Canon EOS 5D Mark III Instruction Manual page 76

AF Area Selection ModesN
AF point expansion (Manual selection, surrounding points)
The manually-selected AF point <S> and adjacent AF points <w> are
used to focus. The AF point expansion is larger than with AF point
expansion (Manual selection
wider area. Effective when it is difficult to track a moving subject with
just one AF point.
AI Servo AF and One-Shot AF work in the same way as with AF point
expansion (Manual selection
Zone AF (Manual selection of zone)
The 61 AF points are divided into nine zones for focusing. All the AF
points in the selected zone are used for the automatic selection of the
point of focus. It makes achieving focus easier than with single-point AF
or AF point expansion and it is effective for moving subjects.
However, since it is inclined to focus the nearest subject, focusing a
specific target is harder than with single-point AF or AF point expansion.
The AF point(s) achieving focus is displayed as <S>.
76
), so the focusing is executed over a
) mode (p.75).