A neutral density (ND) filter is a piece of dark glass or resin that reduces the light entering the lens without changing its color — at least in theory. Less light means you need a longer shutter speed for the same exposure, and that lets you show time passing: water turns smooth, clouds stretch across the sky and moving crowds vanish. ND filters are also used to open up the aperture for shallow depth of field in bright sun, and to keep video at a natural motion blur.
How strong is my filter?
Manufacturers describe strength in three different ways, which causes a lot of confusion. They all describe the same thing:
| Stops | ND number (factor) | Optical density | Light transmitted |
|---|---|---|---|
| 1 | ND2 | 0.3 | 50.0% |
| 2 | ND4 | 0.6 | 25.0% |
| 3 | ND8 | 0.9 | 12.5% |
| 6 | ND64 | 1.8 | 1.56% |
| 10 | ND1000 | 3.0 | 0.098% |
| 15 | ND32000 | 4.5 | 0.0031% |
The stop count is what matters for exposure. The ND number is the factor by which light is reduced (2^stops, rounded: an "ND1000" is really 1,024×). Optical density adds 0.3 for every stop, so a 3.0 filter is 10 stops. When you stack two filters, their stops add: a 3-stop and a 6-stop together make 9 stops. The ND filter chart lists exact stops, density and exposure times for every filter from ND2 to ND100000.
Calculating the exposure
The rule is simple: new time = base time × 2^stops. Every stop doubles the exposure. Starting from 1/125 s:
Notice how quickly the numbers grow: three stops is barely noticeable, six stops reaches half a second, ten stops is several seconds and fifteen stops is over four minutes. Exposures longer than 30 seconds need the camera’s Bulb (B) mode and a remote release or the camera’s built-in Bulb timer. The ND filter calculator includes a countdown timer for exactly this.
Step-by-step: a daylight long exposure
- Set up on a solid tripod and turn off image stabilisation (it can introduce blur on a tripod with some lenses).
- Compose and focus without the filter. Through a 10-stop filter the viewfinder or screen is nearly black and autofocus struggles.
- Switch to manual focus so the camera does not hunt when you press the shutter.
- Meter in manual mode at your chosen aperture (f/8–f/11 for landscapes) and lowest ISO. Note the shutter speed that gives a good exposure, for example 1/60 s.
- Attach the filter carefully without knocking the focus ring.
- Calculate the new time: 1/60 s with 10 stops is about 17 s. Set it, or use Bulb if it is over 30 s.
- Cover the viewfinder on DSLRs, use a 2-second delay or remote, and take the shot.
- Check the histogram and adjust by a third or half a stop if needed — real filters are rarely exactly their nominal strength.
How long should the exposure be?
- Waterfalls and streams: 1/4 s to 2 s keeps texture with a silky look; longer turns the water into pure white mist.
- Waves on a beach: 0.5–2 s shows swirling motion lines; 30 s and more flattens the sea into fog.
- Clouds: 1–4 minutes for visible streaks, depending on wind speed. Shoot towards or away from the wind direction for radiating lines.
- Removing people: several minutes; anyone who keeps moving will not register. Fifteen stops is the usual choice in daylight.
- Video: a shutter of 1 ÷ (2 × frame rate) — 1/50 s at 25 fps — gives natural motion. In sun, that needs 3–6 stops of ND to keep a wide aperture.
Common problems and how to avoid them
Color casts
Many strong ND filters are not perfectly neutral and add a blue, magenta or warm tint. Shoot RAW and correct white balance afterwards; a custom white balance taken through the filter fixes it in camera. Cheaper filters vary more, and the cast often increases with strength.
Light leaks
Light entering through the optical viewfinder or gaps around a square filter holder shows up as hazy streaks or a purple glow during long exposures. Close the eyepiece shutter or cover it, and use the foam gasket of your holder.
Vignetting
Stacking several screw-in filters on a wide-angle lens can darken the corners. Use slim filters, a single stronger filter, or a square holder sized for the lens.
Noise and hot pixels
Long exposures heat the sensor and can show bright dots. Long-exposure noise reduction takes a second "dark frame" of the same length and subtracts it — effective, but it doubles the waiting time.
Vibration and wind
Over seconds or minutes, any tripod movement shows. Hang your bag from the centre column, keep the legs short in wind and avoid touching the camera during the exposure.
Variable and graduated filters
A variable ND is two polarisers that rotate against each other, giving roughly 2–8 stops in one filter. They are popular for video, but at the strong end they can produce an uneven dark "X" across the frame. A graduated ND is dark at the top and clear at the bottom, used to hold back a bright sky rather than to lengthen the exposure — a different tool from the solid filters this guide is about.
Choosing your first filter
If you buy one filter, make it a 6-stop: it is useful for water at any time of day and for clouds at sunrise and sunset. A 10-stop is the classic for daylight seascapes and dramatic cloud streaks. Add a 3-stop for portraits in sun and for fine-tuning by stacking. Work out any combination in the calculator before you leave home.