Long Exposure and ND Filter Calculator

1. Meter the scene without the filter and note the shutter time.
2. Enter that base shutter in seconds.
3. Choose the ND filter you are using.
4. Read the new long-exposure time to set on the camera in bulb mode.

A neutral density, or ND, filter is a dark piece of glass that cuts the light reaching the sensor without changing the colour. It lets you use much longer shutter times in daylight, so moving water turns silky and clouds streak across the sky.

The filter strength is measured in stops. Each stop halves the light, so you must double the shutter time to compensate. A strong 10-stop filter turns a fraction of a second into several seconds, which is where the dramatic long-exposure look comes from. The maths is the only tricky part on location, because once a strong filter is on the lens the scene is too dark for the camera meter to read reliably, so working the new time out in advance is what keeps the shot from being pure trial and error in the field.

1. Meter the scene without the filter for the base shutter.
2. Find the filter strength in stops.
3. Raise two to the number of stops for the multiplier.
4. Multiply the base shutter by that factor for the new time.

A 10-stop filter multiplies the exposure by 2 to the power of 10, which is 1024, so it turns 1/125 second into about 8 seconds. This is why long-exposure photographers memorise a few key filters as simple multipliers, since doubling per stop makes the jumps easy to picture: three stops is eight times, six stops sixty-four, and ten stops just over a thousand.

You meter a bright waterfall at 1/125 second and fit a 10-stop ND filter for a smooth-water effect. The new time is 1/125 times 1024, about 8 seconds. Switch to a 6-stop filter and the multiplier drops to 64, giving roughly half a second, enough to soften the water without a full long exposure. Both need a tripod to stay sharp.

Filter Strength

Each extra stop doubles the exposure, so a 10-stop filter needs a far longer time than a 3-stop one.

Base Exposure

A darker scene already needs a longer base shutter, so the filtered time is longer still.

Aperture and ISO

Changing aperture or ISO shifts the base exposure, which then scales with the filter.

Stacking Filters

Stacking two ND filters adds their stops together, multiplying the exposure further.

The multiplier and the time from a 1/125 second base.

Very strong filters push exposures into minutes, ideal for daytime skies but demanding a sturdy tripod and a locked-off camera.

Silky Water

Work out the time to smooth a waterfall, river or sea.

Streaking Clouds

Plan a long enough exposure to blur clouds across the sky.

Removing Crowds

Use a very long exposure so moving people vanish from a scene.

1. Metering Through the Filter

Meter without the filter first; strong ND glass fools the meter.

2. Forgetting the Tripod

Long exposures need a solid tripod and often a remote or timer to avoid shake.

3. Ignoring Bulb Mode

Times beyond 30 seconds usually need bulb mode and a timer or intervalometer.

4. Light Leaks

Very long exposures can pick up light through the viewfinder; cover it if needed.

The stop maths is exact, matching how filters and cameras handle light. Real filters vary slightly, and very long exposures can drift.

Multiply the base shutter by two to the power of the filter stops. A 1/125 second base with a 10-stop ND filter becomes about 8 seconds.

An ND1000 filter is a 10-stop filter, blocking about 1000 times the light, which is 2 to the power of 10.

A 6 to 10 stop filter suits smoothing water in daylight. Six stops softens it; ten stops gives a fully silky, misty look.

Strong ND glass blocks so much light that the camera meter struggles. Meter the scene first, then apply the stops.

Yes. Stacking a 6-stop and a 3-stop filter gives 9 stops in total, multiplying the exposure accordingly.

It is how many times the exposure lengthens. A 10-stop filter has a factor of 1024, so the shutter is 1024 times longer.

Yes. Any exposure beyond a fraction of a second needs a solid tripod, and often a remote or timer to avoid camera shake.

For times beyond your camera maximum shutter, usually 30 seconds, use bulb mode with a timer or intervalometer.

No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Neutral density filter (Wikipedia).
• Long-exposure photography (Wikipedia).
• Exposure value (Wikipedia).