Yes, I've been using the lens with an adapter on the EOS R for a few months now. I'd been using it on the 5D for 2 years before that and so far I can't see any difference. Autofocus is fast and the optics are super sharp, whether on the R or the 5D.

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You can. You simply can't use the APS-C lens on a full-frame camera. The other way round, however, you can. This one would then become one (24mm x1.6 = 38.4mm).