You should probably consider getting a thin (1mm) CPL, such as the Hoya Pro 1s or a brass ring B&W. Some people report vignetting on ultrawides with standard thickness CPLs, so if you have or plan to get something like a 10-20mm or 12-24mm in the future you'll want to stay safe with a thinner filter. Also, I have a Hoya Pro1 CPL, and it is *ridiculously* hard to get off of my Sigma 10-20mm and my aluminum step up/down rings. I've heard that CPLs that are constructed of brass instead of aluminum are easier to get off, such as B&W filters. If I could do it over again, I probably would not get a filter made of aluminum.