Canon 650D - Manual (Page 96)

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f : Changing the Autofocus OperationN
This AF operation is suited for moving subjects when the focusing
distance keeps changing. While you hold down the shutter button
halfway, the subject will be focused continuously.
The exposure is set at the moment the picture is taken.
When the AF point selection (p.97) is automatic, the camera first
uses the center AF point to focus. During autofocusing, if the subject
moves away from the center AF point, focus tracking continues as
long as the subject is covered by another AF point.
AI Focus AF switches the AF operation from One-Shot AF to AI
Servo AF automatically if a still subject starts moving.
After the subject is focused in One-Shot AF operation, if the subject
starts moving, the camera will detect the movement, change the AF
operation automatically to AI Servo AF, and keep tracking the
moving subject.
AI Servo AF for Moving Subjects
AI Focus AF for Automatic Switching of AF Operation
If focus cannot be achieved, the focus confirmation light in the
viewfinder will blink. If this occurs, the picture cannot be taken even if the
shutter button is pressed completely. Recompose the picture and try to
focus again. Or see “Subjects Difficult to Focus” (p.100).
If [z1: Beep] is set to [Disable], the beeper will not sound when focus
is achieved.
With AI Servo AF, the beeper will not sound even when focus is achieved.
Also, the focus confirmation light in the viewfinder will not light up.
When focus is achieved in the AI Focus AF operation with the Servo mode
active, the beeper will sound softly. However, the focus confirmation light
in the viewfinder will not light up. Note that focus will not be locked in
this case.
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