be a heavy one. The film must beextremely fine grained with excellentacutance, and the slightest over-expo-sure will destroy the image quality.Tests show that a half stop overexpo-sure (as referenced by the optimum ex-posure) will degrade the image. Any na-ture photographer can tell you theadditional precautions that are neces-sary to ensure a stable platform.

At smaller apertures of f/4,0 and lessall four 35 mm lenses perform admira-bly. Theoretically the better micro-con-trast of the ASPH lenses should be-come visible in the definition of thefine details. But handheld shooting andthe grain limit of the films mostly usedwill diminish this possible advantage.

Conclusion:

The Summilux-M at the wider aper-tures is a bit overstretched in its capa-bilities. Stopped down it is a good per-former, but that is not the reason youwill buy a 1,4 design.

The Summicron-M was and is an ex-cellent lens. It is a tribute to the design-ers of that lens that it took Leica 20years and the most modern design-and production technology to bring op-tical quality to a higher level.

Summicron-M ASPH and Summilux-MASPH are the more modern designsand are capable of higher image qualitythan the Summilux-M and theSummicron-M. The differences are vis-ible, but you need to compare the im-ages side by side to see it convincingly.The Summicron-M ASPH is a superbgeneral-purpose lens with a very evenand excellent performance. The

Summilux-M ASPH is a lens that bringsSummicron quality into a f/1.4 design.The Summicron-M ASPH and theSummilux-M ASPH have a different fin-gerprint and photographic capabilitiesand therefore a different audience. Ifyou need the best performance avail-able at f/1.4 there is no alternative. The1,4 design is more susceptible to flareand has less flatness of field. If f/2.0 isenough for you the flavor ofSummicron-M ASPH and its price/vol-ume are very attractive. In performanceit and its sibling Summilux-M ASPH arein the same league.

The photographer willing to exploitthe superior qualities of the ASPHlenses, however must be willing to up-grade his technique also.

The current Summilux-M ASPH from1994 (with one aspherical surface) hasbeen preceded by the Summilux-MAspherical with two aspherical surfaces(from 1990).

The performance of this first versionis almost identical to the second ver-sion. The MTF graphs show small dif-ferences that should not be studied tooclosely. In the center the first versionshows slightly higher contrast, but inthe field the second version has an ad-vantage. I doubt if these theoretical dif-ferences are perceptible.

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35mm f/1,4 Summilux-M

At full aperture this lenshas low overall contrastwith a modest definitionof fine details and subjectoutlines. Stopping down,the improvement is com-mendable, becoming ex-cellent around f/8. Theoverall performance char-acteristic should be put inthe context of its age andsmall volume.

[1,4] [8,0]

[38] Leica M Lenses

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35mm f/1,4 Summilux-M (aspherical)

This design is a mile-stone in optical construc-tion, because of its nega-tive front and rear surfaceand the two asphericalsurfaces. Its full apertureperformance is superbwith a high contrast im-age and a clear definitionof extremely fine details.

[1,4] [4,0]

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35mm f/1,4 Summilux-M ASPH

The redesign with oneprecision pressformedaspherical surface can beproduced more economi-cally. Superior imaging atfull aperture, with aslightly different finger-print than predecessor.At f/1.4 it is a close matchto the performance of theSummicron 50mm atf/2.0.

[1,4] [4,0]

[40] Leica M Lenses

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35mm f/2 Summicron-M ASPH

Outstanding image qual-ity at full aperture andover most of the picturearea are its prominentcharacteristics. This lens,with its very low propen-sity for flare, combinedwith its crisp rendition ofextremely fine texturaldetails, is suitable forboth the fine-art photog-rapher and the documen-tary photojournalist.

[2,0] [4,0]

Leica M Lenses [41]

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35mm f/2 Summicron-M

This lens type has beenthe epitome of Leica pho-tography for more than40 years. It is the primechoice of many users be-cause of its small volumeand excellent perform-ance. At full aperture theimage quality on axis isfront-rank, but it drops vis-ibly in the field.

[2,0] [8,0]

[42] Leica M Lenses

5 0 m m l e n s e s

50 mm lensesThe first lens used for the Ur-Leica had

a focal length of 42mm. Commercialmodels (from 1925) were fitted with theAnastigmat/Elmax f/3,5/50mm. This de-sign had 5 elements, presumably toavoid patent conflicts with the Zeisscompany. There is much speculationwhy Barnack/Berek had chosen this par-ticular focal length. The most popularexplanation is the alleged correspond-ence between the angle of view of hu-man vision and that of the 50/52 mm fo-cal length: ± 46º. Now the human eyehas several angles of view, dependingon several criteria. They range from 6ºto 150º and there is no compelling argu-ment that the eye favors the 46º angle.The binocular field of vision is 130º. Letus stay on safer ground. There is asound optical reason for the choice ofthe 50mm focal length.

It so happens that the 50mm focallength is a solid base for excellent per-formance. Adopting this focal length willensure very good optical corrections, sodearly needed for the success of theoriginal Leica.

50mm f/2.8 Elmar

The Elmar f/2,8/50mm was introducedin 1957, almost 33 years after the Elmarf/3,5/50mm. In its day, the Elmar f/2,8was famous for its very good imagequality in the center. It was also slightlyahead of perennial competitor Zeisswith the Tessar. The different positionof the aperture (between the first andsecond element) was the main advan-tage of the Elmar. On test the Elmarperforms acceptably, at full aperturewith a low overall contrast and clearlyrendered fine details with smooth (moreaccurately: soft) edges. Fine details arevisible in the center but become fuzzy inthe field. Stopping down to f/4.0 mark-edly improves overall contrast andbrings in the corners. Very fine detailsnow become more crisp and a small

amount of extremely fine details is de-tectable in the center. At f/5,6 optimumperformance is reached.

The Summicron f/2,0 (7 element ver-sion) at full aperture has much bettercenter performance than the Elmar atf/2,8. Only in the outer zones of the field

the Elmar shows better imagery. Overallthe original Elmar at apertures of f/5,6and smaller is a good performer. Imagequality at the wider apertures is a bit be-low the aura bestowed on it by benigncollectors and users.

50mm f/2,8 Elmar-M

The redesigned Elmar-M f/2.8 at full ap-erture provides a medium to high con-trast image with very fine details verycrisply rendered over most of the field.Generally the new one is a stop or twoahead of the predecessor. More impor-tant for the overall image quality is amuch-improved micro contrast that addsto the very fine textural details a sparklingclarity. At f/5,6 the Elmar-M is close tothe performance of the current Summi-cron-M f/2.0/50mm. Close-up perform-ance of the Elmar-M is excellent at fullaperture. The four elements of the Elmargive less latitude for aberration-correctionthan the six of the Summicron. Thelesser number of elements on the otherhand produce a very pleasing, almostbrittle rendition of very fine details.

At f/2,8 the Elmar-M has the same fin-gerprint as the current Summicron-M atfull aperture (f/2.0), be it with slightlyless overall contrast.

50m f/2 Summicron-M (current)

The aperture of f/2.0 has been theworkhorse of 35 mm snapshot/report-age photography since the early thirties.The earliest version for the M-serieswas the 50mm f/2.0 Summicron with 7elements. This lens was the first to ben-efit from advanced glass research andimproved computations. The designproved to be sensitive to production tol-erances. Medium to high contrast in thecenter with fine details clearly resolvedwas combined with a relatively sharpdrop of performance in the field andouter zones. The higher level of aberra-tions in the field had its negative effectson the overall image quality. At full aper-ture this lens is prone to flare in not verysevere lighting situations. Stoppeddown to f/4.0 this lens performs verywell with a high contrast image overmost of the field and a slightly soft ren-dition of edges of very fine details.Compared to the current 50mm f/2.0

Leica M Lenses [43]

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Summicron-M at the same aperture off/4.0 we note a very different finger-print. The current Summicron-M rendersexceedingly fine details with sparklingclarity. Subtle color shades are cleanlyand smoothly differentiated in bothshadow details and highlight details.Within specular highlights minuscule de-tails are still visible, a proof that flareand veiling glare are very well control-led.

At f/4.0 the Summicron-M performs atits optimum. It is a characteristic of mod-ern lenses that the gap between the im-age quality at full aperture and at the opti-mum aperture is moderately small. At fullaperture the current Summicron-M is at adifferent performance level compared tothe first Summicron seven-element lens,that performs quite commendably. Witha high contrast image over most of thefield and extremely fine details clearlyand crisply rendered in the center this ap-erture can be used for all but the mostexacting demands. Stopped down tof/2,8 this performance extends over mostof the field and exceedingly fine detailsare now crisply rendered with goodclarity.

At f/5.6 the contrast of very fine de-tails begins to drop, a characteristic thatshould be discernible in very criticalwork. The full aperture performance ofthe f/2.0 Summicron-M is difficult toequal, let alone to surpass. The new90mm f/2 Apo-Summicron-M ASPHgives an improved imagery at full aper-ture (the clarity and edge contrast ofoutlines and finest textural details), butone must remind oneself that with 27º,the angle of field is much less.

Close-up performance of theSummicron-M is on the same level asthe performance at infinity setting (actu-ally infinity is that distance where the in-coming light rays are parallel to the opti-cal axis. For most lenses this conditionis satisfied at about 100 times the focallength).

50mm f/2 Summicron-M(previous)

The 1969 version of the Summicron50mm lens has about the same centerperformance as the current (1979) ver-sion. In the field however, the 1969 de-sign has quite low edge contrast. Onstopping down the field does not im-prove much. Extremely fine details aremarkedly lacking over most of the field.

Center performance however is aboutequal to that of the current version, al-beit with a bit lower overall contrast.This fingerprint of the Summicron ver-sion of 1969 at full aperture and per-formance when stopping down has initi-ated the impression that Leica M lensesare deliberately optimized for center‘sharpness’ with a certain neglect of thefield. It has been suggested that thisbehavior has been designed specificallyfor the rangefinder camera, as focusingand composing is done mostly in thecenter part of the field. The reportagestyle of M photography should also ben-efit from these characteristics.

I would be a bit hesitant to supportthese propositions. A higher amount ofspherical aberration in the 1969 versionis mostly responsible for its opticalbehavior. The designers in those dayscould not reduce the aberration withoutresorting to a bigger or more expensivedesign.

