Canon EOS 90D DSLR Camera with 18-135mm Lens - Manual (Page 202)

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Still Image Aspect Ratio
You can change the image’s aspect ratio.
1 Select [ z : Still img aspect ratio].
2 Set the aspect ratio.

Select an aspect ratio, then press <
0
>.
 JPEG images
The images will be recorded with the set aspect ratio.
 RAW images
The images will always be recorded in the [3:2] aspect ratio. The selected
aspect ratio information is added to the RAW image fi le. When you
process the RAW image with Digital Photo Professional (EOS software),
this allows you to generate an image with the same aspect ratio set for
shooting.