What Is An “ND” Filter And How Does It Work?

What Is An “ND” Filter And How Does It Work?

By William Lulow

First, “ND” stands for neutral density. This means that the filter simply limits the amount of light entering the camera by specific quantities. It usually fits in front of the lens and cuts down on the light passing through it. Other filters do similar things to light, but these filters not only limit light, but they limit the light from ALL wavelengths, not just specific colors. So, the effect is to cut down on exposure without changing any colors.

In the heyday of black & white photography, different filters were used to create often dramatic effects. Since a colored filter will transmit light of its own color and absorb all the others, certain colored filters can be used to obtain certain effects. If you wish to darken a blue sky, for example (in a black & white photograph), you would use a filter of an opposite color, in this case something on the red side of the spectrum. So, a red filter, placed over the lens when taking an image in MONOCHROME mode, will darken colors in the blue wavelengths. Here is an example:

This image of a palm tree shot against a blue sky, was made with a red filter over the lens.

Many landscape photographers making images on black & white film, would often use filters to enhance their photographs. Green filters, for example, would block light from the red end of the light spectrum, thereby darkening red objects in black & white. You might use a filter like this if you are shooting apples against a green background say.

Here is another example of how a red filter enhances the clouds and sky in a black & white image.

Now neutral density filters can enhance images shot in various COLOR modes with digital cameras. Since they cut down on light entering the camera, the photographer must compensate for that by increasing the ISO number, slowing down the shutter speed, decreasing the aperture or a combination of all three. This technique comes in handy when you want to use long exposures, for example. I have often made use of this little “trick” when I’m photographing scenes in which there are a lot of people. You can make the people and other moving objects effectively “disappear” from an image if you use a long enough exposure and everyone keeps moving. Here’s an example:

This image was made of Christmas decorations in Florida while many people and vehicles were streaming past. It was actually made with an infrared filter in monochrome mode. The exposure was ISO 1000, f/18 at 30 seconds.

Neutral density filters come in 1 stop, 2 stops and 3 stop versions and you can even combine them to really cut down on the light. Here is the same image in color made with a 2 stop ND filter:

You can see the branches of the tree in the upper left are blurry due to the long exposure.

There is another filter which I use a lot for my scenic images. It is a neutral density graduated filter. It looks like this:

You can see it has an ND filter on top and a clear portion on the bottom. This enables me to darken the sky or the upper portion of my image while keeping the lower portion normal. Here is an example of the effect:

Various filters should be an integral part of your camera equipment. They come in quite handy when doing any kind of landscape photography. I normally don’t use them in the studio, because lighting takes care of details I wish to see. But, outdoors, they enhance your photographs a great deal.