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B&W neutral density filters are made out of neutral gray Schott NG-glass. They block the entire visible spectrum evenly and allow light reduction without influencing color rendition or contrast. Sometimes it is necessary to cut down the amount of light with neutral density filters when working with high-speed films. For example, the B&W 102 filter allows exposures with ISO 400/27 degree film as if it were 100/21 degree film. It is often used to portray a subject against an out of focus background. A neutral density filter comes in handy if you would like to increase the aperture more than the light situation and camera capabilities permit. They are also good for the long exposures which are necessary for a blur-effect or zooming during the shot.