50mm f/1.4 Summilux

The first 50mm Summilux design(1959) closely resembles the perform-ance characteristics of the Summilux 35mm. A low contrast lens at full aperturewith fine details just resolved, this de-sign closely resembles the 50mm f/1.5Summarit in construction and perform-ance. Stopping down to f/2,0 improvesthe center performance to a level justabove the Summicron (7-element ver-sion) but not yet to the level of the sec-ond generation 50mm f/1.5 Summicron.

50mm f/1.4 Summilux-M

The current Summilux-M version wasintroduced in 1962, making the pred-ecessor one of the shortest lived lensesin the Leica history. At full aperture wenote a high contrast image on axis witha fairly quick drop of contrast in thefield. Fine details are clearly visible, butthe edges are very soft.

On stopping down the overall contrastimproves rapidly. Very fine details arerendered crisply in the center only, andrendition in the field is quite dull, if notfuzzy You need to stop down to f/8 toget very good image quality over thewhole field. The fingerprint of this lensis identical to the one of the secondgeneration Summicron. Excellent centerperformance already at full aperture,with a rapid drop off axis from about6mm image height. Close-up perform-ance shows crisp rendering of fine de-tails from f/2.8. Close-ups at full aper-ture are to be avoided if very goodimagery is required. Night pictures withbright small light spots produce a fainthalo around these point sources. Ob-lique light rays however generate a veil-ing glare and an overall softening in therendition of details.

The 35mm f/1.4 Summilux ASPH is tobe preferred when state-of-the-art fullaperture performance is needed. Aper-ture for aperture the 35mm SummiluxASPH shows improved image quality incomparison to the 50mm Summilux-M.

Performance in the outer zones is a bitlow at all apertures, which are notedeasily as the center is of such excellentquality.

[44] Leica M Lenses

5 0 m m l e n s e s

Flare

When I discuss flare (secondary reflec-tions and flare spots) and veiling glare, Irefer to the way specular highlights arerecorded (with or without reflections inthe small details). When veiling glare ispresent, one will notice that the shadowdetails are gray because of unfocusedstray light through the whole lens andthe way strong light sources just out-side or in the far corners dilute the satu-ration of colors and wash out fine tex-tural details.

The commonly used method of shoot-ing straight into the sun or a light sourcewill quite often produce secondary re-flections of the primary source. In thesecases the observer should look at pres-ervation of image details in heavily over-exposed areas. One of the best meth-ods to study inherent flare tendenciesof a lens is to take pictures of trees withmany branches against a bright sky. Theway the light spills over into the dark sil-houetted branches is a good indicationof sensitivity to flare.

Optical character

There is much speculation in the Leicacommunity whether the different opticalcharacteristics noted sometimes (highimage quality in the center and lowerperformance in the field versus some-what lower performance in the centerbut even coverage over the whole im-age field) are deliberately designed tosupport certain types or styles of pho-tography.

I cannot support this position. Any de-signer needs to balance aberrations toget the image quality that is required.As soon as advances can be made, highimage quality over the whole field is theprime requirement. See for example the35mm f/1.4 Summilux ASPH or the90mm f/2.0 Apo-Summicron ASPH.

Noctilux design considerations

One of the curses of high-speedlenses based on the double Gauss de-sign is the stubborn presence of anovercorrecting oblique spherical aberra-tion. This aberration is particularly prob-lematic as it affects the whole image

area at full aperture and it is still busydeviating rays when it is stopped down.One of the design possibilities is split-ting the single last meniscus lens ele-ment. That is the classical Summilux de-sign (seven lens elements), that we alsohave in the 50mm f/1.0 Noctilux-M.

Still, sagittal oblique spherical aberra-tion cannot be avoided. Add this charac-teristic to the lower performance of theSummilux in the field and we see someof the arguments for the emergence ofthe Noctilux f/1.2/50mm. The demandfrom the market to produce an evenwider aperture than f/1.4 seems a bitstrange nowadays. But the emulsiontechnology then was not as advancedas it is now. In these days reportagestyle photography in the darkest cornersof the human living space was quitepopular. The ubiquitous fill-in flash weare used to now, was not available inthose days or pictorially speaking,anathema.

50mm f/1.2 Noctilux

The Noctilux design with twoaspherical surfaces (front surface of firstelement and back surface of last ele-ment) aimed at reducing spherical aber-ration and enhancing image quality inthe field. Both design aims could bemet. At full aperture this lens has me-dium to high contrast with fine detailsvisibly resolved with slightly fuzzyedges. Very fine details are just re-solved, but its micro-contrast is quitelow. On coarse-grained film this level ofrendition of details will be lost in thenoise level of the grain clumps. Withmodern fine grain B&W films these de-tails are visible but quite fuzzy. Stoppingdown to f/1.4 improves overall contrastto medium/high. Fine details hardly im-prove in micro-contrast. At f/2.8 wehave a high contrast image with crisplyresolved very fine details in the center.The field is still lagging behind. Theseareas improve after stopping down tof/5.6 and f/8. Compared to theSummilux f/1.4, the Noctilux f/1.2 is notas good as the Summilux at f/1.4 andf/2.0. At f/2.8 both lenses are compara-ble and from f/4 the Noctilux is ahead bya small margin.

The Noctilux f/1.2 is very sensitive tothe correct distance from lens flange to

film plane. All Leica lenses are cali-brated and adjusted to a maximum con-trast transfer at 20 lp/mm. The perform-ance of the Noctilux f/1.2 drops quicklywhen the distance is not within a0.02 mm maximum tolerance. That isthe reason behind the recommendationthat the Noctilux f/1.2 should be indi-vidually paired to a body.

The laborious production of theaspherical surfaces (only one speciallyconstructed grinding machine was avail-able and it had to be operated manu-ally), the high level of out-of -tolerancesurfaces and the realization that this de-sign did not solve all problems of ultra-high-speed lenses are arguments forthe next player in high speed designs:the 50mm f/1 Noctilux-M.

50mm f/1 Noctilux-M

At full aperture (f/1) this lens has lowto medium overall contrast with fine de-tails clearly visible, but with soft edges.Very fine details are just visible on axisand barely visible in the field. Meridionaland sagittal structures are equally wellrecorded, a sign of a high level of aberra-tion correction. Looking at overall con-trast one dimensionally the previous ver-sion of the Noctilux has a highercontrast at f/1.2, but then that is a halfstop smaller. The better correction ofaberrations in the sagittal plane givesthe current Noctilux a small perform-ance edge.

At f/1.4 the current Noctilux has thesame high contrast as the predecessor.The micro-contrast of very fine detailsare also much improved. Still in the mar-

Leica M Lenses [45]

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ginal zones the previous Noctilux stillhas an advantage. Compared to theSummilux f/1.4 at its full aperture, theNoctilux has lower contrast but a moreeven coverage.

At f/2 the Noctilux improves again witha marked crispening of the fine detailson axis over an image circle of ±14mm.Compared to the current 50mmSummicron, the image quality of thelatter is in a different league. Where theSummicron has the ability to record ex-ceedingly fine details with clarity andsmooth internal gradation over a largepart of the image area, the Noctilux isable to record fine details with verysmooth internal gradation, but fuzzyedges.

At f/4.0 and f/5,6 the Noctilux is anoutstanding performer and is compara-ble to the Summicron at f/2.8 and f/4.The very wide aperture however givesrise to aberrations that cannot be as rig-orously corrected as in the Summicrondesign. This will be visible in the differ-ent fingerprint of this lens. One couldsay that the Summicron draws with avery sharply pointed pen and theNoctilux with a slanted pen to producebroader and smoother strokes.

The finely reproduced internal grada-tion of small textural details in combina-tion with medium overall contrast pro-duces images with a special character.

The Noctilux-M is a lens that defies asimple characterization. At full aperturethe blur circles of the light patches arequite large. It is difficult to detect smallobject details with crisply rendered out-lines. This lens should be employed inquite demanding low level light situa-tions to record larger object shapes withfinely graded internal textures. Here itscapabilities can be advantageously ex-ploited. At all apertures from 1,0 to 2,0overall contrast is lower and the record-ing of very fine details more fuzzy thanthat provided by its companion lenses(Summilux and Summicron). A specialcharacteristic of the Noctilux is its shapepreservation in out-of-focus- areas,bringing a remarkable depth of vision.

From f/4 the Noctilux can be usedwithout hesitation for quite demanding

photographic tasks. The severe vignet-ting at full aperture might be a problemwhen used for evenly and brightly lit ob-jects. The Noctilux then excels in situa-tions of very low light levels and/or vastbrightness differences to record thefeeling and ambiance of the scene. Itsflare reduction is second to none andeven better than that of the Summicron.Its penetrating power in ‘unavailable’light produces stunning images thatshow finely graded details in lowly lit ar-eas of the scene. Daylight pictures inthe night, one would dare to remark.Close-up pictures (±1 meter pr 3’3”) atfull aperture should be considered care-fully, as the very shallow depth of fieldwill produce a razor thin sharpnessplane with very fuzzy out of focusplanes.

There is no need to worry about therangefinder accuracy in combinationwith the reduced depth of field of thef/1.0 aperture even at larger distances.

[46] Leica M Lenses

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50mm f/2.8 Elmar

At full aperture the lowto medium contrast ofthis lens produces aslightly dull image andfine details in the field arerecorded with blurrededges. At f/4 the imagemarkedly crispens andthe on-axis performanceis excellent, with theouter zones trailing be-hind in image quality.

[2,8] [5,6]

Leica M Lenses [47]

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50mm f/2.8 Elmar-M

This redesign has beensignificantly improved. Atfull aperture very fine de-tails are recorded withvery crisp edges andgood clarity. This com-pact (collapsible) lens is afront-rank design for veryhigh requirements.

[2,8] [5,6]

[48] Leica M Lenses

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50mm f/2 Summicron-M

At full aperture this lensexhibits high contrast andvery small image detailsare defined with excellentclarity. In the field and inthe outer zones the imagequality drops a little. Stop-ping down leads to a sig-nificant improvement inthe center of the picturearea, with the outer zonestrailing behind.

