Neutral density filters cut the light entering the lens so you can use long shutter speeds in bright conditions, smoothing water, blurring clouds, and erasing crowds. This calculator converts your base exposure into the long-exposure time you need once the filter is fitted.
How it works
An ND filter blocks light by a fixed number of stops, and each stop doubles the exposure time:
New shutter time = base shutter time × 2^(total stops)
ND filters are usually marketed by an “ND number” where the number of stops is the base-2 logarithm of that number:
stops = log₂(ND number)
So ND2 is 1 stop, ND8 is 3 stops, ND1000 is about 10 stops, and ND100000 is roughly 16.6 stops. When you stack two filters, the stops simply add together.
Worked example
A scene meters at 1/250 second without a filter. Fit a 10-stop ND1000:
- Factor = 2¹⁰ = 1024
- New time = (1/250) × 1024 ≈ 4.1 seconds
Stack an extra 3-stop ND8 on top and you reach 13 stops, factor 8192, giving about 33 seconds — now you need Bulb mode.
Choosing the right ND strength
The filter you need depends on the base shutter speed and the target exposure time:
| Effect | Typical target exposure | Likely filter in daylight |
|---|---|---|
| Silky water (waterfall) | 0.5 – 2 seconds | 6 – 8 stop (ND64 – ND256) |
| Smooth river / surf | 3 – 10 seconds | 8 – 10 stop (ND256 – ND1000) |
| Blurred cloud motion | 30 – 60 seconds | 10 stop (ND1000) |
| Ghostly long exposure | 2 – 5 minutes | 10 – 13 stop (ND1000 stacked) |
| Empty street / crowds | 1 – 4 minutes | 10 – 13 stop |
These ranges assume a sunny-day base exposure around 1/200 – 1/500 s. On an overcast day or at dusk you start with a longer base and need fewer stops.
Common mistakes
Metering after fitting the filter. In-camera metering through a very dark filter can give inaccurate readings. Meter first without the filter, note the exposure, then calculate the new shutter time and switch to manual.
Forgetting Bulb mode limits. Most cameras cap the timed shutter at 30 seconds. The calculator will show the mathematically correct time, but you need to switch to Bulb and use a remote shutter release or intervalometer for anything longer.
Colour cast with cheap filters. A 10-stop filter from a budget brand introduces a noticeable colour cast (often magenta or blue). Plan to correct white balance in post, or shoot RAW. Premium filters (e.g. formulated glass) minimise the cast but cost more.
Tips and notes
- Compose, focus, and meter before fitting a strong ND, because 10-stop filters make the viewfinder nearly black. Some photographers focus, then tape the focus ring before fitting the filter.
- Stacking filters adds stops but can also add vignetting and a colour cast that you may need to correct in post. A combined 10+3 stop stack is often cleaner than stacking three smaller filters.
- For any result longer than 30 seconds, switch to Bulb mode and time the shutter with a release.
- All maths runs locally in your browser; nothing about your shoot is uploaded.