I have always found this a strange term, why not simply call it Depth of Focus?
Anyway, it matters not, what does matter is how it affects your photography and how you can use it to your benefit.
You will often see it abbreviated to DoF. Depth of Field refers to the distance (front to back) in your scene, that is in focus.
You will often see lovely portrait work where the person is in focus, and the background is just nicely blurred.
This is the result of Depth of Field. The blurry background is outside the area of focus,
therefore outside the Depth of Field and as a result, it is not in focus.
[hide][top]So how does depth of field work?
It's all to do with how much light enters your camera and the type of lens that you use.
Basically, there are 3 factors that determine the depth of field in your images
[hide][top]Focal length of the lens
To put it simply, the shorter the focal length, the greater the DOF (or more of the image will be sharp).
I.e., 16mm lens = More in focus, 400mm lens = less in focus.
[hide][top]Distance between camera and subject
If you photograph your subject sitting on a wall about 20-30 meters away or more, and using a wide angle
or standard lens, you can almost guarantee that a lot of your image will be in focus whatever the aperture (within reason).
However, bring the subject closer to say 2 meters with the wall 20-30 metres behind them,
and the camera will focus on the subject but will more than likely throw the background into blurred oblivion.
[hide][top]Aperture setting (usually marked as A or Av mode)
The aperture setting has the largest factor in determining the depth of field of your images.
Just remember that f4, 3.5 or 2.8 (or bigger) will have shallow or little DOF whereas F8, 11, 16 or smaller,
will have greater DOF. This is particularly true if you are doing close up work, a large aperture close up will have
very little in focus. It takes a little getting used to but the smaller numbers are the larger apertures.
So f2.8 is a bigger aperture than F5.6, which is bigger than f11 etc.
Aperture is based on a diaphragm inside your lenses. That diaphragm can be opened up and closed
down using the aperture setting on your camera, either to let more light through the lens to the sensor,
or less light, depending on what Aperture setting you select. So f2.8 will have the aperture open really
wide, letting a lot of light in, and f22 is closed down, letting only a small amount of light in.
So when you are out and about next, instead of setting the camera to auto, experiment with depth of field
and get a bit creative. Used in the right context a large or very shallow DOF can be very effective.
Set your camera to the largest aperture (smallest number) f2.8/f4.0 etc) and find a suitable subject
that you can get close reasonably close to, and take some shots, then adjust the aperture to f/12 or similar
and take some more shots, then go to f22 and take a few more. Load them up on your computer and look at the result.
Look at what happens to the background in the shots. This is Depth of Field! The smaller the F number,
the smaller the DOF. Use this camera feature creatively and you will find your photography improving.
This page Depth of Field and Aperture - why is it so?
explains why aperture controls DoF.