Canon EOS 7D - Manual (Page 93)

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The AF areas are explained starting with the smallest one.
Although this is the same as single-point AF, the
selected AF point <O> covers a smaller pinpoint
area to focus. Effective for pinpoint focusing
overlapping subjects such as an animal in a cage.
Since Spot AF covers a very small area, focusing
might be difficult during hand-held shooting or for a
moving subject.
Select one AF point <S> to be used to focus.
The manually-selected AF point <S> and adjacent AF points <w> are
used to focus. Effective when it is difficult to track a moving subject with
just one AF point.
With AI Servo AF, the manually-selected AF point <S> must focus-
track the subject first. However, it is easier to focus the target subject
than with Zone AF.
With One-Shot AF, when focus is achieved with the expanded AF point,
the expanded AF point <S> will also be displayed along with the
manually-selected AF point <S>.
AF Area Selection Modes
(1) Spot AF (Manual selection) C.Fn III -6
(2) Single-point AF (Manual selection)
(3) AF point expansion (Manual selection) C.Fn III -6