Hello folks. I just upgraded my camera from a T3i to a EOS RP and have a 2 year old sigma lens which is a Sigma DC 18-250 macro hsm. I purchased the r Mount which I was told I would be able to use my old lens with my new camera. I connected it and finding that there is only a circle showing when looking through the lens and when a photo is taken. I tried changing the lens abborration and settings and still just getting a circle. Is there any way to get it to a rectangular shape so I get the full image?
Like just about every third party lens manufacturer out there, Sigma only uses EF mounts on the lenses, regardless if the lens is made for FF or APS-C camera bodes.
The RF bodies are capable of detecting an EF-S mount, but your Sigma most likely has an EF mount. The camera is not automatically switching to "crop mode". You will need set crop mode in the camera body.
Be aware that crop mode will significantly reduce resolution downward from 20 MP. If RF bodies work anything like the 50MP %Ds bodies, the RAW file size will remain the same, but the cropping info in the EXIF will be the only difference.