Camera filters are an excellent way to make good photography great.
Camera filters are an excellent way to obtain more flexibility with, among other things, the color and exposure of your images. Using a neutral density filter is especially useful because such filters affect the quantity of light without changing the quality or color.
Function
Neutral density, or ND, filters reduce the amount of light passing through the lens of your camera, exposing your film or digital sensor. In photography, you change the ISO (the sensitivity of your film or digital sensor), the shutter speed (the length of the exposure in time, generally measured in seconds or fractions of seconds) or the aperture (how large the lens opening is) to control your exposure. A neutral density filter gives you more control over your exposure by allowing you to decrease the amount of light reaching your film or sensor.
Identification
A neutral density filter is a circular, gray piece of glass or gelatin that screws into the end of a camera lens. The darker the gray of the filter, the less light that comes through the lens to reach the film plane, which is the location of your film or digital sensor.
Uses
Use a slow shutter speed with a neutral density filter on a bright day to achieve a look of motion in a photograph.
Neutral density filters "offer an alternative method of controlling exposure when the conventional aperture and shutter controls are inadequate," according to the book "Basic Photographic Materials and Processes: Second Edition," by Leslie Stroebel, John Compton, Ira Current and Richard Zakia. For example, a neutral density filter is perfect on sunny days when you want to isolate your subject with a shallow depth of field using a wide aperture. You also can use neutral density filters with a faster shutter speed on a bright day to obtain a blurred effect in your images.
Density Gradings
There are two systems used to measure light reduction by a neutral density filter. The first system measures the physical density of the filter in millimeters to express the amount of reduced light hitting the film plane. The second system uses mathematical notations. Which system is used depends on the filter manufacturer.
Considerations
Avoid using a neutral density filter for a scene that is not consistently lit; for example, while framing your image at a shadowy area on a bright day. While everything in direct sunlight would be exposed correctly, you likely would lose detail in the shady areas.
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