ABSTRACT
An optical filter is a transparent flat sheet of coloured material that is placed in front of the camera lens so that the exposure takes place through it. Optical quality is important that a camera-quality optical filter is used in an image-forming system rather than any convenient coloured piece of glass or other material. Optical filters may be positioned in front of or behind a lens, the rear fitting allowing smaller diameter filters to be used. Neutral density filters are available in graduated form to give continuous light control over a given range of attenuation, or to give attenuation to a part of the scene only, e.g. the sky region but not the foreground. A set of dichroic or interference filters, of complementary yellow, magenta and cyan hues, are used routinely in a ‘colour head’ equipped with a suitable illumination system.
