Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 101)

101
AF Sensor
These focusing sensors are geared to obtain
higher precision focusing for lenses with a
maximum aperture of f/2.8 or faster. A diagonal
cross pattern makes it easier to focus subjects
that may be difficult to focus. They cover the five
AF points vertically aligned at the center.
These focusing sensors are geared to obtain
high-precision focusing for lenses with a
maximum aperture of f/4 or faster. Since they
have a horizontal pattern, they can detect
vertical lines.
These focusing sensors are geared for lenses
with a maximum aperture of f/5.6 or faster
(greater than f/5.6 but not exceeding f/8 with an
Extender attached). Since they have a
horizontal pattern, they can detect vertical lines.
They cover the three columns of AF points at
the viewfinder center.
These focusing sensors are geared for lenses
with a maximum aperture of f/5.6 or faster
(greater than f/5.6 but not exceeding f/8 with an
Extender attached). Since they have a vertical
pattern, they can detect horizontal lines. They
cover all 61 AF points.