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Depth of field is one of the most crucial determinants of the look and feel
of a photograph, and is arguably one of the least appreciated. "Depth of
field" refers to the zone on either side of your focus point, that is from
near to far acceptably sharp. Things outside this zone of crisp focus, be they
closer to you or farther away, will be noticeably blurry in your picture.
You, the photographic artist, should choose the depth of field that fulfills
your artistic interpretation of a scene. A narrow depth of field will keep only
a thin slab of the world in focus, riveting the viewer's attention only to objects
within that narrow space;
a large depth of field can render everything in your photo sharp so that the
viewer sees everything in the scene. This sketch, in which the camera is focused
on the middle flower, helps illustrate the point:
Four factors control depth of field: 1) lens aperture, 2) lens focal length,
3) distance to the subject, and 4) the size of the film or digital sensor in
the camera. You usually choose focal length and distance for the best composition,
and you are surely stuck with the film or sensor in your camera. In practice,
therefore, aperture is the primary control you exert over depth of field.
Tip 1: Aperture Control
A large aperture (small f/number) creates a narrow depth of field, a small aperture
(large f/number) creates a greater depth of field. To bring attention to one
primary subject, shoot wide f/numbers such as f/2.8, f/4, or f/5.6. Isolating
a subject in this way is called "selective focus."
Conversely, to keep the whole scene sharp, shoot with the lens stopped down
to small apertures like f/16 or f/22. I usually do this in nature scenics. The
pair of pictures of Agawa Falls in Ontario illustrates the creative control
that selecting depth of field provides: one has the foreground leaves blurry,
and one has them sharp.
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Tip 2: Avoid Excessive Depth Of Field
Lens sharpness tends to suffer at the smallest apertures, so don’t stop
down all the way unnecessarily. Stop down just enough for the depth of field
you need, and no more. Some lenses have depth of field engravings on the barrel;
see my website (www.edbergphoto.com)
for a discussion of how to use these markings. In the photo of the asters, I
used f/11—just enough to give me sharpness everywhere while keeping as
much lens resolution as possible. Be advised that what constitutes "acceptable
sharpness" when defining depth of field is a judgment call and also depends
on the final print size. So take any aperture calculations as a guide—not
a rule.
Tip 3: Focus Into The Scene
The depth of field zone extends farther behind the point of optimum focus than
in front. This means you'll make the most out of your available depth of field
if you focus about a third of the way, give or take, into the scene. In the
photograph of the snow-covered boulders, for example, I focused on the tree
to keep everything sharp.
Tip 4: Use A Tripod
If you stop down the lens to gain depth of field, you'll be forced to use longer
shutter speeds to maintain proper exposure. A tripod provides a stable shooting
platform when using shutter speeds too slow to hand hold. I took the Sierra
primrose picture without using a tripod, and you can see the inadequate depth
of field I was forced to accept because of the need for a fast shutter speed
(and therefore a wide aperture).
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