Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 265)

4.
Set the shutter speed (1) and aperture value (2).
Press the shutter button halfway and check the exposure level
indicator.
To set the shutter speed, turn the < > dial. To set the aperture
value, turn the < > dial.
The available shutter speeds vary depending on the frame rate.
5.
Focus and record the movie.
The procedure is the same as steps 3 and 4 for Program AE .
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