Canon EOS-1D Mark II Digial Instruction Manual page 152

C.Fn-16
Safety shift can be set for the shutter-priority AE and aperture-priority
AE modes.
1: If the subject's brightness changes suddenly and the current
shutter-priority AE or aperture-priority AE setting becomes
unsuitable, the shutter speed or aperture is shifted to obtain a
suitable exposure automatically.
C.Fn-17
1: The AF point activation area expands by one point all around the
manually-selected AF point. A total of 7 AF points become active.
This setting is effective for when only one manually-selected AF
point is unable to focus track a subject moving erratically.
2: The camera automatically sets the AF point activation area to 7 or
13 points to suit the lens focal length and AF mode.This is effective
when the subjectís movement is unpredictable.
¡
The AF point activation area centers on the selected AF point.
Therefore, if a peripheral AF point is selected, the AF point
activation area will be smaller as shown below.
7-point
activation area
13-point
activation area
¡
Setting C.Fn-13 will also expand the AF point activation area.
152
Safety shift setting
AF point activation area
Selected AF point
Activation area