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About this tool
Convert a metered shutter speed into the filtered exposure for any ND strength, including stacked filters, optical density and bulb timing.
ND Filter Exposure Calculator converts a shutter speed metered without a filter into the exposure needed behind neutral density, using the rule that each stop halves the light so the shutter must double: filtered time = metered time × 2^stops. It also reports the filter factor, the optical density (stops × log10 2, so 10 stops is density 3.0) and the transmittance, and adds the stops when you stack filters. Aimed at long-exposure landscape, architecture and daytime video work.
Open ND Filter Exposure Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Filter factors multiply, so stops add — the tool sums them instead of you guessing.
Stops, filter factor and optical density shown together, since manufacturers use all of them.
Flags exposures past 30 s and past 1 s, where bulb mode and reciprocity failure start to matter.
Multiply the metered time by 2^10, which is 1024. A meter reading of 1/125 s becomes about 8.2 seconds; 1/60 s becomes about 17 seconds; 1/15 s becomes about 68 seconds, which needs bulb mode.
It is a filter with roughly a 1000× light-reduction factor, which is 10 stops and an optical density of 3.0. The true factor of 10 stops is 1024, so the printed 1000 is a rounded marketing figure — the density number on the ring is the precise specification.
Yes, and their stops simply add because the factors multiply: a 6-stop plus a 3-stop gives 9 stops, a factor of 512. Stacking thick filters on a wide lens risks vignetting, and each extra glass surface adds a little flare and colour cast.
Because of reciprocity failure — beyond about one second, film loses effective sensitivity and needs extra time on top of the calculated figure. The correction is stock-specific, so use the manufacturer's chart. Digital sensors are linear and need no such correction.
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