A camera lens can only focus precisely at one distance at a time. However, things that are closer or more distant than the subject can still look sharp. This zone of apparent sharpness can be so shallow it’s virtually non-existent, or can stretch to a distant horizon.
‘Zone of apparent sharpness’ is a good working definition of depth of field. Only perfect focusing at one distance can create pin-point sharpness, but things on either side may still appear sharp – any blur is too slight to be perceptible. It’s apparent sharpness that counts in the real world. If something looks sharp, even if theoretically it isn’t, it’s sharp enough for our purposes.
For landscape photography, we normally try to get everything sharp, from the grass at the tripod feet to the distant hills. But it’s a matter of choice, not a law. In sports or portraiture, a shallow depth of field is often welcome, because it throws distracting backgrounds and foregrounds out of focus.
Depth of field varies enormously, and is governed by three factors. The first is aperture – the size of the opening created by the lens’s iris. Wide apertures give smaller depths of field. Remember that aperture numbers are fractions, so f/16 is a narrow option and f/4 is wide.
Altering aperture is easy, but it won’t always give the right results. Fortunately, there’s also the lens’s focal length. A longer focal length reduces depth of field. A wide-angle setting (such as the 18mm end of a standard zoom) is good for keeping everything sharp. To blur the background, use a long lens.
The third factor is the distance between the camera and subject. The smaller this is, the less depth of field you’ll have. This is clear with macro subjects, when depth of field all but disappears; you can focus on a specific feature. But depth of field extends beyond the point of focus, as well as in front – focusing on the most distant point won’t give you the maximum depth of field.
Unfortunately, the three factors controlling depth of field don’t always work in harmony. You might decide to swap to a wide-angle lens to improve depth of field. This makes your subject look smaller in the frame. To fix this, you’d move closer, but this reduces depth of field.