Canon EOS-1D X Mark III Advanced User's Manual page 111

Caution
If the "30"" shutter speed blinks, it indicates underexposure.
Turn the <
shutter speed blinking stops or set a higher ISO speed.
If the "8000" shutter speed blinks, it indicates overexposure.
Turn the <
shutter speed blinking stops or set a lower ISO speed.
Note
Aperture value display
The higher the value, the smaller the aperture opening will be. The aperture value
displayed varies depending on the lens. If no lens is attached to the camera, "00"
will be displayed for the aperture.
> dial to decrease the aperture value (open the aperture) until the
> dial to increase aperture value (close the aperture) until the
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