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In
photography Photography is the visual arts, art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is empl ...
and
cinematography Cinematography () is the art of motion picture (and more recently, electronic video camera) photography. Cinematographers use a lens (optics), lens to focus reflected light from objects into a real image that is transferred to some image sen ...
, a filter is a
camera A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...
accessory consisting of an
optical filter An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...
that can be inserted into the
optical path Optical path (OP) is the trajectory that a light ray follows as it propagates through an optical medium. The geometrical optical-path length or simply geometrical path length (GPD) is the length of a segment in a given OP, i.e., the Euclidean dis ...
. 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 A camera lens, photographic lens or photographic objective is an optical lens (optics), lens or assembly of lenses (compound lens) used in conjunction with a camera body and mechanism to Imaging, make images of objects either on photographic film ...
. 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 Monochrome photography is photography where each position on an image can record and show a different ''amount'' of light (Value (color), value), but not a different color (hue). The majority of monochrome photographs produced today are black- ...
, 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 infra red (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 Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...
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 In photography and optics, vignetting ( ) is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word '' vignette'', from the same root as ''vine'', originally referred to a decorative b ...
when using wide-angle lenses.


Nomenclature

There is no universal or reliably standard naming or labelling system for filters. The
Wratten number Wratten numbers are a labeling system for optical filters, usually for photographic use comprising a number sometimes followed by a letter. The number denotes the color of the filter and its spectral characteristics, and these numbers can be g ...
s adopted in the early twentieth century by
Kodak The Eastman Kodak Company, referred to simply as Kodak (), is an American public company that produces various products related to its historic basis in film photography. The company is headquartered in Rochester, New York, and is incorporated i ...
, 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 Color correction is a process used in stage lighting, photography, television, cinematography, and other disciplines, which uses color gels, or filters, to alter the overall color of the light. Typically the light color is measured on a scale kno ...
filters are identified by a code of the form CC''b'', for example, CC50Y: * CC = type (for colour correction) * = 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 filter An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...
s'' are used in various areas of science, including in particular
astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
; 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 curve The transmission curve or transmission characteristic is the mathematical function or graph that describes the transmission fraction of an optical or electronic filter as a function of frequency or wavelength. It is an instance of a transfer func ...
s 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 Clear may refer to: * Transparency, the physical property of allowing light to pass through Arts and entertainment Music Groups * Clear (Christian band), an American CCM group from Cambridge, Minnesota * Clear (hardcore band), a vegan straight ...
**
Infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
** Polarizing ** Neutral density, including the
graduated neutral density filter A graduated neutral-density filter, also known as a graduated ND filter, split neutral-density filter, or just a graduated filter, is an optical filter that has a variable light transmission. Typically half of the filter is of neutral density wh ...
and solar filter * Color ** Color conversion (or
color balance In photography and image processing, color balance is the global adjustment of the intensities of the colors (typically red, green, and blue primary colors). An important goal of this adjustment is to render specific colors – particularly neu ...
) **
Color correction Color correction is a process used in stage lighting, photography, television, cinematography, and other disciplines, which uses color gels, or filters, to alter the overall color of the light. Typically the light color is measured on a scale k ...
** Color separation, also called
color subtraction Subtractive color or subtractive color mixing predicts the spectral power distribution of light after it passes through successive layers of partially absorbing media. This idealized model is the essential principle of how dyes and pigments are ...
**
Contrast enhancement A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radioph ...
*
Special effects Special effects (often abbreviated as F/X or simply FX) are illusions or visual tricks used in the theatre, film, television, video game, amusement park and simulator industries to simulate the fictional events in a story or virtual world. ...
of various kinds, including ** Graduated color, called color grads ** Cross screen and star diffractors **
Diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
and contrast reduction **
Close-up A close-up or closeup in filmmaking, television production A television show, TV program (), or simply a TV show, is the general reference to any content produced for viewing on a television set that is broadcast via over-the-air, s ...
or macro diopters, and split diopters or split focus ** Multi-image ** Spot


