Canon EOS 5D Mark III Instruction Manual page 77

61-point automatic selection AF
All the AF points are used to focus. This mode is set automatically in the
<A> mode.
With 61-point automatic selection AF or Zone AF, the active AF point

<S> 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 may not be able to track the subject. Also, in low temperatures,
the tracking response is slower.
With Single-point Spot AF, focusing with the Speedlite's AF-assist beam

may be difficult.
If the camera cannot focus with the EOS-dedicated Speedlite's AF-assist

beam, set the AF area selection mode to Single-point AF (Manual
selection) and select the center AF point to focus.
When the AF point(s) light up, part or all of the viewfinder may light up in

red. This is a characteristic of AF point display (using liquid crystal).
In low temperatures, the AF point's blinking (p.74) may be difficult to see.

This is a characteristic of AF point display (using liquid crystal).
If you set [24: Orientation linked AF point] to [

AF points], you can set the AF area selection mode and manually-
selected AF point (or Zone) separately for vertical and horizontal
shooting (p.101).
With [24: Selectable AF point], you can change the number of

manually selectable AF points (p.98).
With One-Shot AF, pressing the shutter button
halfway will display the AF point(s) <S> that
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.74) AF
point <S> is used first to achieve focus. The AF
point(s) achieving focus is displayed as <S>.
AF Area Selection ModesN
Select separate
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