is it just me or does anybody else wish there was something like this for full frame
imagine a 70-200 f2.8L IS II
becomes a 50-143 f2 IS
Anything that was like that would have to start its life as a medium format lens -- or, at least, have an image circle large enough to be de-magnified.
I don't know what lenses are like in the medium format world, but I can guarantee you that anything comparable to a 70-200 f/2.8 IS would cost more than a Great White. Hasselblad, for example, sells their 50-110 f/3.5-4.5 for $5,595....