Canon EOS-1D X Mark III Advanced User's Manual page 847

Add cropping information
Adding cropping information displays vertical lines for the aspect ratio specified in Live View
shooting, so that you can compose shots as if shooting with a medium- or large-format
camera (6×6 cm, 4×5 inch, and so on).
When you shoot, instead of cropping images recorded to the card, the camera adds aspect
ratio information to images for cropping in the Digital Photo Professional (EOS software).
You can import images to Digital Photo Professional on a computer and easily crop images
to the aspect ratio set at the time of shooting.
OFF: Off (aspect ratio 3:2)
6:6: Aspect ratio 6:6
3:4: Aspect ratio 3:4
4:5: Aspect ratio 4:5
6:7: Aspect ratio 6:7
5:6: Aspect ratio 10:12
5:7: Aspect ratio 5:7
Caution
Cropping information is also added in viewfinder shooting, but the cropping range is
not displayed.
JPEG or HEIF images are not saved at the cropped size if you use the camera to
process RAW images with cropping information (
produces JPEG or HEIF images with cropping information.
Note
Vertical lines are displayed in the viewfinder when set to [Aspect ratio 6:6].
). In this case, RAW processing
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