Find the shutter speed after adding an ND filter
This ND filter calculator tells you how long to expose once a neutral density filter is on the lens. Type the shutter speed you would use without the filter, then give the filter as an ND number such as ND8 or ND1000, as stops, or as an optical density like 0.9. You get the new shutter speed, the light transmitted and a full chart for your starting exposure.
It also works backwards. Enter the shutter speed you want, for example 30 seconds for silky water or 1/48 for 24 fps video, and it shows how many stops you need and which standard filter is closest. A built-in timer counts down a long exposure so you do not have to watch a clock. Everything runs in your browser.
How to use the ND filter calculator
- Set your aperture and ISO first, then enter the shutter speed without the filter that your camera meter suggests.
- Pick a common filter, or choose how you will type the strength: ND number, stops or optical density.
- Read the new shutter speed and the stats. Use the chart to compare other filters at a glance.
- To work backwards, type a target shutter speed. For video, enter the frame rate and press 180° to use the standard shutter angle.
- On the camera, set bulb or a manual time and press the timer button to count it down.
ND Filter Exposure Calculator features
Three ways to name a filter
ND number, stops or optical density, all converted for you.
Filter you need
Stops required for a target time and the nearest standard filter.
Chart and timer
ND2 to ND100000 at your starting exposure, plus a countdown for long exposures.
The maths: stops, ND number and density
An ND filter transmits a fixed fraction of the light, T = 10−d, where d is the optical density. One stop halves the light, so the strength in stops is log₂(1 ÷ T) = d ÷ log₁₀2, about d ÷ 0.301. The filter factor, or ND number, is 1 ÷ T. The shutter time must grow by the same factor to keep the picture as bright: t′ = t × 2stops = t × ND number.
So ND8 is a factor of 8, three stops and density 0.9, and turns 1/125 s into 8/125, about 1/16 s. ND1000 is marketed as ten stops but is exactly log₂1000 = 9.97 stops, while ND1024 is ten. The difference is a few hundredths of a stop, well inside normal error. Filters stack: add the stops, or multiply the ND numbers. The table follows the naming on Wikipedia’s “Neutral-density filter” page.
Video and the 180 degree rule
Film-style motion blur comes from a shutter open for half of each frame, so the shutter speed is 1 ÷ (2 × frame rate): 1/48 s at 24 fps, 1/60 s at 30 fps. In bright light with a wide aperture the camera would need a much faster shutter, so an ND filter lets you keep the shutter angle and the aperture. If the target needs 3.4 stops, a 3-stop filter leaves you slightly over-exposed and a 4-stop filter slightly under; adjust ISO or aperture a third of a stop to land it.
Limits
Metering through a dark filter can fail, so meter first without it. Exposures longer than a few seconds suffer reciprocity failure on film and extra noise on sensors, so add a little time beyond the calculated value. Cheap filters can add a colour cast and many ND filters pass infrared light, which can leak into the picture; check a test frame. Real filters vary by a fraction of a stop from their marking.
ND Filter Exposure Calculator FAQ
How do I calculate the shutter speed with an ND filter?
Multiply the unfiltered shutter time by the ND number, or by 2 raised to the number of stops. ND64 on 1/60 s gives about one second.
How many stops is ND1000?
About 10 stops (exactly 9.97). ND8 is 3 stops, ND64 is 6 stops and ND1000 is 10 stops.
What ND filter do I need for video?
Set the 180° shutter (1 ÷ 2 × fps), find your exposure at that shutter speed without a filter, and the stops needed is log₂ of the ratio. Enter the target shutter above to see it.
What is the difference between ND 0.9 and ND8?
None. 0.9 is the optical density and 8 is the filter factor. Both are three stops.
Do I need to change ISO or aperture?
No. The filter replaces them: keep aperture and ISO, and slow the shutter speed by the filter factor.
Where do I start with the exposure itself?
Use the exposure value calculator to get the base exposure, then add the filter here.