Special-purpose filters for use with black-and-white film are those filters not classified specifically as contrast or correction filters. Some of the special-purpose filters can be used with both black-and-white and color film. Two of these special-purpose filters are intended primarily for use with color film.

NEUTRAL DENSITY FILTERS

Neutral density (ND) filters reduce the amount of light passing through a camera lens without changing the reproduction of colors in the scene. These filters are nonselective in their absorption of colors of light and therefore uniformly reduce the various colors of light in the spectrum. Thus white light and colored light are transmitted through an ND filter with only the intensity of the light being affected. These filters can be used with both black-and-white and color film. Neutral density filters are gray in appearance. These filters may be needed for pictures being made of a brilliant subject in bright sunlight. When you have set the fastest shutter speed and the smallest f/stop and still cannot make the picture without overexposing the film, you can use an ND filter to further reduce the exposure. Neutral density filters manufactured by Kodak are called Wratten Neutral Density Filters and are available in several densities. The ten most popular densities, with the

Table 3-5. Neutral Density Filters

amount of exposure reduction provided, are shown in table 3-5.

When you desire to reduce the depth of field but maintain a given shutter speed, ND filters permit the use of a larger f/stop which in turn, reduces the depth of field. Neutral density filters are used extensively in motion-picture photography where depth of field is usually quite deep. ND filters are also used with mirror type of lenses where there is no aperture control.

HAZE FILTERS

Suspended in the earth's atmosphere are minute particles of vapor and dust that cause a veil-like appearance called haze. This haze is most apparent in distant scenes. Haze is the result of sunlight being scattered by minute particles of matter that are present in the air. The amount of haze can vary due to

atmospheric conditions. Haze should not be confused with mist, fog, smog, smoke, or clouds. These conditions can also produce a veil-like appearance, but filters have no effect.

When sunlight is scattered, both green light and red light are also scattered by the ever-present haze, but not nearly as much as ultraviolet radiation, violet, and blue light.

When filters are used to absorb scattered sunlight, penetration of the haze is possible. A haze filter is any filter that absorbs atmospherically scattered sunlight. A haze filter includes contrast and correction filters. When contrast and correction filters are used for haze penetration, they may be considered special-purpose filters. Although contrast filters can be used for cutting haze, these filters affect the gray tone rendering of colored objects. The contrast and correction filters that absorb the shorter wavelengths are the most effective. The recommended contrast and correction filter colors, in the order of greatest to least effective for haze penetration, are as follows:

Red

Orange

Yellow

Green

The use of an infrared sensitive black-and-white film with an infrared filter provides the greatest haze penetration of all. Special, visually opaque infrared filters completely absorb the scattered ultraviolet radiation and the visible light that produce haze. This absorption by an infrared filter allows the scene to be photographed entirely with unscattered infrared radiation. An infrared sensitive black-and-white film without an infrared filter, or at least a red contrast filter, is not effective for haze penetration. Infrared black-and-white film is sensitive to ultraviolet radiation, violet, and blue light as well as infrared radiation and red light. The gray tone rendering of a colored subject in a black-and-white print produced from an infrared negative is greatly distorted or contrasty.

The visually opaque infrared filters are identified by numbers as follows:

87

87A

87B

87C

88A

89B

When the effect of haze is to be reduced with an equal change to the gray tone rendering of all colored objects in a black-and-white print, filters that primarily absorb ultraviolet (UV) radiation are required. These filters have a very pale pink or yellow tint and may be identified by numbers as follows:

2A

2B 2C

2E

Colorless haze or ultraviolet absorbing filters are often used to protect the front element of a lens from damage. It is much cheaper to replace a filter than it is to repair or replace a lens.

The polarizing filter is another type of special-purpose filter that can be used to reduce the effects of haze.