do you think we'll have an RF to E mount adaptor in the near future?
The latest Canon glass is RF, I have the 50 1.2 RF and it's outstanding.
The next few lenses will include the 85 1.2 RF and one with DS too, for smoother bokeh
And a stubby looking 70-200 f2.8 IS
Could we go RF to EF using the EF adaptor or the EF control ring adaptor, then EF to E mount?
obviously that would not be ideal long term, but I'm curious.
Not possible, apart the famous R "wide throat" that would poses a lot of challenges with Sony's "narrow throat", flange distance won't allow it, Canon R's is only 2mm longer than Sony's, there won't be any physical space for an AF adapter.
It tends to be down to the mount flange distance whether or not you can typically adapt lenses. You can find a list of lens mounts and their flange distance at https://en.wikipedia.org/wiki/Lens_mount
As mirrorless cameras mounts are much closer to the sensor than an SLR (as they dont need space for the mirror mechanism) you can generally easily find adaptors for SLR designed lenses to put on a mirrorless body.
For the mounts you asked about we are looking at these figures:
Canon EF: 44mm
Canon RF: 20mm
Sony E: 18mm
So you can go EF to E pretty easily and lots of people have done so when switching from Canon to Sony.
It's possible to go from RF to E but there is only 2mm space for the adapter to fit in. Someone (iirc Voigtlander) are bringing out an E/FE lenses to Nikon Z cameras (which is 16mm, again a 2mm space). https://nikonrumors.com/2019/02/27/voigtlander-announced-new-lens-adapters-for-nikon-z-mount-mirrorless-cameras.aspx/ As Tirpitz666 has said though the width of the mount and thus the path the light travels from the end of the lens to the sensor may preclude this from being possible.
As Sony E Mount lenses are designed to be closer to the camera sensor than Canon RF, it doesn't seem very likely we'll get an adapter for that scenario. However both the Canon and Nikon mirrorless mounts are wider than Sony E, so it might be possible to recess an adapter so the lens sits a bit inside the mount. I wouldn't hold your breath on this happening though.
Again as Canon RF needs to be a lot closer than a Canon EF lens, you aren't going to be able to use Canon RF lenses on a Canon EF body.
So to sum it up, you can typically find an adapter for a lens mount that has a larger flanger distance to put it onto a camera with a shorter flange distance, but typically not the other way around. (There are exceptions but you will get limitations with them, like a limited focusing range or some adapters have an element of glass in them and you'll lose quality.)
Let's just say we're back to the old DSLR day. Theoretically, you can do it, but it will be costly and sub-optimal. For MF lenses, it's doable. But for AF lenses, with Canikon is not sharing their mount info, mounting a lens on another camera is just a small problem.
