Canon EOS-1DS Mark III Instruction Manual page 160

Custom Function Settings
0: Disable (use center AF point)
1: Enable (use active AF point)
Enables spot metering linked to the selected AF point. With automatic
AF point selection, spot metering will be at the viewfinder center.
0: Disable
1: Enable (TvtAv)
This works in the shutter-priority AE (Tv) and aperture-priority AE (Av)
modes. When the subject's brightness changes erratically and the
correct autoexposure cannot be obtained, the camera will change the
exposure setting automatically to obtain a correct exposure.
2: Enable (_SO speed}
This works in the Program AE, shutteropriority AE, and aperture-priority
AE modes. When the subject's brightness changes erratically and the
correct autoexposure cannot be obtained, the camera will change the
ISO speed within 100 - 1600 automatically to obtain a correct exposure.
@ If 2 _s set. the maximum burst during continuous shooting (p.54) will
decrease. However. depending on the ISO speed during shoottng, the
actual maximum burst might be higher than indicated _n the wewfinder.
Even if the settabIe ISO speed range has been changed with C.Fn i -
3.12/13. the safety shift will override it when necessary to ou[a - _ne
correct exposure.
With settings 1 and 2. safety shift also works with flash.
Disable: Aft the shooting modes (M, Tv, Av, P, Bulb) wi!J be selectable.
Enable: Only the shooting modes set with [Register] will be selectable.
Register: To make a shooting mode unselectabJe, uncheck the checkmark
< v' >. After entering the settings, select [Apply].
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