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"I have had really bad experiences with Sigma, ..."
A lot of people did, back in the day. At one point, the better photography stores were refusing to carry them, because they had so many complaints. Back when people were still using Minolta's SRT series film cameras, Sigma's sloppy lens tolerances actually destroyed quite a few cameras. You see, Minolta's SRT series SLRs had abnormally close tolerances and the leading edge of the reflex mirror would almost graze the back of the lens when you tripped the shutter. In a good many cases, if you had a Sigma lens in there (or even worse, a Quantaray), the mirror would HIT the back of the lens and it would chip, crack or break. Now imagine what happens to a camera that has abnormally close tolerances when it has tiny glass chips floating around in it. I'm told they are a lot better now, but I do not trust them and I will never buy a Sigma or Quantaray lens for as long as I live.