Keith over at Northlight-Images has uncovered a patent that showcases some of the capabilities of the RF mount as related to accessories such as extenders, converters and whatever else Canon can come up with.
As you may know, one of the biggest advancements in the RF mount over the EF mount is the high-speed communication capabilities, and it's like we've only scratched the surface as to what is possible for the RF mount.
We have heard some rumblings that RF mount extenders are coming, and I wouldn't be surprised to see an EF to RF teleconverter with a control ring for super-telephoto use.
I also believe some RF 70-200mm f/2.8L IS USM photographers would like to see some RF mount teleconverters as well.
The image below will give you some idea of how things flow between the camera, accessory, and lens. (Click for larger)
A tilt/shift adapter could do this, but few EF lenses have the image circle, except existing TS-E lenses. and a second T/S mechanism seems unlikely?
In the recent patent application it shows optical elements as an example - a range of extenders and extenders/adapters would seem the first things we'll see.
Having a single EF->RF extender/adapter would seem a mechanically good idea rather than stacking the extender and adapter. It's possible that the short flange distance might allow optical designs that work better than the current EF extenders - certainly a 1.4x extender adapter with control ring might be very popular.
* a EF-to-RF adaptor with no lens (just a high quality spacer to fit the back focus).
* a EF-to-RF adaptor & teleconverter with 1.4x or 2x (etc) lens magnification
* a RF-to-RF teleconverter with 1.4x or 2x (etc) magnification. This is the only one I'd be interested in, and *only* if it produced a better final image quality then just using no converter and cropping in-camera & optionally up-scaling the image in post.
Yes, here's one I created earlier... ;-)
I suspect the final version (for 1.4x) might not be a lot longer than the EF->RF adapter
From Teleconverters and shift lenses
I know we'd probably get more extreme light fall off but field curvature and edge smearing should be much better in RF mount.
If someone can only afford the one TS-E the use of the 1.4x TC is a common solution.
Personally I find the TS-E24 + TC giving ~34mm with ±17mm of shift an occasionally useful option given the next TS-E up from the 24mm is the 50mm (OK, the 45 is still available)
The article linked in the original post specifically looks at the TC option
An RF TS-E would be interesting. I've been experimenting with adapted medium format lenses on an RP and they work decidedly better in terms of vignetting and range of movements than an EF adapter.
Then you have to consider the only person who notices any differences would be me....
#2 with control ring (or drop in filter) makes sense to marry EF super telephotos until RF versions are released
#3 is needed when the 100-500mm lens is released :)
RF-EF adapter with control ring or ND/CP filter already exist but the CP is seriously expensive for what it is unless you need it for the 8-15mm or 11-24mm. Weirdly enough, it is easy to find either the RF-EF for ND or CP but getting an additional drop in seems tricky to find.
The CPL adapter is also great for macro and the MP-E 65mm, a regular front filter tends to pick up light from the flash. It's even more expensive than you realize, since you need to buy the clear filter as well if you don't change the adapter when you don't want a CPL.
The RF mount makes for far more flexibility with adapted lenses - my M645 lenses have had a new burst of usefulness (10º of tilt and ±15mm of shift)
I might take a trip to the Photography Show this year (for the 1st time in a very long time) to have a look at my options. A Cambo Actus G together with something like a RODENSTOCK Aperture Stop HR Digaron S 23mm lens would be my dream. With a 70mm image circle it would be great with a mirrorless body however the flange distance on this lens is only 44mm so not sure if it would work?
But if Canon eventually release a high MP body and update the 16/24mm TS-E and add those nice auto correction features it would make kit like the Cambo/Rodenstocks less appealing.