Long exposure calculator
Add an ND filter and keep the exposure right: pick your base shutter speed, dial in the filter in stops or by its ND number, and get the shutter speed you need. Or work backwards from a target time to the filter you should buy.
Base exposure
Type a fraction like 1/60 or a plain number of seconds like 30.
Filter strength
Picking a filter sets the stop slider. Choose “Custom stops” for fractional strengths.
in stops - 1 to 16, in half-stop steps.
Reverse (optional)
Result
How it works
An ND filter multiplies your shutter speed by its density factor. Expressed in stops, each stop doubles the time, which gives the simple exponential rule:
The calculator always starts from the shutter speed that gives the right exposure without the filter, then applies the chosen strength. Under 1 second the result is shown as a shutter fraction; at a second or more it is shown as a duration. If you work in reverse, the required stops come from stops = log₂(target ÷ base), and the nearest standard ND is suggested from the common range.
Common ND filters and their stops
| Filter | Stops (approx) | Example from 1/60 s |
|---|---|---|
| ND2 | 1 | 1/30 s |
| ND4 | 2 | 1/15 s |
| ND8 | 3 | 1/8 s |
| ND16 | 4 | 1/4 s |
| ND32 | 5 | 1/2 s |
| ND64 | 6 | 1 s |
| ND128 | 7 | 2 s |
| ND256 | 8 | 4 s |
| ND400 | ≈8.6 | ≈6 s |
| ND500 | ≈9 | ≈8.5 s |
| ND1000 | ≈10 | ≈17 s |
| ND2000 | ≈11 | ≈34 s |
| ND4000 | ≈12 | ≈1 min 8 s |
| ND64000 | ≈16 | ≈18 min |
A filter like ND1000 is dense enough that your camera meter and autofocus will struggle - compose and focus before you fit it, and check the histogram on the first frame.
Example
You meter a waterfall scene at 1/60 s with no filter. Adding a 10-stop filter (ND1000) gives 1/60 × 210 = 17.07 s - set the camera to 17 s (or the nearest available setting and re-check the histogram). Working backwards: to reach a silky 30 s from the same 1/60 s base you need a factor of about 1800×, which is roughly 10.8 stops - the nearest standard filter is an ND2000 (≈11 stops), giving about 33 s.
Related tools
This tool is the same maths as the ND filter calculator and the broader exposure triangle calculator. Planning a night sequence? See the star trail calculator.
Frequently asked questions
What does "stops" mean for an ND filter?
Each stop of an ND filter halves the light reaching the sensor. A 10-stop filter (roughly ND1000) lets through 1/1000 of the light, so a 1/60 s exposure becomes about 17 s. The relationship is exponential: new time = base time × 2 to the power of the stops.
Why is ND1000 called 10 stops when it is not exactly 2 to the power 10?
ND numbers are density factors, and 2^10 = 1024, not 1000. Manufacturers round for convenience, so ND1000 is really about 9.97 stops. In practice the difference is invisible - treat ND1000 as a 10-stop filter and use your histogram to fine-tune.
How do I meter with a heavy ND filter?
Compose and focus first, then fit the filter. Either meter without the filter and add the filter stops (what this calculator does), or use live view / a phone app through the filter. Autofocus and the viewfinder are often too dark to use once a strong ND is fitted.
What strength should I buy?
Start with a 6-stop (ND64) and a 10-stop (ND1000). A 6-stop is enough for silky water on most days; a 10-stop lets you shoot 15–30 s exposures in daylight and works well with the reciprocal maths above.