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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.
Pick your 'Metered shutter speed (no filter)' and 'ND filter' strength, optionally adding a 'Stacked second filter' or 'Extra stops (polariser, grad, bellows)'.
The 'Filtered exposure' recomputes instantly, listing total stops, filter factor, optical density, transmittance and whether you need bulb mode (over 30 s).
Use 'How much ND do I need?' by entering 'Exposure time I want (seconds)' to see the stops required and the closest filter, then press 'Copy result'.
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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