Choosing an ND Filter and Calculating Exposure
How ND numbers relate to stops, the formula for the new exposure time, and rough targets for common long-exposure effects. No product names or prices.
What is an ND filter?
A neutral density (ND) filter is a gray filter that reduces the amount of light entering the lens without shifting its color. Cutting the light lets you use a slower shutter speed or a wider aperture even in bright scenes, which is how long exposures that blur moving water or clouds are made, and it also gives more control over depth of field outdoors.
How ND number relates to stops
An ND number represents the inverse of how much light passes through, and it relates to stops through ND number = 2 to the power of the stop count. Each stop halves the amount of light: 1 stop cuts it in half, 2 stops to a quarter, 3 stops to an eighth. ND8, for example, is 2 to the 3rd power, or 8, so it's a 3-stop filter; ND64 is 2 to the 6th, or 64, a 6-stop filter. ND1000 is a rounded label for 2 to the 10th power, or 1024, roughly a 10-stop filter.
How to calculate the new exposure time
With an ND filter attached, the new shutter speed equals the original shutter speed times 2 to the power of the stop count. For example, starting from 1/125 second and adding a 10-stop filter (an ND1000-equivalent), the new exposure is 1/125 x 2 to the 10th, or 1/125 x 1024, which comes out to roughly 8.2 seconds. Recalculate this multiplication whenever the stop count changes.
Rough exposure-time targets for common effects
Smoothing moving water in a river or waterfall into a silky look typically uses exposures from a fraction of a second up to a few seconds. Streaking clouds or a moving sea into soft blur usually needs tens of seconds to a few minutes, which in bright daylight calls for a stronger stop count to get the shutter that slow. Both effects depend heavily on the subject and the available light, so test shots and adjustment are part of the normal process.
Things to watch for when using ND filters
A very dark ND filter can make autofocus hunt or fail, so it helps to focus first and switch to manual focus before attaching the filter. Long exposures are sensitive to even small vibrations, so pairing a tripod with a remote release or self-timer is standard practice. Stacking multiple filters can also introduce vignetting — darkened corners — especially on wide-angle lenses.
Variable ND vs. fixed ND
A variable ND filter lets you dial in a continuous range of stops from a single piece of glass, which is convenient, but pushing it too far toward its darkest setting can introduce an uneven X-shaped pattern across the frame. A fixed ND filter needs one filter per stop count, but it doesn't have that unevenness issue and tends to behave more predictably on wide-angle lenses.
Frequently asked questions
How many stops is an ND1000 filter?
It's a rounded label for 2 to the 10th power, or 1024, so roughly 10 stops.
How do I calculate the new shutter speed with an ND filter on?
Multiply the original shutter speed by 2 to the power of the stop count: 1024x for 10 stops, 64x for 6 stops.
Why does autofocus struggle with an ND filter attached?
The filter cuts the light reaching the autofocus sensor by a large amount. Focusing before attaching the filter and switching to manual focus avoids the problem.