Focus and Depth of Field
What exactly is sharpness, and how much can it be controlled? In theory, a lens can
only focus on a single distance at a time (the plane of critical focus) and objects at
all other distances will be less sharp. However, in most cases, part of the scene will
be acceptably sharp both in front of and behind the most sharply focused plane.
Objects will gradually become more and more out of focus the farther they are from
the most sharply focused area.
Depth of field is the part of a scene that appears
acceptably sharp in a photograph. Depth of field can
be shallow, with only a narrow band across the
scene appearing to be sharp, or it can be deep, with
everything sharp from nearest to farthest. To a
large extent, you can control how much of it will
be sharp. There are no definite endings to the
depth of field; objects gradually change from sharp
to soft the farther they are from the focused dis-
tance.Controlling Depth of Field
Evaluating and controlling depth of field is more
important in some situations than in others:
■ If you are relatively far from the subject, the
depth of field (the distance between the nearest and farthest points in a scene
that appear sharp in a photograph) will be greater than if you are up close.58 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
Physically small aper-
tures such as f/22 produce
greater depth of field. Large aper-
tures such as f/2.8 produce shal-
low depth of field (see Figure
4.12).FIGURE 4.12
Photo taken at
f/2.8 with a
300mm lens.