Neutral density (ND) filters reduce the amount of light entering the camera, allowing for longer exposure times than would be possible without them. The key is that they reduce light uniformly, so contrast and dynamic range are not affected – unlike a graduated neutral density (GND) filter. The primary purpose of ND filters is blocking light from reaching the image sensor.

ND filters are nothing more than darkened glass placed in front of the lens to absorb a percentage of the incoming light. They are available in different “strengths” usually designated by either the number of stops it slows down the exposure or in terms of optical density strength (see the ND filter strength conversion table below). A 3-stop or 0.9 density ND is ideal for waterfalls in bright sunlight, slowing the exposure to a second or so, depending on the f-stop and ISO used. A strong 10-stop or 3.0 ND filter can blur clouds over several minutes, even on a bright sunny day.

The image below, Iguazu Falls on the border of Brazil and Argentina, would not have been possible without the help of a neutral density filter. Here is the exposure data from this image: 4.0 seconds, f/22, ISO 100, 6-stop ND filter. The correct exposure data without the ND filter would have been the same except for the shutter speed, which would have been 1/15 of a second (6 stops faster). As you can see, I am already at the smallest f-stop (f/22) and the lowest ISO (100) possible for my camera and lens.

There’s nothing else I can do in camera to slow things down to achieve the desired effect. Without the ND filter, the correct exposure would have been 1/8 second so a 4-second exposure would have badly overexposed the image. There is simply no way to create the smooth, silky water I desired under those bright, sunny skies without the ND filter absorbing some of the light that was reaching my camera’s sensor. Unless, of course, I wished to wait for less intense light once the sun went down or a cloud passed overhead. But then there’s that pretty little rainbow I would have missed.