[2,0] [5,6]

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50mm f/2 Summicron-M (current)

The classical doubleGauss design, stretchedto the limit, delivers imagequality of a very high or-der. High contrast, veryclean and crisp definitionof tiny details, clear sub-ject outlines at full aper-ture are the hallmarks ofthis lens that only needsto be stopped down oneor two stops for cutting-edge image quality.

[2,0] [4,0]

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50mm f/1.4 Summilux-M

With quite low contrast atfull aperture, this lens canrecord subject outlines andsmall details with good vis-ibility. Stopping down twostops markedly improvesthe on-axis performance,with the outer zones lag-ging behind significantly.Even in its day it did notset a record performance.

[2,0] [5,6]

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50mm f/1.4 Summilux-M (current)

This redesign shows theimage character of theclassical high-speed lens:high contrast, excellentcenter performance thatdrops markedly when ap-proaching the outer zonesof the picture area. The im-age quality becomes out-standing when stoppeddown to f/8. This lens isclearly challenged by morerecent designs.

[2,0] [5,6]

[52] Leica M Lenses

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50mm f/1.2 Noctilux-M

Arguably the most fa-mous of all high-speedlenses, its design incorpo-rated two aspherical sur-faces. Wide open it re-corded subject outlineswith good clarity and its ef-fective flare suppressionhelped the definition offine details. The lens im-proves by stopping downand the on-axis perform-ance at f/2.8 is superb.

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50mm f/1.0 Noctilux-M

The fingerprint of thislens is unique. At full aper-ture it shows superior flaresuppression, a mediumoverall contrast and a cleanrendition of the finer tex-tures in subject details inshadows and highlights.This is a lens for the con-noisseur, and not a substi-tute for the 50mm f/2Summicron.

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[54] Leica M Lenses

T r i - E l m a r - M

TRI-ELMAR-M

This lens is very important in severalways. Its three focal length (28-35-50)selection brings zooming convenienceto the M-rangefinder line-up. It takes awhile to become accustomed to itsthree ring handling (aperture selection-distance selection and focal length se-lection). The focal length selection ringis close to the distance setting ring andwhen in a hurry it is easy to mix thingsup. After some use (it took me a day)your fingers ‘know’ the right locationsintuitively. The convenience of havingthree focal lengths at swift disposalwould be useless for the critical Leica Muser if the optical performance werebelow par.

General optical performance

I tested the Tri-Elmar-M in comparisonwith the 28mm Elmarit-M current andthird generation, the 35mm ASPH andlast non-ASPH version and the current50mm Summicron. As I always use thesame test method I can easily refer tothe older generations as well. First sur-prise: the Tri-Elmar-M hardly improveson stopping down and this statement istrue for all three focal lengths. This

behavior is only possible in a very wellcorrected optical system.

It also means that the Tri-Elmar-M ex-hibits excellent optical performance atits full aperture. Admittedly not as wideas its fixed focal length brothers and sis-ters, but we will take up this issue atthe end of this section.

How excellent is theperformance?

Not every person will like the conclu-sion, but the Tri-Elmar-M is clearly supe-rior in all optical parameters to manyLeica lenses of the 28, 35 and 50mm fo-cal lengths. With the exception of theaforementioned 5 lenses (28 currentand 3rd generation, 35 ASPH and imme-diate predecessor: (the 7 elementSummicron) and the current 50mmSummicron) the Tri-Elmar-M will out-class any other Leica lens of the 28, 35and 50 focal length of previous genera-tions by a large margin. Second sur-prise: in many picture-taking situationsits performance is equal to that of cur-rent Leica lenses of 28, 35 and 50mmfocal length. There are obvious and vis-ible differences between the Tri-Elmar-M and its current fixed focal lengthcompanion lenses. To appreciate therelative importance of these differencesI would like to draw a distinction be-tween two types of Leica M use. Notethat this a distinction between styles ofuse and not between users. The sameperson in one situation will demand su-perior optical performance and in an-other situation this person will be moreconcerned with capturing the fleetingmoments of passion and life. Leica Musers are fortunate that their equipmentsupports both styles eminently.

The performance of the Tri-Elmar-M at50mm: the lens gives a high contrastimage with fine and very fine detailsrendered crisply. Extremely fine detailshave somewhat softer edges, but are

still quite visible. This performance ex-tends over a circular image area with adiameter of 12mm (the center area). Inthe outer zones (the image circle from 9to 16mm from the midpoint) the con-trast drops a little and the very fine de-tails become slightly softer. Some astig-matism lowers the contrast here. Theextreme outer area and corners are softwith fine details just visible. Stoppingdown to f/5.6 brings somewhat morecontrast and better definition of ex-tremely fine details.

This performance level continues untilafter f/11 where diffraction softens thedetails and lowers the contrast. Close-up capabilities (1.2 meter) are very goodwith a contrast image showing crisplyrendered fine details over the whole im-age field. At 35 mm: at full aperture thecontrast now is a bit lower and very finedetails are a bit softer.

Extremely fine details are just visible inthe center, but in the outer zone barelyso. The corners are on the same levelas they are in the 50 mm setting. Quiteremarkable here is the uniform perform-ance over the total image field. Theclose-up performance again shows ahigh contrast image with excellent ren-dition of details over the whole imagefield. At 28mm: Leica states that the 50position of the Tri-Elmar-M gives thebest performance, and a slightly lowerperformance at the 28 setting. Indeeddistortion is a bit greater than it is at the35 and 50 settings. When photograph-ing flat objects like walls, some barreldistortion is clearly noticeable. Whenpicturing architectural objects withdepth, this effect mostly vanishes. Atfull aperture fine details are renderedwith medium to high contrast in thecenter and contrast is a little lower inthe outer zone. Very fine details areclearly visible and become somewhatsofter in the outer zones. At close-updistances the image is of the same high

Leica M Lenses [55]

T r i - E l m a r - M

contrast and uniformity of field as it is atthe other settings. Here, as with the 35and 50 settings, stopping down in-creases contrast but the correction ofaberrations is already of such a highlevel that image details and textural de-tails only improve a little.

Comparison to thefixed focal lengths.

These lenses excel of course with ex-cellent to outstanding performance atthe wide apertures of 2.0 and 2.8. Atf/4.0 they are at thier optimum and thenthey are comparable to the Tri-Elmar-Mat nearby its optimum. For all focallengths we can give this conclusion,based on the f/4,0 performance.

The fixed focal lenses perform a littlebetter than the Tri-Elmar-M in the imagequality at the level of extremely fine de-tails and the performance in the outerzones and far corners. The overall con-trast of the fixed focal lenses too is ashade better, giving the pictures slightlymore clarity.

Very careful comparison of the pic-tures (low speed slide film at 30 x)taken with the Tri-Elmar-M and its com-panions shows these performance dif-

ferentials in contrast and the quality atthe level of extremely fine details. TheTri-Elmar-M shows remarkable suppres-sion of flare and night shots taken onthe 28 position give very good clarity ofhighlights and shadows with good rendi-tion of details and only faintly visiblecoma in the extreme outer zone.

For most users, who will shoot casu-ally or who do not demand the utmostof enlargements or projection distances,the performance differences are imma-terial and will not be of any importance.The very demanding user might notethe differences but it is a matter of per-sonal preference how to rate thesequality differences.

Conclusion.

Is the Tri-Elmar-M a replacement forthree top class fixed focal lengthlenses? The answer is obviously noteasy. Its full aperture of f/4.0 has itslimitations. Especially when using lowspeed films. Its compact design consti-tutes an attractive alternative to threepopular focal lengths and in this respectit performs outstandingly well. Thesmooth and quick changing of focal

length brings many picture-taking oppor-tunities that would be lost if you had tochange several lenses. And the criticalLeica user can use these new possibili-ties in the secure knowledge that theresulting pictures will show all the quali-ties for which Leica lenses are famous.And it may even captivate the most criti-cal users of older generations of Leicalenses in the 28 to 50mm focal lengthgroup. Weighing only 330 grams (lessthan 12 ounces), it is a very convenientlens with excellent performance thatthe older lenses simply cannot match.The modern and current generations areable but hard-pressed to surpass thislevel of performance at f/4 and smaller.The Leica user who needs outstandingperformance at apertures wider thanf/4,0 and/or big enlargements showingthe smallest image details with greatclarity and contrast needs to changelenses and after many years might wearout the bayonet flange.

[56] Leica M Lenses

T r i - E l m a r - M

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28mm f/4 Tri-Elmar-M ASPH.

One of the very bestlenses available for theM, its image quality at allthree focal lengths is im-peccable at full aperture.The maximum aperture atf/4 is sufficient for manysituations and the smoothand easy change of focallength helps capturingelusive picture opportuni-ties.

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35mm f/4 Tri-Elmar-M ASPH.

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50mm f/4 Tri-Elmar-M ASPH.

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MTFM T F m e a s u r e m e n t

MTF-measuring instrument to obtain MTF data

[60] Leica M Lenses

7 5 m m l e n s e s

75 mm lensesAt first sight one might ask if a focal

length of 75mm has added value, as itseems quite close to the 50mm and90mm lenses. The angles of field are45 degrees, 33 degrees and 27 de-grees, respectively. If we take a look athistory we note that Leitz offered a73mm f/1.9 Hektor (from 1931 - 1946)and a 85mm f/1.5 Summarex (from1943 - 1960). Both offered the widestaperture on the market at its time.

Leitz introduced the Summilux 75 asthe new incarnation of the time-honored high-speed lens for candidphotography and non-posed portraitphotography. The maximum apertureof f/1.4 posed some constraints onweight and volume and so the focallength of 75mm was adopted.

The 75mm Summilux was designedin 1980 as were the 90mm f/2Summicron-M, the 35mm f/2Summicron-M and the 21mm f/2.8Elmarit-M and they may be consideredto be the last designs of the classicalperiod. This series of lenses accompa-nied the introduction of the M4-P, theP standing for Professional, indicatingquite clearly the role of the new M-model as the dedicated tool for candidwork in preciously little available light.