Colorless / Neutral


Clear and ultraviolet

Clear filters, also known as window glass filters or optical flats, are transparent and (ideally) perform no filtering of incoming light. The only use of a clear filter is to protect the front of a lens. Clear glass will absorb some UV. UV filters are used to block invisible
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
light, to which most photographic sensors and film are at least slightly sensitive. The UV is typically recorded as if it were blue light, so this non-human UV sensitivity can result in an unwanted exaggeration of the bluish tint of atmospheric haze or, even more unnaturally, of subjects in open shade lit by the ultraviolet-rich sky. Normally, the glass or plastic of a camera lens is practically opaque to short-wavelength UV, but transparent to long-wavelength (near-visible) UV. A
UV filter UV filters are compounds, mixtures, or materials that block or absorb ultraviolet (UV) light. One of the major applications of UV filters is their use as sunscreens to protect skin from sunburn and other sunlight- or UV-related damage. After the ...
passes all or nearly all of the
visible spectrum The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optica ...
but blocks virtually all ultraviolet radiation. (Most spectral manipulation filters are named for the radiation they pass; green and infrared filters ''pass'' their named colors, but a UV filter ''blocks'' UV.) It can be left on the lens for nearly all shots: UV filters are often used mainly for lens protection in the same way as 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 has a pale yellow color; these strong filters are more effective at cutting haze, reduce
purple fringing In photography (particularly digital photography), purple fringing (sometimes called PF) is the term for an unfocused purple or magenta "ghost" image on a photograph. This optical aberration is generally most visible as a coloring and lightenin ...
in
digital camera A digital camera, also called a digicam, is a camera that captures photographs in Digital data storage, digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film or film stock. Dig ...
s, and can subtly darken pale blue skies – which improves contrast between sky and clouds. Strong UV filters are also sometimes used for warming color photos taken in shade with daylight-type film. They were originally developed to increase contrast in airborne surveillance photography, and were adopted by
mountaineering Mountaineering, mountain climbing, or alpinism is a set of outdoor activities that involves ascending mountains. Mountaineering-related activities include traditional outdoor climbing, skiing, and traversing via ferratas that have become mounta ...
photographers to remedy the strong UV at high altitude. 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. * The filter can be cleaned frequently without damage to the lens surface or coatings; a filter scratched by cleaning is much less expensive to replace than a lens. * If there is blowing sand the filter will protect the front of the lens from abrasion and nicks. * A few lenses, such as some of Canon's L series lenses, require the use of a filter to complete their weather sealing. Arguments against their use include: * Adding another element may degrade image quality if its surfaces are less than perfectly flat and parallel. Filters from reputable makers are very unlikely to cause any problems, but some "bargain" products are optically inferior. * The two additional reflections at air-glass interfaces inevitably result in some light loss – at least four percent at each interface, if the surfaces are uncoated; they also increase the potential for
lens flare A lens flare happens when light is scattered, or ''flared'', in a lens system, often in response to a bright light, producing a sometimes undesirable artifact in the image. This happens through light scattered by the imaging mechanism itself, ...
problems. * Low-quality filters may cause problems with autofocus. * A filter may be incompatible with the use of a
lens hood In photography, a lens hood or lens shade is a device used on the front end of a photographic lens, lens to block the Sun or other light source(s) to prevent glare (vision), glare and lens flare. Lens hoods may also be used to protect the lens fr ...
, since not all filters have the required threading for a screw-in hood or will allow a clip-on hood to be attached. Adding a lens hood on top of one or more filters may space the hood away from the lens enough to cause some
vignetting In photography and optics, vignetting ( ) is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word '' vignette'', from the same root as ''vine'', originally referred to a decorative b ...
. There is a wide variation in the spectral UV blocking by filters described as ultraviolet.


