Photographic filter

In photography, 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 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.

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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,^[1][2] and can reduce purple fringing in digital cameras.^[3] 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 and against use of protection filters incude:^[4]

For:

• 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.

Against:

• 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.

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.^[5] Also in lower quality filters, problems with autofocus and image degradation have been noted.

Color correction

A major use is to compensate 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.

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 proliferated.

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

A polarizing filter, used both in color and black and white photography, can be used to darken overly light skies. Because the clouds are relatively unchanged, the contrast between the clouds and the sky is increased. Atmospheric haze and reflected sunlight are also reduced, and in color photographs overall color saturation is increased. Polarizers are often used to deal with situations involving reflections, such as those involving water or glass, including pictures taken through glass windows (this uses the phenomenon of Brewster's angle) .

Polarizers are the type of filter whose use is least affected by digital photography; while effects that may visually resemble the results of a polarizing filter can be simulated with software post-processing, many of the optical properties of polarization control at the time of capture simply cannot be replicated, particularly those involving reflections.

There are two types of polarizing filters. A linear polarizer filter transmits one of two states of linearly polarized light. A circular polarizer (sometimes called a CPL filter) similarly selects a linear state but then converts it to circularly polarized light, by adding a birefringent layer (typically a quarter-wave plate) to the filter after the linear polarizer. The metering and auto-focus sensors in certain cameras, including virtually all SLRs, will not work properly with linear polarizers, both because of the mirror and because of the beam-splitters used to split off the light for focusing and metering. Circular polarizers will work with all types of cameras.^{[citation needed]}

Neutral Density

A Neutral Density (ND) filter creates a reduction in light that is neutral and equal for the film or sensor area. This filter is often used to allow for longer exposure times whenever a longer exposure would normally create over exposure in the camera.

A Graduated Neutral Density (GND) filter is a neutral density filter that varies the effect with a gradient so it can be used to compress dynamic range across the entire scene. This can be beneficial when the difference between highlights and shadows of a scene are too great to allow for proper exposure for both.

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. However, this 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 provide a very precise degree of control of the level of effect, however the "look" may be noticeably different. Additionally, 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

Various widths, colors (often black or white), and grid shapes (typically diamonds or squares) and spacings of netting, usually made from nylon, are used to provide diffusion effects. These are used both for the "dreamy" look and for contrast reduction. The homebrew approach to this sort of effect is generally to stretch a piece of pantyhose material in front of the lens.

Diopters and split diopters

Some argument could be made as to whether these are technically filters at all, or actual accessory lenses, however they are sold by filter manufacturers as part of their product lines, using the same holders and attachment systems. Diopters are simple single or two-element lenses used to assist in close-up and macro photography. They provide some number of positive optical diopters, which magnify the subject and allow objects very close to the lens to be brought into focus. They are sometimes sold singly, and sometimes sold in kits of +1, +2, and +4 diopters, which allows them to be combined to produce a range from +1 to +7.

A split diopter is a diopter in which only half of the camera's lens area is covered by the filter. A round split diopter has a usual filter ring, but is filled with only a semicircle of glass (or plastic). This allows the photographer to photograph an object which is very close against a background much further away, effectively extending depth of field. Careful composition is required to make effective use of this device.

Materials and construction

Photo filters are commonly made from glass, resin plastics similar to those used for eyeglasses (such as CR39), polyester and polycarbonate; sometimes acetate is used. Historically, filters were often made from gelatin, and color gels, also called gelatin or simply gel filters are still used, but these are no longer actually made from gelatin, generally being instead made from one of the plastics mentioned above.

Sometimes a color is blended throughout the filter material, in other cases the filter is a sandwich composed of a thin sheet of material surrounded and supported by two pieces of clear glass or plastic.

Certain kinds of filters use other materials inside a glass sandwich; for example, polarizers often use various special films, netting filters have nylon netting, and so forth.

The rings on screw-on filters are most often made of aluminum, though in more expensive filters brass 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. (See "Stuck filter removal" below.)

High quality filters have multiple layers of optical coating to reduce reflections and to allow more light to pass through the filter. Uncoated filters can block up to 9% of the light, while multi coated filters can allow for up to 99.7% of the light to pass through. Manufacturers brand their high-end multi coated filters with different labels, for example:

• Hoya: HMC (Hoya Multi Coating)
• B+W: MRC (Multi Resistant Coating)

Reflections can lead to flare and reduced contrast. Multi-layer coatings, which reduce this effect, are highly desirable in any filter.^[6] Exceptions to this rule are infrared and ultraviolet photography, where uncoated filters are usually used; multi-coated filters have a tendency to reflect more wavelengths outside the visible spectrum, making them unsuitable for such purposes.

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 mm, 37 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, 112 mm and 127 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.

Filter diameter for a particular lens is commonly identified on the lens face by the ligature "ø". For example, a lens marking may indicate "ø 55mm."

Square filters

For square filters, 2" x 2", 3" x 3" and 4" x 4" were historically very common and are still made by some manufacturers. 100 mm x 100 mm is very close to 4"x4", 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 industry. 75 mm x 75 mm is very close to 3" x 3" and while less common today, was much in vogue in the 1990s.

A French manufacturer called 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 "system" range is still made (as of 2005) by 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 and Singh Ray also make square / rectangular filters in the 100 x 100 and Cokin "P" sizes.

Gel filters are very common in square form, rarely being used in circular form. These are thin flexible sheets of 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 (100 mm, 67 mm, 84 mm, etc.) are often made rectangular, 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 and Hasselblad, have created their own systems of bayonet mount 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

From the 1930s through the late 1970s, filters were also made in a sizing system knows as a series mount. The filters themselves were round pieces of glass (or occasionally other materials) with no threads or rings attached. Instead, the filter was placed between two rings; the mount ring either screwed into the lens threads or was slipped over the lens barrel and the retaining ring screws into the mounting ring to hold the filter in place. The series designations are generally written as Roman numerals, I through IX, with the interesting exception of the series 4.5 filter. Retaining Ring sizes include:

• Series VII - 54.346 mm diameter, 36 tpi thread pitch

Series filter number to mm conversion:

IV = 20.6 mm
4.5 = 25.5 mm
V = 30.2 mm
5.5 = 35.9 mm
VI = 41.3 mm
VII = 50.8 mm
7.5L = 57mm
VIII = 63.5mm
8.5/5.5L = 74.8 x 5.6mm
8.5/8mm = 74.8 x 8mm
IX = 82.6 mm
93 = 93mm
103 = 103mm
107 = 107 mm
119 = 119mm
125 = 125 mm
138 = 138 mm

Stuck filter removal

Filter rings are generally made from either aluminum or brass. Lens barrels, particularly the threads to which filters attach, are usually made from aluminum. Filter rings, particularly aluminum ones, can sometimes "bind" to the aluminum lens threads and be difficult to remove. Aluminum is a relatively soft metal; attempting to remove a stuck filter by squeezing with the hand generally puts a lot of inward pressure on just the two areas being gripped; this can bend and deform both the filter ring and the lens threads, permanently weakening or damaging both and making the filter even more difficult to remove. Methods should be employed that apply pressure evenly around the filter ring. Typically this is achieved either by use of a filter wrench or by cupping the filter ring and front of the lens with a piece of fabric to protect them and provide friction, then pressing the combination against a hard surface and twisting the lens barrel. Other aids to stuck filter removal include using either a tightened rubber band or shoelace around the rim of the filter to improve grip.

See also

• Filter (optics)
• List of photographic equipment makers

References