Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 342)

: Expand AF area:
Focuses using one AF point < > and AF points outlined here in blue. Effective for moving
subjects, which are difficult to track with 1-point AF.
Focusing on your preferred subject is easier than with Zone AF.
When Servo AF is used, first you will focus using an AF point <
>.
: Expand AF area: Around
Focuses using one AF point < > and surrounding AF points outlined here in blue, which
makes it easier to focus on moving subjects than with AF point expansion ( ). Servo AF
operation is the same as for AF point expansion ( ).
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