Canon EOS-1D X Mark III Advanced User's Manual page 832

C.Fn2
Safety shift
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 the standard exposure. [Shutter speed/Aperture] applies to <
mode. [ISO speed] applies to <
OFF: Disable
Tv/Av: Shutter speed/Aperture
ISO: ISO speed
Note
When [ISO speed] is set, safety shift also applies to AEB shooting in <
Safety shift overrides any changes to [Range for stills] or [Min. shutter spd.] from
default settings in [
obtained.
The minimum and maximum limits for the safety shift with the ISO speed are
determined by [Auto range] (
the [Auto range], the safety shift will take effect up or down to the manually set
ISO speed.
Safety shift will take effect as necessary even when flash is used.
>, < >, or <
: ISO speed settings] if standard exposure cannot be
). However, if the manually set ISO speed exceeds
832
> or <
> mode.
>
> mode.