Canon EOS-1D X Mark III Advanced User's Manual page 868

: One-Touch Image Quality Setting
Pressing the button assigned to this function enables you to switch to the image size set
here. While the camera switches the image size, icons blink in the viewfinder and
the image size blinks on the rear LCD panel. One-touch image quality setting is canceled
after you finish shooting, and the camera reverts to the original image size.
* 7: To set the image size for this function, press the < > button when the setting screen is
displayed.
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