Infrared

Unlike ultraviolet filters, which are suitable for general photography as they are designed to attenuate shorter ultraviolet wavelengths and pass visible wavelengths, filters for
infrared photography In infrared photography, the photographic film or image sensor used is sensitive to infrared light. The part of the spectrum used is referred to as near-infrared to distinguish it from far-infrared, which is the domain of thermal imaging. Wav ...
are designed to block portions of the visible spectrum while passing longer wavelengths of light in the infrared spectrum, and hence they may appear dark red to black in color. Historically, the
Wratten number Wratten numbers are a labeling system for optical filters, usually for photographic use comprising a number sometimes followed by a letter. The number denotes the color of the filter and its spectral characteristics, and these numbers can be g ...
has been used to describe the spectral absorption characteristics of filters used with infrared photography. Common types include filters in the Wratten #87, 88, and 89 series; since Wratten numbers were assigned sequentially, there is no consistent logic (for instance, the #89B filter has a transition wavelength where the filter achieves 50% transmittance at approximately 720 nm, while #87 has its transition wavelength at approximately 795 nm. Because black-and-white infrared film retains significant sensitivity to blue wavelengths, sometimes red and orange filters are used to decrease contrast. Other manufacturers may embed the transition wavelength in the name of the filter. For example, the Hoya R72 (720 nm) and RM90 (900 nm) are intended for infrared photography, corresponding to Wratten No. 89B and 87B, respectively. For use with color infrared film, some manufacturers advise filters which restrict blue and green visible wavelengths, but pass most of the red spectrum, with a transition wavelength around 550 nm.


Polarizer

A
polarizing filter A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well ...
, used for both
color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...
and
black-and-white photography Monochrome photography is photography where each position on an image can record and show a different ''amount'' of light ( value), but not a different color ( hue). The majority of monochrome photographs produced today are black-and-white, ei ...
, is colourless and does not affect colour balance, but filters out light with a particular direction of polarisation. This reduces oblique reflections from non-metallic surfaces, can darken the sky in colour photography (in monochrome photography colour filters are more effective), and can saturate the image more by eliminating unwanted reflections. Linear polarising filters, while effective, can interfere with metering and auto-focus mechanisms when mirrors or beam-splitters are in the light path, as in the digital single lens reflex camera; a
circular polarizer A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well ...
is also effective, and does not affect metering or auto-focus.


Neutral density

A
neutral density filter In photography and optics, a neutral-density filter, or ND filter, is a photographic filter, filter that reduces or modifies the intensity of all wavelengths, or colors, of light equally, giving no changes in hue of color rendition. It can be a ...
(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 otherwise required for correct exposure in the prevailing light conditions, without changing the tonal balance of the photograph. A
graduated neutral density filter A graduated neutral-density filter, also known as a graduated ND filter, split neutral-density filter, or just a graduated filter, is an optical filter that has a variable light transmission. Typically half of the filter is of neutral density wh ...
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 would be burnt out.


Color filters


Color conversion

Appropriate color conversion filters are used to compensate for colour casts caused by lighting not balanced for the film stock's rated
color temperature Color temperature is a parameter describing the color of a visible light source by comparing it to the color of light emitted by an idealized opaque, non-reflective body. The temperature of the ideal emitter that matches the color most clos ...
, which is usually 3200–3400 K for use with professional incandescent light sources and 5500–5700 K for daylight. Color conversion filters attenuate a range of visible wavelengths to shift the perceived color temperature. The need for these filters has been greatly reduced by the widespread adoption of digital photography, since
color balance In photography and image processing, color balance is the global adjustment of the intensities of the colors (typically red, green, and blue primary colors). An important goal of this adjustment is to render specific colors – particularly neu ...
may be corrected with camera settings as the image is captured, or by software manipulation afterwards. These color conversion filters are identified by non-standardised numbers which vary from manufacturer to manufacturer. Many filter manufacturers use the
Wratten number Wratten numbers are a labeling system for optical filters, usually for photographic use comprising a number sometimes followed by a letter. The number denotes the color of the filter and its spectral characteristics, and these numbers can be g ...
or make reference to it. The Wratten numbers were assigned sequentially as applications were created (80''x'' and 82''x'' for blue cooling filters, 81''x'' and 85''x'' for amber warming filters), so there is no systematic logic that ties the number to its effect: for example, the 80A filter has the strongest "cooling" effect, followed by the 80B, and both are stronger than the 82C, which is stronger in turn than the 82B. The 80/85 series are regarded as "color conversion" filters, while the corresponding 82/81 series are "light balancing filters" which generally have a weaker effect than the 80/85 series. Typically, the 80A blue filter used with film for daylight use corrects the perceived orange/reddish cast of incandescent photographic photoflood lights, and significantly improves the stronger cast produced by lower-temperature household
incandescent lighting An incandescent light bulb, also known as an incandescent lamp or incandescent light globe, is an electric light that produces illumination by Joule heating a filament until it glows. The filament is enclosed in a glass bulb that is either ...
, while the 85B amber filter will correct the bluish cast of daylight photographs on
tungsten film Tungsten film is photographic film designed to accurately represent colors as perceived by humans under tungsten light; the more usual color films are "daylight films", balanced to produce accurate colours under direct light from the sun or electro ...
. To avoid confusion and to assist photographers in selecting the appropriate filter, some manufacturers, including B+W, Rodenstock, and Hoya, include or use the
mired Contracted from the term micro reciprocal degree, the mired () is a unit of measurement used to express color temperature. Values in mireds are calculated by the formula: : M = \frac, where ''T'' is the colour temperature in units of kelvins an ...
shift to name their filters, which quantifies the effect of a color conversion filter. The mired value associated with a given color temperature is computed as the reciprocal of the color temperature, in Kelvin, multiplied by 10^6: M = \frac The shift is the difference in the mired values of the film and light source. Sometimes the decamired is used, where 10 mired = 1 decamired, as the smallest perceptible color temperature change is from a 10 mired shift. \Delta M = M_ - M_ = \frac - \frac
From the equation, when the film has a higher color temperature than the light source, a negative mired shift is required, which calls for a "cooling" filter; these have a perceptible blue color, and the more saturated the color, the stronger the cooling effect. Likewise, when the film has a lower color temperature than the light source, a positive mired shift is required, which calls for an amber "warming" filter. Stacking color conversion filters creates an additive mired shift: for example, stacking a Wratten 80A (-130 mired) with a Wratten 82C (-60 mired) results in a total mired shift of -190. A typical set of color conversion filters has a geometric sequence, e.g. ±15, ±30, ±60, and ±120 mired, which corresponds approximately to the pattern of the Wratten filters, and allows intermediate values to be obtained by stacking.


