Canon EOS-1 D X Mark II Instruction Manual page 128

3 Customizing AF Functions
If [EOS iTR AF (Face priority)] or [EOS iTR AF] is set, the camera may
take longer to focus than when [Disable] is set.
Even if you set [EOS iTR AF (Face priority)] or [EOS iTR AF], the
desired result may not be obtained depending on the shooting conditions
and subject.
Under light so low that the EOS-dedicated, external Speedlite emits the
AF-assist beam automatically, AF points are selected automatically
based only on AF information.
Face detection may not work if the face is small or under low-light
conditions.
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