What shutter speed do you need with an ND filter?
Enter the shutter speed your camera meters without a filter and pick an ND filter, or stack two. You get the new shutter speed and a table for every common filter.
How the tool works
Each stop of ND halves the light, so the shutter time doubles. The new time is the metered time multiplied by 2 to the power of the stops. An ND8 is 3 stops (8x), ND64 is 6 stops, ND1000 is 10 stops (1024x in practice) and a 15 stop filter is 32768x. Optical density is 0.3 per stop, so ND 0.9 is 3 stops, 1.8 is 6, 3.0 is 10 and 4.5 is 15. Stacked filters add their stops together.
A polariser costs roughly 1 to 2 stops, so the calculator uses 1.5 as an average. Meter without the filter first, at your base ISO and chosen aperture, then apply the result. If the answer is longer than 30 s, set the camera to Bulb and use an intervalometer or remote release. Stacking filters can add a colour cast and vignetting on wide lenses, and light can leak in through the viewfinder, so cover it.
The film option applies the power law correction Tc = Tm to the power of p, with an exponent you enter from the film datasheet. Treat the answer as an estimate, since reciprocity failure differs between films and the law only holds for a limited range of times. Digital sensors do not need this correction.
Questions
How do I calculate exposure time with an ND filter?
Multiply the metered time by 2 to the power of the filter stops. At 1/125 s, a 10 stop filter gives 1/125 x 1024, about 8.2 s.
How many stops is ND1000?
ND1000 is 10 stops, strictly 1000x, which is about 9.97 stops. Most photographers treat it as 10 stops and 1024x.
What do I do when the time is over 30 seconds?
Switch the camera to Bulb mode and use an intervalometer or remote release to open and close the shutter for the time shown.
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