Canon EOS-3 Instructions Manual page 121

Even if the exposure settings you have set are not suitable, the camera
will automatically alter the settings to obtain a standard exposure. This
feature is convenient when the scene's brightness changes suddenly.
This setting is suited for subjects whose focusing distance changes
irregularly.
The focusing point selection range expands by one point all around the
manually-selected focusing point. This setting is effective when the
manually-selected focusing point is unable to focus track a subject
moving irregularly.
As shown in the table below, the camera sets the focusing point selection
range automatically to suit the lens focal length, AF mode, and subject
movement during predictive AF. This feature is useful when the subject
movement is unpredictable.
AF Mode
One-Shot AF
The focusing point selection range
does not expand automatically.
AI Servo AF
For a slow-
moving subject.
Remarks
Lens Focal Length
Shorter than 300mm
For a fast-
moving subject.
Custom Function Settings
300mm or Longer
The focusing point selection range
expands by 1 point.
For a slow- For a fast-
moving subject. moving subject.
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