Is depth of field spread evenly in front of and behind the point you are focused on?
Almost never. In most normal situations, depth of field extends further behind the plane of focus than in front of it. Often the difference will be dramatic.
When shooting with a wide-angle lens and a small aperture, there will typically be a metre of depth of field your side of the focused point, but everything behind as far as the eye can see will also be sharp.
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What about photographing macro subjects?
Depth of field at very close distances is evenly spread in front of and behind the focus point (but perhaps just a millimetre or two in each direction).
So what do I have to do to maximise my area of focus for deep depth of field?
First, set a narrow aperture (such as f/22). It may be necessary to use a tripod, or to increase the ISO to make small apertures feasible in anything but bright light.
Next, use the widest lens you can get away with for the subject, and stand as far away as possible. Finally, focus on a point about a third of the way up the frame to ensure as much of the shot as possible is sharp.
And what do I have to do to minimise depth of field and to maximise blur?
Use the longest lens you can get away with, and/or get as close to the subject as possible. Now set the maximum aperture available on your lens.
'Fast' lenses with wider-than-average maximum apertures are much better at this effect. Cameras that come with larger sensors also offer more restricted depth of field than those with smaller sensors.
In other words, all DSLRs are better than point-and-shoot digital cameras, but full-frame DSLRs are better than budget-priced DSLRs.
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Learning the lingo around depth of field
Depth of field
A measure of how much of a picture is in focus, from the nearest point in the scene to the camera that looks sharp, to the furthermost point that looks sharp.
Depth of Field Preview
A feature on some cameras that enables you to see the viewfinder image at the actual aperture you'll be using for the exposure. This will give you a visual indication of how much depth of field there is.
Circle of confusion
A point on a subject only appears as a point on the image if it's perfectly in focus. If it is not, the point appears as a circle in the picture. The more blurred the picture, the bigger the circle.
If the circles are small enough, however, that part of the picture will still look sharp. The largest-sized disk that looks like a point is known as the 'least circle of confusion'; this scientific measurement is fundamental to accurate calculation of depth of field.
Infinity
Optical term to describe objects so far away that light from them reaches the lens as parallel rays. It is generally used to describe objects that are on or near the horizon.
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