Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 264)

Manual Exposure Recording
You can manually set the shutter speed, aperture value, and ISO speed for movie recording.
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.
3.
Set the ISO speed.
Press the < > button.
Turn the < > or < > dial to set it.
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