How an ND filter changes the exposure time
A neutral density filter lets through only part of the light. Every stop halves the light, so the exposure time doubles to keep the brightness. A filter of n stops multiplies the exposure time by 2 to the power of n.
t with filter = t without filter × 2^stops
ND numbers, stops and optical density
Manufacturers describe the same strength in different ways. Some names are rounded: ND1000 is sold as a 10-stop filter (factor 1024), NiSi lists a 9-stop filter as ND500 and a 15-stop filter as ND32000, and Kase calls a 12-stop filter ND4000. Check the stops on the packaging when a name is unclear.
- Stops: every stop halves the light. 6 stops is a factor of 2^6 = 64.
- ND number: the factor by which the light is reduced. ND64 is 6 stops, ND8 is 3 stops.
- Optical density: 0.3 per stop. 1.8 is 6 stops, 3.0 is 10 stops.
Example calculations
At 1/30 s without a filter, a 10-stop filter (ND1000) gives 1/30 s × 1024, about 34 s. A 6-stop filter (ND64) turns 1 s into 64 s, which is 1 min 4 s. Filters add up: ND8 (3 stops) and ND1000 (10 stops) make 13 stops, a factor of 8192.
Limits
The strength printed on a filter is nominal, and a real filter can deviate by a fraction of a stop. A test shot confirms the result. The calculator only applies the stops; it does not correct for anything else that changes with very long exposures.
FAQ
How do I calculate the exposure time with an ND filter?
Multiply the exposure time without a filter by 2 for every stop of the filter. An ND64 filter (6 stops) turns 1 s into 64 s, an ND1000 filter (10 stops) turns 1/30 s into about 34 s.
How many stops is an ND1000 filter?
An ND1000 filter is a 10-stop filter with an optical density of 3.0. Strictly, 10 stops is a factor of 1024, which the name rounds to 1000.
What does ND64 mean in stops and density?
ND64 reduces the light by a factor of 64. That is 6 stops and an optical density of 1.8.
Can I combine several ND filters?
Yes. The stops add up and the factors multiply: ND8 (3 stops) and ND64 (6 stops) make 9 stops, a factor of 512. Enter up to three filters in the calculator.
Why does my camera stop at 30 seconds?
Many cameras offer fixed exposure times up to 30 s. Longer times are set in Bulb mode, where the shutter stays open while the release is held or until it is pressed a second time. The countdown timer on this page helps with the timing.
Do I change the aperture or ISO when I attach the filter?
Not necessarily. The filter only reduces the light, and the longer exposure time makes up for it, while aperture and ISO keep their values. If the exposure time would be too long, the exposure triangle calculator shows how an aperture or ISO change offsets it.