I use the Metabones EF to R 4K Speedbooster on the EOS R and you can do some pretty interesting things with it in 1080 and older Gold Ring Canon EF lenses. Shooting 1080 with these lenses you get a wider image circle of the lense so in this example the EF 85 1.8 becomes a 60ish 1.2 with swirly edges kinda anamorphic - like. With stabilization enabled the image is cropped enough to almost cut out ll the vignetting in 2.4:1. Most of the shots in this video I didn't crop out the vignetting.
I've been experimenting with the R in 4k since the release of the R5 and with the Speedbooster and st