Long Exposure Time Calculator
Calculates the correct long-exposure shutter speed when stacking ND filters, accounting for reciprocity failure. Use it when shooting waterfalls, light trails, or seascapes with neutral density filters.
About this calculator
Each stop of ND filtration halves the amount of light reaching the sensor, requiring the shutter to stay open twice as long. When stacking multiple ND filters, their stops add together, so the total light reduction is 2^(ndFilter + stackedFilters) times the base exposure. The adjusted exposure time is: exposureTime = baseShutter × 2^(ndFilter + stackedFilters) × reciprocityFactor. The reciprocityFactor compensates for reciprocity failure — a film (and to a lesser extent digital sensor) characteristic where very long exposures require additional time beyond what the math predicts. A factor of 1.0 means no compensation is needed; film shooters often use values of 1.2–2.0 for exposures beyond 1 second. Stacking filters multiplies convenience but can introduce color casts, so high-quality filters and post-processing corrections are recommended.
How to use
Your metered base shutter speed is 1/125 s (0.008 s). You attach a 10-stop ND filter and a 3-stop ND filter (stackedFilters = 3) and estimate no reciprocity adjustment (reciprocityFactor = 1). Total stops = 10 + 3 = 13. exposureTime = 0.008 × 2^13 × 1 = 0.008 × 8192 = 65.5 seconds. Set your camera to bulb mode and expose for approximately 65–66 seconds. If using film, apply your reciprocity failure factor — for example, reciprocityFactor = 1.3 gives 0.008 × 8192 × 1.3 ≈ 85 seconds.
Frequently asked questions
How do I calculate exposure time when stacking two ND filters?
When you stack two ND filters, their stop values add together. An ND 10-stop filter combined with an ND 3-stop filter gives a total of 13 stops of light reduction. You then multiply your base (unfiltered) shutter speed by 2 raised to the power of the total stops. So a base speed of 1/125 s becomes approximately 65 seconds with 13 total stops. Always measure your base exposure without the filters in place first, then apply the formula.
What is reciprocity failure and how does it affect long exposures?
Reciprocity failure is the phenomenon where film (and some sensors) do not respond linearly to light at very long exposures. At exposures beyond 1 second, you may need to increase the calculated time by a factor of 1.2 to 2.0 or more to achieve the correct brightness. Each film emulsion has a unique reciprocity curve, and manufacturers publish compensation tables. Digital sensors exhibit a much milder form of this issue, but noise accumulation at long exposures can produce similar underexposure-like effects in shadows. Using the reciprocityFactor field lets you bake this correction directly into the calculated exposure.
What ND filter strength should I use for silky waterfall photos?
For smooth, silky water effects you typically need an exposure of at least 0.5 to 2 seconds. In bright daylight at ISO 100 and f/11, your base exposure might be around 1/250 s, so you need roughly 7–8 stops of ND to reach 1 second (2^7 = 128, and 1/250 × 128 ≈ 0.5 s). A 10-stop ND filter is the most popular choice for waterfall work as it gives you flexibility across a range of lighting conditions. In overcast light, a 6-stop filter may be sufficient without the color cast risks that come with very dense glass.