Canon EOS REBEL T7i Instruction Manual page 180

3 Reducing FlickerN
Under [54: Custom Functions (C.Fn)], if you set [10: Mirror lockup] to

[1:Enable], the [Anti-flicker shoot.] setting will automatically switch to
[Disable].
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 in the
viewfinder.
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, set [Flicker detection] to

[Show] under [52: Viewfinder display] (p.75). When you shoot with
flicker reduction, <G> will light up. Under a light source that does
not flicker or with no flicker detected, <G> will not be displayed.
If [Flicker detection] is set to [Show] and [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 to [Enable] before
shooting is recommended.
In Basic Zone modes, <G> will not be displayed, but the effects of

flicker will be reduced when you shoot.
Flicker reduction also works with flash. However, the expected result

may not be obtained for wireless flash photography.
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