Very good said, Airy! There is always imminent compromise between geometrical distortions and sheer resolution of any optical system. Fully improved for GD lens will never be absolutely sharp to extreme corners. There are couple lenses fully improved, at all, but price... Otus I mean. All of 3 of them, as of on today... And, yes, Chambeshi, Bjorn was absolutely correct. My copy was acceptable on D700, D3, D4 bodies at 5.6. But, on D800-810, at 5.6, the lens is eaten alive by very demanding sensors. So, as you can see, f8.0 to f9,5, my copy is working very well. THX! LZ