Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 351)

3.
Aim the camera at the subject.
An AF point is displayed around their eye.
To choose an eye to focus on, press the < > button to change the
AF point to < >, then use < >. As you use < >, the AF point
changes again to < >.
You can also tap the screen to choose an eye.
If your selected eye is not detected, an eye to focus on is selected
automatically.
4.
Take the picture.
Caution
Subject eyes may not be detected correctly, depending on the subject and shooting
conditions.
Note
To switch to [ Eye Detection AF : Disable ] without using menu operations, press
the < > button and then the < > button. To switch to [ Eye Detection AF :
Enable ], press the < > button again.
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