I use ND filters primarily for smoothing out water in streams and oceans. I use a big stopper, 10 stop filter. I usually shoot at around 30 seconds on a very, very sturdy tripod. I use a Tiffen filter which also tends to enrich the shot for colors. I also use Minolta AF lenses that further enrich the colors of the shot. The nice thing about ND filters each and every shot you take comes out differently cause the water moves differently each and every time. My ten stopper is very easy to calculate, without the filter I get my exposure to come out a 1/30 of a second, that way, when I put my filter on, it becomes a 30 second exposure. Things to keep in mind, make sure your eyepiece is covered as stray light will get in and effect your exposure, also once you focus without the filter attached move lens to manual focus so it won't hunt during your long exposure. Enjoy and keep on shooting until the end.