Canon EOS Rebel T6i EF-S 18-55mm IS STM Lens Kit Instruction Manual - Page 207

Using AF to Focus (AF Method)  You can also tap on the LCD monitor screen to select the face or subject. If the subject is not a face, will be displayed.  If no faces can be detected, or if you tap on the LCD monitor but do not select any face or subject, the camera will switch to [FlexiZone Multi] with automatic selection (p.208). 3 Focus on the subject.  Press the shutter button halfway to focus.  When focus is achieved, the AF point will turn green and the beeper will sound.  If focus is not achieved, the AF point will turn orange. 4 Take the picture.  Check the focus and exposure, then press the shutter button completely to take the picture (p.192).  If the subject's face is significantly out of focus, face detection will not be possible. In such a case, set the lens's focus mode switch to and 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.  The may cover only a part of the face. 207