In photography and videography, a filter is a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or oblong 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 or clipped onto 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 filter use is impractical.
Many filters are identified by their Wratten number.
Uses of filters in photography
Filters 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 ultraviolet
Clear 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.
While in certain cases (such as harsh environments) a protection filter may be necessary, there are also downsides to this practice. Arguments for the use of protection filters include:
- If the lens is dropped, the filter may well suffer scratches or breakage instead of the front lens element.
- One can clean the filter frequently without having to worry about damaging the lens coatings; a filter scratched by cleaning is much less expensive to replace than a lens.
- If there is blowing sand the filter may protect the lens from abrasion from sand
- Some lenses like Canon's L series lenses require the use of a filter to complete the weather sealing
And arguments against their use include:
- Adding another element degrades image quality due to aberration and flare.
- It may reduce the use of lens hoods, since threading a lens hood on top of the clear filter might cause vignetting on some lenses, and since not all clear filters would even have threads allowing a hood to be attached.
- If the ring of the filter is struck in a drop it may shatter the filter and cause scratches to the front element rather than a bend in the filter ring of the lens barrel
Additionally, users of UV filters must be careful about the quality of such filters. There is a wide variance in the performance of these filters with respect to their ability to block UV light. Also in lower quality filters, problems with autofocus and image degradation have been noted.
Color correction
A major use is to compensate for the effects of lighting not balanced for the film stock's rated color temperature (usually 3200 K for professional tungsten lights and 5500 K for daylight): e.g., the 80A blue filter used with daylight film corrects the orange/reddish cast of household tungsten lighting, while the 85B used with tungsten film will correct the bluish cast of daylight. Color correction filters are identified by numbers which sometimes vary from manufacturer to manufacturer. The use of these filters has been greatly reduced by the widespread adoption of digital photography, since color balance problems are now often addressed with software after the image is captured, or with camera settings as the image is captured.
Color subtraction
Color subtraction filters work by absorbing certain colors of light, letting the remaining colors through. They can be used to demonstrate the primary colors that make up an image. They are perhaps most frequently used in the printing industry for color separations, and again, use has diminished as digital solutions have become more advanced and abundant.
Contrast enhancement
Filters are commonly used in black and white photography to manipulate contrast. For example a yellow filter will enhance the contrast between clouds and sky by darkening the latter. Orange and red filters will have a stronger effect. A deep green filter will darken the sky too but will lighten green foliage and will make it stand out against the sky. Also see diffusion filters, which are used to reduce contrast.
Polarizer
Main article: Polarizing filterNeutral density
A neutral density filter (ND filter) is a filter of uniform density which attenuates light of all colors equally. It is used to allow a longer exposure (to create blur) or larger aperture (for selective focus) than required for correct exposure in the prevailing light conditions, without changing the tonal balance of the photograph.
A graduated neutral density filter is a neutral density filter with different attenuation at different points, typically clear in one half shading into a higher density in the other. It can be used, for example, to photograph a scene with part in deep shadow and part brightly lit, where otherwise either the shadows would have no detail or the highlights burnt out.
Cross screen
A cross screen filter , also known as a star filter , creates a star pattern, in which lines radiate outward from bright objects. The star pattern is generated by a very fine diffraction grating embedded in the filter, or sometimes by the use of prisms in the filter. The number of stars varies by the construction of the filter, as does the number of points each star has.
Diffusion
A diffusion filter (also called a softening filter ) softens subjects and generates a dreamy haze ( see photon diffusion ). This is most often used for portraits. It also has the effect of reducing contrast, and the filters are designed, labeled, sold, and used for that purpose too. There are many ways of accomplishing this effect, and thus filters from different manufacturers vary significantly. The two primary approaches are to use some form of grid or netting in the filter, or to use something which is transparent but not optically sharp.
Both effects can be achieved in software, which can in principle provide a very precise degree of control of the level of effect, however the "look" may be noticeably different. If there is too much contrast in a scene, the dynamic range of the digital image sensor or film may be exceeded, which post-processing cannot compensate for, so contrast reduction at the time of image capture may be called for.
Transparent diffusion
Zeiss manufactures a widely noted Softar diffusion filter which is made of many tiny globs of acrylic deposited on one surface which act as microlenses to diffuse the light. In some versions the globs are on the inside of the filter (facing the photographer) while on others they face outwards (towards the subject). In various versions the globs vary in number and diameter, from approximately 97 to 150 globs each 1 mm to 3 mm wide.
Homebrew approaches to transparent diffusion filters are generally based on modifying a clear or UV filter by placing various materials on it; the most popular choices are petroleum jelly, optical cement, and nail polish. Transparent filters are more commonly used for the "dreamy" or "misty" effect than for contrast reduction.
Grid or netting
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