An ND filter (neutral density filter) is a lens attachment that reduces the amount of light entering a camera without changing the color of the image. Think of it like sunglasses for your lens. It blocks light evenly across the entire visible spectrum, so colors stay accurate while the exposure drops by a predictable amount. Photographers and videographers use ND filters to shoot with slower shutter speeds or wider apertures in bright conditions that would otherwise blow out the image.
How ND Filters Work
Every ND filter does the same basic job: it sits between the scene and your camera sensor, cutting down the light that passes through. The “neutral” part is key. Unlike a colored filter, which blocks specific wavelengths, an ND filter reduces all wavelengths equally. Your reds, greens, and blues all dim by the same amount, so the image looks natural, just darker.
The two main construction methods are reflective and absorptive. Reflective filters use thin metallic coatings on the glass surface to bounce light away before it enters the lens. Absorptive filters change the composition of the glass itself so the material soaks up light as it passes through. Both achieve the same result, but reflective designs sometimes create unwanted reflections between filter surfaces, while absorptive designs handle stray light more predictably.
Why You’d Use One
The most common reason is creative control over shutter speed and aperture in bright light. Say you want to photograph a waterfall with that smooth, silky look to the water. That effect requires a slow shutter speed, maybe half a second or longer. On a sunny day, even at your lowest ISO and smallest aperture, half a second would massively overexpose the shot. An ND filter solves this by cutting enough light that a long exposure becomes possible without blowing out the image.
For video, ND filters are nearly essential. Most filmmakers keep their shutter speed at double the frame rate (the “180-degree shutter rule”) to get natural-looking motion blur. At 24 frames per second, that means a shutter speed of 1/50. On a bright day, 1/50 lets in far too much light, especially if you also want a wide aperture for shallow depth of field. Without an ND filter, you’d be forced to either stop down to a tiny aperture or increase the shutter speed, both of which change the look of your footage in ways you may not want.
Types of ND Filters
Fixed ND Filters
A fixed ND filter blocks one specific amount of light. You buy a 3-stop, a 6-stop, or a 10-stop filter, and that’s what you get. The glass has a single uniform coating, and the light reduction never changes. These tend to produce the cleanest image quality because the construction is simple: one piece of coated glass or resin with nothing moving inside.
Fixed filters work best when you have time to set up your shot and the lighting isn’t changing. Landscape photographers gravitate toward them for exactly this reason. If you’re shooting a single scene at golden hour and the light is consistent, a fixed filter gives you the sharpest, most color-accurate result. The tradeoff is flexibility. If conditions change, you need to physically swap filters.
Variable ND Filters
Variable ND filters let you dial in a range of light reduction by rotating a ring on the filter. They work by stacking two polarizing layers on top of each other. As you rotate the outer layer, the combined polarization effect blocks more or less light. A single variable filter might cover anywhere from 1.5 to 5 stops, or 2 to 8 stops, depending on the model.
This adjustability makes them popular with videographers, travel shooters, and anyone working in rapidly changing light. Documentary filmmakers and run-and-gun video shooters rely on them because you can adjust exposure on the fly without swapping glass or interrupting a take. The downside is that variable filters are thicker than fixed ones, which can cause dark corners (vignetting) on wide-angle lenses.
Variable ND filters also have a known artifact called the “X pattern” or cross polarization. When you push the filter toward its maximum density, two intersecting dark bands can appear across the frame, forming an X shape from corner to corner. This happens because of how the two polarizing layers interact at extreme rotation angles. The problem gets worse with wider lenses: a 12mm lens might start showing the artifact at just 5 stops of reduction, while focal lengths past 35mm are rarely affected even at maximum density. Staying a stop or two below the filter’s rated maximum usually avoids it entirely.
Graduated ND Filters
Graduated ND filters are dark on one half and clear on the other, with a smooth or hard transition between the two zones. They’re designed for scenes where one part of the frame is much brighter than the other, like a bright sky over a darker landscape. The dark half tames the sky exposure while the clear half lets the foreground expose normally. These are rectangular filters that slide into a holder, so you can position the transition line exactly where the horizon falls in your composition.
Understanding ND Filter Numbers
ND filter labeling can be confusing because manufacturers use three different systems, sometimes on the same product. Here’s what they mean:
- Stops (e.g., 3-stop, 6-stop, 10-stop): The most intuitive system. Each stop halves the light. A 3-stop filter lets in 1/8 of the original light. A 10-stop filter lets in roughly 1/1000.
- ND factor (e.g., ND8, ND64, ND1000): The number tells you the light-division factor. ND8 means the light is divided by 8 (3 stops). ND1000 means divided by 1000 (about 10 stops).
- Optical density (e.g., 0.9, 1.8, 3.0): A logarithmic scale used in optics. An OD of 0.3 equals 1 stop. OD 0.9 equals 3 stops. OD 3.0 equals 10 stops.
You can also stack ND filters to combine their effect. If you put a 3-stop filter on top of a 6-stop filter, you get 9 stops of reduction. In optical density terms, you simply add the numbers: 0.9 + 1.8 = 2.7.
Glass vs. Resin Filters
ND filters are made from either optical glass or resin (a type of hardened plastic), and the material affects image quality more than most beginners expect.
Optical glass transmits light more accurately, producing better color reproduction and sharper images from edge to edge. Glass maintains a flat surface under normal handling, so the focal plane doesn’t shift when you attach the filter. You won’t need to refocus after adding the filter. Glass is also harder and more scratch-resistant than resin, and manufacturers can apply water-repellent and anti-scratch coatings that don’t adhere to resin surfaces. The main risk with glass is breakage: drop it on a rock and it can crack or shatter.
Resin filters are lighter, cheaper, and survive drops better since the material flexes instead of shattering. But resin is softer and scratches easily, and those scratches degrade image quality over time. Resin filters are also known for introducing color casts, typically blue, magenta, or yellow depending on the brand. The material can flex slightly under pressure or temperature changes, which shifts the focal plane and reduces sharpness, particularly at the edges of the frame.
Built-In ND Filters for Video Cameras
Some professional video cameras skip external filters entirely by building electronic variable ND systems directly into the camera body. Sony’s camcorder lineup, including models like the FX6 and FX9, features an internal electronic ND that adjusts seamlessly from 2 to 7 stops of reduction. The adjustment is stepless, meaning you can smoothly transition between densities mid-shot without visible jumps in exposure. This eliminates the X-pattern artifacts, vignetting, and color shifts that external variable ND filters can introduce.
Color Shifts and Infrared Issues
Not all ND filters are perfectly neutral. Cheaper filters often introduce a slight color cast that you’ll need to correct in editing. More importantly, many ND filters block visible light effectively but still allow infrared light to pass through. Your eyes can’t see infrared, but your camera sensor can. When visible light is heavily reduced by a strong ND filter (6 stops or more), the infrared light that sneaks through becomes a larger proportion of what hits the sensor. This creates unusual color casts, often a reddish or magenta tint, that are difficult to correct in post-production. Higher-end filters include infrared-blocking coatings specifically to prevent this problem, which is why a 10-stop filter from a reputable manufacturer costs significantly more than a budget option.