Steve Baggett , Apr 15, 2003; 02:34 p.m.
My advice is to get an ND1.8 (6 stops) or an ND3.0 (10 stops). The best exposure time range for blurring waterfalls (IMO) is 2-seconds to 8-seconds. (Beyond 8 seconds, the water tends to look too "wispy" for my tastes.) If you are shooting early with 50/100ASA film, then the 6 stops ND using f/16 or f/22 should get you into that time range. If you are shooting under brighter conditions, the 10 stops ND might be necessary. Rough calcuation: 100ASA-1/125@f16 (sunny 16) will require 10 stops to get down to 8 seconds required exposure at f/16. I wouldn't stack ND filters, however, as the more glass you have in front of the lens the more flare you could get.