Hi Mrc,

While I cannot answer Misha's question, I know that Steve Kossack, whom I just went out to Death Valley with, is using Singh Ray's filters. Here are some reviews that he did:

http://www.stevekossack.com/tips/filters.htm

A primer on NDG Filters:
http://www.stevekossack.com/notes/NDGrads.htm

Cheers!
James

Manhattan, New York, NY Pet Photographer