In 2019, I am still using the 5D mk2, but it's starting to really show it's age in the footage. Stuck and hot pixels won't go away and banding artifacts are getting worse. Thinking about getting the EOS R as a replacement. I use the Sigma 18-35 on my C100 and those two paired are amazing.
Does anyone have any experience with the Sigma 18-35 and the EOS R?
I do not have that combo, but I do have a C100. The Sigma lens is made for crop sensor bodies. The C100 has a Super 35 image sensor, not a full frame sensor. The EOS R has a crop mode when it detects EF-S lens mounts, but the Sigma uses a regular EF mount.
I would expect that you would need to put the camera in its "crop mode", which really isn't much different than changing the Aspect Ratio. A full frame image will still be captured, but they EXIF will contain cropping data. In other words, the RAW files will be same size, but the JPG outputs will be smaller because of the reduced resolution from the cropping.