Canon EOS Rebel SL3 DSLR Camera with 18-55mm Lens (Black) - Manual (Page 172)

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z If [ z : Auto Lighting Optimizer] ( = 136) is set to any setting other than
[Disable] , the image may still look bright even if a decreased flash exposure
compensation is set.
z If flash exposure compensation is set with an external Speedlite (sold
separately, = 174), you cannot set the flash exposure compensation with
the camera (Quick Control or External flash function settings). If it is set with
both the camera and external Speedlite, the Speedlite’s setting overrides the
camera’s.
z The compensation amount will be retained even after you set the power
switch to <
2
>.