Canon EOS Rebel SL3 DSLR Camera with 18-55mm Lens (Black) - Manual (Page 199)

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z Focusing on a subject other than a human face
Once the AF point <
z
> achieves focus, the AF point <
z
> will also
move to track the subject if you change the composition or if the subject
moves.
z If the subject’s face is significantly out of focus, face detection will not
be possible. Adjust the focus manually ( = 212) so that the face can be
detected, then perform AF.
z An object other than a human face may be detected as a face.
z Face detection will not work if the face is very small or large in the picture, too
bright or too dark, or partially hidden.
z The <
p
> may cover only a part of the face, not the whole face.
z The size of the AF point changes depending on the subject.