RF with Older EF Lenses


If only I knew what I was doing.....
CR Pro
Sep 8, 2012
Been looking around for some information and only getting bits and pieces........

I looked at numerous forums (like FM) and saw a video using an R5 with EF 400 f/2.8 and adapter and it looks very promising. I am contemplating replacing some EF glass in time. Were I to use an EF 300 f/2.8 II with an EF-RF adapter and possibly stack an EF 1.4x or 2.0X in that setup, does anyone have a feel for the autofocus/maximum frame rate/battery life with that kind of setup? I know I'll likely lose some focus points. According to Canon, this lens is very capable of some pretty good FPS on an R5 but nobody is talking about how the battery holds up "pushing" that kind of focusing hardware in a birding type of scenario.

All in all, this is an amazing camera for stills but I am thinking about the future using existing lenses versus replacements (which would be a significant stretch on my budget :rolleyes:).

Thanks in advance


I'm New Here
Oct 9, 2014
I use my old 500mm f4 plus the 1.4x TC attached with no problems. It works as well (or better considering the new eye focus system) as on my 7D MKII. The only caveat so far is trying to refocus on something in a busy background situation, but hopefully that will get straightened out and is not a lens issue.

Battery life does take a bit of a hit compared to a non-mirrorless system, but that is to be expected. In my opinion, I think the IS probably uses more power than the focus motor since it is constantly working while composing as opposed to the focus motor. I can still get several hundred photos per battery with it, just take a spare or two. I have no intention in the near future of buying newer RF lenses, since the EF-RF adapter works seamlessly so far. Maybe after a while I may look at newer glass. I have only had to retrain myself to remove/replace lenses using the release on the adapter instead of on the camera body.
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