Canon EOS-1D X Mark III Af Setting Manualbook page 73

aperture f/number.
When using a lens that has a maximum aperture of f/2.8,
you can select from 155 cross-type AF points, plus a center
dual-cross AF point.
When using a lens that has a maximum aperture of f/4,
cross-type AF with all 155 points is possible.
When using a lens that has a maximum aperture of f/5.6,
65 cross-type AF points towards the center are available
to use. For each AF point that is not cross-type, horizontal-
line focusing AF is used.
Number of AF points available, AF point patterns, Area AF
frame shapes, and other details vary with lenses, which
are classified into 10 groups (A–J). Visit the Canon website
or others and see Group Classification of Lenses for lenses
you will use.
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