Why is there so much noise and softness from my Sigma 10-20mm?

Could be a bad copy of the lens. I had one (for Canon) with the same problem, sent it back and my replacement was 100 times better. Third party lenses are a crap shoot. The same can be said with brand lenses, but it happens at a much lower rate. There is a reason beyond the name for the price difference. Not saying can or should be able to afford Canon gl all the time. I can't either, but you do get what you pay for with lenses.