Colour Correction Filters
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 not be possible without them. In monochrome photography, coloured filters affect the relative brightness of different colours; red lipstick may be rendered almost white to almost black with additional filters. Others change the colour balance of images so that photographs under incandescent lighting show colours as perceived rather than with a reddish tinge. Some filters distort the image in the desired way, diffusing an otherwise sharp image, adding a starry effect, etc. Linear and circular polarising filters reduce oblique reflections from non-metallic surfaces.
Many filters absorb part of the light available, necessitating more prolonged 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.
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, but the degree of correspondence between the filters and the number labels is approximate. A code of the form CCaab somewhat more accurately identifies most colour correction filters, e.g. CC50Y – CC for colour correction, aa = 50 for the strength of the filter (50%), and b = Y for yellow.
Optical filters are used in various areas of science, including 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 colour enhancing filters and high-quality photographic filters, like sharp cut-off UV filters.
Colour subtraction filters work by absorbing specific colours of light, letting the remaining colours through. They can be used to demonstrate the primary colours that make up an image. They are perhaps most frequently used in the printing industry for colour separations, and again, use has diminished as digital solutions have become more advanced and abundant.
Didymium filters, sold as “colour enhancement” or “fall colour” filters act similarly: They remove a narrow (or broad) band of colour in the yellow part of the spectrum (589 nm).[a] Some astronomical filters similarly use didymium in heavier concentrations. Even astronomical filters which don’t use didymium typically are some narrow pass-band colour filters.
Effects of using a polariser and a red filter in black-and-white photography
Coloured filters are commonly used in black and white photography to alter the effect of different colours in the scene, changing the contrast recorded in black and white.
For example, a yellow filter or, more dramatically, an orange or red filter will enhance the contrast between clouds and sky by darkening the blue sky while leaving the clouds bright (after exposure compensation). A deep green filter will also darken the sky and lighten green foliage, making it stand out against the sky. Light yellowish-green filters were used as standard portrait filters for panchromatic film since they render skin tones as light to dark grey while darkening deep reds and blues to nearly black.
A sky-blue filter (cyan) mimics the effect of older orthochromatic film – or with a “true blue” filter, even older film only sensitive to blue light – rendering blue as light and red and green as dark, showing blue skies the same as overcast, with no contrast between sky and clouds, darkening blond hair, making blue eyes nearly white, and red lips nearly black.
Diffusion filters have the opposite, contrast-reducing effect; in addition, they “soften” focus, making minor blemishes invisible.