Canon EOS-1 Settings Manual page 22

AI Servo AF AI Servo AF tracking method 2
C.Fn III-4
Important C.Fn
Shift focus to the subject in the foreground with Main
focus point priority
Effective for extremely fast moving subjects and sports with random movements
With Custom Function C.Fn III-4 'AF tracking method,' the default
setting is option 0: 'Main focus point priority.' However, when actually
shooting with AI Servo AF, there are many situations where option 1:
'Continuous AF track priority' is appropriate. So, what type of
situation is appropriate for the Main focus point priority setting? With
the Main focus point priority option selected, when a subject other
than the primary subject moves into the area of the manually selected
AF point (usually in the center of the expanded area so it is called the
main focus point), you can specify whether the focus should switch to
the secondary subject.
With characteristics like this, by continuing to take advantage of area
expansion (athletes in sports), 'Main focus point priority' is effective
when you want to continuously switch between targeted subjects. At
track events, when you want to switch the focus to an athlete that
comes into the foreground, or when you want to continuously switch
focus between cyclists in a bicycle road race, this setting can be highly
effective.
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