Canon EOS REBEL T8i Advanced User's Manual page 180

2.
Focus and take the picture.
Tapping a face for focus
Tapping a face or subject to focus on changes the AF point to [
tap.
Even if the face or subject moves on the screen, the AF point [
Caution
If the subject's face is significantly out of focus, face detection will not be possible.
Adjust the focus manually (
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.
AF cannot detect subjects or people's faces at the edges of the screen.
Recompose the shot to center the subject or bring the subject closer to the center.
Note
The active [
The size of the AF point changes depending on the subject.
Once you press the shutter button halfway and the subject is in focus,
the AF point turns green and the camera beeps.
An orange AF point indicates that the camera could not focus on
subjects.
) so that the face can be detected, then perform AF.
] may cover only a part of the face, not the whole face.
180
] and focuses where you
] moves to track it.