Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 96)

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For pinpoint focusing over a narrower area than
with single-point AF. Select one AF point <
O > to
focus.
Effective for pinpoint focusing or focusing on
overlapping subjects such as an animal in a cage.
Since Single-point Spot AF covers a very small
area, focusing may be difficult during hand-held
shooting or for a moving subject.
Select one AF point < S > to be used for focusing.
The manually-selected AF point < S > and adjacent AF points < w >
(above, below, on the left, and on the right) are used to focus. Effective
when it is difficult to track a moving subject with just one AF point.
With AI Servo AF, the initial manually-selected AF point <
S > must
focus-track the subject first. However, it is superior to Zone AF in
focusing on the target subject.
With One-Shot AF, when focus is achieved with expanded AF points,
the expanded AF points <
S > will also be displayed along with the
manually-selected AF point < S >.
AF Area Selection Modes
Single-point Spot AF (Manual selection)
Single-point AF (Manual selection)
AF point expansion (Manual selection )