Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 485)

3.
Set the cropping frame.
The image area within the cropping frame will be cropped.
Resizing the cropping frame size
Turn the < > dial to resize the cropping frame size. The smaller the
cropping frame, the more magnified the cropped image will look.
Changing the cropping frame aspect ratio and orientation
Turn the < > dial to change the cropping frame aspect ratio and
orientation.
Moving the cropping frame
Use < > to move the cropping frame vertically or horizontally.
Correcting tilt
You can correct image tilt by ±10°. Press the < > button. While
checking tilt relative to the grid, turn the < > dial (in 0.1° increments)
or tap the left or right arrow (in 0.5° increments) in the upper left of the
screen to correct tilt. After completing the tilt correction, press <
>.
4.
Check the image area to be cropped.
Press the < > button. The image area to crop is displayed.
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