Color correction

Color conversion and light balancing (LB) filters must be distinguished from color correction filters (CC filters), which filter out a particular color cast that may have various causes, including reflections from colored surfaces,
fluorescent lighting A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, to produce ultraviolet and make a phosphor ...
(which has an unbalanced spectrum),
underwater photography Underwater photography is the practice of capturing images beneath the surface of the water, often done while scuba diving, but can also be done while diving on surface supply, snorkeling, swimming, from a submersible or remotely operated und ...
, or the Schwarzschild effect (also known as
reciprocity failure In photography, reciprocity is the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material. Within a normal exposure range for film stock, for example, the reciprocity law states ...
). In general, CC filters are supplied in densities varying between 5 and 50% in primary colors, both additive (red, green, and blue) and subtractive (cyan, magenta, and yellow). They may be used for graphic effect or to compensate for differences in color balance between film batches for critical work. Fluorescent filters generally have a magenta hue, selectively absorbing excessive green light, and have a name which includes the letters FL, such as FL-D for use with daylight balanced film.


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 Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ...
industry for
color separation Color printing or colour printing is the reproduction of an image or text in color (as opposed to simpler black and white or monochrome printing). History of color printing Woodblock printing on textiles preceded printing on paper in both Ea ...
s, and again, use has diminished as digital solutions have become more advanced and abundant. Didymium filters, sold as "color enhancement" or "fall color" filters act similarly: They remove a narrow (or broad) band of color in the yellow part of the spectrum ( 589 nm). Some astronomical filters similarly use didymium in heavier concentration. Even astronomical filters which don't use didymium typically are some kind of narrow pass-band color filter.


Contrast enhancement

Colored filters are commonly used in
black and white Black-and-white (B&W or B/W) images combine black and white to produce a range of achromatic brightnesses of grey. It is also known as greyscale in technical settings. Media The history of various visual media began with black and white, ...
photography to alter the effect of different colors in the scene, changing contrast recorded in black and white of the different colours. The standard rule-of-thumb is a colored filter will selectively lighten its color, while darkening other colors, especially the
complementary color Complementary colors are pairs of colors which, when combined or color mixing, mixed, cancel each other out (lose Colorfulness, chroma) by producing a grayscale color like white or black. When placed next to each other, they create the stronge ...
, as the filter passes that color while attenuating others. 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 additionally lighten green foliage, making it stand out against the sky. Light yellowish-green filters were used as standard portrait filters for
panchromatic film A panchromatic emulsion is a type of photographic emulsion that is sensitive to all wavelengths of visible light, and produces a monochrome photograph—typically black and white. Most modern commercially available film is panchromatic, and the ...
, since they render skin-tones as light to dark grey, while darkening deep reds and blues to nearly black. A sky-blue filter (
cyan Cyan () is the color between blue and green on the visible spectrum of light. It is evoked by light with a predominant wavelength between 500 and 520 nm, between the wavelengths of green and blue. In the subtractive color system, or CMYK c ...
) 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 small blemishes invisible.


