I picked up a kit that had a UV, a variable ND filter and a circular polarizer. I threw on the VND - and even after making adjustments - at just about all stops - the effect has, what looks like, an inverted bell curve. (Where the center is darker than the sides. Anybody else using a variable ND filter have this affect? Or did I pick up a bad brand? (Promaster Quad Filter Kit for Phantom 3) If there's another brand with a variable ND filter that does better - suggestions would be appreciated. Otherwise, I'll probably trade it back for a 8 ND.