Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 145)

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When Autofocus Fails
1
Set the lens’s focus mode switch
to <MF>.
2
Focus on the subject.
Focus by turning the lens’s focusing
ring until the subject looks sharp in
the viewfinder.
MF: Manual Focus
Focusing ring
If you manually focus while pressing the shutter button halfway, the focus
indicator <
o > will light up when focus is achieved.
With Automatic selection AF, when the center AF point achieves focus,
the focus indicator <
o > will light up.