Canon EOS Rebel T1i Instruction Manual page 186

3 Custom Function SettingsN
C.Fn-6
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, noise in the shadow areas may be slightly more than usual.
With setting 1, the settable ISO speed range will be 200 - 3200.
Also, the <A> icon will be displayed on the LCD monitor and in the
viewfinder when highlight tone priority is enabled.
C.Fn-7
If the images comes out dark or the contrast is
low, the brightness and contrast are corrected
automatically.
The settings made with the camera can be
reflected in RAW images when the images are
processed with Digital Photo Professional
(provided software).
0: Standard
1: Low
2: Strong
3: Disable
Depending on the shooting conditions, noise might increase.
This function will not work if manual exposure or bulb exposure has been
set.
In the Basic Zone modes, [Standard] is set automatically.
186
Highlight tone priority
Auto Lighting Optimizer