Canon EOS-1D X Mark III Af Setting Manualbook page 64

Face Detection and
Head Detection AF
[Subject tracking settings] is the
algorithm used to determine AF point
auto switching during [Auto selection
AF], [ + Tracking AF], [Large Zone AF],
and [Zone AF].
By detecting people's faces and heads,
extremely accurate subject detection and
tracking is possible.
Detects people's faces and heads and
achieves accurate tracking
Face detection and Head detection AF is active when [AF
priority (People): Enable] is set in the [AF4] tab*, [Subject
tracking settings]. It is effective when [Zone AF], [Large Zone
AF], or [Auto selection AF] [
and AF points are selected automatically based on face and
head information. Face detection and head detection
automatically switches in scenes where the face appears
and disappears due to movement of the subject such as
+ Tracking AF] is selected,
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