Metro Wilanowska
An ND filter is one of the simplest tools in a camera bag, and one of the most often misunderstood. Our customers regularly ask us what the difference is between ND8 and ND64, why one filter says “ND 0.9” while another says “ND8”, and which one to take on a sunny-day shoot. In this article we explain how neutral density filters work, what their markings mean and how to choose the right one for the conditions on set.
What is an ND filter?
ND stands for neutral density. An ND filter is simply a piece of darkened glass that evenly reduces the amount of light entering the lens. “Neutral” means a good filter should not change colours or contrast, only let less light reach the sensor. Think of it as sunglasses for your camera.
Why use an ND filter for video?
The 180-degree rule and natural motion blur
Filmmaking follows a simple rule: your shutter speed should be roughly one over twice your frame rate. At 25 fps that is 1/50 s, at 50 fps it is 1/100 s. This gives you the natural motion blur audiences are used to from cinema. The problem comes in bright sun: to avoid overexposure, the camera would have to drop to 1/1000 s or faster. Motion then looks crisp and choppy, especially in fast pans and action camera footage. An ND filter lets you stay at 1/50 s even in full sun.
Shallow depth of field in bright light
The second reason is aperture. If you want an outdoor portrait with a creamy background at f/1.4 or f/2, on a sunny day without a filter you will simply blow out the image, even at base ISO while sticking to the 180-degree rule. An ND filter takes away the excess light, so you can shoot wide open and keep the same cinematic look in any conditions. In stills photography, ND filters, especially very strong ones, are also used for long exposures, for example to smooth out water or clouds in daylight.
What do ND8, ND16 and ND64 mean?
The number on the filter is a factor that tells you how many times it reduces the light. ND8 lets through 1/8 of the light, ND16 1/16, and ND64 just 1/64. In photography and filmmaking we measure exposure in stops (often loosely called f-stops or EV). Each stop doubles or halves the amount of light. That is why ND2 is 1 stop, ND4 is 2 stops (2 × 2) and ND8 is 3 stops (2 × 2 × 2 = 8). ND16 is 4 stops, ND32 is 5 and ND64 is 6 stops. Just check which power of two the number on the filter is.
Optical density: ND 0.3, ND 0.6, ND 0.9
On cinema filters, such as rectangular 4×5.65 filters for a matte box, you will see a different notation: optical density. Here every 0.3 equals 1 stop. ND 0.3 is 1 stop (equivalent to ND2), ND 0.6 is 2 stops (ND4), ND 0.9 is 3 stops (ND8) and ND 1.2 is 4 stops (ND16). The 0.3 is no coincidence: optical density is the base-10 logarithm of the attenuation factor, and log₁₀(2) ≈ 0.3. Both notations describe exactly the same thing, just on a different scale.
| Stops | Factor (ND) | Optical density | Light transmission |
|---|---|---|---|
| 1 | ND2 | 0.3 | 50% |
| 2 | ND4 | 0.6 | 25% |
| 3 | ND8 | 0.9 | 12.5% |
| 4 | ND16 | 1.2 | 6.25% |
| 5 | ND32 | 1.5 | approx. 3.1% |
| 6 | ND64 | 1.8 | approx. 1.6% |
| 7 | ND128 | 2.1 | approx. 0.8% |
| 8 | ND256 | 2.4 | approx. 0.4% |
| approx. 10 | ND1000 | 3.0 | 0.1% |
How does it work in practice?
Example 1: without a filter the camera needs 1/400 s. Each stop doubles the shutter time: 1/400 → 1/200 → 1/100 → 1/50. That is 3 stops, so with an ND8 you can shoot the same frame at 1/50 s with the same image brightness. Example 2: according to the classic “Sunny 16” rule, correct exposure in full sun at ISO 100 is around f/16 at 1/100 s. Want to shoot at f/2.8 and 1/50 s? Going from f/16 to f/2.8 is 5 stops, and from 1/100 to 1/50 s is 1 more stop. That is 6 stops in total, so an ND64 (ND 1.8).
Which ND filter for which conditions?
The exact value depends on your aperture, ISO and frame rate, but when shooting to the 180-degree rule you can use this as a rough guide:
- Overcast day, shade, early morning: ND4–ND8 (ND 0.6–0.9).
- Typical sunny day: ND16–ND32 (ND 1.2–1.5).
- Harsh sun, snow, water, beach: ND64 or stronger (ND 1.8+).
- Wide-open aperture (f/1.4–f/2) in full sun: often 7–8 stops or more, e.g. ND64 + ND4 or a strong variable ND.
- Long-exposure stills (smooth water, streaking clouds): ND1000 (around 10 stops) and stronger.
Action cameras and smartphones usually have a fixed aperture, so an ND filter is practically the only way to get a slower shutter speed in bright sun.
Types of ND filters
Fixed ND filters
This is the simplest and usually the optically best option: one filter, one value, e.g. ND8. Fixed NDs give the most predictable colour and the most even darkening. The downside is that you have to swap filters when the light changes. They come as round screw-in filters, rectangular 4×5.65 or 4×4 filters for a matte box, and as dedicated clip-on filters for action cameras.
Variable ND filters (VND)
A variable ND is made of two polarising layers. By turning the ring you smoothly adjust the density, for example from 2 to 8 stops, without taking the filter off. That is a huge advantage in documentary and run-and-gun work, where the light changes constantly. Watch out at the extremes, though: at maximum density a dark cross can appear in the frame (the so-called X-pattern), and on wide lenses the sky may darken unevenly. Better filters have hard stops that keep you out of that range.
Soft ND (graduated) filters
A soft ND darkens only the top part of the frame, with a gentle transition into the clear part. It is used when the sky is much brighter than the foreground, for example in landscapes or at sunset. Instead of a blown-out sky you get detail in the clouds and a well-exposed foreground. They are easiest to use in a matte box, where you can slide the filter up and down.
ND/PL filters and built-in ND
An ND/PL filter combines neutral density with polarisation, so it also cuts reflections from water and glass and deepens the blue of the sky. Some cinema cameras have ND built into the body, for example the Sony FX6 with its electronic variable ND. That is very convenient, but mirrorless cameras, action cameras and smartphones still need an external filter.
Things to watch out for
- Infrared contamination: strong ND filters (from around 6 stops) cut visible light more than infrared. Blacks, especially in synthetic fabrics, can then turn brown or maroon. IRND filters solve this.
- Colour cast: cheap filters can add a green or magenta tint. Set your white balance with the filter already on.
- Stacking filters: every extra piece of glass adds a risk of flare and vignetting, especially on wide lenses.
- Lenses with a bulbous front element: some wide-angle lenses cannot take a screw-in filter. That is where a matte box and rectangular filters come in.
- Autofocus and monitoring: with very strong filters, autofocus may slow down and the viewfinder or screen image gets darker.
Rent ND filters at CamCam
At our rental house you will find a wide range of filters: Tiffen, NiSi and SmallRig variable NDs in various diameters, rectangular 4×5.65 ND 0.3–1.2 filters for matte boxes, soft ND filters and polarisers, as well as ND kits for DJI and GoPro action cameras. We also stock matte boxes, including the Tilta Mirage kit with a VND filter. If you are not sure which filter suits your project, get in touch: we work on film sets ourselves and are happy to advise. You can also book a free test session and try the filters on your own camera.
