Canon EOS 5DS Instruction Manual page 192

3 Reducing FlickerN
If the subject is against a dark background or if there is a bright light in
the image, flicker may not be detected.
Under certain special types of lighting, the camera may not be able to
reduce the effects of the flicker even when <G> is displayed.
Depending on the light source, flicker may not be detected properly.
If you recompose a shot, <G> may appear and disappear
intermittently.
Depending on the light sources or shooting conditions, expected result
may not be obtained even if you use this function.
Taking test shots is recommended.
If <G> is not displayed in the viewfinder, checkmark [Flicker
detection] in [Show/hide in viewfinder] (p.77). When the camera
reduces the effects of the flicker when you shoot, <G> will light.
Under a light source which does not flicker, or if no flicker is detected,
<G> will not be displayed.
If [Flicker detection] is checkmarked and [z4: Anti-flicker shoot.] is
set to [Disable], metering under a flickering light source will cause
<G> to blink in the viewfinder as a warning. Setting [Enable]
before shooting is recommended.
In the <A> mode, the effects of flickering light will be reduced when you
shoot, but <G> will not be displayed.
Anti-flicker shooting also works with flash. However, the expected result
may not be obtained during wireless flash shooting.
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