Canon EOS 5D Mark III Instruction Manual (en)

Using AF to Focus
Focus the subject.
Press the shutter button halfway and 
the camera will focus the face 
covered by the <p> frame.
When focus is achieved, the AF point 
will turn green and the beeper will 
If focus is not achieved, the AF point 
will turn orange.
If a face cannot be detected, the AF 
point < > will be displayed and AF 
will be executed at the center.
Take the picture.
Check the focus and exposure, then 
press the shutter button completely to 
take the picture (p.200).
If the focus is far off, face detection will not be possible. If the lens 
enables manual focusing even while the lens focus mode switch is set to 
<AF>, turn the focusing ring to attain rough focus. The face will then be 
detected and <p> will be displayed.
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, titled horizontally or diagonally, or partially hidden.
The <p> focusing frame may cover only part of the face.
When you press <
> straight down or press <
0>, the AF mode will 
switch to the Live mode (p.209). You can tilt <
9> to move the AF point. 
Press <
9> straight down again or press <0> to return to the u (face 
detection) Live mode.
Since AF is not possible with a face detected near the edge of the 
picture, the <p> will be grayed out. Then if you press the shutter button 
halfway, the center AF point < > will be used to focus.