How to Calculate Long Exposures with ND Filters
Neutral Density (ND) filters are essential tools in landscape and architectural photography. They reduce the amount of light entering the lens, allowing you to use dramatically slower shutter speeds to capture the passage of time—turning rough oceans into mist and moving clouds into streaks.
The Long Exposure Formula
Every time you add 1 stop of ND filtering, you cut the light in half, meaning your shutter speed needs to be doubled. The formula is: Final Shutter Speed = Base Shutter Speed × 2^(ND Stops).
Because cameras automatically measure light poorly when heavy filters (like a 10-stop ND1000) are attached, the best practice is to:
- Focus and meter the scene without the filter to get your base shutter speed.
- Switch your lens to Manual Focus so it doesn't hunt in the dark.
- Attach the ND filter and use this calculator to find the correct new exposure time.
- Switch your camera to Bulb (B) mode if the calculated time exceeds 30 seconds.
Common ND Filter Naming Conventions
| Stops of Light Reduction | ND Factor Number | Optical Density (OD) | Common Usage |
|---|---|---|---|
| 3 Stops | ND8 | 0.9 | Waterfalls and flowing rivers |
| 6 Stops | ND64 | 1.8 | Golden hour seascapes, traffic trails |
| 10 Stops | ND1000 | 3.0 | Daylight long exposures, removing crowds, smoothing clouds |