From 1980 to 1993 no new designswere developed for the M-series. Since1993 the 21, 35 and 90mm have beencompletely overhauled, with new de-signs incorporating Leica’s expertise ofaspherical technology. It is a tribute tothe excellence of the Summilux designthat, when shifting production fromCanada to Germany, only a redesign ofthe lensmount (lighter by 40grams)was deemed necessary. The weight re-duction is much appreciated, as theearlier versions were a bit on the heavyside after a few hours continuous use.

75mm f/1.4 Summilux-M

At full aperture the lens exhibits a me-dium to high overall contrast, with ex-tremely fine details quite visibly re-corded. Very fine details are clearlyresolved with some softness at theedges. Some astigmatism is visible inthe outer zones, which softens the fin-est possible textural details. This per-formance holds over most of the imagefield, with a detectable reduction in theoutermost zone. The corners, althoughmuch softer, still record very fine detailswith good visibility. Stopping down tof/2.0 achieves the high overall contrastneeded to record extremely fine detailswith clarity and crispness. Higher con-trast generally gives the fine detailsmore clarity and sharper edges. Theouter zones now also improve and onlythe extreme corners lag a bit beyondthis performance.

At f/2.8 the contrast is slightly higheryet and now the micro contrast is at itstop, allowing the clear and crisp rendi-tion of exceedingly small details. Now atripod is most needed to record the fin-est possible details. We are talkingabout small details with a diameter ofabout 0.3mm in the image, photo-

graphed at a distance of 7.5 meters!You need to view the real object at re-ally close distances to see what thelens/film combination can record. Thisperformance level is maintained fromf/2 and f/2.8 to f/8 and the choice of ap-erture needs only to be justified ondepth-of-field arguments. Stoppingdown to f/11 slightly lowers the overallcontrast and the definition of very finedetails also loses a bit of its crispness.At f/16 a marked reduction of contrastoccurs.

Flare is very well suppressed but theuse of the built-in shade is mandatory!When taking pictures against stronglightsources (for instance when record-ing fashion shows) the fine light raysthrough smoke and dust hoveringaround lamps are rendered with subtlegradations.

The rendition of highlight details is un-commonly good: highlights hold their in-ternal gradations and separation of fineluminance differences is also very good.The objects (including catch lights) arerendered very life-like and the extremesharpness gives a special tactility toevery object. The very shallow depth offield at large apertures/medium dis-tances (at 2 meters distances we have±6cm) requires careful focusing at thelimit of the mechanical/optical precisionof the M6-rangefinder. But candid por-traits and full figures are clearly isolatedfrom the background.

Historical comparison betweenthe Summilux-M and theSummarex f/1.5/85mm.

At full aperture the Summarex has alow to very low overall contrast, withfine details rendered quite soft. Outlinesof larger subject details show fuzzyedges. This performance holds over thewhole image field from center to the

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outer zonal regions. The extreme cor-ners are very soft.

Stopping down to f/2.0 brings a verymarked jump in overall contrast and onaxis performance now is of surprisinglyhigh level with very fine details ren-dered with good clarity. In the field how-ever the recording of fine details mightbe described as close to sharp.

At f/2.8 the central performance nowextends over a large portion of the im-age field. Stopping down to f/4.0 bringsin very fine details with good visibility,but without the crispness and sparklingclarity we are accustomed to see in cur-rent designs.

Performance improves up to f/8 afterwhich the inevitable drop (diffraction)softens the whole image that by nowhas a very even coverage.

The time span between the Summiluxand the Summarex covers more than 30years of optical progress, which is mostevident in the imagery with wider aper-tures. The inherently higher aberrationcontent is visible in the somewhat flatand dull rendition of fine details atsmaller apertures. At the wider aper-tures the fine details show quite fuzzyedges and overall contrast is low. Flareis copiously present in adverse situa-tions.

Comparing the 75 mm Summilux-M to the 90mm Summicron-M

(of the same generation).

The Summilux-M 75 stopped down tof/2 has a higher contrast image with aclean and crisp rendition of extremelyfine details over the larger part of theimage field (excepting the outermostzones and the corners) than the com-panion 90mm f/2 Summicron-M at itsfull aperture (f/2). But the Summilux ataperture f/1.4 is not as good overall asthe Summicron-M at f/2. This behaviorillustrates the general rule when com-paring the f/1.4 and f/2 pair of lenses or

the f/2 and f/2.8 pair of lenses (of samefocal length of course). The f/2 (f/2.8)provides higher image quality at maxi-mum aperture than the f/1.4 (f/2) ver-sion, but stopped down one stop thehigher aperture lens improves to a levelgenerally above the quality of thesmaller aperture version.

There are finer differences to be notedwhen comparing the full aperture per-formance of the Summilux at f/1.4 andthe Summicron at f/2. The Summiluxstays on the same quality level fromcenter to corner, with only a verygradual reduction. The Summicron onthe other hand drops quite a bit in thezonal area starting about 7mm from thecenter, but improves in the corners.When taking a portrait or a human-inter-est scene (camera horizontal) and plac-ing the face/person in the middle, theweaker zone of the Summicron coin-cides with the out-of-focus zone. Thebehavior of the out-of -focus image isthen both influenced by the inherent im-age quality in this zone and the out-of-focus-blur because of the sharpnessplane located at the face/person. Shoot-ing the same scene with the 75mm f/1.4 Summilux will produce a differentout-of-focus impression, again becauseof the different definition and the largerout-of-focus blur size. The wider aper-ture and the shorter focal length willcompensate here a bit, but still thefuzzy background will be quite differentin character.

There is a long and fruitless debate inthe Leica community as to which lensmore closely represents the naturalviewing angle (and perspective) of theeye. The 35 mm and 50mm were bothappointed as candidates. Some verystrong arguments have been given forthe 90mm as providing the most naturalperspective. Whatever the truth (if it canbe found in this case), the 75mm per-spective is very pleasing for close-rangeportraits and medium-range candid

shots. Using a 90mm or a 50mm andjust stepping back or forward, will givethe same magnification of course. Butthe foreground-background relation (indepth of space) will change quite con-spicuously and the visual effect is alsoaltered. The perspective relation be-tween the 75mm and 90mm is closelyrelated with the 21mm and 24mm.

The Summilux 75mm has its specificstrengths when used in the photo-graphic domain for which it has beendesigned. If you need the maximum ap-erture of f/1.4 and/or the pictorial ef-fects and special style of imagery possi-ble with the 75mm Summilux-M, thislens should be in your tool kit. Stoppeddown to f/5,6 the Summilux equals theperformance of the 90mm f/2 Apo-Summicron-M ASPH at this same aper-ture, which is some act.

[62] Leica M Lenses

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85mm f/1.5 Summarex M

Stopped down to mediumapertures, this lens from1943-1960 performs re-markably well. At full aper-ture the contrast is very lowand subject outlines arerendered with good visibil-ity. A significant improve-ment can be seen whenstopping down to f/2. Nowof only historical interest, itclearly shows the opticaladvances made in the lasttwo decades.

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75mm f/1.4 Summilux M

An outstanding lens fordistinctive location por-traiture and human-inter-est photography. At fullaperture the lens has amedium/high overall con-trast and renders fine de-tails crisply and with goodclarity of subject outlines.Stopped down to f/2brings in very fine detailswith considerably highercontrast.

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[64] Leica M Lenses

9 0 m m l e n s e s

90 mm lensesIf we would add the several types of

the Elmar 90, the Thambar 90, theElmarit 90and Tele-Elmarit 90 versionsand the Summicron 90, we count 14 dif-ferent designs, signifying the impor-tance of this focal length for the range-finder Leica.

The first 90mm, the 9mm f/4 Elmarwas announced in 1931 and stayed inproduction till 1964, spanning the devel-opment of the Leica from the begin-nings till the landmark design of the M3.Before 1930 the only emulsion availablefor the Leica was the Perutz ‘Feinkorn-Spezial-Fliegerfilm’. It had an acceptablegrain structure, but big enlargementswere not possible. The basic premisethat the small Leica negative should beenlarged 3 times (to compete with thepopular rollfilm print of 6 x 9 cm), mightbe influenced too by the limited poten-tial for enlargement of the emulsion.The well-known Circle of Confusion of0.033mm is also based on this enlarge-ment factor of 3. This measure of 0.033mm is hopelessly inadequate for currentdemands, but no one wishes to changethe calculations. Anyway, after 1930,improved emulsions with anti-halo back-ing and improved sharpness becameavailable. Still, the amount of details thatcould be recorded and made visible af-ter enlargement was quite modest. Theargument for a focal length larger thanthe 50mm standard lens was the abilityto get larger details on film, assumingthe same picture-taking distance. The90mm focal length did not start its lifeas a portrait lens, but as a lens to cap-ture details too small for the emulsionto record. Significantly the 105 and 135mm focal lengths were introduced atabout the same time.

As noted in the 50mm review, the 46ºangle of view has no special relation tothe characteristics of the eye. The mostused camera, the 6x9cm rollfilm had alens with a 53º angle of view and as thiscamera was the natural competition forthe Leica, it would seem natural to

adopt this angle too. The restriction to46º was made on optical arguments(less aberrations to correct in the field).

The human field of view is an ellipseabout 150º high and 210º wide. The ex-tent of binocular overlap is about 130º,the field of view that seems to do thebinocular processing is about 40º, andthe extent of eye movement without acompensating head movement is about20º.

The 27º angle of view is a most naturalone for many objects, as the field ofview can be viewed without headmovement. The popularity of the 90mmis based on the flexibility of use. In ef-fect, one could shoot many photo-graphic assignments with only this focallength. The concentration on the maintopic is quite demanding for a good

composition and requires a careful se-lection of which elements to include inthe picture. The M-rangefinder gives the90mm frame within a larger environ-ment. Anticipating the moment that allpieces fall into a meaningful pattern iseasier than it is with a SLR, where theviewfinder isolates the photographerfrom the scene. The Leica M and a90mm lens form a very fine partnership.The rangefinder accuracy is muchhigher than is needed for the exact loca-tion of the sharpness plane, even at adistance of 20 meters, where depth offield will cover the occasional focus er-ror anyway. Still there is one and onlyone plane of best sharpness and theuse of the hyperfocal distance setting isnot recommended.

