ND filters (Neutral density) are filters that cut the light intensity without altering the color,etc. of the scene. I don't remember the math, but I think a ND8 will cut your exposure time 4 stops (from, lets say 1/4 sec to 2 sec) - they're usefull when, for example, you want to show the blur of a waterfall in a fairly bright situation but the slowest shutter speed you can get is, say, 1/60 sec (a really bright day!) the solution is to put a pair of "sunglasses" on your lens (ND filters) - cuts back on the amount of light, allowing for slower shutter speeds, ND8 would give you 1/8 sec (there again, don't remember the math...help?) A blank piece of film over the lens would cut out a bunch of light, but the exposure time would be a matter of experimentation (used to use a couple of chunks in the viewfinder to shoot eclipses (sp?)...or ignore this - I've had a couple of beers (x7) :^)