I have a Sigma 15-30 Ex zoom and it is probably the most used lens I own. Sharp all over from 5.6 down to about f16 when diffraction starts to take over. Compared to the lies of a Nikon 17/35 it is no match but there again it is about 1/4 of the price even 2nd hand. I will get a 12x16 colour print from it anytime ant any focal length.