In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a glass or plastic disk in a metal or plastic ring frame, which can be screwed into the front of or clipped onto the camera lens. Filters modify the images recorded. Sometimes they are used to make only subtle changes to images; other times the image would simply not be possible without them. In monochrome photography, coloured filters affect the relative brightness of different colours; red lipstick may be rendered as anything from almost white to almost black with different filters. Others change the colour balance of images, so that photographs under incandescent lighting show colours as they are perceived, rather than with a reddish tinge. There are filters that distort the image in a desired way, diffusing an otherwise sharp image, adding a starry effect, etc. Linear and circular polarising filters reduce oblique reflections from non-metallic surfaces.
Overview Many filters absorb part of the light available, necessitating longer exposure. As the filter is in the optical path, any imperfections – non-flat or non-parallel surfaces, reflections (minimised by optical coating), scratches, dirt – affect the image. In digital photography the majority of filters used with film cameras have been rendered redundant by digital filters applied either in-camera or during post processing. Exceptions include the ultraviolet (UV) filter typically used to protect the front surface of the lens, the neutral density (ND) filter, the polarising filter, color-enhancing filters, and the infrared (IR) filter. The neutral density filter permits effects requiring wide apertures or long exposures to be applied to brightly lit scenes, while the graduated neutral density filter is useful in situations where the scene's dynamic range exceeds the capability of the sensor. Not using optical filters in front of the lens has the advantage of avoiding the reduction of image quality caused by the presence of an extra optical element in the light path and may be necessary to avoid vignetting when using wide-angle lenses.
Nomenclature
There is no universal or reliably standard naming or labelling system for filters. The Wratten numbers adopted in the early twentieth century by Kodak, then a dominant force in film photography, are used by several manufacturers, including B+W, but the actual spectral characteristics of a filter may vary by manufacturer, despite having the same Wratten number. In addition, the Wratten numbers are sometimes used interchangeably with alternative names; for example, the Wratten filter number 6 is also named K1, while #11 is also named X1.
Some manufacturers use a combination of Wratten numbers and wavelengths to identify filters. For example, Nikon offers four UV / skylight filters: L1A, L1B, L37, and L39; the L1A and L1B correspond to Wratten numbers 1A and 1B, while L37 and L39 include the wavelength cutoffs of 370 nm and 390 nm, respectively. Colored filters used to enhance contrast for black and white photography include a letter (Y, O, or R) and a similar wavelength cutoff: for example, R60 is a red filter with a step-like transmission function at 600 nm. For other filters, the alternate Wratten name is used (for example, X0 and X1 for green filters). Many colour correction filters are identified by a code of the form CCaab, for example, CC50Y:
CC = type (for colour correction) aa = strength or density of the filter (50 = 50%) b = color (in this case, Y for yellow) While the same information may be present, the specific sequence of colour and density may vary by manufacturer.
Scientific uses
Optical filters are used in various areas of science, including in particular astronomy; photographic filters are roughly the same as "optical" filters, but in practice optical filters often need far more accurately controlled optical properties and precisely defined transmission curves than filters only made for general photography. Photographic filters sell in larger quantities, at correspondingly lower prices, than many laboratory filters. The article on optical filters has information relevant to photographic filters, particularly special-purpose photographic filters like color enhancing filters and high-quality photographic filters, like sharp cut-off UV filters.
Photographic uses
Filters in photography can be classified according to their visible color and use:
Colorless / Neutral Clear and ultraviolet Infrared Polarizing Neutral density, including the graduated neutral density filter and solar filter Color Color conversion (or color balance) Color correction Color separation, also called color subtraction Contrast enhancement Special effects of various kinds, including Graduated color, called color grads Cross screen and star diffractors Diffusion and contrast reduction Close-up or macro diopters, and split diopters or split focus Multi-image Spot
Colorless / Neutral
Clear and ultraviolet
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