Canon MARK IV User Manual (en)

Custom Function Settings
C.Fn III -8     AF expansion with selected point
During manual AF point selection, it is possible to activate the AF points 
surrounding the selected AF point for AF. This function works in both the 
One-Shot AF and AI Servo AF modes.
Convenient when it is difficult to track a moving subject with just one 
manually-selected AF point. If the manually-selected AF point cannot 
focus, focusing will be done with the expanded AF points.
0: Disable
1: Left/right AF point
The points on the immediate left and right (or top and bottom for a 
vertical shot) of the manually-selected AF point become active.
2: Surrounding AF points
The adjacent AF points surrounding the manually-selected AF point 
become active.
3: All 45 points area
In the AI Servo AF mode, the autofocus will center on the manually-
selected AF point with the row of six AF points right above and below the 
AF point and the three AF points on the immediate left and right will also 
become active (total of eighteen AF points). If the subject moves to 
another AF point within the expanded AF points, the expanded AF will 
center on that AF point. Since all 45 AF points will operate, as long as 
the subject is within the Area AF frame, it will be focused at all times.
In the One-Shot AF mode, if focus cannot be achieved with the manually-
selected AF point, focusing will be done with the remaining 44 AF points.
Even when C.Fn III -10-1/2/3/4 is set, the AF point expansion will take effect.
In AI Servo AF mode, first focus with the manually-selected AF point.
If there are two manually-selected AF points and setting 1 is set, the AF point 
on the left and right of the two AF points will become active. If setting 2 is set, 
up to eight AF points adjacent to the two AF points will become active.
The AF point expansion centers on the selected AF point. Therefore, if a peripheral 
AF point is selected, the AF point expansion will be smaller as shown below.
Expanded by one 
AF point all around
Selected AF point
Expansion points
Expanded by one AF 
point on the left/right
All 45 AF points 
can be used