Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 288)

Focusing with AF (AF Method)
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Take the picture.
Check the focus and exposure, then
press the shutter button completely to
take the picture (p.272).
AF Operation
Even when focus is achieved, pressing the shutter button halfway
will focus again.
The image brightness may change during and after the AF
operation.
If the light source changes while the Live View image is displayed,
the screen may flicker and focusing may be difficult. If this happens,
exit Live View shooting and perform AF under the actual light source
under which you are shooting.
Notes for AF
If you cannot achieve focus with AF, set the lens’s focus mode switch to
<
MF > and focus manually (p.291).
If you shoot the subject at the periphery and it is slightly out of focus,
recompose to move the subject (and AF point) toward the screen center,
focus again, then take the picture.
The external Speedlite will not emit the AF-assist beam. However, if an
EX-series Speedlite (sold separately) equipped with an LED light is used,
the LED light will turn on for AF-assist when necessary.
With certain lenses, it may take more time to achieve focus with
autofocus, or accurate focusing may not be achieved.