More about filters for the Sigma 8-16mm lens can be found here: Sigma 8-16mm filter adapter options, DIY and commercial.

Rgds/Stefan

My opinion.
If you looking for a 500mm reach zoom lens at a reasonable prize then there isn't much of a competition for the Sigma 150-500 OS!
At 500mm you will need to use a bit of technique and practice to get sharp pictures especially if you haven't been using optically stabilized
lenses before (Half-press the shutter button, wait a while until stabilized and shoot a series of 3 or more to be able to pick the sharpest one afterwards).
This is because of the fact that OS/VR/IS or whatever does fail sometimes and more often if you don't use it properly.

The Sigma 150-500mm OS isn't the sharpest knife in the PRO lens drawer but or many people it will be the best option.
Properly used you will be able to catch many useable pics with this lens that you wouldn't be able to get with a shorter lens.

Some image samples and a full review can be found here: http://www.stockholmviews.com/sigma_150-500_os/index.html

Sigma 150-500mm @ 500mm F8 shuterspeed 1/500 iso 100

Sigma APO 150-500mm f/5-6.3 DG OS HSM review
Performed by stockholmviews Can be found here :0)
The review is kind of home made but still give a hint of the performance of the 150-500 and how to use it.

(S