I post too Much on Here!!
- Jul 21, 2010
What you're suggesting is not really feasible.This isn't quite what I was getting at in my original proposition - I doubt RF users would be that concerned with EF-M glass and I realise going the other way round, to use EF-M on RF isn't at all practical or cost-effective.
My thought was what if RF-S was simply EF-M with a new name, so you could use existing EF-M lenses and also RF lenses via an adaptor - exactly the same way as EOSM can use EF and EF-S lenses. Of course, this does mean you wouldn't be able to take your RF-S glass with you if you wanted to "upgrade" to FF. Similar (compatibility wise, not physically) to EF-S and EF in that regard.
Using existing EF-M lenses on an RF mount would require an adapter with optics, and likely result in substantial image degradation.
EF and EF-S lenses have a longer flange distance (44mm) than RF (20mm) or EF-M (18mm). That allows room for an adapter that is really just a spacer, without optics. Mounting an RF lens on an M body means a 2mm adapter, not really practical to use and no one has made one (nor, I suspect, will anyone). Mounting an EF-M lens on an R body would require the lens to sit 2mm inside the body, and that's not possible. Thus, any adapter would need optical elements like the old FD-to-EOS adapter.