Originally Posted by filorp 
i would like to ask you what you mean "slight decentering" ill show you my samples if you could judge them and tell me if there is any problem, those are very corner crops..
this is the link:
Fotoalbum.eu - Stwórz swój fotoalbum online
I presume that photo 1 has the chimney pot in one corner and in photo 2 it was the opposite corner?
If so, then yes yours is decentered, as one corner is a lot better than the other corner. Mine cleaned itself by F4, which is why I described it as "slightly" decentered.
It is important to have reasonable expectations though - a wide-angle zoom is never going to be pin-sharp in the extreme corners, particularly at F2.8
HTH
R