In photography, depth of field (DOF) refers to the range of distances from the camera that appear to be in focus. A lens can only focus at one distance. However, the sharpness as one moves closer or farther away from that distance diminishes gradually and within a certain range nobody notices it.
Perhaps you want to take a photo of a friend standing in front of another object. If you have a narrow (or short) DOF, your friend may be in focus while the foreground and background appear out of focus. At the other extreme, a wide (or long) DOF could result in the entire image appearing to be in focus.
There’s a good example of effective DOF control here.

Photographers manipulate their DOF to get the effect that they desire. A narrow DOF is often used to place emphasis on the subject or part of the subject. (With the right conditions, I can have your nose in-focus and your face out of focus). A wide DOF is often desired in landscape photography so that the entire image appears in focus.
Depth of field (DOF) is a factor of
– Aperture: The smaller the aperature (larger f number), the greater the DOF.
– Focal length: The smaller the focal length, the greater the DOF. Wide angle lenses therefore generally have a larger DOF than telephoto lenses at the same aperture.
– Distance: The greater the distance between subject and camera, the larger the DOF.
So if your intent is to throw the background out of focus, you want to use a long lens, a small aperture, have your subject close to you, and the background farther away. If you want both your friend and the historic building behind him or her in focus, you want your friend as close as possible to the building. If you’re using a telephoto lens you want to be further away, or use a wider lens. Also select a smaller (larger f number) aperture.
When you see a landscape photographer with their camera on a tripod, it may be to help with precise composition or because the camera is heavy — but it’s also likely they are working with a small aperture (f22 or even smaller) and as a result need a longer exposure time.
Some cameras have a DOF preview button that close the aperture down to what it will be when the exposure is taken so that you can judge what will be in focus and what won’t. Of course with a digital SLR, you can also take a shot and have a look — just make sure you zoom in and don’t trust the small screen on the camera.
Some lenses (especially “normal” lenses) also have a DOF scale right on them that indicates the DOF range for each aperture and makes your job much easier.
DOF is often a problem for those who run their cameras in a fully automatic mode when using a flash. Cameras generally select a large aperture to take full advantage of available and flash light, resulting in a narrow DOF. If you’re shooting groups or the background is important, get into the habit of using an aperture priority mode. That lets you choose the aperture you want, and the camera sets the corresponding shutter speed. In fact, unless I’m shooting a quick moving object (where shutter speed is critical), my camera is usually in aperture priority mode. Just make sure you keep an eye on the shutter speed so that it doesn’t drop too low and result in blurry images.
If you’re looking for more technical stuff, there’s a long article on DOF including some diagrams on Wikipedia.