Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 199)

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3 Reducing Flicker
If the subject is against a dark background or if there is a bright light in
the image, flicker may not be properly 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, the expected
result may not be obtained even if you use this function.
Taking test shots in advance is recommended.
If < G > is not displayed in the viewfinder, add a checkmark to
[ Flicker detection ] in [ Show/hide in viewfinder ] (p.84). When the
camera reduces the effects of the flicker when you shoot, < G > will
light up. Under a light source which does not flicker, or if no flicker is
detected, <
G > will not be displayed.
If a checkmark is added to [ Flicker detection ] and [ z 3: 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.
Flicker reduction also works with flash. However, the expected result
may not be obtained during wireless flash shooting.