|Polarising and Skylight filters|
for 52mm filter thread fitting
image by AWCam (Image rights)
A filter is an optical device that can be added to the lens of a camera. It consists of one or two glass (or, more recently, plastic or gelatine) plates, mounted in a frame that is fitted to the lens of a camera. Attachment may be push-fit, bayonet, or threaded. Also a filter may be built in to the camera or lens.
Filters can also be large sheets of acetate, for example covering the windows of a house to correct the colour balance and / or intensity of external light when taking interior photographs. Sheet filters can also be put in front of the light source (flash or tungsten).
Filters are used in colour enlargers for colour balancing.
Colour filters reduce ("filter") light of a particular colour. Early colour filters were cuvettes which could be filled with coloured water. Nowadays colour filters consist of only one glass plate which is coated with a transparent glass colour.
|Voigtländer Bessa with hinged yellow filter|
image by Mark O'Brien (Image rights)
In black and white photography colour filters can be used to improve contrast.
Filters can be grouped as: contrast enhancement, colour correction, neutral, and special effect.
Adding a poor quality filter can greatly degrade lens resolution, widely misunderstood is the high cost of optically flat filters with proper coatings necessary to preserve lens resolution. A high quality filter can cost more than a poor quality lens.
In black-and-white photography coloured filters can be used to improve modelling of skin tones (green or yellow-green filters), or to darken washed-out skies and show cloud detail (e.g. yellow filter). Many box-type cameras, and some more sophisticated models, had built in yellow or green filters for these purposes. These filters work because black and white film does not have equal sensitivity for all colours.
For colour photography the main contrast enhancing filter is a polarising filter. In bright sunlight, especially when the sun is high, there are many reflections that will reduce contrast and, since reflections are plane polarised, a polarising filter can greatly reduce reflected light and enhance colour contrast as well as darkening blue sky.
Polarising filters come in two types, plane polarising and "circular" polarising (photographic circular polarising filters are not true circular polarising filters). Plane polarising filters, due to the single plane of light, cause problems for any camera with secondary mirrors for light measurement. Circular polarising filters, much more difficult to manufacture, polarise in varying planes eliminating secondary problems (simple "Polaroid" dark glasses cause very distracting patterns in car windscreens, circular polarised glasses do not).
So-called skylight filters, which block ultra-violet (UV) light and so improve colour rendering, are also contrast enhancing. However, many lenses already include UV coating (film is sensitive to UV, eyes are not). Digital sensors are more sensitive to UV than film; when using older lenses on a digital camera a UV filter can be important to reduce unwanted colour fringing.
The primary use of colour correction filters is to correct the colour balance of artificial light when taking photographs indoors or under tungsten lighting, such as on a film set or stage lighting. Early studio lighting was also tungsten and is still preferred by some photographers for its soft tones.
Blue filters are used to correct daylight colour film for tungsten lighting.
Correction filters are also available for tube (neon) lighting, however due to the nature of tube light they can only be partially successful.
Tinted filters can partly correct the very cold, blue, light typical of overcast conditions.
Neutral density filters (ND filters) merely reduce the level of transmitted light, without changing the colour. They are used in very bright conditions to allow a larger aperture or shorter exposure time.
Special effects filters are extremely varied, such as soft-focus and "sparkling" highlights. Such filters often degrade image quality more than standard filters.
Filters on wide angle lenses
On wide angle lenses light entering the edges of the filter at an angle will travel further through filter material than at the centre, where the light rays are perpendicular. This further increases vignetting from which all lenses suffer, especially wide angle. Worse still, the filter mount itself may obstruct light, especially if more than one filter is mounted.
A special, and unusual, neutral correction filter graduated from darker at the centre to clear at the edges can be used to reduce vignetting.
Extreme wide angle lenses are sometimes made with built in filters. The filters are fitted in a disc located near the optical centre of the lens having much less effect on vignetting. A rotary dial on the lens is used to select clear or, most usually, orange and red monochrome contrast enhancement filters.
Filters on telephoto lenses
Large aperture telephoto lenses have large front apertures, making filters very expensive and, in extreme cases, impractical. Some extreme telephoto lenses are fitted with a rear filter slide in attachment. Various filters with a special mount may be fitted and a clear filter must be in place if no other filter is fitted since the filter is part of the optical path.
Dichroic filters have special coatings to reflect, rather than absorb, unwanted light wavelengths. They can be made to transmit a very narrow range of wavelengths ("narrow pass") and can withstand the heat of powerful enlarger lamps. However, they are expensive to manufacture and will only filter light at particular incident angles making them unsuitable for general photography.
Dichroic filters are used in many colour enlargers, especially commercial enlargers.
Many lens mounted filters require exposure compensation to ensure correct exposure since the filter may absorb a significant amount of light. When using TTL measurement / exposure the built in light meter will automatically allow for any necessary correction, but when using a separate meter compensation must be applied.
Filter factor is marked on filters indicating correction. 1 means no correction needed, 2 means 1 stop additional exposure, 4 means 2 stops additional exposure, and so on. This can be applied manually as a correction of the metered exposure value, or (rarely) the meter may incorporate exposure compensation (exposure compensation is common on cameras but rare on separate meters).
Filter factor is the reciprocal of light reduction (-1 stop = ½ intensity, -2 stops = ¼ intensity). It is an arithmetic scale with each step doubling: 1, 2, 4, 16, 32, etc.