Bought this to go on my new Sigma 10-20mm. The front element is quite large and also bulges out because of the wide angle of the lens. It is alot cheaper to replace a filter if you scratch it rather than the front of the lens.
Went with the Hoya Pro range as they are very thin which is what I needed for a wide angle lens to reduce vignetting. This is a quality filter, the ring is made from a metal so has a bit of weight to it which adds to its solid construction. The filter is so clear that when I removed it from the case and held it up, i honestly thought there was no glass in it, it looks as though you could put your finger through as there were no reflections. marks or dust on it.
I originally was going to buy the Hoya Pro UV filter, not because its a UV filter but just to protect the lens. Then I saw this one which was under half the price of the UV filter so snapped it up straight away. Highly recommended to protect your expensive lens.