Canon EOS 7D - Manual (Page 94)

94
AF Area Selection Modes
The 19 AF points are divided into five zones for focusing. All the AF
points in the selected zone are used for the automatic selection of the
AF point. 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.
With One-Shot AF, the AF point <S> which achieved focus will be
displayed.
All the AF points are used to focus. Automatically set in fully-automatic
modes (1/C).
With One-Shot AF, pressing the shutter button
halfway will display the AF point(s) <S> which
achieved focus. If multiple AF points are displayed,
it means they all have achieved focus. This mode
tends to focus the nearest subject.
With AI Servo AF, the manually-selected (p.92) AF
point <S> is used first to achieve focus. This
cannot be set in fully-automatic modes (1/C).
(4) Zone AF (Manual selection of zone)
(5) 19-point AF auto selection
With 19-point AF and Zone AF, the active AF point will keep switching to
track the subject in AI Servo AF mode. However, under certain shooting
conditions (such as when the subject is small), it might not be able to track
the subject. Also, in low temperatures, the tracking response is slower.