The Japanese patent office published last night a Canon patent – originally filed in May 2019 – for a mount adapter that contains not just a teleconverter, but one with variable magnification, and presumably variable effects on the effective aperture. The news was first picked up by Canon News.

Interestingly, the patent is not a lens design patent, as one sees normally with a teleconverter patent, but rather focuses on the mechanism of the variable magnification itself. This mechanism provides a lever mechanically coupled to a lens group inside the adapter, pushing the group forward and back. Forum dwellers at sites like Canon Rumors may recall conversations about this sort of idea back when rumors of new super telephoto lenses were being discussed.