Canon EOS 40D - Manual (Page 157)

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3 Custom Function SettingsN
C.Fn II -3 Highlight tone priority
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.
C.Fn III: Auto focus/Drive
C.Fn III -1 Lens drive when AF impossible
If autofocus is executed, but focus cannot be achieved, the camera can
either keep trying to focus or stop.
0: Focus search on
1: Focus search off
Prevents the camera from becoming grossly out of focus as it attempts
to focus again. Especially convenient with super telephoto lenses which
can become extremely out of focus.
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 - 1600.
Also, the ISO speed displayed on the LCD panel and in the viewfinder, will
have the “0” displayed as a smaller character such as “2oo”. When the
image’s shooting information (p.117) is displayed, the ISO speed’s “0” will
also be displayed as a smaller character.