Canon EOS 5D Mark II Instruction Manual page 80

f: Selecting the AF ModeN
When the AF point selection (p.81) is automatic, the camera first
uses the center AF point to focus. Within the spot metering circle,
there are six invisible Assist AF points ( in diagram) that function in
the AI SERVO AF mode. Therefore, even if the subject moves away
from the center AF point during autofocusing, focusing can still
continue. Also, even if the subject moves far away from the center
AF point, focus tracking continues as long as the subject is covered
by another AF point.
A manually-selected AF point will focus track the subject in the AI
Servo AF mode.
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.
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. However, the focus confirmation light
<o> in the viewfinder will not light.
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80
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.