If, like me, you’re in the market (as I have been for quite a while!) for an Ultra Wide Angle lens for your Canon 20D / 30D and were always left scratching your head as to why the Canon EF-S 10 – 20 was worth approx $200 more than the Sigma 10 – 20, well then this article should go a good way toward explaining the differences, or, more to the point, the fact that there pretty much is none, and there’s no reason at all why you should pay the extra money for the Canon lens.