Canon 5D Mark III User Manual (en)

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.
About the AF Sensor
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/4 horizontal + f/5.6 vertical
Dual cross-type focusing: 
f/2.8 right diagonal + f/2.8 left diagonal
f/5.6 vertical + f/5.6 horizontal
Cross-type focusing: 
f/5.6 vertical + f/5.6 horizontal
f/5.6 vertical