What's Depth of Field?

Whenever you take a picture, your camera focuses somewhere some distance away from the camera.  We always hope that this "somewhere" is the subject we want in focus.  Thankfully there is always some distance in front of and behind the place where the camera is focusing that appears to be in focus.  That distance is called depth of field.  The following diagram illustrates the concept.

Wherever your camera is focused, there is a distance closer to and farther way from that point that appears to be in focus.

Notice in the above photograph, the grass at the bottom of the picture in front of the river otter is out of focus.  The grass at the top of the picture behind the otter is also out of focus.  Even the back of the otter is a little out of focus.  My depth of field for this photograph was just wide enough to cover the otter's head and neck.

The depth of field of your photograph will change depending on three factors:

1. F/Stop.  The above photograph was shot at f/5.6.  If I had chosen f/16, the depth of field would be much greater, and more of the frame would be in focus.
2. Distance from subject.  The greater the distance between your camera and the place you are focusing, the greater the depth of field you will have in your photograph.
3. Focal length of lens.  The longer your lens, the less depth of field you will have in your photograph.

Now 2 and 3 work together.  For instance, in the above photograph, if I were to cut the distance between my camera and the otter (and the otter magically didn't move) and shot at the same focal length, I would have less depth of field in the photograph, but the otter would be much larger in the frame.  If I were to widen the focal length of the lens so that the river otter was the same size as in the above photograph, the depth of field would be exactly the same as in the above photo, assuming I shot the photograph at the same f/stop.

If you shoot with a large enough f/stop, a wide enough lens, or are far enough away from your subject (or some combination of the three), your depth of field can be so great that it will extend behind the focus point to infinity.  That is, the entire image behind the plane of focus will be sharp.

Note:  For more detail on the affects of distance, focal length and aperture on depth of field, if you shoot the same subject at the same f/stop many times, varying the focal length of the lens and your distance from the subject so that the subject is always the same size in the frame, the depth of field will be exactly the same in every photograph.  However, there is still significance in choosing whether to shoot your subject with a wide angle lens close up or a long lens far away.

1. With longer lenses, it's easier to get your subject separated from the background.  Longer lenses have a narrower angle of view, so the background of your image will cover much less area than shorter lenses.  Wide angle lenses have wider backgrounds that cover a greater area.  This means that it is easier to compose your photograph with a background that is not distracting to your subject.  If I shot the above photograph at 100mm, I would have had to include much more stuff in the background, stuff that I wouldn't want in my photograph, like trees and water.  Because I shot at 400mm, I was able to shoot this picture with only grass in the background, making (for me) a more pleasing and simple composition.  And, because the background covers less space at 400mm, the background grasses are larger in the frame than they would be if I had shot at 100mm.  Even though they would be out of focus whether I shot at 100mm or 400mm (with the otter the same size), the out of focus grass in the background is larger in the frame at 400mm, so it's blurriness is more apparent to the viewer.  The subject will appear to stand out from the background a little more when shot with a longer lens.
2. Wide angle lenses create a sense of drama and depth.  If  you use a wide angle lens and make a small subject large in the frame, you may still have an entire mountain in the background.  But since the photograph is a flat, 2-D artifact, the disparity between the relative size of the small subject in the foreground and the large mountain in the background can create drama and interest.  Beyond this, it will also be easier to ensure that the distant background is in focus along with your subject.