Originally posted by: Alyx
I've got the sigma 10-20mm lens and I've never liked my results with polarizers.
The extreme wide angle makes the polarizing effect change over the course of the picture so if the sky is in the shot it will turn out very strange.
If you are looking to buy filters the best prices I've found are
http://2filter.com/ I've bought quite a few things with them. Personally I don't use filters with my 10-20mm normally, I just be extra careful not to scratch it since I don't keep a UV filter on it. If you're worried a UV filter is a great investment to protect the lens though.
Here's an example of the effect of a polarizer on a Wide angle, notice how the exposure of the sky changes a lot.
http://photos.anands.net/Album...wy1/slides/Hwy1000.jpg