Canon EOS-1DS MARK III Instruction Manual page 63

Custom Function Settings
In the AI Serve AF or One-Shot AF mode with manual AF point selection,
you can increase the number of AF points by using the Assist AF points.
Effective when it is difficult to track a moving subject with just one AF point.
O: Disable
1: Enable (left/right Assist AF points)
The points on the immediate left and right (or top and bottom for a
vertical shot) of the user-selected AF point become active:
2: Enable (surrounding Assist AF points)
The points immediately surrounding the user-selected AF point become
active.
The AF point expansion centers on the selected AF point. Therefore, f a
peripheral AF point is selected the AF point expansion wili be smalter as
snown below.
Assist AFeel1[ e xpanded by I pcntonleftandright Assis[AFpointexpanded b y I _ }In[all ar0una
N SetectedAFpoint [] Expansionpoints
Even when C.Fn UJ1-9-1/2 is set. the exl: ansion will take effect.
0:19 points
1: Inner g points
The user-selectable AF points wi!l be limited to the inner 9 points.
2: Outer 9 points
The user-seJectable AF points will be Jimited to the outer 9 points.
17O