Canon EOS-1DX Af Setting Manualbook page 110

AF area selection modes such as [Single-point AF] and
[AF point expansion] are effective for switching according
to the size and type of subjects. While looking through
the viewfinder it can be difficult to change the mode
while tracking the subject. However, by assigning the AF
area selection modes you want to use, to specific
buttons, you can switch instantly while continuing
shooting. There are five buttons that can be assigned.
Think about the characteristics of the sports and subjects
you want to shoot beforehand, and assign the AF area
selection modes you think you will use. By assigning
different modes to each of the five buttons therefore 5
modes, you can control up to five modes at will. In
addition, aside from the AF area selection modes, various
functions can be registered and applied, so by making
settings as needed, you have the flexibility to handle
conditions as they change.
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