Canon EOS 20D - Manual (Page 66)

f Selecting the AF ModeN
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* About Predictive 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, 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.
¡ With a manually selected AF point, the selected AF point will focus
track the subject.
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 Focus AF for Automatic Switching of AF Mode
When focus is achieved in the AI Focus AF mode with the Servo mode
active, the beeper will sound softly. The focus confirmation light <
o> in the
viewfinder will not light.