Canon EOS 5D Mark III Instruction Manual page 102

3 Customizing AF FunctionsN
Orientation linked AF point
You can set the AF area selection mode and manually-selected AF
point separately for vertical shooting and horizontal shooting.
: Select separate AF points
The AF area selection mode and manually-selected AF point (or Zone)
can be set separately for each camera orientation (1. Horizontal, 2.
Vertical with the camera grip at the top, 3. Vertical with the camera grip
at the bottom). Convenient when, for instance, you want to keep using
the right AF point during all camera orientations.
When you manually select the AF area selection mode and AF point (or
Zone with Zone AF) for each of the three camera orientations, they will
be set for the respective orientation. Whenever you change the camera
orientation, the camera will switch to the AF area selection mode and
manually-selected AF point (or Zone) set for that orientation.
If you clear the camera settings to their defaults (p.56), the setting will be
[Same for both vert/horiz]. Also, your settings for the three camera
orientations will be cleared and all three will revert to Single-point AF
(Manual selection) with the center AF point selected.
If you set this and later attach a lens from a different AF group (p.79-85,
especially Group H), the setting may be cleared.
102
: Same for both vert/horiz
The same AF area selection mode and
manually-selected AF point (or Zone) are
used for both vertical shooting and
horizontal shooting.