Special effects


Cross

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 Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
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. The pattern of the diffraction grating can affect the shape of the resulting highlights as well.


Diffusion

A diffusion filter (also called a softening filter) softens subjects and generates a dreamy haze (''see
photon diffusion Photon diffusion is a situation where photons travel through a material without being absorbed, but rather undergoing repeated scattering events which change the direction of their path. The path of any given photon is then effectively a random wal ...
''). This is most often used for portraits, providing an effect similar to that of a dedicated
soft focus In photography, soft focus is a lens flaw, in which the lens forms images that are blurred due to uncorrected spherical aberration. A soft focus lens deliberately introduces spherical aberration which blurs fine texture in the image while reta ...
lens. 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 Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...
of the digital
image sensor An image sensor or imager is a sensor that detects and conveys information used to form an image. It does so by converting the variable attenuation of light waves (as they refraction, pass through or reflection (physics), reflect off objects) into s ...
or film may be exceeded, which post-processing cannot compensate for, so contrast reduction at the time of image capture may be called for.


Close-up and split diopter lenses

A close-up lens is not technically a filter but
accessory lens In photography, a secondary lens or accessory lens is a lens designed to be used in conjunction with another lens, called the ''primary lens''. A secondary lens may be designed to be used either in front of the primary lens, between it and the s ...
which attaches to a lens like a filter, hence the alternative but misleading term "close-up filter". They are often sold by filter manufacturers as part of their product lines, using the same holders and attachment systems. A close-up lens is a single or two-element converging
lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
used for close-up and
macro photography Macro photography (or photomacrography or macrography, and sometimes macrophotography) is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is g ...
, and works in the same way as
spectacles Glasses, also known as eyeglasses (American English), spectacles (Commonwealth English), or colloquially as specs, are Visual perception, vision eyewear with clear or tinted lens (optics), lenses mounted in a frame that holds them in front ...
used for reading. The insertion of a converging lens in front of the taking lens reduces the
focal length The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...
of the combination.
Close-up lens In photography, a close-up lens (sometimes referred to as ''close-up filter'' or a ''macro filter'') is a simple secondary lens used to enable macro photography without requiring a specialised primary lens. They work like reading glasses, allo ...
es are usually specified by their optical power, the
reciprocal Reciprocal may refer to: In mathematics * Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal'' * Reciprocal polynomial, a polynomial obtained from another pol ...
of the focal length in meters. Several close-up lenses may be used in combination; the optical power of the combination is the sum of the optical powers of the component lenses; a set of lenses of +1, +2, and +4 diopters can be combined to provide a range from +1 to +7 in steps of one. A split diopter has just a semicircular half of a close-up lens in a normal filter holder. It can be used to photograph a close object and a much more distant background, with everything in sharp focus; with any non-split lens the
depth of field The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...
would be far too shallow.


Multi-image

A multi-image filter, sometimes called multiple image or kaleidoscopic, uses a faceted lens which generally repeats the central subject one or more times in the periphery; the images may be repeated with a radial or parallel layout.


