Canon EOS 1D - Manual (Page 96)

96
E : Selecting the AF Mode
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.
AF is also possible by pressing the
button.
Focus Tracking with AI Servo AF
If the subject approaches or retreats from the camera at a constant rate,
the camera tracks the subject and predicts the focusing distance
immediately before the picture is taken. This is for obtaining correct
focus at the moment of exposure.
When the AF point selection is automatic (p.97), 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 the Area AF frame.
With a manually selected AF point, the selected AF point will focus
track the subject.
AI Servo AF for Moving Subjects
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.
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