ND Filter Exposure Calculator
A neutral density filter darkens the whole image evenly so you can use a much longer shutter speed in bright light. Because each stop of ND halves the light, you double the shutter time for every stop to hold the same exposure. This calculator takes your metered base shutter speed and the filter's strength in stops and returns the new shutter time in seconds, minutes, and the light-reduction factor. Use it to plan smooth-water and cloud-streak long exposures without guessing at the timer.
ND exposure formula
New shutter t = base time * 2^(stops)
Light reduction factor = 2^(stops)
minutes = seconds / 60
ND factor to stops = log2(factor); density to stops = density / 0.3
Each stop doubles the required time. A 0.008 second base (1/125) with 10 stops becomes about 8.2 seconds.
Long exposure context
- One stop of ND halves the light, so the shutter time doubles per stop.
- ND8 is 3 stops, ND64 is 6 stops, ND1000 is about 10 stops.
- Stacking filters adds their stops together.
- Very long exposures may need a slight extra adjustment for reciprocity or sensor warming.
- Use a tripod and remote or timer release for multi-second exposures.
ND filter: frequently asked questions
How does a neutral density filter change shutter speed?
Each stop of ND halves the light, so to keep the same exposure you double the shutter time per stop. The new time equals the base time times 2 raised to the number of stops: t_new = t_base times 2 to the power of stops. A 1 second base with a 10-stop filter becomes 1,024 seconds, about 17 minutes.
How do ND numbers relate to stops?
ND filters are labelled different ways. An ND factor (like ND8) means stops equal the base-2 log of the factor, so ND8 is 3 stops. An optical density (like 0.9) means stops equal density divided by 0.3, so 0.9 is 3 stops. This calculator takes the number of stops directly, the common photographic measure.
What is a 10-stop ND filter used for?
A 10-stop filter (often labelled ND1000 or 3.0 density) cuts light by a factor of about 1,024, turning a fast daylight exposure into minutes. It is used to blur moving water into a smooth sheet, streak clouds, and remove moving people from a scene in bright conditions.
Can I stack ND filters?
Yes. Stacking adds their stops, so a 6-stop plus a 3-stop filter gives 9 stops total. Add the stop counts and enter the sum. Be aware that stacking can introduce vignetting on wide lenses and may add a slight colour cast.
Why does my long exposure still look off?
Very long exposures can show reciprocity effects on film and sensor noise or warming on digital. The pure ND math here is exact for the light reduction; if the image is slightly under or over, adjust by a fraction of a stop. Also recheck focus and use a tripod and remote release.
Official sources
- NIST: optical density and photometry metrology.
- CIPA: Camera and Imaging Products Association standards.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.