Canon EOS REBEL T4I Instruction Manual page 99

Custom Function Settings*
C,Fnll:image
0: Disable
1: Enable
Improves the highlight detail. The dynamic range is expanded from the
standard 18% gray to bright highlights. The gradation between the grays
and highlights becomes smoother.
_[_ With setting 1 the Auto Lighting Optimizer (p.123) is automatical!'y set to
[Disable] and the setting cannot be changed.
With setting !, noise (grainy image, banding,etc.) may become slightly
more pronounced than with setting 0.
With setting 1 the settable range will be ISO 200 - 12800 (up to ISO 6400
for movies).
Also. the <D+> icon will be displayed on the LCD monitor and in the
viewfinder when highlight tone priority is enabled.
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