Neutral Density Filters

Neutral density (ie, ND) filters reduce the amount of light that can pass through a lens. These filters have the effect of reducing the exposure over part of the image being taken in either a graduated or abruptly graduated (split) way.

They are used to filter the entire visible spectrum evenly, allowing light reduction, without influencing color or contrast.

Patterned Neutral Density Filters

They are sometimes called ND filters or gray filters. Attenuation is accomplished using light-absorbing glass or a thin-film metal coating that combines both absorption and reflection. Metallic type neutral density filters deposit a metal alloy coating onto a specific type of substrate, to obtain their optical density which is determined by the wavelength region of interest.  Neutral density filters are sensitive to angles but are more forgiving than interference filters.

ND filters attenuate spectral regions selected from 250 to 2500 nm. The level of attenuation can be specified from optical density 0.04 to 4.0.

Transmission is reduced by the Neutral Density Filter by either reflection or absorption. A reflective neutral density filter uses partial reflection to reduce light transmission evenly, while absorptive neutral density filters rduce light transmission using partial absorption.

A variable neutral density filters change the transmission linearly over time.

Neutral density filters can be with all types of film, but are optimal when working with high-speed films or long-exposure motion applications.  Neutral density filters can be used when a wide aperture is required or there is too much light.

Distracting backgrounds can be diminished by permitting the use of a wider aperture to reduce depth of field as required to throw the background out of focus while maintaining a focud subject, thus allowing for emphasis on the subject by controlling depth of field or creating motion effect with subject blur.

Neutral density filters are available with different levels of tint for fine-tuning exposures.  Neutral density filters of higher values are required for observation of intense light sources.

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