§ 11 — Tools / 02Light & exposure● Runs in your browser
ND filter calculator
Enter your metered shutter speed and filter strength to get the long-exposure time, then run the countdown from the same screen.
All tools- Exposure time
- 8 s1/125 s × 1000
- Filter strength
- 9.97 stopsND1000 · density 3
Ready. Starts when you press Start. Length 8 s.
How it works
The method behind the numbers.
A neutral density filter blocks a fixed amount of light without changing colour, so you can use a slower shutter in daylight. Each stop of filter doubles the exposure time needed for the same brightness.
t(new) = t(metered) × 2^stops
factor = 2^stops
stops = log2(factor) = density ÷ log10(2)- ND8 is 3 stops, ND64 is 6, ND1000 is about 10. Optical density is the log10 of the factor, so one stop is 0.301 (usually rounded to 0.3). A 0.9 filter is 2.99 stops, a factor of 7.9, which makers sell as ND8.
- Stacked filters add their stops. ND8 plus ND64 is 3 + 6 = 9 stops, a factor of 512. Stacking can add vignetting and colour casts that this tool does not model.
- Meter and set the exposure without the filter, then read the new time here. Past 30 s most cameras need bulb mode, so time it with the countdown or a remote.
- Film and some sensors need extra time at very long exposures (reciprocity failure). That depends on the stock or camera and is not included.