90mm f/2,8 Elmarit-M

This, the current version of the 90mmwith a 2.8 aperture, was introduced in1995 for the M-line. Its design closelyresembles the one used in the 90mm f/2,8 Elmarit-R, introduced in 1983.

The optical performance of theElmarit-M at full aperture is outstanding.The overall contrast is high to very high.On axis, over a circular area of about12mm diameter (radius 6mm fromcenter) extremely fine details are ren-dered crisply. In the field (the outerzones) there is some softening becauseof curvature of field and some astigma-tism. The lower contrast reduces theability to record the finest structures oftextural details and stopping down to f/4is required to bring in this level of de-tails. The overall rendition crispens vis-ibly, with only the outer zones lagging abit behind. As this area will be unsharpin most cases because it is in the back-ground, it will only be noticeable by amost critical observer when studying anextended object filling the whole frame.The optimum aperture is reached bystopping down to f/5.6 and now theframe is covered with exceedingly finedetails from center to corners.

At this aperture and with proper tech-nique (correct exposure, proper vibra-

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tion control and accurate focus) theElmarit-M will outperform the capabili-ties of almost any film on the market.

Very critical study of the performancedetails of this lens, compared to thepredecessors (the 90mm f/2.8 Elmaritof 1959, and the 90mm f/2.8 Tele-Elmarit of 1964 and the Tele-Elmarit-Mof 1974) proves two points: the per-formance improves almost linearly overthe period, with the ‘Tele’ versions pro-ducing a slightly flatter image, a bit dullperhaps. The second point is the rela-tion emulsion performance and opticalperformance. On the same film only thecurrent Elmarit-M can record the finesttextural details, only visible when en-larging more than 30 times. The otherlenses do not show this level of rendi-tion of details when recording the identi-cal object. The film is not the limitingfactor in most cases. It is the lens. Inthe sixties however the (Tele)-Elmaritlenses would have been better than thefilm emulsions that were available atthat time, with some exceptions.

Vignetting is hardly visible, but moreimportant is its absence of flare. Flare isone of the worst image degrading fac-tors. It is most visible when strong lightsources are present in the object area,but, contrary to common sense, willalso operate when a large area of highluminance (as an overcast sky) is part ofor close to the object.. The mechanicalconstruction needs to be tuned to re-duce the stray light within the lenswhen the light energy passes throughthe lens elements. The particular clarityand crispness of very fine details of theElmarit-M is a tribute to its optical andmechanical design.

Close-up performance at full apertureis already excellent and can be usedeven for critical demands.

The natural companion lens for the90mm Elmarit-M is the Tri-Elmar. Withonly two lenses, the Leica M user getsa high performance package of greatflexibility (covering 28 to 90mm) and ata very modest weight.

90mm f/2.8 Tele-Elmarit-M

This lens is a Canadian design of 1974(the current Elmarit is a Leitz Germanydesign) and it utilizes four separate lenselements, as does the current one, butin a different configuration. The outwardappearance of the Tele-Elmarit-M hasmuch in common with the Elmar-C(1973) for the Leica CL. It is clearly de-signed as a very compact lens and witha weight of only 225 grams (less than 8ounces) – the Elmarit-M weighs 380grams (just over 13 ounces) - it can becarried in a pocket. There is a Leicamaxim that states that optical perform-ance requires volume. Reduce the vol-ume below a certain threshold and per-formance starts to suffer. Relativelyspeaking, of course, as the design goalsof the Leica optical department are lo-cated at Olympic heights.

At full aperture the contrast of theTele-Elmarit-M is medium. Very fine de-tails are rendered with slightly softedges over most of the image field. Atthe edges and corners the image isquite soft, and fine details, while visible,have fuzzy outlines.

Stopping down brings a very markedimprovement, the edges of very fine de-tails improve noticeably and now covermost of the image field. For the casualviewer, the performance approachesthe value of the Elmarit-M. Still, a sideby side comparison reveals the differ-ences: the Tele-Elmarit-M gives a flat-ter, more dull image, due to a lowercontrast and the softness of the veryfine textural details, which give a pictureits sparkle and clarity. Stopped down tof/8 the Tele-Elmarit equals the imagequality of the Elmarit-M.

Close-up performance is good, but forexacting demands an aperture of f/5,6might be preferable.

When ease of travel and compactnessare of overriding importance, this lens isa good choice. Its fine overall imagequality makes it suitable for many pic-ture-taking situations. Stopped down tof/4 or f/5,6 it is an excellent performer.

90mm f/2 Summicron-M

The first 90mm Summicron (withoutthe suffix -M) arrived on the scene in1958, and with a weight of 680 gramsand a length of 99mm from bayonetflange to front rim was a physical heavy-weight. Its performance at full aperturewas moderate and so the ever-creativesales people invented the notion of aportrait lens. The softness of theSummicron at full aperture would sup-port the romantic portraiture of women,and the lower contrast would help tak-ing reportage style pictures in high con-trast lightning situations.

These notions are still en vogue todayand the full aperture of the Summicron90mm lenses is mostly described in thiscontext.

The 90mm f/2 Summicron-M was in-troduced in 1980 in a mount that lostmuch weight (460 grams) and with 5lens elements (6 for the predecessor).Two lens surfaces are plane, which re-duces cost, but you also loose possibili-ties for additional optical correction. Atfull aperture the 90mm Summicron-Mhas medium-high overall contrast andvery fine details are registered withfuzzy edges over most of the imagearea. This behavior has been describedas ‘smooth sharpness’ (an oxymoronlike ‘military intelligence’). Stoppingdown to f/2.8 brings a very marked im-provement of contrast of the subjectoutlines and a crispening of the edgesof very fine details. In its overall impres-sion, it is now comparable to the 90mmf/2.8 Elmarit-M at full aperture. TheElmarit however is able to record ex-ceedingly fine detail structures that are

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beyond the capabilities of theSummicron. At f/4 the Summicron im-proves markedly again and now the ren-dition of the very finest details isbrought into the picture. Compared tothe Elmarit-M, the higher flare level ofthe 90 mm Summicron softens theedges a bit, but at this aperture theSummicron gives an excellent perform-ance over most of the image field. Theedges of the image (the zonal area cov-ering the outermost 4 to 5 millimeters inthe horizontal direction) lag a bit behindthe rest of the image. But in most pic-tures this part is automatically coveredby the unsharpness of the out-of-focuszone.

At f/5.6 the Summicron-M reaches itsoptimum with outstanding performanceon axis and slightly lower performancein the outer zones of the field. At f/8these outer zones become slightlycrisper and the whole picture area isnow covered with a high contrast imageand a crisp rendition of extremely finedetails.

Close-up performance is much im-proved, when compared to the first ver-sion of the Summicron, which had to bestopped down to f/5,6 to get decentquality. At smaller apertures theSummicron 90mm is a very strong per-former. Its full aperture quality is accept-able. The best test for a wide aperturelens is a picture of a night scene withlots of street lamps and neon light ad-vertising. Flare, veiling glare, halosaround bright spots, contrast in thedarker parts of the scene and the clearseparation of closely spaced highlightsare easily spotted. The Summicron-Mperforms commendably in this type ofscenery, but its image quality is not fullyconvincing. Choosing the Summicronand not the Elmarit would be justified bythe performance at full aperture f/2. Inmany cases you might consider the75mm f/1.4 Summilux.

90mm f/2 APO-Summicron-MASPH.

Many people underestimate thegrowth of aberration content when thedesigner has to open up a lens onemore stop. The step from f/2.8 to f/2seems a small one. In fact some aberra-tions increase by a factor of nine and

that amount of aberration is not easy tocontrol and correct.

The designer needs to accept a re-duced image quality or if possible hehas to aim for a higher level of correc-tion. Leica designers have been wellaware of the absolute and relative per-formance of the 90mm Summicron andits production life of almost 20 yearsshows that a better correction had towait for new tools.

When designing lenses for the M cam-era, the designer has a few additionalparameters to give attention to. Weightand volume are the obvious limiting fac-tors. If volume and weight were not anissue, the designer could use heavyspecial glass and more glass elementsfor correctional purposes.

With 500 grams, the APO-SummicronASPH is a fraction heavier than its pre-decessor, but dimensionally the two arealmost equal.

The creativity and expertise of the de-signer can be assessed by looking atthe specifications in relation to the per-formance. One quite complex asphericalsurface and two special glass typeswere used to achieve the requiredapochromatic correction from the coreof a highly evolved optical system.

At full aperture (2.0) the lens exhibits ahigh contrast image with extremely finedetails rendered with excellent clarityand contrast. On axis (center) and in thefield (outer zones) and extending to thevery corners, minuscule details are re-corded impeccably. The faintest trace ofsoftness at the edges of very fine de-tails can be detected. Outlines of imagedetails have superb edge contrast. Atf/2,8 the contrast improves a bit and thewhole image becomes somewhat morecrisp, bringing exceptionally fine detailsabove the threshold of visibility. Fromf/2.8 to f/5.6 we find an enhanced ca-pacity for recording the finest possibledetails with the crystal-clear clarity andexcellent edge contrast that is the hall-mark of the New Design Principles setforth by Mr. Kölsch.

Perfect centering, only the faintesttrace of astigmatism and no curvatureof field added by painstaking engineer-ing make this lens the one to use.

Stopping down after 2.8 only improvesdepth of field. After f/16 we notice aslight softening of edges and a drop in

overall contrast as diffraction effects be-come visible.

Gone are the days when one had toexcuse the quality at wide open aper-ture with the argument that image deg-radation had to be expected. The APO-Summicron-M ASPH is one of the veryfew 90mm lenses to offer stunningquality already at f/2,0. Most high speedlenses loose a bit of punch when stop-ping down due to focus shift, caused byresidual zonal spherical aberration.Again this lens has a very good correc-tion of this aberration and after stoppingdown, image degradation is very slight,if at all perceptible.

Close up performance at full aperturedelivers very crisp pictures over thewhole image field. The very high level ofcorrection of this lens brings a rapiddrop to the unsharpness area, whereoutlines of objects are preserved. De-tails however are washed out. Thisbehavior in the sharpness-unsharpnessgradient, a very rapid and abrupt changefrom the plane of focus to the unsharp-ness zone, is typical of the current LeicaM lenses.

