Neutral Density Filters
Thrown together by David Tamés, August 28, 2023, revised February 10, 2026
When shooting at 24p or 30p with wide apertures outdoors, it becomes impossible to maintain both the correct shutter speed and a wide aperture. The solution to this problem is sunglasses for your camera in the form of Neutral Density (ND) filters. The core problem is that for natural-looking motion blur, you want your shutter angle at roughly 180°, which means your shutter speed should be approximately double your frame rate: 1/48 at 24p, and 1/60 at 30p. Outdoors in bright sunlight, even at ISO 100 or the base ISO of a camera like the Sony FX3 (ISO 800 in S-Log3) makes this problem even more acute.
You simply cannot open up to f/2.8 while maintaining that shutter speed without excessive overexposure. Stopping down to f/11 or f/16 to compensate defeats the purpose if you want shallow depth of field for foreground/background separation. ND filters solve this problem by reducing the amount of light entering the lens without affecting color, letting you maintain your desired aperture and shutter speed independently.
The Sony a7iii video kits comes with three ND filters: ND8 (3 Stop), ND64 (6 Stop), ND1000 (10 Stop). Be very careful unscrewing and screwing on filters onto the threads on the front of the lens. If you feel any resistance the filter and lens threads are cross threaded, unscrew (counter-clockwise) and try screwing on again to avoid cross-threading the filter threads.
The Sony FX3 video kits available from the CAMD Media Center come with a NiSi True Color Variable ND filter (1-5 ND) and an add-on 4 stop filter that snaps on when you need more than 5 stops of ND.
Fixed ND filters come in specific densities, typically measured in stops of light reduction. Common ratings include ND 0.3 (1 stop), ND 0.6 (2 stops), ND 0.9 (3 stops), ND 1.2 (4 stops), and ND 1.8 (6 stops). A good fixed ND is just a uniform piece of glass with no moving parts, so there is no risk of uneven density, color shift, or cross-polarization artifacts. The disadvantage is obvious: if lighting conditions change (clouds rolling in and out, moving between shade and sun), you have to physically swap filters, which takes time and interrupts shooting. You may choose to carry a set of two or three fixed NDs and stack them as needed, though stacking introduces the possibility of vignetting on wider focal lengths and additional glass surfaces that can cause flare.
Variable ND filters use two polarizing elements that rotate relative to each other. As you rotate the outer ring, the combined polarization progressively blocks more light, giving you a continuous range of density. Typically the range is something like 2 to 8 stops in a single filter. The appeal for run-and-gun documentary and micro-budget production is that you dial in your exposure on the fly without stopping to change the filters. However, there are real tradeoffs. At the extremes of rotation (particularly the darkest settings), variable NDs can produce an “X” pattern or cross-shaped artifact in the image caused by cross-polarization. This is especially visible against sky or bright uniform surfaces. Lower-quality variable NDs also introduce color shifts, typically a warm or magenta or green cast that you will then have to correct in the grade. Because they are polarizers, variable NDs will also affect reflections and sky rendering in ways that a fixed ND will not. Water surfaces, glass, and sky contrast will change as you rotate, which may or may not be desirable. Higher-end variable NDs like NiSi or PolarPro minimize these issues, however, they do not eliminate them completely.
For controlled narrative work where you have full control of the lighting, fixed NDs are preferred and will provide you with cleaner, more predictable results. For cinéma-vérité, or any situation where lighting conditions change rapidly and you do not want to stop to swap filters, a quality variable ND is the practical choice.
| Optical Density | stops | ND Number | Light Reduction | Combination |
|---|---|---|---|---|
| 0.3 ND | 1 | ND101 | 1/2 | (1) |
| 0.6 ND | 2 | ND102 | 1/4 | (2) |
| 0.9 ND | 3 | ND103 | 1/8 | (3) |
| 1.2 ND | 4 | ND104 | 1/16 | (3+1) |
| 1.5 ND | 5 | ND105 | 1/32 | (3+2) |
| 1.8 ND | 6 | ND106 | 1/64 | (6) |
| 2.1 ND | 7 | ND107 | 1/128 | (6+1) |
| 2.4 ND | 8 | ND108 | 1/256 | (6+2) |
| 2.7 ND | 9 | ND109 | 1/512 | (6+3) |
| 3.0 ND | 10 | ND110 | 1/1024 | (10) a.k.a. ND1000 |
Sometimes filters are labeled with the optical density, which for this set would be 0.9, 1.5, and 3, respectively. Optical Density is a measure of the transmittance through an optical medium. Optical density equals the log to the base 10 of the reciprocal of the transmittance.
Common filter sizes used in matte boxes: 4” x 4” 4” x 5.65”
In order to reduce the number of filters in a kit, you only need four filters, 1, 2, 3, 6, 10, in order to be able to choose any ND value from 1 through 10 and beyond. In fact, if you’ll never need to go over ND 12, you only need ND 1, 2, 3, 6 in your kit, assuming you don’t mind stacking filters. Most matte boxes allow for two filters.
For more information see A Guide to Neutral Density Filters (B&H Explora)
Image: ND Filter Demo by Robert Emperley (CC BY-SA 2.0).