ND Filter & Long-Exposure Calculator

Frequently Asked Questions

What is the difference between ND number and stops?

The ND number is the light-reduction factor - ND8 blocks 8x the light, which is 3 stops. Stops are log2 of the ND number. Always check which system your filter is labeled in before calculating.

Does digital photography have reciprocity failure?

No - digital sensors do not suffer the reciprocity failure that affects film at long exposures. However, long digital exposures do accumulate thermal (hot pixel) noise, especially in warm conditions.

How do I combine stacked ND filters?

Add their stop values together. A 6-stop ND plus a 3-stop ND equals 9 stops total - not 18, and not an average. This multiplies the time by 2^9 = 512x.

Why does my ND filter show a color cast?

Budget resin filters absorb different wavelengths unequally. This is more visible when stacking multiple filters. Quality glass NDs from reputable brands minimize this, but shooting raw allows easy correction in post.

Can I use a variable ND to replace multiple fixed NDs?

Yes, but variable NDs often introduce cross-polarization artifacts (an 'X' pattern) at extreme settings and color shifts. They are convenient for run-and-gun work but rarely match the optical quality of fixed glass for critical long exposures.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

A neutral-density (ND) filter blocks a fixed fraction of light measured in stops. Each stop halves the light, so to preserve the same exposure, the shutter must stay open twice as long for every stop added. The core formula is: new_shutter = base_shutter x 2^(total_stops).

ND filter strength is labeled three different ways, and this calculator accepts all of them. The stop rating (e.g., 10-stop) is the most intuitive. The ND number is the light-reduction factor - ND8 blocks 8x the light, which equals 3 stops (log2(8) = 3); ND1000 blocks approximately 10 stops (log2(1000) = 9.97). Optical density is the logarithmic measurement used in scientific filters: density / 0.3 = stops (each 0.3 density step is one stop). Stacked filters simply add their stop values.

The workflow is: meter the scene without the filter to get the base shutter speed, enter it here, specify the filter(s), and read the corrected exposure time.

Use Cases

ND filters are used to extend shutter speed beyond what would otherwise be possible in a given light:

Waterfalls and seascapes: 6-10 stop NDs to blur water into smooth silk in daylight

Cityscapes free of people: 10+ stop NDs for multi-second exposures that erase moving crowds

Cloud streaks and sky motion: 6-8 stop NDs to show wind movement in clouds across a wide landscape

Architecture and tourism: removing people and vehicles from busy scenes with long exposures

Video: 1-3 stop variable NDs to hold 180-degree shutter (1/50 s at 24 fps) with a wide aperture in bright sun

Tips

Most cameras cannot meter through a very dark ND - meter first without the filter, record the base shutter, then mount the filter and apply this calculation.

ND8 is 3 stops, not 8 - the number is the light-reduction factor; ND1000 is roughly 10 stops. Confusing number with stops is a very common mistake.

Cheap resin ND filters often add a strong color cast, especially when stacked. Shoot raw and correct white balance in post, or invest in quality multi-coated glass.

For critical exposures, bracket one stop either side of the calculated time to account for filter inaccuracies and reciprocity effects on film.

Long exposures increase sensor heat and thermal noise. On hot days, shorter exposures with multiple stacked frames can give cleaner results than a single very long shot.

FAQ