I simply could not accept the idea that I was so wrong about this lens.. and that those couple hundreds of bucks were thrown through the window.
So I decided to try another trick:
Instead of using it's internal diaphragm, I placed in front of the lens the black piece of paper with round opening, so the effective f number was 5.6, the same as last night and gave it another go.
The result was astonishing: it is hard to believe that the same lens can perform so drastically better.
The crop from upper right corner is attached...
I am a happy man after all
