Canon EOS-1D X Mark III Advanced User's Manual page 377

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Subject tracking settings
You can specify conditions for automatic selection of the main subject to track, as well as
ease of AF point switching to track other subjects.
Applies to Zone AF/Large Zone AF/Auto selection AF area selection modes in viewfinder
shooting, and to
Live View shooting.
AF priority (People)
• Disable
Main subjects to track are determined based on subject information.
• Enable
Prioritizes people's faces or heads as the main subjects to track, which are
determined based on subject information.
When a person's face or head is not detected, the camera may track other body
parts.
+Tracking/Zone AF/Large Zone AF (vertical or horizontal) AF methods in
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