Definition
A neutral density (ND) filter is a darkened piece of glass or resin placed over the lens that cuts the light reaching the sensor without changing its colour. Its strength is given in stops: each stop halves the light, so a 6-stop filter needs 64 times the exposure and a 10-stop needs 1,024 times.
A waterfall meters at 1/125s, f/11, ISO 100. A 6-stop ND multiplies that by 64, giving about 1/2s, which softens the water. A 10-stop ND multiplies it by 1,024, giving about 8 seconds, which turns it to mist. Check the result in the calculator, then use a tripod and a remote release.
Calculate it for your own shot with the ND Filter Calculator.
Also used in: Long Exposure Timer
An ND filter buys shutter speed when the light will not give it. In daylight, even at ISO 100 and f/16, a scene often meters at 1/60s or faster, and nothing in the exposure triangle gets you to the multi-second exposures that turn moving water to mist or clouds to streaks. A 6-stop filter turns 1/60s into about one second; a 10-stop turns it into roughly fifteen.
The strength has to match the intent, because guessing wastes light. Reach for a 10-stop on a partly cloudy afternoon and the exposure runs so long that a break in the clouds blows out mid-frame before the shutter closes. ND filters also let a portrait photographer hold f/1.4 wide open in bright sun without passing the flash sync speed, which is a second, less obvious reason to keep one in the bag.
A waterfall in open shade meters at 1/125s, f/11, ISO 100, which freezes the water into hard white ribbons. A 6-stop ND filter drops the shutter to about 1/2 second, enough to soften the flow while keeping some texture in the spray. A 10-stop filter on the same scene stretches the exposure to around 8 seconds, smoothing the water completely and blurring any moving leaves at the frame edge, at which point a tripod and a remote release stop being optional.
Stacking a 3-stop and a 6-stop gives 9 stops and about 4 seconds, though budget stacked filters can add a colour cast that has to be pulled out in editing. For a seascape at sunset where the light is already dropping fast, a 3-stop is often enough, because the base exposure is a second or longer on its own.
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