ND Filter Calculator

How ND filter exposure calculation works

A neutral density (ND) filter blocks a fixed fraction of light from reaching your sensor without changing color, letting you use a much slower shutter speed than the ambient light would otherwise allow — the basis of most silky-water, cloud-streak, and daytime long-exposure photography. Each stop of ND halves the light reaching the sensor, so to keep the same exposure you must double the shutter speed for every stop. The math is simple: new shutter speed = base shutter speed × 2^stops. A 10-stop ND (like the popular ND1000) turns a 1/8s daylight exposure into 128 seconds — over two minutes — which is exactly the kind of number this calculator formats as "2 min 8 sec" instead of a hard-to-read decimal.

Reading ND filter names

ND filters are labeled two different ways that both describe the same thing: an "ND number" (the fraction of light transmitted, e.g. ND1000 transmits 1/1000th of the light) and a "stop count" (how many stops of light that removes). The two aren't always simply related by clean round numbers — manufacturers round differently — so this calculator uses the commonly published stop-equivalents in the table below rather than recomputing from the ND number every time.

Using the countdown timer

Once you calculate an exposure, a countdown timer appears below the result. Press Start the instant you open your shutter (via a remote release or your camera's bulb mode), and the timer counts down to zero and vibrates when it's time to close the shutter. Because phones normally dim and lock the screen after a short idle period — which would leave you guessing whether the timer is still running — this page requests the browser's Screen Wake Lock API when you start the timer, so your screen stays on for the full exposure. Not every browser supports wake lock; if yours doesn't, the page tells you so and you should keep the screen on manually (or increase your device's auto-lock timeout before a long session).

FAQ

Why use an ND filter at all?

To force a slow shutter speed in bright light for creative effect — silky-smooth waterfalls and rivers, streaking clouds, blurred-out moving people or traffic to leave an "empty" scene, or simply to use a wide aperture in daylight without overexposing.

How do I avoid star trails becoming streaks instead of points in very long exposures?

The Earth's rotation smears stars into short trails once your exposure gets long enough — roughly the "500 rule" (500 ÷ your full-frame-equivalent focal length = max seconds for pinpoint stars). For anything longer than a few seconds at typical focal lengths, either drop to a lighter ND (or none) and raise ISO instead, or accept intentional star trails as the creative effect. Very strong ND filters (8+ stops) are generally not used for astrophotography.

What is reciprocity failure?

On film (not digital sensors), very long exposures become progressively less sensitive than the simple 2× formula predicts, so film shooters doing multi-minute ND exposures often need to add extra time beyond this calculator's result — consult your specific film stock's reciprocity chart. Digital sensors don't exhibit meaningful reciprocity failure, so this doesn't apply to digital cameras.

Does this account for filter color cast?

No — this tool only calculates exposure time. Cheaper ND filters (especially very strong ones) can introduce a color cast, most often a magenta or blue tint, which you correct in white balance or post-processing.

Is it free?

Yes — free, no account, and every calculation runs in your browser.

Informational tool for photographers — always verify exposure with a test shot, especially with strong ND filters where metering through the filter can be unreliable.