Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 234)

f : Aperture-Priority AE
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If the “ 30" ” shutter speed blinks, it indicates
underexposure.
Turn the <
6 > dial to set a faster aperture (lower
f/number) until the shutter speed blinking stops or
set a higher ISO speed.
If the “ 8000 ” shutter speed blinks, it indicates
overexposure.
Turn the <
6 > dial to set a slower aperture
(higher f/number) until the shutter speed blinking
stops or set a lower ISO speed.
Aperture Value Display
The higher the f/number, the smaller the aperture opening will be. The
f/number displayed will differ depending on the lens. If no lens is attached to
the camera, “
00 ” will be displayed for the aperture.