The APO-Summicron-M ASPH is veryflare resistant. One should have no illu-sions here. It is always possible to forcebright patches and secondary (ghost)images into a lens. The APO-Summicron-M ASPH is very stable, butnot immune to these effects.

When taking pictures that containbright light sources, the use of filters isdiscouraged unless one can control thedirection of the light reaching the lens toa high degree.

Users who upgrade from the previousSummicron-M 90 to the current one willnotice the excellent suppression of veil-ing glare. The Apo version showsblacker shadows than the previous one.When using high speed lenses, quite of-ten a certain amount of veiling glare willilluminate the shadow areas, producingan impression of shadow penetration. Infact what happens is a graying of theshadows by the stray light. No detailswill be visible in the shadows. It justlooks as if the lens /film combinationhas a very high speed. The first pictureswith the Apo version produce blackershadows, and so the user is inclined tothink that he has underexposed or thatthe lens does not give the true speed.The truth is that the user now has a bet-

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ter-corrected lens in his hands and anew learning experience is required.One has to adjust to the new character-istics.

Conclusion

Both 90mm lenses, the Elmarit-M andthe Apo-Summicron-M ASPH representthe current state-of-the-art of Leica lensdesign and define the limit of imagequality at this focal length and aper-tures. The specification of the 90mmf/2.8 version is not a limiting case andthe Elmarit-M can employ a computa-tion from 1983 to present superb imagequality that is difficult to surpass. Theearlier versions of the Elmarit type arevery good lenses too. The commitmentof Leica to exceed the bounds of thefeasible optical performance at any timeis clearly documented in the case of the90mm lenses. Five recomputations forthe Elmarit between 1959 and 1983document the relentless drive forward.

The Apo-Summicron-M ASPH, now inits third recomputation, defines the per-formance level for the next generationof Summicron design. It has stunningperformance at all apertures, distancesand over the whole image field.

The choice between both lenses is aneconomical one. The Elmarit-M ischeaper and has less volume, and itsoptical performance is only marginallysurpassed by the Summicron version.The price-performance relation of theApo-Summicron-M ASPH is temptingand if one wishes to enjoy breathtakingfull aperture performance, this lensopens a new vista.

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90mm f/2.8 Tele-Elmarit M

This very compact andlightweight lens deliversexcellent imagery overall.Wide open the contrast ismedium and the defini-tion of very small picturedetails is slightly soft.Stop down two stops andthe image quality over thewhole picture area be-comes of a very high or-der.

[2,8] [5,6]

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90mm f/2.8 Elmarit M

Before the arrival of thenew 90mm f/2 Apo-Summicron ASPH thislens was the best me-dium telephoto lens everdesigned for the M. Atfull aperture extremelyfine textures and picturedetails are rendered withhigh fidelity and excellentclarity. Stopping down im-proves a bit on thismarvelous performance.

[2,8] [5,6]

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90mm f/2 Summicron M

Stopped down to f/4 andsmaller, this classical lensdelivers a first-class per-formance. The full aper-ture performance that ischaracterized by mediumcontrast, somewhat fuzzyoutlines of finer details,and a trace of veiling glaredo indicate that the designis a bit overcharged.

[2,0] [5,6]

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90mm f/2 APO Summicron-M ASPH

A masterful lens, settinga new standard of excel-lence, this one shows inwhat direction the Leicadesigners are thinking. Atfull aperture extremelysmall details are renderedwith very good clarity overthe whole image field.Flare is hardly detectable,as is vignetting. A superblens.

[2,0] [4,0]

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135 mm lensesBarnack had the vision and the engi-

neering creativity to design his Leicawith a coupled rangefinder to accommo-date lenses of different focal length.The 135 mm focal length sets the me-chanical limit for the M-rangefinder, notthe accuracy limit. The M rangefinder isbased on the principle of vernier acuity.This and the long base ensure that therequired accuracy at all distances iswithin very tight tolerances. To explainthe mechanical limit, consider the fol-lowing. The movement of the range-finder roller is about 2mm, which staysthe same, irrespective of the lens cou-pled to the body. The 50mm lensmoves 2mm when focusing from 1 me-ter to infinity. So the relation is 1:1. The135 mm lens has a movement of about18mm to focus from 1.5 meter to infin-ity. This 18mm has to be converted tothe 2mm movement of the roller. Thatis a reduction of 1:9. Any small error inthe mechanical coupling will thereforebe ‘enlarged’ 9 times. The mechanismhas to be built to a very high degree ofprecision to ensure that this ‘error’ stayswithin the required overall tolerance.

The frame of the 135 lens in the finderis small, but just big enough to allowframing. A tight composition however isnot feasible. One should give somemargins around the motive.

The 135mm f/2.8 Elmarit-M with spec-tacles tried to overcome the limit of theframe, but added a cumbersome deviceto the Leica body.

With the Tele-Elmar-M from 1965 thedesign reached the theoretical optimumattainable in these days and reigned un-challenged for more then 30 years. In-deed the optical performance of theTele-Elmar-M in its various redesignedmounts (three times) stayed the same,as the computation was not changed.

Even today it delivers outstanding im-age quality. Its field of view and fore-ground-background relation can be usedvery advantageously for reportage pho-

tography and fine-art studies alike. It is avery versatile focal length with a longtradition of classical images. It is a pitythat the 135 mm focal length does not

get the attention it deserves pictorially.The new 135mm f/3.4 Apo-Telyt-Mmight change this undeserved Cinde-rella status.

most of the image field and they softena little in the outer zones. .

The subject outlines are sharply deline-ated and give the image a high sharp-ness impression. Stopped down to 5.6the contrast improves somewhat, butthe outer zones still lag behind. Afterf/8,0 the contrast of the very fine objectdetails diminish a bit. Stopping downfurther softens the edges of fine detailsslightly more. This performance holdsfrom infinity to about 3 meters.

For close-up pictures at its closest dis-tance (1.5 meter) one should stop downto get the optimum performance.

Centering is perfect for the older ver-sion I tested, and some curvature offield and a trace of astigmatism can benoted on the bench. This lens is at itstop at f/5.6 and by stopping down fur-ther, overall contrast and edge contrastof very fine details are reduced a frac-tion. The overall performance is of avery high standing. This lens is alsocommendably flare-free. When using itat the limit of its performance, oneshould use a tripod at least once to re-ally appreciate its image quality.

135mm f/2.8 Elmarit-M

This lens has been introduced in 1963.It weighs more than 700 grams, and it isa bit bulky. Its assigned task is photo-journalism in available light, that is i.e. at

135mm f/4 Tele-Elmar-M

The Tele-Elmar-M has an optical lay-out, consisting of 5 lenses in 3 groups.In comparison the Apo-Telyt also hasfive lenses, but now in four groups. TheTele-Elmar-M is optically unchangedsince 1965 and has been given severalfacelifts. The optical performance is,even from today’s high standards, out-standing.

At full aperture the whole image fieldfrom center to the outermost cornersgives a high contrast image. Extremelyfine details are rendered crisply over

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low light levels. Its usefulness howeveris much broader, as it can be used in allsituations where accurate framing is re-quired.

At full aperture the lens produces animage of low to medium contrast. Theobject outlines are recorded withslightly soft edges with very fine details

clearly visible over the whole imagefield. Its wide-open performance sug-gests that is a bit overstretched opti-cally. Stopped down to f/4 image qualityimproves strikingly and approaches theperformance level of the Tele-Elmar-M.At f/5.6 and smaller apertures theElmarit-M inches towards the imagequality of the Tele-Elmar-M, withoutrivaling that level.

135mm f/3.4 APO-Telyt-M

Lenses with a focal length larger thanthe 50mm standard lens enlarge thesubject details, when the picture istaken from the same position of course.But enlarging the object has a nastydrawback. The aberrations are also en-larged. Especially lateral and longitudinalchromatic aberrations will degrade theimage quality as fine details and outlinesalike are recorded with color fringes.The designer will opt for an apochro-matic correction to reduce the second-ary spectrum. But optics is optics andbehind the secondary spectrum loomsthe tertiary spectrum. So perfect im-

agery is not yet attained. The Apo-Telyt-M is a very fine example of a designthat combines the special demands ofthe M series (lightweight and small vol-ume) with that other characteristic ofthe M lenses: impeccable optical per-formance. With only five lens elements(to reduce weight) the designer hascomputed a masterpiece, supported bythe engineers of the production depart-ment.

The Apo-Telyt at full aperture (f/3.4)produces a high contrast image with ex-ceptionally fine details very crisply ren-dered over the whole image field fromcenter to corners. Stopped down tof/4.0 the Apo-Telyt improves visibly onthe Tele-Elmar-M on its ability to renderthe finest possible details with excellent

contrast and clarity. Stopping downthis level of performance holds to

the aperture f/8, and stopping downfurther only very small losses in edgecontrast can be detected.

This APO-Telyt -M shifts the perform-ance level of M-lenses to a higher pla-teau. It represents current thinkingabout optical performance as imple-mented by Leica. At wider aperturesand closer distances the unsharpnessarea sets in abruptly and the shapes ofobjects rapidly lose its details. For mepersonally this behavior is excellent, butbokeh aficionados might be less happy.

The distinctive characteristic of theApo-Telyt is its superior clarity of ex-ceedingly fine details that give the Apo-Telyt images a new look. While forsome purposes the Tele-Elmar-M givesadequate performance, the Apo-Telytoffers a lucidity of fine color hues and al-most lifelike rendition of very small sub-ject details. In direct comparison therendering of the same fine details bythe Tele-Elmar-M is dull, or when goingto the edge soft or washed out. Whenreproducing still smaller details the Tele-Elmar-M produces noise where theApo-Telyt still records these details withauthority.

This level of optical performance isvery sensitive to manufacturing toler-ances. Computer diagrams show theloss of performance when focus isshifted away from its optimum position.Lavish, some would say excessive at-tention to production tolerances is in-deed needed here.

Conclusion.

We may note that Leica M users arevery well served in the medium tel-ephoto lens field and now can produceimages that are the envy of R-users,who long had the advantage in this field.

