How to use it
- Type the Exposure without the filter (seconds) that your meter gives, as a decimal.
- Choose the Neutral-density filter you have, such as ND8.
- Read the Exposure time with the filter. Above 30 seconds it is written as minutes:seconds.
- Use the table to compare the filters by their stops, their density and the time they need.
How the exposure time is worked out
An ND filter cuts the light by a fixed factor. The Neutral-density filter article on Wikipedia describes the filter by its optical density d: the share of light that passes is 10 to the power of −d. The factor by which the light is cut is therefore 10 to the power of d, and d is the base-10 logarithm of the factor.
The same light needs the same exposure, so the time is multiplied by the factor. Stops are the base-2 logarithm of the factor, and each stop is one doubling of the time. ND1000 is a density of 3.0 and 9.97 stops, which the article rounds to about 10 stops.
Exposure with the filter = exposure without the filter × factor
Optical density d = log10(factor)
Stops = log2(factor) = d × 3.32
Light that gets through = 100 ÷ factor, in per centCodes and brands
The Neutral-density filter article notes that the same filter can carry different codes: Hoya, B+W and Cokin print ND2 or ND2x, while Lee and Tiffen print 0.3ND. The density in the code is the number to compare, so 0.3ND and ND2 are the same filter.
What the calculator does not cover
- Long exposures and film or sensor response (reciprocity): the calculator multiplies the time and does not correct for it. Use the figures in your camera or film manual for very long exposures.
- Star movement in a night photo: for the longest exposure before stars trail, use the Star trail rule calculator.
Worked example: a 2-second exposure through ND8, ND64 and ND1000
A 2-second exposure through ND1000 is 2 × 1000 = 2,000 seconds, which is 33 minutes and 20 seconds (33:20). ND1000 has a density of 3.0 and cuts the light by 9.97 stops.
The same 2 seconds through ND8 is 16 seconds, and through ND64 it is 128 seconds, written 2:08. Each filter multiplies the time by its own factor.
| Filter | Stops | Density | Exposure time |
|---|---|---|---|
| ND8 | 3.00 | 0.90 | 16 s |
| ND64 | 6.00 | 1.81 | 2:08 |
| ND1000 | 9.97 | 3.00 | 33:20 |
Frequently asked questions
What does a neutral-density filter do?
A neutral-density filter reduces the light that reaches the lens, so you can use a longer exposure in bright light. The Neutral-density filter article describes each filter by its optical density or by its f-stop reduction.
What does ND1000 mean?
The number is the factor: ND1000 lets 1 part in 1000 of the light through, which is 0.1 %. Its optical density is 3.0, and it cuts the light by about 10 stops, exactly 9.97.
How are stops and optical density related?
The stops are the optical density multiplied by log2(10), which is about 3.32. A density of 3.0 is therefore 9.97 stops, and a density of 0.3 is one stop.
Why are long exposures written as minutes and seconds?
The calculator shows times over 30 seconds as minutes:seconds so that they are easy to read, for example 33:20. Above 30 seconds the camera usually needs bulb mode, which the calculator notes.
Do I multiply the exposure time by the filter's factor?
Yes. The exposure with the filter is the time without it times the factor, so an ND8 turns 1 second into 8 seconds, the same as three stops more exposure.
Why do filters carry different codes for the same strength?
The Neutral-density filter article notes that brands use different codes. Hoya, B+W and Cokin print ND2, while Lee and Tiffen print 0.3ND; both describe the same filter with a density of 0.3.
Sources
- Wikipedia: Neutral-density filter (optical density, factors, stops and the table of codes)
- Wikipedia: Exposure value (one EV is one stop: twice or half the light)
Figures and text last checked on Tuesday, November 10, 2026.