I'm certain they could have designed a better 800/5.6 than they made by putting a 2x TC within the 400/2.8. The EF MkII supertele lenses were noticeably better optically than the MkI versions (whereas the MkII to MkIII upgrade for the 400/600 seemed to be about weight reduction rather than optical improvement). The EF 800/5.6 never got the MkII treatment, so even analogous changes to what the 300/400/500/600 received would have yielded an RF 800/5.6 better than we have.I am remain to be convinced that Canon could not have done better by designing an 800mm from scratch instead of putting a 2xTC on a 400mm.
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