ND filter exposure calculator
Meter the scene without the filter, enter the shutter speed and the strength of your ND filter, and get the longer exposure time that keeps the same brightness. Useful for silky water and long daytime exposures.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Set your camera to the exposure you want without the filter and note the shutter speed.
- Enter it in seconds (1/125 s is 0.008) and the strength of the filter, as a factor or in stops.
- Set the new shutter speed shown. Keep the aperture and ISO the same.
How it works
An ND filter of factor F removes log₂(F) stops of light. Each stop halves the light, so the exposure time must double per stop:
new time = metered time × 2^stops = metered time × F
Common filters: ND2 = 1 stop, ND4 = 2, ND8 = 3, ND16 = 4, ND64 = 6, ND1000 ≈ 10 stops.
Worked example
Metered exposure 1/125 s (0.008 s) with an ND8 filter: 3 stops, so 0.008 × 2³ = 0.064 s, about 1/15 s. With an ND1000 filter and a base of 0.5 s the result is 500 s, or 8 minutes 20 seconds.
Tips
- Frame and focus before putting on a dark filter, or use live view with the filter off.
- For exposures over about 30 seconds many cameras need Bulb mode and a timer or remote.
- Stacking filters adds their stops: ND8 + ND64 is 3 + 6 = 9 stops.
Frequently asked questions
How many stops is ND8, ND64 or ND1000?
Stops = log₂ of the factor. ND8 is 3 stops, ND64 is 6 stops, ND1000 is about 10 stops (9.97). Each stop doubles the exposure time.
How do I enter a shutter speed like 1/125?
Divide 1 by the number: 1/125 s is 0.008 seconds, 1/60 is about 0.0167, and 2 seconds is 2.
Do I need to add extra time for reciprocity failure?
Digital sensors usually do not need it for exposures of a few minutes. Some films do, and so do a few very long exposures, so check your film's data sheet. This calculator gives the plain doubling-per-stop result.
Some ND filters are labelled in optical density. How do I convert?
Density 0.3 equals 1 stop, so factor = 10 to the power of the density: 0.9 is ND8 (3 stops), 3.0 is ND1000. Enter the factor or the stops.