Absolute Beginner's Guide to Digital Photography

CHAPTER 4 THE IMPORTANCE OF LENSES 55

FIGURE 4.8
Begin panning
before a fast-
moving subject
is in front of
you. This is
always prefer-
able to playing
catch-up.

a bracket that covers the subject you want to be sharp. When you press the shutter,

the camera focuses on the selected area. This feature allows you to maintain your

framing without autofocus locking and reframing each picture. It also allows you to

shoot fast-moving subjects that are not in the center of the frame (see Figure 4.8).

Some wide-area autofocus systems use the light reflected off your eyeball to deter-

mine where you are looking in the viewfinder, and therefore what aspect of the pic-

ture to bring into sharp focus. Other systems automatically select the nearest subject

in the picture and focus there.

Types of Autofocus

Your camera might let you select among manual, single-shot, and continuous focus.

With single-shot autofocus, sometimes called focus priority, the camera will not let

you take a picture until it has focused. After the lens locks in the focus, you must

either expose the picture or let up on the shutter-release button. This assures you of

a high percentage of sharp pictures, but can prevent you from capturing a critical

moment.

With continuous focus, the camera constantly searches for the correct focus when

you partially depress the shutter button. The camera revises its focus as the subject

moves closer or farther away. Continuous focus allows you to make an exposure

whenever you like. This approach works well for shooting action but does not guar-

antee a sharp picture every time.