A reader writes,

On recent vacation, I really did take many terrific shots, but a bummer being a group shot of four kids against some rocks at the beach. The colour and lighting are fine, but three kids are in crystal clear focus and the fourth, blurry.

There are a few things that could cause that problem.  Autofocus isn’t magic, and can’t read your mind, so it sometimes ends up focusing on something other than what you want.  Autofocus mechanisms look for a straight line, so, for example, if a person is standing in front of a fence, it’s not unusual for the camera to autofocus on the fence rather than the person. Digital SLRs often have multiple autofocus modes, and it is critical that you understand the characteristics of the mode you’re using.

Since some of the kids are in focus, he camera is most likely focusing on one of the kids, and at least one other is at a different enough distance to be out of focus. However, it could also be focusing on another object and some of the kids are close enough to appear in sharp focus, while others are not.  Chances are that you’re struggling with a classic depth of field issue.

Depth of Field refers to the range of distances that are in sharp focus.  The range depends upon the focal length of the lens, your distance from the subjects, and the aperture of the lens.  As a general rule, the problem you have will be decreased with a shorter focal length, small aperture, and with the subjects farther away from you.

If you’re setting up a group shot, try to have all your main subjects roughly the same distance from the camera.  I usually put my camera into the autofocus mode that causes it to focus on whatever is in the center of the viewfinder, and force the camera to focus on either the closest subject or the middle subject, spending on the circumstances.  I center the subject I want the camera to focus on, lightly press the release until the camera focuses, and then, holding the release in the "half-press" position, recompose the image and take the shot.  It may seem awkward at first, but with a bit of practice it will become virtually automatic.

More sophisticated SLRs often have a "preview" button that closes the lens down to the chosen aperature so that you can visually check your depth of field, and that might help as well.

If you’re shooting in bright sunlight, you might want to try putting your camera in "aperture priority" mode and setting the aperture to f11 or f8.  That will give you a larger depth of field.

Note that if you are shooting a subject and want to intentionally put the background out of focus, you’ll want to do the reverse — a large aperture, a long focal length, and move the subject as far away from the background (ie. close to you) as possible.