Physical design


Materials and construction

Photo filters are commonly made from
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
,
resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
plastics similar to those used for eyeglasses (such as CR-39),
polyester Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include some natura ...
and
polycarbonate Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate ester, carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, toughness, tough materials, and some grades are optically transp ...
; sometimes
acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic, or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...
is used. Historically, filters were often made from
gelatin Gelatin or gelatine () is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also be referred to as hydrolyzed collagen, coll ...
, and
color gel A color gel or color filter ( Commonwealth spelling: colour gel or colour filter), also known as lighting gel or simply gel, is a transparent colored material that is used in theater, event production, photography, videography and cinematogr ...
s. While some filters are still described as gelatin or gel filters, they are no longer actually made from gelatin but from one of the plastics mentioned above. Sometimes the filter is dyed in the mass, in other cases the filter is a thin sheet of material sandwiched between two pieces of clear glass or plastic. Certain kinds of filters use other materials inside a glass sandwich; for example,
polarizers A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of wel ...
often use various special films, netting filters have
nylon Nylon is a family of synthetic polymers characterised by amide linkages, typically connecting aliphatic or Polyamide#Classification, semi-aromatic groups. Nylons are generally brownish in color and can possess a soft texture, with some varieti ...
netting, and so forth. The rings on screw-on filters are often made of aluminum, though in more expensive filters
brass Brass is an alloy of copper and zinc, in proportions which can be varied to achieve different colours and mechanical, electrical, acoustic and chemical properties, but copper typically has the larger proportion, generally copper and zinc. I ...
is used. Aluminum filter rings are much lighter in weight, but can "bind" to the aluminum lens threads they are screwed in to, requiring the use of a filter wrench to get the filter off of the lens. Aluminum also dents or deforms more easily. High quality filters are multi-coated, with multiple-layer
optical coating An optical coating is one or more thin-film optics, thin layers of material deposited on an optical component such as a lens (optics), lens, prism (optics), prism or mirror, which alters the way in which the optic reflection (physics), reflects a ...
s to reduce reflections. Uncoated filters can reflect up to 12% of the light, single-coated filter can reduce this considerably, and multi-coated filters can allow up to 99.8% of the light to pass through (0.2% unwanted reflection); the loss of light is not important, but part of the light is reflected inside the camera, producing
flare A flare, also sometimes called a fusée, fusee, or bengala, bengalo in several European countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illuminatio ...
and reducing the contrast of the image.


Filter sizes and mountings

Manufacturers of lenses and filters have standardized on several different sets of sizes over the years.


Threaded round filters

The most common standard filter sizes for circular filters include 30.5 mm, 35.5 mm, 37 mm, 39 mm, 40.5 mm, 43 mm, 46 mm, 49 mm, 52 mm, 55 mm, 58 mm, 62 mm, 67 mm, 72 mm, 77 mm, 82 mm, 86 mm, 95 mm, 105 mm, 112 mm 122 mm, 127 mm. The filter diameter has a steady increase from 43 to 58 mm every 3 mm and from 62 to 82 mm every 5 mm. Other filter sizes within this range may be hard to find since the filter size may be non-standard or may be rarely used on camera lenses. The specified diameter of the filter in millimeters indicates the diameter of the male threads on the filter housing. The thread pitch is 0.5 mm, 0.75 mm or 1.0 mm, depending on the ring size. A few sizes (e.g. 30.5 mm) come in more than one pitch. Most filters have a 0.75 mm pitch thread, some manufacturers use a 1.0 mm pitch thread; filters with thread pitches are incompatible with lenses with a different thread pitch. The filter diameter for a particular lens is commonly identified on the lens face by the
In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for ...
symbol. For example, a lens marking may indicate: “⌀55 mm” or “55⌀” meaning it would accept a 55 mm filter or
lens hood In photography, a lens hood or lens shade is a device used on the front end of a photographic lens, lens to block the Sun or other light source(s) to prevent glare (vision), glare and lens flare. Lens hoods may also be used to protect the lens fr ...
.


