Canon EOS 70D Instruction Manual page 235

Using AF to Focus (AF Method)
If the subject's face is significantly out of focus, face detection will not be
possible. You can prevent this by setting [Continuous AF] to [Enable].
An object other than a human face may be detected as a face.
Face detection will not work if the face is very small or large in the
picture, too bright or too dark, or partially hidden.
The <p> may cover only part of the face.
You can press <0> or the <L> button to display the AF point < > at
the center and use <9> to move the AF point.
Since AF is not possible with a face detected near the edge of the
picture, the <p> will be grayed out. If you press the shutter button
halfway, the subject will be focused in FlexiZone - Multi method with
automatic selection.
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