Autofocus

All digital cameras come with autofocus (AF). In autofocus mode the camera automatically focuses on the subject in the focus area in the center of the LCD/viewfinder. Many prosumer and all professional digital cameras allow you to select additional autofocus areas which are indicated on the LCD/viewfinder.

Example of a camera with a multi selector button (extreme right) to select the AF area spot. The selected area spot is indicated on the main LCD by a red bracket.

In "single AF" mode, the camera will focus when the shutter release button is pressed halfway. Some cameras offer "continuous AF" mode whereby the camera focuses continuously until you press the shutter release button halfway. This shortens the lag time, but reduces battery life. Normally a focus confirmation light will stop blinking once the subject in focus. Autofocus is usually based on detecting contrast and therefore works best on contrasty subjects and less well in low light conditions, in which case the use of an AF assist lamp is very useful. Some cameras also feature manual focus.

See also AF Servo.
This article is written by Vincent Bockaert

Source : http://www.dpreview.com