Canon EOS-1D X Mark III Advanced User's Manual page 909

Group E
Autofocusing with 191 points is possible. All the AF area selection modes are selectable.
With an extender attached to extendable lenses, AF is available when maximum aperture is
f/8 (in a range of f/5.6–f/8).
: Cross-type AF point. Offers excellent subject tracking and high-precision focusing.
: AF points sensitive to horizontal lines.
Caution
If the maximum aperture value is slower than f/5.6 (greater than f/5.6 but not
exceeding f/8), focus may not be achieved with AF when shooting low-contrast or
low-light subjects.
If the maximum aperture value is slower than f/8 (greater than f/8), AF is not
possible during viewfinder shooting.
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