Canon EOS 5D Mark IV L-series Zoom Lens Kit Owner's Manual

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3
 Changing the File Name
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The first character cannot be an underscore “_”.
User setting2
When you select the “*** + image size” registered with User setting2 and 
take pictures, the image-recording quality character will be automatically 
appended as the file name’s fourth character from the left. The meaning of 
the image-recording quality characters is as follows:
“***L” = 73 / 83 / 1
“***M” = 74 / 84 / 41
“***S” = 7a/8a/61
“***T” =  b
“***U” = c
When the image is transferred to a computer, you can refer to the fourth 
character of the image file name to identify the image size (image-recording 
quality) without opening the image. RAW or JPEG images can be 
distinguished with the extension.
 
If you cannot enter text in step 3, press the <Q> button and use the 
character palette when the blue frame appears.
 
The extension will be “.JPG” for JPEG images, “.CR2” for RAW images, 
and “.MOV” or “.MP4” for movies.
 
When you shoot a movie with User setting2, the file name’s fourth 
character will be an underscore “_”.