Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - 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. ) ends 104