Is a variable density filter like the Singh-Ray Vari-ND essentially two polarized filters in rotation? If so can I attain the same effect by adding another 77mm CPL to my existing one? I've read as much, and it matches my rusty understanding of paired polarizing filters, but wanted to ask here for opinions. [URL]http://www.singh-ray.com/varind.html[/URL] Are there any advantages to a combo unit like the Vari-ND besides convinience (?) and the index?