Depth of Field and Photography: Simply put, depth-of-field is how much of a photograph is in sharp focus from front to back. Whenever possible you should take control of aperture selection and use either an aperture-priority or manual mode.
One technique that many photographers use to add a ‘wow factor’ to their images is to shoot with a shallow depth of field. In doing so they isolate part of the shot which is nicely in focus while throwing elements in the background (and sometimes the foreground) out of focus and into a lovely blur.
There are three main factors that can be used to control depth-of-field:
The lens aperture.
There's a simple, direct relationship between aperture and depth-of-field - the smaller the aperture, the more extensive the depth-of-field. So if you want to keep as much as possible sharp, you should set as small an aperture as possible - preferably f/16, or even f/22 if your lens has the capability.
If, however, you want to concentrate the viewers attention on just one part of the scene, and throw the rest out-of-focus, you should select a large aperture. Exactly how large will depend on the maximum aperture of the lens you're using. For example. on a 50mm standard lens it will be f/1.7, f/1.8 or f/2, but on a standard zoom it will typically be f/3.5 or f/4.5.For general picture-taking, when you want most of the picture to be in focus, you might want to set a middling aperture of around f/8 to f/11.
The focal length of the lens.
Attach a wide-angle lens and you'll benefit from extensive depth-of-field, which makes it easy to keep everything in focus. The wider the angle-of-view, the greater the depth-of-field. Choose a telephoto lens and the depth-of-field is immediately more limited. The longer the focal length, the more restricted the zone of sharpness is.
The Camera-to-Subject Distance
For various technical reasons, the closer you get to the subject the more limited the depth becomes. In fact, when shooting close-up subjects it can extend to just a few millimeters in front of and behind the subject.
Four common ways for using depth-of-field
Everything sharp.
With subjects such as landscapes, groups, interiors and travel you'll usually want to keep everything sharp. Using a wide-angle lens set to a small aperture will give you extensive depth-of-field, perhaps from around one meter through to infinity. But there will be times when foreground interest is closer than that. Here you need to resort to a neat little technique called hyperfocal focusing that allows you to increase the depth-of-field.
As a rule-of-thumb, there's twice more depth-of-field behind the subject than in front of it. So if you photograph a distant subject such as a landscape and focus on infinity you waste lots of depth-of-field. By focusing a little closer, you'll extend the depth-of-field at the front so it comes nearer to the camera, while still making sure that infinity falls within the depth-of-field behind the actual point of focus.
Main subject sharp with background completely out-of-focus.
There are some subjects where you want the main subject to stand out strongly from an out-of-focus background. Portraiture, where the emphasis is on the person, rather than the location, is probably the most popular area. What you need here is a use a telephoto lens at its widest aperture. It's also worth moving the subject as far away as possible from the background - in cramped studio situations this is often impossible but outside against a wall or foliage it's usually easier.
Main subject sharp, with background out-of-focus but still recognizable.
Sometimes throwing the background completely out-of-focus is going too far. You want to show the subject in its natural environment, but with the background toned down to it doesn't compete for attention. A person on the beach, for instance, an animal in the zoo, or a flower in a garden. Here a standard to short telephoto lens, somewhere from 50mm to 135mm, is ideal - especially if it's coupled with a middle range aperture of around f/8.
Zone of sharpness deliberately limited.
Occasionally you may want to limit the depth-of-field to a very specific zone. Maybe in a portrait you want just the eyes in focus, and not even the ears or the tip of the nose. Here you may want to check your depth-of-field preview on your camera, This will give a visual indication of what will and won't be in focus by manually stopping down the lens.
Summary
Overall, the practical use of depth-of-field can be summed up as follows: To maximise depth-of-field and have as much sharp as possible use a wide-angle lens, set a small aperture and stand back from your subject. To minimise depth-of-field with only a small zone of the scene sharp use a telephoto lens, set a large aperture, and get closer to your subject.