An ND filter reduces light so you can use a longer exposure or another desired setting. Choose strength from the exposure change you actually need, not from the most extreme option on the shelf.
What to do
Establish a usable exposure without the filter. Calculate the intended stop change, fit a compatible filter and verify the new result. Check focus, color and any corner shading.
Begin by deciding the effect you want, such as smooth water, blurred clouds or a wide aperture in bright sun. Then work out how much slower the shutter has to be to produce it.
Converting strength to stops
Filter strength is labeled in different ways, so learn the language of your filters:
- Stops: each stop halves the light, so three stops multiplies the exposure time by eight.
- Density numbers: some filters use a density value, while others use a factor. Check the maker’s chart to convert them to stops.
- Light conditions: bright sun may call for a stronger filter than an overcast morning.
- Variable filters: a rotating filter offers a range, but can show uneven darkening at the extremes, so check the frame.
- Stacking: combining filters adds their strengths, but also adds the chance of color cast and vignetting.
Pick a strength that gets you the effect with a shutter speed you can still work with, and keep a weaker filter in the bag for variety.
A mistake to avoid
A stronger filter is not automatically more useful. It can make focusing, exposure calculation and moving-subject control harder. In dim conditions, a very strong filter makes the viewfinder nearly black.
A detail worth checking
Account for the filter holder or stacked filters in a wide-angle setup. Corner shading can come from the physical arrangement rather than exposure strength, so inspect the edges before committing to a long sequence.
Try this practical check
Moving from 1/125 second by three stops leads to approximately 1/15 second. Treat that as a calculation example, then test the actual filter and camera combination. Focus and compose before fitting the filter when the screen gets too dark.
Common questions
Will an ND filter change the color of my photos?
Some add a slight tint, particularly strong ones. Check a test shot and correct white balance if needed.
Do I need a tripod?
For long exposures, yes or another firm support, since any camera movement shows up as blur.
Can I fake ND with editing?
Not for the long exposure itself. Software can blend frames to imitate it, but a filter records the real motion.
Related reading
UV, CPL and ND Filters: What Each Does (2026)
