Canon EOS-1 Settings Manual page 16

AI Servo AF AI Servo Tracking Sensitivity
C.Fn III-8
Important C.Fn
Capture quickly moving objects with
AF point area expansion
Expanded by 1 point all around assists the photographer's selected AF point
There are times when it is difficult to focus accurately using the
selected AF point when the subject is agile and moves around
extremely quickly. In situations like this, using C.Fn III-8 'AF Expansion
with selected point' will increase the number of active AF points for
steadier and more consistent tracking by using the AF points
surrounding the manually selected AF point. When shooting primary
subjects that move rapidly up and down, or left and right, C.Fn III-8
option 2: 'manually selected AF point expanded by 1 assist AF point
all around' is effective. For example, when the center AF point is
selected, a total of 6 points, upper diagonal left and right, same row
left and right, and lower diagonal left and right are available. With
this setting, when it is not possible to capture extreme movement
with the selected AF point, the expanded points will function,
increasing the odds of capturing the desired subject. This option is
most effective when shooting dynamic scenes such as sports with
rapid up/down or left/right movement.
16
1 point selected
2 points selected
Manually selected AF point
AF point area expansion
(Red SI appears in AI Servo
when SW-1 is active.)
When capturing dynamic vertical
movement, such as this skier leaping
from a mogul, expanding the manually
selected AF point by 1 Assist AF point
all around can be effective.
EF300mm f/2.8L IS USM 1/4000 sec.
f/6.3 ISO400