Canon EOS-1D X Mark III Advanced User's Manual page 654

2.
Select (1) Still photo shooting button.
3.
Set the lens's focus mode switch to <
4.
Display the Live View image.
To activate or deactivate Live View, select (26) Live View shooting
button.
For more responsive Live View image display, select (27) Live View
image quality toggle button, which will reduce Live View image quality
to improve responsiveness. Select it again to restore the original image
quality.
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