Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 106)

Lenses and Usable AF Points
106
Autofocusing with only the 13 points shown in the diagram is possible.
Only the following AF area selection modes are selectable: Single-point
Spot AF, Single-point AF, AF point expansion (manual selection ),
Zone AF (Manual selection of zone), and 13-point automatic selection
AF.
If an Extender is attached (Extender compatible lenses only) and the
maximum aperture is f/8 (greater than f/5.6 but not exceeding f/8), AF
will be possible.
: Cross-type AF point. Subject
tracking is superior and high-
precision focusing is achieved.
: AF points sensitive to horizontal lines
(AF points in line extending left and
right through center AF point) or
vertical lines (top and bottom AF
points adjacent to the center AF
point).
: Disabled AF points (not displayed).
Group I
Even when AF point expansion (manual selection ) is set, AF point
expansion will be applied to the 13 AF points. If the manually selected AF
point does not have all four AF points on the top, bottom, left, or right, it
will expand only to those active AF points that are adjacent to it.
If the maximum aperture 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 is slower than f/8 (exceeding f/8), AF is not
possible during viewfinder shooting.