Canon EOS 5DS Instruction Manual page 98

AF Area Selection ModesN
When AI Servo AF mode is set with 61-point automatic selection AF, or
Zone AF (manual selection of zone), the active AF point <S> will keep
switching to track the subject. 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 may
become slower.
With Single-point Spot AF (manual selection), focusing with an EOS-
dedicated, external Speedlite's AF-assist beam may be difficult.
If a peripheral AF point or a wide-angle lens is used, achieving focus may
be difficult with an EOS-dedicated, external Speedlite's AF-assist beam.
In such a case, use an AF point closer to the center.
When the AF point(s) light up, part or all of the viewfinder may light up in
red. Also, if [z4: Crop/aspect ratio] is set, the semi-transparent,
masked area may light up in red. This is a characteristic of the AF point
display (using liquid crystal).
In low temperatures, the AF point's display may be difficult to see. This is
a characteristic of AF point display (using liquid crystal).
With [24: Orientation linked AF point], you can set the AF area
selection mode + AF point (or only the AF point) separately for the
horizontal and vertical orientations (p.126).
With [24: Selectable AF point], you can change the number of
manually selectable AF points (p.124).
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