An ND filter (neutral density filter) is a piece of optical glass or resin placed in front of the lens that reduces the amount of light reaching the sensor without affecting color. Properly made ND filters are spectrally neutral across the visible range, so the only effect on the image is darker exposure. They give the photographer access to slower shutter speeds or wider apertures in conditions that would otherwise force a fast shutter or a small f-stop.
NDs are rated in stops of light reduction or in optical density. A 3-stop ND is also labeled ND8 (transmits 1/8 of the light) or density 0.9. A 6-stop is ND64 or 1.8. A 10-stop is ND1000 or 3.0. A 15-stop is ND32000 or 4.5. The mapping is logarithmic: each additional stop halves transmission and adds 0.3 to the optical density value. Manufacturers do not always agree on labels, so the actual measured strength of a filter can vary by a third of a stop or more between brands.
Three primary uses dominate. First, long exposures in daylight: a 10-stop ND turns a 1/30 second base exposure into a 30-second exposure, smoothing water and clouds into the dreamlike blurs that define modern fine-art landscape work. Second, wide apertures in bright sun for shallow depth of field: a 3- or 6-stop ND lets a portrait shooter open up to f/1.4 at midday without exceeding the camera’s maximum shutter speed or invoking electronic shutter rolling artifacts. Third, matching shutter angle in video: cinematographers maintain a 1/48 or 1/50 second shutter for a 180-degree shutter angle at 24p, and ND is the only way to hit that in daylight.
Two physical formats exist. Screw-in circular NDs thread directly onto the front filter ring and stack cleanly. Square or rectangular drop-in NDs slide into a holder mounted on the lens and can be combined with graduated NDs that darken only part of the frame, useful for balancing sky and foreground. Variable NDs use two stacked polarizers; rotating the front element changes the angle between them and varies density continuously, typically across a 2- to 8-stop range, though they can introduce color casts and an X-shaped pattern at the extremes.
Quality varies enormously. Cheap NDs introduce a magenta, green, or warm color cast that requires correction in post and can also reduce lens sharpness. Premium brands like B+W, Hoya, Lee, Nisi, Breakthrough Photography, and Haida produce filters with neutral color and high optical quality, at correspondingly higher prices. Multi-coating reduces flare and reflections, important when shooting toward the sun or with bright point lights in frame.
Working with strong NDs requires technique adjustments. With a 10-stop on the lens, the viewfinder image is too dark to compose; the photographer composes and focuses without the filter, then mounts it and triggers an exposure with a cable release or app. Autofocus often fails through a 10-stop, so manual focus set in advance is standard. Calculating exposure manually or with a phone app (a 1/250 second metered scene at base ISO becomes a 4-second exposure with a 10-stop ND) is part of the workflow.