Canon EOS-1 Settings Manual page 31

Countermeasures for focus shifting to the background
Use AF Expansion with selected point (C.Fn III-8) to make it
difficult for focus to shift to the background
EF400mm f/2.8L IS USM 1/2500 sec. f/4 ISO800
Shoot with C.Fn III-8 option 2: Assist AF
point expanded by Surrounding AF points
Set C.Fn III-8 Area expansion to option
2: Surrounding AF points, and capture
the subject with the selected AF point
plus the Surrounding AF points.
When using AI Servo AF for continuous shooting
of moving subjects, one of the most common
issues is the focus shifting to the background
instead of the targeted subject. Focus shifting to
the background usually occurs because the
subject is moving extremely quickly, and it is
caused by the selected AF point not being able to
continue tracking the subject. When the AF point
is on the background instead of the subject, focus
naturally shifts to the background.
Custom Function C.Fn III-8 'AF point expansion'
can reduce occurrences of this issue. When option
2: 'Surrounding AF point' is selected, it increases
the number of active AF points. The subject is
easier to track because it is more likely to remain
within the area covered by the active focusing
points. Also, setting C.Fn III-2 'Tracking Sensitivity'
to one of the slower options can improve the
stability of subject tracking.
OK
A case of continuously
tracking the subject
Even with subjects that move
extremely quickly it is possible to
s h o o t c o n t i n u o u s b u r s t o f
i m a g e s w i t h o u t t h e f o c u s
shifting to the background.
31
For illustration, all AF points are shown together. The blue assist AF points are for illustrative purposes only, and they do not actually light up in blue.