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The filter can be a square or rectangle shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which can be screwed in front of the lens. Filters allow added control for the photographer of the images being produced. Sometimes they are used to make only subtle changes to images; other times the image would simply not be possible without them. The negative aspects of using filters, though often negligible, include the possibility of loss of image definition if using dirty or scratched filters, and increased exposure required by the reduction in light transmitted. The former is best avoided by careful use and maintenance of filters, while the latter is a matter of technique; it usually will not be a problem if planned out properly, but in some situations does make filter use impractical. Many filters are identified by their Wratten number.
Uses of filters in photographyFilters in photography can be classified according to their use:
Clear and ultraviolet;
Color correction, also called "color conversion" or "white balance correction";
Color separation, also called Color Subtraction;
Contrast enhancement;
Infrared;
Neutral Density, including the Graduated ND filter and Solar filter;
Polarizing;
Special Effects of various kinds, including
Graduated color, called color grads;
Cross screen and Star diffractors;
Diffusion and contrast reduction;
Sepia tone;
Spot;
Close-up or macro diopters, and split diopters or split focus;
Clear and ultravioletClear filters, also known as window glass filters or optical flats, are completely transparent, and (ideally) perform no filtering of incoming light at all. The only use of a clear filter is to protect the front of a lens. UV filters are used to reduce haziness created by ultraviolet light. A UV filter is mostly transparent to visible light, and can be left on the lens for nearly all shots. UV filters are often used for lens protection, much like clear filters. A strong UV filter, such as a Haze-2A or UV17, cuts off some visible light in the violet part of the spectrum, and so has a pale yellow color; these strong filters are more effective at cutting haze, and can reduce purple fringing in digital cameras. Strong UV filters are also sometimes used for warming color photos taken in shade with daylight-type film.
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