Filippo, I have used the much maligned 18~35 Sigma for quite some time
now. Initially I though I had made a bad purchase because of the
negative posts on this lens. However, with the cost of the Nikon lens
of similar size and even the cost of the Tokina, there was no other
option. I have since successfully shot several jobs which included
slides done with this lens and have had no problems with sharpness or
crisp color rendition.
I've handled the Tokina and outside of the greater speed of the lens,
see no reason to spend twice the money that I did on this one - I would
buy the Nikon, just because it IS The Nikon, but can't afford that and
so I am content with the results of my purchase!
As to an 18~70 Sigma f2.8 ... I wonder if that's coming out along with
the 20~600 f4 <tongue now removed from cheek>
