Not sure this is possible, but you can try. If you can't select it from the menu its likely not possible. The camera will likely detect the crop lens and switch for you. The camera may not allow Full setting with an APS-C lens.
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That is exactly what happens. I tested it at B&H today. I'm requesting that this be changed in firmware to allow the choice. I don't believe it will ever happen for technical reasons and more so for marketing reasons. They don't want cheaper RF-S lenses used on full frame cameras. But I'm hoping they prove me wrong!
But you can use RF-S lenses on full frame cameras, so not following your logic.
As I understand it, the OP does not want the FF camera to automatically reduce the recording area to that of an APS-C sensor when a RF-S lens is connected - as it does now. This is done to make the camera produce a non-vignetted image, but the request is to allow that feature to be bypassed for an artistic effect.
Ah, gotcha. I think it would be a long shot though for Canon to update firmware to allow that. Do agree with others in doing this work in post. The resultant images wouldn't of course have the same resolution, but unless very large prints are needed, that shouldn't be an issue.
Actually, if very large prints would be needed (and if the subject is stationary), a single-row or multi-row panorama could be created and then in post, add the vignetting.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.