Jernej LeticaPhotography Tools & Stories
Photography Calculators

Work out a long exposure with ND filters

Convert ND stops, filter factors or optical density into one shutter time, with a countdown for exposures over 30 seconds.

Built and written by Jernej Letica

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Filters, stacked (leave a filter off to skip it)

How to calculate a long exposure with ND filters

  1. Enter the base shutter speed, which is the exposure the scene needs without the filter, such as 1/60 or 0.5.
  2. Choose how the filter is labelled: stops, filter factor (1000 for ND1000) or optical density (3.0).
  3. Add a second or third filter to stack them. Their stops add up.
  4. Read the shutter time. For an exposure over 30 seconds, a countdown appears for the bulb.

Stops, filter factors and optical density

A filter with a factor of 8 passes one eighth of the light, which is 3 stops, because stops are the base-2 logarithm of the factor. ND1000 has a factor of 1000 and reduces the light by 9.97 stops. Optical density is a base-10 logarithm, so each unit of density is log2(10), or 3.32 stops. An optical density of 3.0 is the same filter as ND1000, and it is usually rounded to 10 stops.

Stacking filters

Stacked filters multiply their light reduction, so their stops add. A 3-stop filter with a 6-stop filter gives 9 stops, a factor of 512. The base shutter of 1/60 s becomes 512 / 60 = 8.53 s. The calculator adds the stops and does not model colour cast or vignetting from the glass.

Exposures over 30 seconds

Above 30 seconds the calculator shows a countdown from the calculated time. It can be paused and reset. The countdown runs in the browser and does not control the camera, so set the camera to bulb or timer mode and check the frame on the camera display.

Tips for better results

  • Measure the base shutter with the filter off, in the same mode and at the same ISO you will shoot with.
  • Read the number printed on the filter, because manufacturers label stops, factors and optical density differently.
  • The result is the arithmetic from the labels. Real filters can differ slightly, so test one frame and adjust.

Privacy

Your entered content never leaves your device. The calculations and the countdown run locally in your browser.

Frequently asked questions

Is ND1000 the same as 10 stops?

Not exactly. ND1000 reduces light by a factor of 1000, which is 9.97 stops. Ten stops is a factor of 1024. The two are usually given the same name, so the calculator shows ND1000 as 9.97 stops.

How is an optical density entered?

Enter the number printed as OD on the filter, such as 0.9, 1.2 or 3.0. Each unit of optical density is 3.32 stops, so 0.9 is 2.99 stops and 3.0 is 9.97 stops.

Why does the countdown appear only above 30 seconds?

Shorter exposures can be read from the camera display and timed directly. Above 30 seconds, a bulb exposure needs a timer, so the countdown shows the time that remains.

Does the calculator include a polarising filter?

No. A polarising filter is not an ND filter and reduces light by an amount that depends on the angle. Meter the base shutter with the polariser fitted and enter that value.

Does the result include reciprocity failure?

No. The result is the stop arithmetic only. Reciprocity failure changes the exposure for some film and very long exposures, so check the data for the film or sensor before relying on a time of several minutes.

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