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| p.1 #3 · New Canon Teleconverter IV? |
The 200-400 TC can be simply removed out of the optical path. That means that without it in place, the lens still focuses light on the film plane. If you were to design such a TC for "normal" lenses, it would be akin putting an extension tube between the body and lens, thus not focusing the light on the film plane at infinity anymore.
So in other words the lens has to be specifically crafted to support this. Normal lenses won't work.