Nope, untrue. A telephoto extender goes between the rear element of the lens and the image sensor and tries to do its magic while remaining in the same system. That is where most of the stress comes from. To repeat, this technology is different because it utilizes the difference in the flange focal distances of an SLR system and a mirrorless.
Originally Posted by E. von Hoegh
If introducing a lens between (1) source of light, and (2) the image sensor necessarily introduces attendant problems and aberrations, then why even bother taking a photograph in the first place? I mean, all your Summicron is doing is introducing problems and aberrations, no?
Here's the deal.
You have light rays emitting from the rear of your lens. It's going to hit the sensor plane after ~45mm and create an image circle that will cover FF.
It is theoretically very sound and possible to introduce a set of optics within those ~45mm that will converge & concentrate those light rays so that after those ~45mm, the image circle will be smaller and just cover APS-C without significantly reducing the image quality.
Like it's been said, this concept has been used before most notably in astrophotography (of the best kind, I may add). If it's such a degrading "snake-oil" concept/technology, why would some of the most expensive astrophotographic imaging equipment use it?
Now, wether or not the actual execution of the concept here is entirely satisfactory remains to be seen. But seeing that one of the most talented lens designers living today is involved in the development of the product, my bet would be on him than people who apprently don't understand some basic principles of physics.