Canon EOS REBEL XS Instruction Manual page 63

Custom Function Settings*
0: Enable
if the image comes out too dark or the contrast is too low, the image
brightness and contrast are corrected automatically. (Depending on the
shooting conditions, image noise may increase.) This function will not
work with manual exposures nor _ and _+_ [. images.
in the Basic Zone modes, the Auto Lighting Optimizer will operate
automatically.
1: Disable
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