About The Af Sensor - Canon EOS 5D Mark III Instruction Manual

About the AF Sensor

The camera's AF sensor has 61 AF points. The illustration below shows
the AF sensor pattern corresponding to each AF point. With f/2.8 or
larger maximum aperture lenses, high-precision AF is possible at the
viewfinder center.
Depending on the lens attached to the camera, the number of
usable AF points and AF pattern will differ. For details, see pages
79 to 84.
Diagram
78
Cross-type focusing: f/4 horizontal + f/5.6 vertical
f/5.6 vertical
focusing
The focusing sensor is geared to obtain higher precision
focusing with f/2.8 or larger maximum aperture lenses. A
diagonal cross pattern makes it easier to focus subjects
that may be difficult to focus. It covers the five vertical AF
points at the center.
The focusing sensor is geared to obtain high-precision
focusing with f/4 or larger maximum aperture lenses. Since
it has a horizontal pattern, it can detect vertical lines.
The focusing sensor is geared for f/5.6 or larger maximum
aperture lenses. Since it has a horizontal pattern, it can
detect vertical lines. It covers three columns of AF points at
the viewfinder's center.
The focusing sensor is geared for f/5.6 or larger maximum
aperture lenses. It can detect horizontal lines and covers all
61 AF points in a vertical pattern.
Cross-type focusing:
f/5.6 vertical + f/5.6 horizontal
Dual cross-type focusing:
f/2.8 right diagonal + f/2.8 left diagonal
f/5.6 vertical + f/5.6 horizontal