I have Canon Mount Adapter EF - EOS R with control ring for EOS R5 and it's working with all my EF lenses (7 lenses) except my trusty EF 100-400 L MKI. It's working for 1-2 shots and then I'm getting Err 01. 100-400 is not my most used lens so upgrade to RF 100-500 is not for me. I still using it on my old 5D Mark III and love it. Wanted to try it with most simple adapter (without control ring) so after reading positive reviews ordered Meike MK-EFTR-A adapter, got it and Voila!!! The magic happened - my Canon EF 100-400 L MKI now working on Canon R5. Thank you, Meike!!!