Canon EOS-1DX Instruction Manual page 329

Safety shift
OFF: Disable
Tv/Av: Shutter speed/Aperture
This function takes effect in the shutter-priority AE (s) and aperture-
priority AE (f) modes. If the subject's 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: ISO speed
This function takes effect in the Program AE (d), shutter-priority AE
(s), and aperture-priority AE (f) modes. If the subject's 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.
If the safety shift automatically sets ISO 32000 or higher (when the camera's
internal temperature is low, ISO 20000 or higher), when [ISO speed] is set,
the maximum continuous shooting speed at <o> will be approx. 10 shots/
sec.
Under [z2: 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 sensitivities of the safety shift using the ISO
speed will be determined by the [Auto ISO range] setting (p.133).
However, if the manually set ISO speed exceeds the [Auto ISO range],
the safety shift will extend up to the manually set ISO speed.
If [Shutter speed/Aperture] or [ISO speed] is set, safety shift will take
effect if necessary even when flash is used.
3 Custom Function Settings
329