Long Exposure Calculator: Shutter Speed with ND Filters
A neutral density (ND) filter reduces the amount of light reaching your camera sensor without changing colour. By blocking a precise number of stops of light, an ND filter lets you use a much slower shutter speed than would otherwise be possible in bright conditions. This enables creative effects such as silky smooth waterfalls, cloud streaks across a bright sky, or motion blur in busy streets. The calculation is straightforward: each stop of ND filter doubles the required exposure time, so the new shutter speed is the base shutter speed multiplied by 2 raised to the power of the filter's stop value. For example, a 10-stop ND1000 filter on a base exposure of 1/250 s (0.004 s) gives a new exposure of 0.004 x 1,024 = 4.10 seconds. Enter your base shutter speed and select your ND filter below. The calculator shows the new exposure in a readable format: fractions of a second for short exposures, minutes and seconds for longer ones, and hours and minutes for very long exposures. Common ND filter choices and their typical applications are listed in the reference table below.
New exposure time: --
How long exposure time is calculated
The formula for new exposure time with an ND filter is:
new_time = base_time × 2^stops
Where base_time is your original shutter speed in seconds and stops is the number of stops your ND filter blocks. Each stop doubles the exposure time because each stop of ND filter halves the light reaching the sensor.
Worked examples
- ND64 (6 stops) with 1/125 s base: 0.008 x 2^6 = 0.008 x 64 = 0.512 s
- ND1000 (10 stops) with 1/250 s base: 0.004 x 2^10 = 0.004 x 1,024 = 4.10 s
- ND1000 (10 stops) with 1/60 s base: 0.0167 x 1,024 = 17.1 s
- ND32000 (15 stops) with 1/125 s base: 0.008 x 32,768 = 262.1 s = 4 min 22 sec
Note: the ND factor printed on the filter (e.g. ND1000) is an approximation. A true 10-stop filter has a factor of 2^10 = 1,024, not exactly 1,000. This calculator uses the exact power-of-two formula for accuracy.
ND filter reference: stops, factors and uses
| Stops | ND factor | Optical density (OD) | Typical use |
|---|---|---|---|
| 1 | ND2 | 0.3 | Slight light reduction, wide aperture in mild overcast |
| 3 | ND8 | 0.9 | Smooth water in daylight, reduce depth of field in bright sun |
| 6 | ND64 | 1.8 | Strong motion blur in daylight, multi-second exposures in shade |
| 10 | ND1000 | 3.0 | Milky water, cloud streaks, 30-second exposures in full sun |
| 15 | ND32000 | 4.5 | Daytime very long exposures, minutes-long shutter in sunlight |
Optical density (OD) = log10(ND factor). OD 3.0 = 10 stops = ND1000. Source: Wikipedia: Neutral-density filter.
Reciprocity failure (film only)
Reciprocity failure is a phenomenon that affects film (analogue) photography but not digital sensors. When a film is exposed for longer than about 1 second, it loses sensitivity, and the actual exposure is less than the calculated value. To compensate, film photographers must extend the exposure beyond the calculated time, typically by a correction factor that depends on the film type and exposure duration.
For digital cameras, the sensor responds linearly to light across the full range of practical exposure times. No reciprocity correction is needed. The calculated new shutter speed from this tool is the time you should set on your digital camera.
Long exposure calculator: frequently asked questions
What is a neutral density filter?
A neutral density (ND) filter is an optical filter that reduces the intensity of light entering the camera lens without affecting colour. It works like sunglasses for your camera. By blocking a fixed number of stops of light, an ND filter lets you use a much slower shutter speed or wider aperture in bright conditions, enabling effects such as silky water, cloud streaks and motion blur in daylight.
What ND filter should I use for silky waterfalls?
For smooth, silky water in daylight, a 3-stop (ND8) filter is a popular choice, giving you roughly 8 times the base exposure. In brighter conditions, a 6-stop (ND64) filter allows exposures of several seconds. A 10-stop (ND1000) filter is ideal for very long exposures of 30 seconds or more in full sunlight, producing the classic milky water look. The right choice depends on the ambient light level and how much motion blur you want.
How do I stack ND filters?
When you stack two ND filters together, the stops add up. A 3-stop ND8 combined with a 6-stop ND64 gives 9 stops total (equivalent to ND512). To calculate the combined exposure multiplier, multiply the individual ND factors: ND8 x ND64 = ND512. Stacking filters can introduce vignetting (dark corners) on wide-angle lenses, and some combinations cause a colour cast. Variable ND filters offer an adjustable range without stacking.
What is optical density (OD)?
Optical density (OD) is a logarithmic measure of how much light a filter absorbs. It is defined as OD = log10(exposure multiplier). An ND filter with OD 3.0 blocks 1,000 times the light, which equals 10 stops. OD 0.3 corresponds to 1 stop (factor of 2), OD 0.9 to 3 stops (factor of 8), and OD 3.0 to 10 stops (factor of 1,000). OD is printed on many professional filters as an alternative to the stop or ND factor notation.
Can I use ND filters for video?
Yes. ND filters are widely used in video to maintain the 180-degree shutter rule: shutter speed should be approximately twice the frame rate (e.g. 1/50 s at 25 fps). In bright conditions without an ND filter, achieving the correct shutter speed would require a very small aperture, reducing depth of field. ND filters allow videographers to maintain a wide aperture and correct shutter speed regardless of ambient light. Variable ND filters are especially popular for video because they allow continuous adjustment.
Official sources
- Wikipedia: Neutral-density filter.
- B&H Photo Video: Introduction to ND Filters.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only; actual results depend on scene luminance and camera metering.