The teleconverters that I am aware of were meant to be used with lenses from the same company making the teleconverters.They can't make a legal fuss about the use of the RF mount for manual focus lenses - you can't claim intellectual property for accessories that fit on to your patented gizmo. All Canon can do is to prevent the illegal use of their communication protocols, and they can't stop genuine reverse engineering of those protocols. I don't see why there are not TC's from competitors - all they have to do is have direct contacts going the the TC, and all it would fail to do is to report the modified f-number and focal length to the EXIFs - I could live with that.
I assume those will come after third-party lenses.
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