Just thought I'd add a note to this. Singh-Ray now sells a Vari-N-Duo, which is basically their variND filter with an additional rotation point behind the two rotating glass elements, which "adds" a polarizer. I saw one of these last night in a workshop we held, and all it adds is the ability to freely rotate the two glass elements, indicating using this as a polarizer definitely works. If you don't have this duo version, you would have to unscrew the filter to rotate it, but certainly would work as a polarizer.