Canon EOS-1D X Mark III Advanced User's Manual page 380

Selectable AF point
You can change the number of AF points available for manual selection. Note that
regardless of the [Selectable AF point] setting, autofocusing for Zone AF, Large Zone AF,
or Automatic selection AF uses the corresponding AF area selection mode.
[ ] All points
All AF points are available for manual selection.
[ ] Only cross-type AF points
Only cross-type AF points are available for manual selection. The number of AF points
varies depending on the lens used.
[ ] 61 points
The main 61 AF points are available for manual selection.
[ ] 15 points
The main 15 AF points are available for manual selection.
[ ] 9 points
The main 9 AF points are available for manual selection.
Caution
With some lenses, fewer AF points can be manually selected.
Note
Even settings other than [All points] can be used with AF point expansion (Manual
selection ) or AF point expansion (manual selection: surround), Zone AF, and
Large Zone AF .
AF points not available for manual selection are not shown in the viewfinder when
you press the < > button.
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