ND Filter Calculator

Neutral density filters cut the light reaching the sensor without changing its color, so you can use slow shutter speeds in daylight: silky waterfalls, streaking clouds, empty city squares, or wide apertures on a sunny beach. The catch is the arithmetic, especially since filter makers label strength in three different ways. This calculator handles it.

Three ways to label a filter

Stops are the photographer’s unit: each stop halves the light. ND factors such as ND8 or ND1000 say by how much the light is divided: ND8 is three stops, because 2 × 2 × 2 = 8. Optical density, written as 0.3, 0.9 or 3.0, is the scientific measure: every 0.3 is one stop. The calculator accepts any of the three and converts between them, so an ND 1.8 is recognized as ND64, six stops.

Calculating the shutter speed

Meter the scene without the filter and enter that shutter speed. You can type it as 1/125, 0.5, 2 or 1:30 for a minute and a half. The calculator multiplies it by two for every stop of filtration. A 10-stop filter turns 1/125 s into about 8 seconds, a 6-stop filter turns 1/60 s into about a second.

Exact time and camera setting

Results come in two forms: the exact time, and the nearest standard shutter speed your camera offers in third-stop steps. Times beyond 30 seconds usually need bulb mode with a remote or the camera’s built-in long-exposure timer; use the exact time there.

Stacking two filters

Photographers often combine a strong ND with a graduated or a second ND. Enter the second filter’s strength in stops, and the two are added. A 10-stop plus a 3-stop filter gives 13 stops, so 1/125 s becomes about 66 seconds.

Changing aperture or ISO at the same time

If the exposure gets too long, or too short, you can also change aperture or ISO. Enter the original and new values; the calculator includes them. Going from f/8 to f/11 doubles the time again, raising ISO from 100 to 200 halves it.

Practical long-exposure tips

Compose and focus before screwing on a strong filter, since autofocus and the viewfinder struggle in near darkness. Switch to manual focus so the camera does not hunt. Cover the viewfinder eyepiece on DSLRs to stop light leaking in. Use a sturdy tripod and a remote or self-timer. Light changes during long exposures, so check the histogram and adjust.

Color casts

Strong ND filters, especially cheaper ones, can add a slight warm or cool cast. Shoot raw and correct it later, for example with the White Balance Fixer.

Film photographers

For exposures longer than about a second, film needs extra time because of reciprocity failure. The calculator gives the metered time; apply your film’s reciprocity correction on top, as the Pinhole Camera Calculator does for several Ilford films.

Which strength for which subject

Three stops smooth moving water slightly and let you open the aperture on bright days. Six stops blur waves and give soft clouds over a few seconds. Ten stops create the classic minimalist seascape and empty streets, with exposures of 30 seconds to a few minutes. Fifteen stops allow exposures of many minutes in full daylight.

Related tools

Plan shoots at sunrise and sunset with the Sun Angle Calculator. For star photography exposure limits, use the Star Trail Calculator.