You asked
What is the canon rf 5 2mm f 2 8l dual fisheye good for?
The Canon RF 5.2mm f/2.8L Dual Fisheye is a specialist lens. It's a twin-lens 3D VR lens for full-frame Canon RF bodies. It's manual focus only and not stabilised, so it suits people making VR content rather than everyday snapshots. On full frame it frames at 5.2mm, and on APS-C it behaves like about 8.3mm.
If you want to try it, I'd pair it with the Canon EOS R6 Mark II for a well-rounded full-frame setup. The Canon EOS R5 is the one to pick if you want more resolution. The Canon EOS R8 is another full-frame body that takes it. If you're unsure how the crop works on different bodies, the sensor sizes guide explains it.
Written by Claude from this site's pages.