Filters are transparent or translucent optical accessories made from coloured glass, gelatin, or plastic, designed to modify the quality, colour, or character of light passing through them. By selectively absorbing certain wavelengths of light while allowing others to pass through, filters alter the colour content of the light that ultimately reaches the film, image sensor, or photographic paper, enabling the photographer or darkroom printer to exercise precise control over the final appearance of an image.
Filters can be used at two distinct stages of the photographic process. At the camera stage, filters are placed in front of the lens to influence the image at the point of capture. In colour photography this might involve warming or cooling the overall colour balance of a scene, reducing atmospheric haze, or managing the way particular colours are recorded. In black and white photography, coloured filters are used to manipulate tonal contrast - a red filter, for example, will dramatically darken blue skies while lightening red and orange tones, while a green filter can help separate and define foliage from other elements in a landscape.
At the printing stage, filters are used in the darkroom to control the colour balance of an enlarger's light source when making colour prints, or to adjust the contrast of prints made on variable contrast black and white photographic paper. Colour printing filters are typically supplied in sets of primary and complementary colours at varying densities, allowing fine adjustments to be made to the colour balance of a print during the enlarging process.
Filters are available in a wide variety of forms, including screw-in circular filters that attach directly to the front of a lens, square or rectangular resin filters used with modular holder systems, and gelatin filter sheets that can be cut to size for use in filter holders, over light sources, or beneath enlarger lenses. Each material has its own characteristics in terms of optical quality, durability, and cost, with optical glass generally offering the highest quality and gelatin offering the greatest flexibility and range of available colours and densities.