Each stop is the equivalent of cutting the exposure by half or doubling it, depending on the direction. ASA ratings and shutter speeds & angles also work on the principle of doubling or halving -- i.e. 500 ASA is twice as fast as 250 ASA, so if you switched from 250 ASA film to 500 ASA film (one stop more sensitive), and had the same light level, and wanted the same exposure, you'd have to:
Close down the iris (f-stop) by one stop to compensate.
Or close down the shutter angle by half (thus cutting the exposure time per frame by half).
Or double the frame rate (thus cutting the exposure time per frame by half).
Or put an ND.30 filter on the lens.
Or cut the number of footcandles in the lighting down by half.
Or pull-process the 500 ASA film by one stop.
Any single one of those techniques would lose one stop of light (i.e. half the light.) You only need to do one of these things. If you did two of these things, you'd lose two stops of light, right?
Most light meters are programmable in 1/3 stop increments, something like (in ASA / ISO / EI values):
6, 8, 10, 12, 16, 20, 25, 32, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000
So if you switched from a 200 ASA stock to a 500 ASA stock, you'd compensate by 1 1/3 stops. If you rated a 500 ASA stock at 320 ASA, you're overexposing it by 2/3's of a stop. If you put an 85B filter on a camera with 500 ASA film, since the filter loses 2/3's of a stop, you'd set your meter to 320 ASA to compensate for the filter light loss.
If you rate a film stock slower in speed (less sensitive) than recommended, you'd probably be opening up the iris to compensate (shooting at wider apertures). So assuming the light level is the same, and you decide to rate a 500 ASA stock at 250 ASA instead, you'd probably be shooting one stop more open than if you used the 500 ASA rating. So if at 500 ASA, you were at f/4, by changing to 250 ASA, you'd be at f/2.8.
Edited by David Mullen, 04 November 2005 - 11:50 PM.