Canon EOS 5D Mark III Instruction Manual (en)

 Custom Function SettingsN
C.Fn3: Others
Add cropping information
If you set cropping information, vertical lines for the aspect ratio you have 
set will appear on the Live View image. You can then compose the shot as 
if you were shooting with a medium- or large-format camera (6x6 cm, 4x5 
inch, etc.).
When you take a picture, the aspect ratio information for cropping the 
image with the provided software will be appended to the image. (The 
image is recorded to the card without being cropped.)
After the image is transferred to a personal computer, you can use Digital 
Photo Professional (provided software, p.394) to easily crop the image to 
the aspect ratio that was set.
OFF : Off
6:7 : Aspect ratio 6:7
6:6 : Aspect ratio 6:6
5:6 : Aspect ratio 10:12
3:4 : Aspect ratio 3:4
5:7 : Aspect ratio 5:7
4:5 : Aspect ratio 4:5
Default Erase option
During image playback and image review after image capture, when you 
press the <L> button, the erase menu appears (p.283). You can set which 
option, [Cancel] or [Erase], is to be preselected on this screen.
If [Erase] is set, you can just press <
0> to quickly erase the image.
[Cancel] selected
[Erase] selected
If [z4: Aspect ratio] is not set to [3:2], you cannot add cropping 
information to the image.
If cropping information has been added to a 1 image, the image 
cannot be cropped with the camera’s RAW image processing.
If [Erase] is set, be careful not to erase an image accidentally.