Canon EOS-1D X Mark III Advanced User's Manual page 913

Group I
The 15 AF points shown here are available for autofocusing. As the AF area selection mode,
choose from Spot AF, 1-point AF, Expand AF area:
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 (AF points left and right of the center) or vertical
lines (AF points above and below the center).
: Disabled AF points (not displayed).
Caution
AF point expansion (manual selection
Expansion for manually selected AF points that are not surrounded by active AF
points is limited to any available adjacent points.
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.
, or 15-point Automatic selection AF.
) applies to the 15 available AF points.
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