Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 257)

Shutter-Priority AE
< > recording mode enables you to set your preferred shutter speed for movies. ISO
speed and aperture values are set automatically to suit the brightness and obtain standard
exposure.
1.
Set the Live View shooting/Movie shooting switch to < >.
2.
Set the recording mode to < >.
Press the < > button, then turn the < > or < > dial for
selection.
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