Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 110)

2.
Set the desired aperture value.
Watch the top LCD panel as you turn the < > dial.
3.
Focus on the subject.
Press the shutter button halfway.
The shutter speed is set automatically.
4.
Check the display and shoot.
As long as the shutter speed is not blinking, the standard exposure will
be obtained.
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