ND Filter Exposure Calculator
Calculate the correct long-exposure shutter speed when using one or more stacked ND filters. Essential for landscape and waterfall photographers who need to adjust for light reduction and film reciprocity failure.
About this calculator
Neutral density filters reduce the amount of light reaching the sensor by a fixed number of stops. Each stop doubles the required exposure time, so the total new exposure is calculated as: newExposure = baseShutterSpeed × 2^(ndStrength × stackedFilters) × reciprocityFailure. The ndStrength value represents the filter's stop rating (e.g., a 10-stop ND filter has ndStrength = 10). When stacking multiple filters, their stop values add together, multiplying the exposure requirement exponentially. Reciprocity failure is a correction factor applied to film photography (and sometimes digital sensors in extreme exposures) where the sensor or film responds less efficiently to very long exposures, requiring additional time beyond the theoretical value.
How to use
Suppose your base shutter speed without filters is 1/125 s (0.008 s), you are using a single 10-stop ND filter (ndStrength = 10, stackedFilters = 1), and reciprocity failure = 1.0 (no correction needed). Step 1: Calculate the stop multiplier: 2^(10 × 1) = 2^10 = 1,024. Step 2: Multiply by base shutter speed: 0.008 × 1,024 = 8.19 seconds. Step 3: Multiply by reciprocity failure: 8.19 × 1.0 = 8.19 s. Set your camera to approximately 8 seconds to achieve correct exposure.
Frequently asked questions
How do I calculate the correct shutter speed when stacking two ND filters?
When stacking filters, you add their stop values together and use that combined number as the exponent. For example, stacking a 6-stop and a 10-stop filter gives 16 total stops. The formula becomes: newExposure = baseShutterSpeed × 2^(ndStrength × stackedFilters). Make sure ndStrength represents the per-filter stop value and stackedFilters is the count of identical filters, or adjust accordingly if filters have different ratings. Stacking can introduce color cast, so test before committing to a long exposure.
What is reciprocity failure and when do I need to account for it in ND filter calculations?
Reciprocity failure (also called the Schwarzschild effect) occurs when film — or in extreme cases digital sensors — does not respond linearly to very long exposures. At exposures beyond roughly 1 second on film, you must expose longer than the theoretical value to achieve correct density. The reciprocityFailure factor (e.g., 1.2 or 1.5) is multiplied by the theoretical exposure to compensate. For most modern digital cameras, reciprocity failure is negligible and you can set this factor to 1.0. It is most relevant to large-format and medium-format film photographers.
Why does each ND stop double the required exposure time?
Each stop in photography represents a halving or doubling of light. An ND filter rated at N stops blocks 2^N times the light that would otherwise reach the sensor. To maintain the same exposure value (brightness), you must compensate by letting light in for 2^N times longer. This is why the formula uses 2 raised to the power of the stop count. A 3-stop ND requires 8× longer exposure, a 6-stop requires 64×, and a 10-stop requires 1,024× longer — which turns a 1/100 s handheld shot into a roughly 10-second long exposure.