Quote Originally Posted by Tony Bennett View Post
I have heard that stacking two circular polarising filters and varying them relative to each other can produce the same effect as the Vari-ND filter. One polariser cuts out about 2 stops, the other one can remove variably whatever light is left, as a maximum.

Has anyone tried this technique, and if so, does it work?
This is in essence how the Vari-ND works, but without the side effects that John mentioned above (unsure of the science behind it). You can safely go to around 8 stops of attenuation, but going beyond that is likely to produce a moire pattern / "newton ring" type effect. Additionally, they also have a 5-Stop ND "MorSlo" filter that you can attach to the Vari-ND (giving up to 13 stops - hopefully enough for most situations) (or the MorSlo can be used by itself).