Modern DSLR cameras come with a number of focus points, which can usually be seen through the viewfinder. These are known as AF (autofocus) points. DSLRs have a number of these autofocus points, ranging from five to 51 AF points.
What Are Autofocus Points?
Autofocus points are what the camera uses to focus on a subject. You'll probably first notice them when you press the shutter halfway. Many cameras will emit a "beep," and some of the AF points will light up (often in a red or green color) in the viewfinder. When your DSLR is left on automatic AF selection, you'll know where the camera is focusing by which AF points light up.
Using automatic AF selection can work fine, if you're using a big depth of field and aren't shooting anything that's moving. But, with certain types of subjects, the camera can become confused as to where it is meant to be focusing. For example, if you're trying to shoot a butterfly on a leaf with a background filled with contrast, the camera could focus on the more distinct contrasts at the back. So, to be safe, it's sometimes better to use manual AF selection.
What Is Manual AF Selection?
Manual AF selection often means that you can just select a single AF point, which will give you a precise area on which to focus. However, some modern cameras, such as the Canon EOS 7D, have extremely clever AF systems, which allow you to not only pick single points, but also to pick a group or section of the photo on which to focus. AF systems are becoming far more sophisticated, thus reducing the opportunities for the photographer to get his or her focus wrong.
Using a Large Number of AF Points
Having a lot of AF points is particularly useful if you like to take lots of action shots, or if you photograph pets and children ... both of which rarely sit still! With a higher number of AF points, you can cut down on the chances of the subject being away from a point of focus. If you mainly shoot portraits or landscapes, though, you'll probably be happy with a bare minimum of AF points, as you can easily adjust your subjects or your position.