A neutral density (ND) filter is a piece of tinted glass or resin that reduces the amount of light entering a lens without changing color. It exists to solve one specific problem: bright light forces a fast shutter speed or a small aperture, and both of those choices have side effects a shooter doesn’t always want. An ND filter buys back control by cutting light so exposure settings can be chosen for their look rather than forced by the sun.
ND filters come in two main types — fixed and variable — and the choice between them affects workflow more than image quality.
Why ND Filters Matter for Video
Video has a rule photography doesn’t: the 180-degree shutter guideline, which sets shutter speed at roughly double the frame rate for natural-looking motion blur — 1/50s at 24fps, for example. That shutter speed is fixed by the look a shooter wants, not by available light, which means on a bright day the only way to control exposure without blowing out highlights is to close the aperture drastically or add an ND filter. Stopping down the aperture too far introduces diffraction softness and gives up control over depth of field; an ND filter lets the shutter speed and aperture stay where they’re wanted while the ND does the work of cutting light.
Fixed ND Filters
A fixed ND filter has one constant density, expressed either in stops (ND2, ND4, ND8…) or as an optical density number (0.3, 0.6, 0.9…). Each step doubles the light reduction: ND2 cuts one stop, ND4 cuts two stops, ND8 cuts three stops, and so on.
| Filter | Stops of Light Reduction | Common Use |
|---|---|---|
| ND2 | 1 stop | Slight overexposure correction |
| ND4 | 2 stops | Bright overcast conditions |
| ND8 | 3 stops | Sunny conditions, wide aperture work |
| ND64 | 6 stops | Full sun, very wide apertures or long exposures |
| ND1000 | 10 stops | Long-exposure photography, extreme brightness |
Fixed NDs are optically simpler than variable NDs, which generally means less risk of image degradation, but matching the right density to changing light means carrying several filters or swapping them out as conditions change — a real slowdown when the sun goes in and out of clouds mid-shoot.
Variable ND Filters
A variable ND is actually two polarizing filters stacked and rotated against each other; twisting the outer ring changes how much light passes through, giving a continuous range (commonly 2–8 stops or wider) from one physical filter. That’s the entire appeal: one filter covers a whole range of lighting conditions, adjusted on the fly without stopping to swap glass.
The trade-off shows up at the extremes. Twisted too far toward its maximum density, a variable ND commonly produces an “X” or bowtie-shaped dark patch across the frame, especially visible on wide-angle lenses — a side effect of how the two polarizing layers interact with the lens’s field of view. Cheaper variable NDs are also more prone to a slight color cast and softness compared to a good fixed ND, though higher-end variable filters have mostly closed that gap.
Fixed vs. Variable: Which to Buy
| Fixed ND | Variable ND | |
|---|---|---|
| Convenience | Requires swapping filters as light changes | One filter adjusts on the fly |
| Image quality | Generally cleaner, less prone to artifacts | Can show an X-pattern or softness at extreme settings |
| Cost for full range | Higher — needs a set of several filters | Lower — one filter covers a wide range |
| Best for | Controlled shoots, static lighting conditions | Run-and-gun, changing light, solo shooters |
Working DPs on controlled sets often carry a matte box with swappable fixed NDs for consistent, artifact-free results shot to shot. Solo shooters, documentary crews, and anyone working in fast-changing light tend to prefer a single good variable ND for the speed, accepting the small risk of artifacts at extreme settings.
A Note on Polarizers
A circular polarizer is a different tool that’s sometimes confused with ND filters: it cuts reflections and glare and can deepen a blue sky, and it does reduce light slightly (roughly 1–2 stops) as a side effect, but that’s not its purpose. Some filters combine ND and polarizing effects in one piece of glass; it’s worth checking which effect a filter is actually built for before assuming it does both jobs well.
Conclusion
ND filters exist to separate exposure control from creative shutter and aperture choices, which is why they’re standard kit for anyone shooting video outdoors. Fixed filters trade convenience for consistency; variable filters trade a small risk of artifacts for the speed of a single adjustable piece of glass. Most working kits eventually include both.