Overshifting the lens, here at 11-12mm where the recommended maximum is 8mm along the long axis. On the original photo and, to a lesser extent, on this re-sampled view, sharpness drops steeply but only in the very extreme corners at the top. The fish is still sharp enough.
Also, there is some significant vignetting (not corrected).
Df, 35/2.8PC at f/8. Very bad light grazing the lens front, while the wall was in the shadow. Processed in NX Studio (slight increase of contrast and saturation; white balance warmed up a bit; 5:4 with bottom cropped away).
A darn good lens, even though in "normal" usage (unshifted) the Zeiss 35/2 ZF2 is superior. But that's comparing emeralds with diamonds.