Ok I would like to put my 2c's in here. I have a soft copy of this lens, Sigma have confirmed this so I know it is not user error as has been suggested earlier in the post by some. I think Sigma do have an IC problem with this lens
That said I would still recommend this lens to anyone and I would still buy this lens again. Why? because Canon have nothing to compete with this lens in my opinion. It is a great lens its just a shame about the quality control from Sigma.
Cheers Ricky

