Sharpness loss at the edge of frame is mostly a function of the lens itself. It's only apparent to the smaller sensor after using this adapter because it could never see it before, it only saw the "sweet spot". In point of fact, sharpness at the edge of frame is better through the adapter on a smaller sensor than the same lens mounted to a FF camera.
Seems lots of people still aren't bothering to really investigate or grok what they've read so that they can add meaningful discussion.
edit: it cracks me up that, even though the adapter IS performing a kind of magick, its critics now feel compelled to criticize the magick itself as if it just weren't miraculous enough for their standards....
"Yeah, you turned water into wine...but you didn't raise anyone from the dead. Fail. You suck."