long, or "wide," depth of field, thenobjects behind and in front
of thefocal point will also be in focus.
Depth of field is primarily deter-mined by two factors: the
apertureand the focal length of the lens. Sinceaperture concerns
light, we'll dealwith its effect on depth of field in thissection.
Focal length is discussed inmore detail in Chapter 11.
For now, just bear in mind thatdepth of field decreases as
focallength increases. In other words, ifyou set a 50mm and a 100mm
lens atthe same aperture and focus them ona subject at the same
distance, thedepth of field will be greater with the50mm lens.
Objects in the foregroundand background will be in focus forthe
50mm lens that are out of focusfor the 100mm lens.
Aperture, as you'll recall, is the sizeof the opening through
which lightenters the camera. A smaller aper-ture, like a shorter
lens, increasesdepth of field. For a partial explana-tion of why
this is so, consult "FocalPoint: Depth of Field." Fortunately,you
don't really need to understandthe reasons for this fact in order
tomake use of it.
With a small aperture, therefore,the range of acceptable focus
will ex-tend beyond whatever is in precisefocus. Only one exact
distance will be100% focused. (This distance iscalled the focal
point.) However, ob-jects in the fo reground andbackground will
seem to be in focusas well. As the aperture of the lensincreases,
this range of acceptablefocus (depth of field) decreases.
The distance between the focalpoint and the camera also
affectsdepth of field. The greater thisdistance is, the greater the
depth offield will be. For example, a standard50mm lens set at f/8
and focused at
6 feet will have a depth of field from5 to 7 feet. The same lens
at the sameaperture, focused at 20 feet, will alsohave a depth of
field from 10 to 30feet. In the first case, relatively closeto the
camera, the depth of field ex-tends only 2 feet. In the second
case,at a greater distance, it extends 20feet. If you focus on
something 2 feetaway, the depth of field will only beabout 1
inch.
You may, of course, want lessdepth of field. This brings us
tohow depth of field "works" in aphotograph.
Two objects in a photograph thatare both in focus will seem
closetogether, because that's the wayyou're used to seeing things.
Youreyes provide you with very selectivefocusing, so two objects
that aren'tclose together are not both in focusat the same time.
Your brain expectsa photograph to work the same way.
Depth of field; therefore, can dotwo useful things in a
photograph.First, it can convey the impression ofdepth. As you may
recall, a photo-graph is a two-dimensional image
ofthree-dimensional objects. It needs allthe help it can get to
seem three-dimensional. By reproducing someobjects in focus and
some out offocus (imitating normal vision),depth of field helps to
create the il-lusion of a three-dimensional image.
Depth of field can also tell the eyewhere to look, and help it
tell dif-
ferent objects apart. You've probablyseen photographs in which a
treeseems to be growing out of someone'shead. That's one common
result oftoo much depth of field.
Used creatively, limited depth offield helps to single out one
object orone part of an object. The viewer'seye automatically aims
for what is infocus. If that also happens to be theprimary subject
of the photograph the thing the photographer wanted usall to notice
then the photograph islikely to attract interest and attention.
Too much depth of field can haveanother undesirable effect. By
bring-ing too many images to the viewer'sattention, the emotional
or aestheticimpact of the primary subject may bewatered down or
destroyed. Insteadof concentrating on one subject, theeye will get
busy checking out all theothers.
Sometimes, of course, that 'sprecisely the response you want.
Aphotograph of a bustling city, for ex-ample, may best capture the
bustle byhaving many details in focus. With aphotograph of a single
rosebud, onthe other hand, you'll probably wantto be more
selective.
126 The Photographic Eye