The Apo-Telyt is a truly superb lens. Itsoptimum performance is on a level thatrequires users who are willing and ableto exploit it to the fullest their technique

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135mm f/4 Elmar M

This 1965 design fromwill outperform many cur-rent comparable lenses. Itdelivers outstanding per-formance at all aperturesand at f/5.6 it might becalled superb. This focallength is very useful forthe reflective type of pic-ture-taking.

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135mm f/2.8 Elmarit M

Wide open this designexhibits medium overallcontrast that softens finedetails a bit. Stoppeddown to f/5.6 it deliversvery good image quality.The detachable spectacleviewfinder helps theframing of the image.

[2,8] [5,6]

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135mm f/3.4 Apo Telyt M

High overall contrast,outstanding clarity ofsmall picture details, veryclean colors and subjectoutlines with high edgecontrast are the trademarkof apochromatic correc-tion as practiced by Leica.At full aperture the Apo-Telyt extends the picturepossibilities of the M bodywith an image quality thatis unrivaled.

[3,4] [5,6]

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When discussing the origins of theLeica, the credit should go to OskarBarnack, who designed the Ur-Leica andErnst Leitz, who decided to start thecommercial production in 1924. Hemade this decision on his own, as all hisadvisers were against the production.The risks were quite large indeed, be-cause the microscope firm had no expe-rience with photographic cameras. If wesearch for the reasons for the successof the Leica, we need to look at Leicalenses. The first lens for the commercialversion of the Leica was designed byMax Berek. So in a certain sense he isresponsible for the worldwide successof the Leica.

Max Berek was born on August 16,1886 in the small town Ratibor as son ofa millworker. Like so many of his con-temporaries in the late part of the 19thcentury, when Germany experienced acultural and scientific explosion, he at-tended the university to expand hisknowledge. He began his study in math-ematics and mineralogy in Berlin in1907 and finished there in 1911 with afamous crystallographic research.

In 1912 Ernst Leitz invited him to be-come the first scientist employed by theLeica company. We should admireLeitz’ uncanny ability to select top talentfor his firm. Berek stayed with Leitz tillhis death on October 15, 1949.

Berek’s area of research focused onmicroscopy, especially polarization-microscopy. In this area he reachedworld fame and his inventions (theBerek compensator and the formula tocompute depth of field of microscopicvision) are still used today. He wroteseveral books on the principles of micro-scope technology.

He was able to use this backgroundand knowledge, when Ernst Leitz askedhim to design a photographic lens for“Barnack’s camera”. The lens was af/.3,5/50mm triplet with the last threelenses cemented into one unit. It isknown as the Leitz Anastigmat and laterthe Elmax, presumably a concatenationfrom ErnstLeitzMaxBerek. The 5 ele-ments helped to give this lens an out-standing performance and today anMTF measurement would deliver veryhigh marks.

In an interview in 1940 Berek notedthat the choice for the aperture of 3.5was quite deliberate. A wider aperture,he remarked, would have been easyfrom the designer’s point of view. TheLeica camera however, was a newproduct and should succeed in the mar-ket. Therefore the quality of the Leicaimages was of paramount importance.The aperture of 3.5 gave excellent opti-cal performance and more importantly,it had an extended depth of field. Soeven if the Leica user misjudged thedistance a bit, he was assured of highquality images. Berek rightfully as-sumed that the user of this new instru-ment needed to gain experience withthe wide aperture and the focusing. Heshould not be disappointed with the re-sults, even while experimenting andlearning.

The optical correction of the Elmax de-parted from the older generations ofanastigmats. These were corrected forthe green to purple part of the spec-trum, because emulsions of those dayswere sensitive to this part of the spec-trum. Again Berek assumed that theuser would need a lens corrected forthe whole spectrum, so he computed alens where the red part of the spectrumwas also corrected. He noticed that anylens that is well corrected for panchro-matic emulsions is also suitable forcolor film. But panchromatic film needsto be corrected for every wavelength inthe visible spectrum, because anywavelength can produce unsharpnesseffects. For color film the sensitivity ofthe eye enters into the equation and sothe lens should be best corrected forthe yellow part of the spectrum (themiddle part, that is).

These kinds of considerations indicatea very sensitive mind to the needs anddemands of the Leica user and a firm

understanding of the core elements ofthe Leica camera and the Leica philoso-phy. Berek designed 23 lenses for theLeica. The last one of which was the85mm f/1.5 Summarex from 1940. Hereceived a personal Grand Prix in 1937at the Paris World Fair for his accom-plishments. Up to now Leica has pro-duced about 65 different lenses for therangefinder system. Berek alone ac-counts for more than 35% of all Leica rf-lenses and his design considerationsstill can be noted in today’s designs.

In the 1940 interview Berek remarksthat a high quality image is less an issueif the optical performance than of thetechnical expertise of the user. This per-ception is still true today. His ideasabout designing lenses were publishedin a book called “Grundlagen derpraktischen Optik” (subtitle “Analyseund Synthese optischer Systeme”) or“Fundamentals of practical optics (sub-title: Analysis and Synthesis of opticalsystems). It was published in 1930 andmany reprints were made until 1986,when the last version was printed. Thebook is still very interesting for its ap-proach and contents. The handbook byM. von Rohr ‘Die Bilderzeugung inoptischen Instrumenten vomStandpunkt der geometrischen Optik’(The geometrical investigation of forma-tion of images in optical systems), pub-lished in 1920 was the reference in theworld of optics. Berek improved on thetheory of geometrical optics with hisbook. He and Merté (from Zeiss) en-gaged in a small scientific battle aboutthe ‘principles of geometrical optics’.This scientific debate had its parallel inthe Zeiss and Leitz lenses for the twocoupled rangefinder systems of the thir-ties: Contax II and Leica III.

Berek loved to work at night wheneverything was quiet. He would retreatto his room with a pot of tea, his sliderule and, smoking cigars, would tracedifficult rays to compute the correctionshe needed. He was very well versed inthe flute and played in many chambermusic sessions. Given the close rela-tionship between optical and soundwaves, his accomplishments in both ar-eas are not surprising.

Professor Dr. Max Berek

P r o f . D r . M a x B e r e k

[78] Leica M Lenses

G l o s s a r y

Glossary of abberationsThe spherical lens is the fundamental

building block of optical systems. Itstwo most important properties are its in-dex of refraction and the curvature ofits two surfaces. It is a known fact thatthe direction of a light ray changes as itpasses from one medium (like air) intoanother medium (like glass). The extentof this change is governed by a number:the index of refraction. Because this in-dex depends on the wavelength (λ) ofthe light ray, one can define a glass witha series of numbers. This variation of in-dices of refraction is called dispersion.Visible light has a spectral range of 400nm to 780 nm (nm = nanometers). Theindex refraction is greater at 400 nm (forexample: Schott glass BK7: 1.53026)than it is at 700 nm (1.512894). Thischange in the index of refraction is dif-ferent for each type of glass and theamount of change can be greater than4%. A standardized number for themagnitude of dispersion is provided bythe Abbe Number. A large (small)number indicates low (large) dispersion.Glass with an Abbe Number greaterthan 50 is classified as flint glass andglass with an Abbe number lower than50 is called crown glass. There are ex-ceptions, however. Glass manufacturersdivide their catalogs into three sections:Preferred glass (in stock), Standardglass (produced at regular intervals) andSpecial glass (produced upon demand,provided the quantities are sufficient).

When a spherical lens images a pointsource of light (object) on the plane offocus, one would expect a point-shapedimage. The law of refraction states thatthe path of a light ray that strikes theglass surface at a given angle of inci-dence is altered by an angular amountthat is a function of the angle of inci-dence. If that angle is changed, theamount of deviation (=Refraction) willalso change. A curved lens surfacemeans that the angles of incidence forparallel incoming rays will be greater atthe rim than they are at the center ofthe lens, so that rays that come in nearthe edge are bent (=refracted) morestrongly.

The illustration below shows that therefracted rays do not intersect the opti-cal axis at a single image point, but atvarious points along that axis. This erroris the basic spherical error, called

spherical aberration. It is a monochro-matic error, and in this case this meansthat the rays of each wavelength thatcome in at a distance from the opticalaxis form an image point on the axisthat is located ahead of the image pointformed by rays that come in near theaxis. Instead of a point, a patch of dif-fused light (circle of confusion) isformed on the image plane. The rayscoming in near the optical axis come toa focus on a plane that is called theGaussian plane. The point image has abright core surrounded by a halo .

When the lens is stopped down, therays that come in at a distance from theaxis are blocked and the position of theimage point will change, as will its form.The core will become larger and the sur-rounding halo will become smaller. Thischange in the position of the image iscalled focus shift. Because one alwaysfocuses [R-lenses only] wide open, itmakes sense to select a plane of focuson which the contrast is best at full ap-erture. High contrast occurs when thehalo is small, even when the bright corepoint is slightly larger than it was on theGaussian image plane. In practice, thisinvolves displacements by very smallamounts, and their order of magnitudeis the domain of the optical designer’sexpertise. If we imagine an image coreon the Gaussian image plane with a di-ameter of 0.02 mm and a flare rim of0.08 mm, the core on the ideal focusingplane will be slightly larger, namely0.025 mm and the flare diameter willbe reduced to 0.04 mm.

It is obvious that such an image pointhas less flare and better contrast. Theo-retical resolution is reduced by a smallfraction because the image core is a lit-tle larger. In practice this resolution can-not be achieved anyway, because straylight diminishes contrast and the entirepicture will have a flat appearance. Thisexample shows exactly where the opti-cal designer has to apply his or hersskills in order to optimize a system andit also shows how tight the manufactur-ing tolerances have to be.

Spherical aberration affects theimaging of points that are located on orclose to the optical axis (for all light raystraveling parallel to that axis). If the ob-ject point that is being imaged is furtheraway from the optical axis, the bundleof rays coming from that object be-comes skewed and asymmetrical. Thesecond monochromatic error is theasymmetrical error, also called coma.

