ND Filter & Long-Exposure Calculator

Frequently Asked Questions

How does an ND filter change shutter speed?

Each stop of ND doubles the required exposure time: new shutter = base × 2^stops. A 10-stop ND turns 1/60 s into ~17 seconds.

How do ND numbers map to stops?

ND2 = 1 stop, ND4 = 2, ND8 = 3, ND64 = 6, ND1000 ≈ 10 stops (optical density = stops × 0.3). The calculator converts either notation.

Can I stack ND filters?

Yes - stops add: a 6-stop plus a 3-stop gives 9 stops. Stacking can add vignetting and color cast, especially with wide lenses.

What is reciprocity failure?

On film, very long exposures need extra time beyond the math; digital sensors are largely immune but add noise on multi-minute exposures.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

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.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How This Calculator Works

A neutral−density (ND) filter blocks a fixed amount of light measured in stops. Each stop halves the light, so to keep the same exposure the shutter must stay open twice as long for every stop. The core formula is new shutter = base shutter × 2^(total stops). The calculator also converts between the three common ways ND strength is labeled: stop rating (e.g. 10−stop), the ND number (ND1000 ≈ 2^10 = 1024× reduction, so log₂(1000) ≈ 9.97 stops), and optical density (density ÷ 0.3 = stops, because each 0.3 density step is one stop). Stacked filters simply add their stops together.

Worked Example

You meter a scene at 1⁄250 s (0.004 s) without a filter and screw on a 10−stop ND. New time = 0.004 × 2^10 = 0.004 × 1024 ≈ 4.1 s. Add a 3−stop ND on top (13 stops total) and the time becomes 0.004 × 2^13 ≈ 33 s - long enough to blur a waterfall to silk or erase moving crowds. The exposure table below shows the same base time through every common ND so you can preview each look before swapping glass in the field.

Real-World Use

Long−exposure photographers use this to turn daylight into motion: ND8−ND64 for flowing water at slower shutter speeds, ND400−ND1000 for multi−second seascapes, cloud streaks, and architectural shots free of people. Videographers use mild NDs (1−3 stops) to hold a 180° shutter (1⁄50 s at 24 fps) wide−open in bright sun. Because most cameras cannot meter through a very dark ND, the standard workflow is: compose and meter without the filter, read the base shutter, then apply this calculation for the filtered exposure.

Common Mistakes

Reciprocity failure on film. Film loses sensitivity at multi−second exposures, so it needs extra time beyond the 2^stops result - check the emulsion's data sheet. Digital sensors do not suffer this, though long exposures add thermal noise.

Confusing ND number with stops. ND8 is 3 stops, not 8; the number is the light−reduction factor (2^3 = 8×), while ND1000 is roughly 10 stops, not 1000.

Forgetting to add stacked filters. A 6−stop plus a 3−stop is 9 stops, multiplying time by 512× - never average them.

Color cast. Cheap resin NDs add a strong cast that worsens when stacked; correct white balance or shoot raw.

Limitations & Related Tools

This tool assumes aperture and ISO stay fixed and that the filter's rated strength is accurate - many filters are off by a fraction of a stop, so bracket critical shots. It does not model film reciprocity failure, lens vignetting from stacked thick filters, or diffraction softening if you also stop down. For very long exposures, also plan for battery drain and use a sturdy tripod and remote release in bulb mode. Pair this with an exposure−triangle calculator when you want to trade some of the slowdown for a smaller aperture, and a sunny−16 estimator if you have no meter at all.

ND Filter & Long-Exposure Calculator

Frequently Asked Questions

Related Photography Calculators