Canon EOS-1DX Instruction Manual page 171

About Program Shift
In the Program AE mode, you can freely change the shutter speed and
aperture combination (Program) set automatically by the camera while
maintaining the same exposure. This is called Program shift.
To shift the program, press the shutter button down halfway, then turn the
<6> dial until the desired shutter speed or aperture is displayed.
Program shift is canceled automatically after the picture is taken.
Program shift cannot be used with flash.
4
Take the picture.
Compose the shot and press the
shutter button completely.
If the "30"" shutter speed and the maximum
aperture blink, it indicates underexposure.
Increase the ISO speed or use flash.
If the "8000" shutter speed and the minimum
aperture blink, it indicates overexposure.
Lower the ISO speed or use an ND filter (sold
separately) to reduce the amount of light entering
the lens.
d: Program AE
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