This aberration is also caused by thedifferences in the refraction of light raysin relation to the different angles of inci-dence of the rays that strike the curvedglass surface. The illustration aboveshows that the rays coming in from be-low are bent more strongly then the up-per rays. Spherical aberration involves asmall bright core that is surrounded bycircles of diffuse light , all of them hav-ing the same center (the axis). Withcoma, we observe the same structures(core and rings or zones of diffuse light),but now the asymmetry causes everyring to have a different form and to oc-cupy a different position on the imageplane. The image point is drawn out onone side and takes the form of a point

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with a bright core and a triangular blurzone, looking much like a comet.

One can think of the bundle of rays asa cone whose point is positioned pre-cisely on the image plane. That createsa point at that location, a point that issurrounded by zones of diffuse light. Ifthis cone impinges on the image planeat an angle, as shown in the illustrationof coma, the image will no longer havethe shape of a point. The oblique coneof light will be intersected by the curva-ture of the surface of the lens. If youlook at a circular lens from the front, youwill see s full circle. If you now rotatethe lens in a vertical direction, it will be-come an ellipse, like a cat’s eye. It isclear that rays entering the lens in thevertical direction, the longer one, willhave different angles of inclination thanrays entering in the horizontal (theshorter direction). This aberration iscalled astigmatism. ). The bundle of rays

in the horizontal plane focus at a differ-ent location than do the rays in the verti-cal plane. They do not form a point of

light, but a line, one lying in front of theother and mutually perpendicular. All ob-ject points that have a horizontal focus,are located on a tangential plane, whichis not flat but has the shape of a pa-rabola or ellipsoid. In the same pattern,we also have a vertical plane, which iscurved as well. At the optical axis, thesesurfaces coincide, but they divergequite significantly in the outer zones,Between both extremes, we will find aposition where the point has the leastunsharpness.Astigmatism is difficult tovisualize if one doesn’t consider a fourthaberration at the same time.

Because of the spherical shape of thelens surface, the object is also imagedon a curved plane. But the film plane isflat, and that creates another problem.This (monochromatic) imaging error iscalled field curvature. When this aberra-tion is present, sharpness gradually de-creases towards the edges of the im-age. The image is dish-shaped becauseof the spherical forms of the lens. Thatis also the case with astigmatism. Ifboth aberrations are present, we willhave two separate curved fields(dishes). If astigmatism has been elimi-nated, there is still curvature of field tobe dealt with. These three dishes areusually curved in the same (forward) di-rection. The difference in the positionsof the three dishes is on the order of2% of the focal length. An optical de-signer is capable of eliminating astigma-tism, but he or she can also over-correctit in the opposite direction and therebycompensate for curvature of field. Thenthere will only be a residue of less than0.04% if the focal length (to cite a nu-merical example).

All of these aberrations are sharpnesserrors that diminish the sharpness ofthe image. But there are other aberra-tions that only affect the shape of theimage, even if the image points wereabsolutely sharp. This, the fifth aberra-tion is called distortion. An optical sys-tem always depicts an object in a spe-cific size. A 50 mm lens focused at 10meters (32’10”) reduces every object bya factor of 200. But one can expect thatthe reduced image is geometrically ac-curate and that the ratio of reduction re-mains constant across the entire image.This is called scale fidelity. Unfortu-

nately this is not the case with mostlenses, because the reduction scale var-ies within the image area. When thescale increases as the distance from thecenter of the image increases, the re-sult is pincushion distortion. When thescale is reduced towards he edges ofthe image, we get barrel-shaped distor-tion.

These five aberrations are calledmonochromatic aberrations becausethey act on a single wavelength. Be-cause light diffraction (color dispersion)is not the same for blue light as that forred light, these colors are refracted dif-ferently. Blue, for instance, is bent moresharply than red light and they convergeon different focal points. If we place theimage plane in the middle betweenthese two focal points, we will see agreen (or yellow) core with a purplefringe (red plus blue). If we shift the im-age plane, the color of the fringe willchange from blue to red or vice-versa.This imaging error is called longitudinalchromatic aberration and just like spheri-cal aberration, it causes the image to ap-pear flat because it reduces contrast.With this chromatic error the imageplane will be in a different place for eachwavelength. The dispersion of the glasswill also cause a change in the size ofthe color image in each wavelength. Be-cause short-wave light (blue) is re-fracted more strongly, blue rays willconverge at a closer focal point. The ef-fect is similar to that of a lens with ashort focal length, which depicts objectsat a reduced scale. The focal length islinked to the magnification factor, andthat is why a variation in the refractiveindex also causes a variation in magnifi-cation. This error is called lateral chro-matic aberration and it mostly affectsthe reproduction of fine structures. Awhite image point is separated into itscomponent colors and reproduced as astretched rainbow. A dark point with alight background is reproduced with acolor fringe that appears in blue on theupper rim and in red on the lower rim.

Aberrations are often reduced whenthe lens is stopped down, because mar-ginal rays no longer contribute to theimaging errors. Lateral chromatic aberra-tion is not diminished by stopping downthe aperture and it is very difficult to

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correct. Both chromatic aberrations (lat-eral and longitudinal) increase from thecenter of the image towards its edges.

These seven imaging errors are calledthe Seidel aberrations, because Ludwigvon Seidel was the first person to exam-ine them on a scientific basis around1850. There are still other aberrations,which are classified as aberrations ofhigher orders. The index of refraction isat the bottom of all aberrations and cal-culations are based on the sine of therefracted angle. The sine function canalso be expressed as a geometric se-ries: sin p = p - p3/3! + p5/5! – p7/7! +…

Each term in this series is related to agroup of aberrations of a certain order.The first term represents an error-freeimage, as it occurs in the center of apicture. This very small area around theoptical axis is called the paraxial region.It is logical to associate it with aberra-tions of the first order. The next termincorporates the number “3” and itherefore refers to aberrations of thethird order (the Seidel aberrations). Be-cause this series only contains unevennumbers, the next term refers to aber-rations of the fifth order, and so on.

An optical designer can avail himself ofa long list of steps that he or she cantake to determine the imaging perform-ance. The choice of the proper glass is avery important factor and since theproperties of glass affect the correctionof errors to a significant extent, it pro-vides additional room for creativity. Butcertain glasses are 300 times as expen-sive as standard glasses, possibly diffi-cult to process and they may be quiteheavy. Then the selection becomes verycritical. Other possibilities, like the useof aspherical surfaces and apochromaticcorrection are described in the chapterentitled “Core technologies”.

The computed imaging performancewill not be achieved if all the other fac-tors of influence are not under control.One of the most disturbing influences iscaused by decentering. Every lens ele-ment has its own optical center, whichshould be aligned on the main opticalaxis during the assembly of a multi-ele-ment lens. If that is not the case, for in-stance when the lens element is slightlyshifted at a right angle to the opticalaxis, the imaging performance can be

diminished significantly. A lens can alsobe tilted, which means that its centermay well be positioned on the mainoptical axis, but it may be seated atan angle.

Centering errors influence the opticalperformance, specifically contrast andthe reproduction of the finest structureson the image plane outside the centerof the image.

Stray light is another unpleasantly dis-turbing influence. Stray light consists ofthose rays that do not contribute to theformation of the image, but which arereflected by the glass surfaces and dis-persed by the remaining aberrations orreflected inside the mechanical lensmounts or by the blades of the iris dia-phragm. This kind of light produces anoverall veil across the image plane, itbrightens shadow areas, causes halosaround highlights and diminishes con-trast. Therefore stray light is a combinedresult of residual aberrations, mechani-cal construction and assembly, and theproperties of the glass.

Lens coating is a process designed toreduce reflections. A simple anti-reflec-tion substance, for instance lithium fluo-ride, is vaporized and deposited on theglass surface until it reaches a specifiedthickness. The thickness of the coatingdepends on the wavelength that is to becorrected and it amounts to ë/4 (a quar-ter of the amplitude of a wave cycle).This means that interference reducesreflections of this wavelength consider-ably and almost completely for the adja-cent wavelengths. Coatings are alsoused to balance the color rendition oflenses so that they all match. In prac-tice, multiple layer coating systems arealso used for the reduction of broadband reflections. There are very fewrules in this field: antireflection systemsare part of specific optical systems andtheir properties. In the final analysis, itdoes not matter what kind or methodsof anti-reflection measures are taken,what matters is that they should be ef-fective.

Diameter of the circleof least confusion

Even the most highly corrected lensstill has residual aberrations. A pointlight source is depicted as a minisculebright spot. With modern Leica lenses,the order of magnitude of the diameterof that tiny spot is between 0.01 and0.02 mm. As stated earlier, this circle ofdispersion consists of a bright core sur-rounded by rings of dispersion of dimin-ishing brightness. It is not easy to deter-mine the overall diameter of this brightspot, because it depends upon the mini-mum brightness of the last ring that isto be included in that diameter. Even so,a standard circle of least confusion hasbeen defined as having a minimum di-ameter of 0.03 mm. Every bright spoton the image plane with a diameter thatis smaller than 0.03 mm is perceived bythe viewer as a point light, when en-larged 3 times. Therefore an optical re-production can be less sharp than itshould be as a result of the correction ofresidual aberrations in the system. Thisproperty can be used for reproducing(with good sharpness) the object planeon which the lens has been focused, aswell as the planes that are located di-rectly behind and in front of the objectplane. Since most photographic objectsare distributed over a distance from thelens, one should be able to depict a cer-tain depth of space with sufficientsharpness. The diameter of the circle ofconfusion determines the depth of fieldand the depth of focus. The depth offield scale on a lens provides informa-tion about the extent of distances withinwhich objects located in that range willbe rendered with adequate sharpness.The relationship between f-stops, depthof field and distances can easily be readon that scale. Nevertheless, one mustremember the diameter of 0.03 mm,which is the basis for these computa-tions, as they are used for the depth offield scales on Leica lenses. In projec-tion and in enlarging, the depth of fieldis necessarily smaller, because all imagepoints are being enlarged. The usabledepth of field is often one or two f-stopssmaller than indicated. Therefore, whenusing a working aperture of f/4, oneshould use the depth of field indicatedfor an aperture of f/2.8.

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Editor: Erwin Puts, Utrecht NLTechnical support: Peter Karbe, Leica Camera AGTranslation: Rolf Fricke, Rochester USAPhotographs: Erwin Puts, Utrecht NLDesign and Layout: Coremdia, Hamburg

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