Canon EOS-1D X Mark III Advanced User's Manual page 104

Caution
If the "30"" shutter speed and the lowest aperture value blink, it indicates
underexposure.
Increase the ISO speed or use flash.
If the "8000" shutter speed and the highest aperture value blink, it indicates
overexposure.
Lower the ISO speed or use an ND filter (sold separately) to reduce the amount of
light entering the lens.
Note
Program shift
With Program AE, you can freely change the combination (program) of shutter
speed and aperture value set by the camera while maintaining the same exposure.
This is called Program shift.
With Program shift, you can press the shutter button halfway, then turn the <
dial until the desired shutter speed or aperture value is displayed.
Program shift will be canceled automatically when the metering timer (
(exposure setting display turns off).
Program shift cannot be used with flash.
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