A picture in focus is sharp, crisp, and clear, with the details highly visible and cleanly delineated. Focus may be selective rather than universal, meaning only parts of an image are sharp (the areas you care about, we hope). When used judiciously, selective focus can be very effective, drawing your attention to the main subject.

Depending on which digital camera you have, it might offer several different focus options, or only auto-focus. But even if the only choice you have is auto-focus, you still can be the one in charge, rather than relinquishing all photographic control to your camera.

Using auto-focus is a relatively easy matter. Point the camera directly at your subject, making certain that the area that you want to be in focus is in the middle of your viewfinder. Then press the shutter button down halfway and hold it. The lens will automatically focus on what you see in the middle of the viewfinder. As long as you hold the button in halfway, the auto-focus will remain locked on the same distance, allowing you to move the camera to compose your photograph. (Some cameras have an auto-focus lock button, which does the same thing.) Then slowly press the shutter button the rest of the way down when you're ready to snap the shot.

So how does your auto-focus determine what to focus on? Better digital cameras offer several options. Here are some of the focusing modes your digital camera may feature and some advice on how to use each of them.

Spot auto-focus: This mode uses a tiny spot (smaller than the usual center area) to determine the auto-focus point. It is especially useful when there is a very important small section of the subject (perhaps a piece of jewelry or your subject's eyes, which are the optimal focus point for most pictures of people) that you need in sharp focus.

Flex auto-focus: Also called area or sometimes zone auto-focus, this mode allows you to focus on areas other than the center of your frame. It displays a series of small boxes in various locations in the viewfinder. You can either let the camera select the proper focus point or, by using the four-way jog button, you can select the square closest to the subject or section that you want to be in sharp focus. Flex auto-focus is useful when you don't want to have to recompose the photo frequently, as you check the focus of an off-center subject.

Continuous auto-focus: In this mode, which is also known as tracking auto-focus, the lens refocuses continuously—there is no focus lock. Continuous auto-focus is especially useful when you are tracking a moving subject, such as a football player.

Predictive auto-focus: A handful of semi-pro and pro cameras let you select a predetermined focus point (such as the open gate on a fence or the finish line at a racetrack). When the subject reaches that point, the camera will automatically take a picture. Predictive auto-focus, which is best done with a tripod, can be an extremely effective way to get dramatic, razor-sharp action and sports shots.

To overcome auto-focus limitations in low-light settings, some digital cameras include an AF assist feature. This is a light, lamp, or signal that illuminates when you press the shutter button halfway down. The light may be infrared or white, or it may even be a laser beam or, in rare instances, sonar echo. Typically, the lamp is just bright enough to illuminate up to about 8 to 10 feet, which is enough to allow the auto-focus to do its job.

AF assist is a mixed blessing. It does work, so long as you aren't trying to light up a subject farther than 10 feet away. But it's also obvious and obtrusive. Subjects and bystanders can see the lamp on most models, which makes them aware that you're taking photographs. It can be annoying, because to use AF assist properly, you may have to press that shutter halfway a number of times, framing your shots and lining up your subjects, before you actually take the picture. And when you do take the shot, the flash will fire. All this obvious preparation destroys candid moments, because your subjects are alerted to an imminent photograph.

Should you use an AF assist? Yes, if it's the only way you can get the photos you want; no, if you want to shoot natural candids. Fortunately, there's almost always a menu option for turning off this feature.

Many digital cameras have an option called manual focus, or MF. It's especially useful when autofocus won't lock in on the subject you want to shoot, or you wish to control depth of field by focusing the lens in between two areas or subjects.