Use the free ND Filter Exposure Calculator below. Everything runs in your browser, with no sign-up and no upload.
Water turns to silk, clouds stretch into brushstrokes, and a busy street empties of people. All of that comes from one thing: a very long exposure. An ND filter makes long exposures possible in daylight by cutting the light before it reaches the sensor.
What is ND Filter Exposure Calculator?
A neutral density filter reduces light evenly, without changing colour. Its strength is measured in stops: each stop halves the light. A 3-stop filter turns 1/60 second into 1/8 second; a 10-stop filter turns 1/250 second into about 4 seconds. Filters are sold by stops, or by ND number: ND8 is 3 stops, ND64 is 6 stops, ND1000 is 10 stops, because the number is two raised to the power of the stops.
How it works
The calculator multiplies your base shutter speed by two for every stop of filter. That is all the maths there is, but doing it for a 10-stop filter in your head, in the field, is error-prone, especially when the base speed is a fraction like 1/250. The tool accepts stops directly, or an ND number from the common list, and reports the new time in seconds and in minutes and seconds for very long exposures.
How to use it
- Meter or set the exposure without the filter and note the shutter speed.
- Enter that speed and the filter strength in stops, or pick the ND number.
- Read the new shutter speed and set it on the camera.
- Use a tripod, cover the viewfinder if light can leak in, and use a remote or timer release.
Worked example
At the coast you meter 1/125 second at f/11. You screw on a 10-stop ND1000 filter. Ten stops multiply the time by 1,024, so 1/125 becomes about 8 seconds. The waves blur into mist while the rocks stay sharp. With a 6-stop ND64 instead, the same base speed becomes 0.5 seconds: enough to soften moving water, but not enough to erase it. Choosing the filter is choosing the look.
Tips and limits
Long exposures punish vibration, so a solid tripod matters more than an expensive filter. Very dark filters make the viewfinder black, so compose and focus before fitting the filter, then switch the lens to manual focus so it does not hunt. Cheap variable ND filters can show an X-shaped darkening at their strongest settings; fixed filters avoid that. And remember that the calculated time assumes the filter’s stated strength is accurate, which is usually close but not perfect, so check the first frame. If the first frame is slightly off, adjust the shutter time by a third of a stop at a time until the histogram sits where you want it, rather than changing the aperture and losing your chosen depth of field.
Please note: photography exposure tools are educational calculators. Sunny 16 and Zone System figures are starting-point rules of thumb, and a meter reading governs in real shooting. Time-lapse figures assume the camera can sustain the interval, and buffer or card limits may intervene.
Frequently asked questions
What does ND1000 actually mean?
It means the filter passes one thousandth of the light, which is ten stops less. In general, ND number equals two to the power of the stops.
Why is my long exposure too bright or too dark?
Either the filter strength is not exactly as labelled, or light leaked through the viewfinder during the exposure. Cover the eyepiece and re-test.
Can I stack two ND filters?
Yes, and their stops add. A 3-stop and a 6-stop filter together give 9 stops. Watch for vignetting with thick stacked filters on wide lenses.
Try the calculator above with your own numbers, and use the result as a planned starting point for your next shoot. A few minutes of planning before you press the shutter usually saves a whole session of corrections afterwards.