Canon EOS 5D MARK III Instruction Manual page 14

Custom Function Settings _
OFF: Disable
TvtAv: Shutter speed/Aperture
This takes effect in the shutter-priority AE (Tv) and aperture-priority AE
(Av) modes. If the subject brightness changes and the standard
exposure cannot be obtained within the autoexposure range, the
camera will automatically change the manually-selected setting to obtain
a standard exposure.
ISO: _SO speed
This works in the Program AE (P), shutter-priority AE (Tv), and
aperture-priority AE (Av) modes. If the subject brightness changes and
the standard exposure cannot be obtained within the autoexposure
range, the camera will automatically change the manually set ISO speed
to obtain a standard exposure.
Under [It_2: ISO speed settings], even if [ISO speed range] or [Min.
shutter spd.] is changed from the default setting, safety shift will
override it if a standard exposure cannot be obtained.
The minimum and maximum ISO speeds of the safety shift using the ISO
speed will be determined by the [Auto ISO range] setting (p. 127).
However. if the manually set ISO speed exceeds the [Auto ISO range],
the safety shift will take effect up to the manually set ISO speed.
ff [Shutter speedlApertu[e] or [ISO speed] is set, safety shift will take
effect if necessary even when flash is used.
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