Canon EOS 5D Mark III Instruction Manual (en)

Using AF to Focus
AF operation
Focusing will take slightly longer.
Even when focus has been achieved, pressing the shutter button 
halfway will focus again.
The image brightness may change during and after the AF 
If the light source changes while the Live View image is displayed, 
the screen may flicker and focusing may be difficult. If this happens, 
stop the Live View shooting and autofocus under the actual light 
If you press the <
u> button in the Live mode, the image will be 
magnified at the AF point. If focusing is difficult in the magnified view, 
return to the normal view and autofocus. Note that the AF speed 
may differ between the normal and magnified views.
If you autofocus in the Live mode’s normal view and then magnify 
the image, the focus may no longer be correct.
In the u Live mode, pressing the <
u> button will not magnify the 
Live Mode and u (Face Detection) Live Mode Notes
In the Live mode or u (face detection) Live mode, if you shoot a 
peripheral subject and it is slightly out of focus, aim the center AF point 
over the subject to focus, then take the picture.
The external Speedlite will not emit the AF-assist beam. However, if an 
EX-series Speedlite (sold separately) equipped with a LED light is used, 
the LED light will turn on automatically for AF-assist when necessary in 
the Live mode and u (face detection) Live mode.