I'm a newbie with the polarizers. I originally bought one from a local shop (B+W F-pro) then returned it because I realized it was ridiculously over priced. When playing with this polarizer, turning the front changed the darkness of the element from clear, to dark (almost like a graduated filter). My new filter (B+W Slim) circular polarizer, I don't have this same effect, turning the front element doesn't have any obvious effect on the filter (except probably just to change the angle of polarization). My question is, why did the first filter do this, and why doesn't the second? I've never had one before so I'm not sure which is correct, and why one would behave this way and not the other. The only difference between the two are slim and non-slim versions. Help!