Canon eos rebel t1i - Manual (Page 65)

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E : Changing the AF ModeN
This AF mode is 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.66) 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 mode from One-Shot AF to AI Servo
AF automatically if the still subject starts moving.
After the subject is focused in the One-Shot AF mode, if the subject
starts moving, the camera will detect the movement and change the
AF mode automatically to AI Servo AF.
AI Servo AF for Moving Subjects
AI Focus AF for Automatic Switching of AF Mode
If focus cannot be achieved, the focus confirmation light <o> in the
viewfinder will blink. If this occurs, a picture cannot be taken even if the
shutter button is pressed completely. Recompose the picture and try to
focus again. Or see “When Autofocus Fails” (p.192).
If the [1 Beep] menu is set to [Off], 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 <o> in the viewfinder will not light.
When focus is achieved in the AI Focus AF mode with the Servo mode
active, the beeper will sound softly. However, the focus confirmation light
<o> in the viewfinder will not light.