Square filters

For square filters, 2" × 2", 3" × 3" and 4" × 4" were historically very common and are still made by some manufacturers. 100 mm × 100 mm is very close to 4" × 4", allowing use of many of the same holders, and is one of the more popular sizes currently (2006) in use; it is virtually a standard in the
motion picture A film, also known as a movie or motion picture, is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, emotions, or atmosphere through the use of moving images that are generally, since ...
industry. 75 mm x 75 mm is very close to 3" × 3" and while less common today, was much in vogue in the 1990s. The French manufacturer Cokin makes a wide range of filters and holders in three sizes which is collectively known as the Cokin System. "A" (amateur) size is 67 mm wide, "P" (professional) size is 84 mm wide, and "X Pro" is 130 mm wide. Many other manufacturers make filters to fit Cokin holders. Cokin also makes a filter holder for 100 mm filters, which they call the "Z" size. Most of Cokin's filters are made of optical resins such as CR-39. A few round filter elements may be attached to the square/rectangular filter holders, usually polarizers and gradient filters which both need to be rotated and are more expensive to manufacture. Cokin formerly (1980s through mid-1990s) had competition from Hoya's 'Hoyarex' system (75 mm x 75 mm filters mostly made from resin) and also a range made by Ambico, but both have withdrawn from the market. A small (84 mm) "system" range is still made (as of 2012) by Formatt Hitech. In general, square (and sometimes rectangular) filters from one system could be used in another system's holders if the size was correct, but each made a different system of filter holder which could not be used together. Lee, Tiffen, Formatt Hitech and Singh Ray also make square / rectangular filters in the 100 × 100 mm and Cokin "P" sizes. Gel filters are very common in square form, rarely being used in circular form. These are thin flexible sheets of
gelatin Gelatin or gelatine () is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also be referred to as hydrolyzed collagen, coll ...
or plastic which must be held in rigid frames to prevent them from sagging. Gels are made not only for use as photo filters, but also in a wide range of colors for use in lighting applications, particularly for theatrical lighting. Gel holders are available from all of the square “system” makers, but are additionally provided by many camera manufacturers, by manufacturers of gel filters, and by makers of expensive professional camera accessories (particularly those manufacturers which target the movie and television camera markets. Square filter systems often have lens shades available to attach to the filter holders.


Rectangular filters

Graduated filters of a given width (67 mm, 84 mm, 100 mm, etc.) are often made oblong, rather than square, in order to allow the position of the gradation to be moved up or down in the picture. This allows, for example, the red part of a sunset filter to be placed at the horizon. These are used with the "system" holders described above.


Bayonet round filters

Certain manufacturers, most notably
Rollei Rollei () is a German manufacturer of optical instruments founded in 1920 by and in Braunschweig, Lower Saxony, and maker of the Rolleiflex and Rolleicord series of cameras. Later products included specialty and nostalgic type films for the ...
and
Hasselblad Victor Hasselblad AB is a Sweden, Swedish manufacturer of medium format (film), medium format cameras, photographic equipment and image scanners based in Gothenburg, Sweden. The company originally became known for its classic analog medium-for ...
, have created their own systems of
bayonet mount A bayonet mount (mainly as a method of mechanical attachment, such as fitting a lens to a camera using a matching lens mount) or bayonet connector (for electrical use) is a fastening mechanism consisting of a cylindrical male side with one ...
for filters. Each design comes in several sizes, such as Bay I through Bay VIII for Rollei, and Bay 50 through Bay 104 for Hasselblad.


Series filters

Starting in the 1930s, filters were also made in a sizing system known as a Series mount. The Series filters are round pieces of glass (or occasionally other materials) with no threads. Very early Series filters had no rims around the glass, but the more common later production Series filters had the glass mounted in metal rims. The Series size designations are generally written as
Roman numerals Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, eac ...
, I to IX, though there are a few sizes not written that way, such as Series 4.5 and Series 5.5. Most Series filter sizes are now obsolete, production having ceased by the late 1970s. However, Series 9 (IX) became a standard of the motion picture industry and Series 9 filters are still produced and sold today, particularly for professional motion picture cinematography. To mount Series filters on a camera lens, first an appropriate adapter is mounted to the lens, either by threading onto the lens, pushing into the lens, or clamping on to the lens barrel. Then the filter is placed in the adapter, and finally, a retaining ring is threaded into the adapter to secure the filter. In some cases, additional accessories, such as a lens hood or a second filter, can be accommodated in the adapter, or the hood itself may act as the retaining ring. Lenses designed for Series filters have a suitable adapter built-in to the front, and generally require only a retaining ring.


See also

*
Color gel A color gel or color filter ( Commonwealth spelling: colour gel or colour filter), also known as lighting gel or simply gel, is a transparent colored material that is used in theater, event production, photography, videography and cinematogr ...
*
List of photographic equipment makers This list of photographic equipment makers lists companies that manufacture (or license manufacture from other companies) equipment for photography. Camera and lens manufacturers Note that producers whose only presence in the photo industry at ...
*
Optical filter An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optic ...


Footnotes


References


External links


Photography Filters


* ttp://www.lenstip.com/115.4-article-Polarizing_filters_test_Results_and_summary.html Polarizing filters test - Results and summary - Lenstip.com* {{Photography Optical filters Filter