I just wanted to put a quick post together for prospective R7 and R10 users to state that, despite canon not formally supporting the .71x focal reducer for their mirrorless line, I have confirmed that it is an incredibly useful and nearly seamless integration for the system. Please read on for the details:
I use the .71x focal reducer on my canon R5 to get a higher quality full frame equivalent 4k 60p mode. It works incredibly well! The autofocus seems to be 100% as functional as without the focal reducer. I am even theorizing that the focal reducer is using a very similar firmware to the straight-through adapter from canon; one just has a lens element crammed in there.
Eye tracking, head tracking, animal eye af, and vehicle af grab the subject and follow them. It doesn't seem to have any problem with hunting or losing focus. I would go as far as to say that the autofocus performs BETTER in low light scenarios thanks to the extra full stop of light you get from the focal reducer.
Many people would tell you not to use the focal reducer because it's not "ideal" compared to native lenses. I would say that this is true of many third-party speed boosters, but not this one. Canon's first-ever speed booster makes even the old reliable metabones look like a cheap alternative. It is an exceptional performer regarding chromatic aberration, corner sharpness, and contrast: this element they've manufactured has no compromises. The blue flare issue that some have reported when using it has never been an issue for me. Using a lens hood can completely fix this in most situations.
One very minor idiosyncrasy to evaluate is that when booting up the camera with the adapter, you will see the disclaimer that it is not a canon eos mirrorless product. You just acknowledge and move on, they should really just get rid of that disclaimer and formally recognize that the adapter works exceptionally well with their mirrorless line.
I'm not trying to bolster canons marketing or anything, but I would like to point out that the canon R7 with the focal reducer would be a much better option than the R6 for many applications. If you're hesitating to buy the R7 because you'd rather have the full frame look for portraits, here is a VERY solid solution! You can use a straight-through adapter when you'd like more reach (wildlife or sports) and the focal reducer when you want shallow DOF or better low light performance!
If you have any questions regarding the use of the adapter on the mirrorless systems, or if you'd like me to test a specific use case, please fire away!
The Deaf Director on youtube covered this same topic and shows the benefits of R5 crop mode with the 0.71 adapter for video recording specifically.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.