I have the Canon R5 and various RF lenses. I also got the EF 8-15mm f/4 fisheye lens. With the R5 the view is NOT a fully 180 degree view but I have t slightly crop a bit in order to avoid black corners....
I had to read carefully. At 15mm you don't quite get full frame diagonal corner coverage. As if the lens was designed to get the 180 degree diagonal for slide frames and prints with negatives in holders? Except that the lens was released in the digital era. Odd. (I understand you get a full circle at 8mm)...
I'm not sure I understand. I just slipped the 8-15 on my R5. No black corners at 15mm.
Just so people understand, the 8-15mm was designed to be used by all three sensor sizes then available, APS-C, APS-H and Full Frame. On full frame it goes from the classic circular image to a cropped image that fills the frame. There are markings on the side of the lens for APS-C and APS-H. APS-H lines up with 12mm and APS-C lines up 10mm (approximately). There is a zoom limiter that prevents you from going below 10mm if you activate it.
It's a fun lens, but I don't use it all that often. I usually take it to a basketball game once or twice a season and shoot from behind the hoop to get a couple of fun shots. I've also used it to get a picture of a small flower in the foreground with the college's main building in the background. It's best used sparingly and I doubt if I would replace it with an RF version, but it's one lens I will keep even if I sell most of my EF lenses.