Filters
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. Supplementary close-up lenses may be classified as filters. Linear and circular polarising filters reduce oblique reflections from non-metallic surfaces.
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.
There is no universal standard naming 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. Colour correction filters are often identified by a code of the form CC50Y—CC for colour correction, 50 for the strength of the filter, Y for yellow.
Optical filters are used in various areas of science, including in particular astronomy; they are essentially the same as photographic filters, but in practice often need far more accurately controlled optical properties and precisely defined transmission curves than filters exclusively for photographic use. Photographic filters sell in larger quantities at correspondingly lower prices than many laboratory filters. The article on optical filters has material relevant to photographic filters.
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) Ultra typically used to protect the front surface of the lens, the neutral density (ND) filter, the polarising filter 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 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.
Photo filters are commonly made from glass,resin plastics similar to those used for eyeglasses (such as CR-39), polyester and polycarbonate; sometimes acetate is used. Historically, filters were often made from gelatin, and color gels. 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 often use various special films, netting filters have nylon netting, and so forth.
The rings on screw-on filters are 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.
High quality filters are multi-coated, with multiple-layer optical coatings 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 and reducing the contrast of the image. Manufacturers brand their high-end multi-coated filters with different labels, for example:
B+W: MRC (Multi Resistant Coating), MRC nano (99.5% transmission, for XS-Pro series) Hoya: HMC (Hoya Multi Coating), HD (8-layer coating, 99.35% transmission) Heliopan: SH-PMC (8-layer coating, 99.8% transmission)
Manufacturers of lenses and filters have standardized on several different sets of sizes over the years.
The most common standard filter sizes for circular filters include 30.5 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. Thethread 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.
The filter diameter for a particular lens is commonly identified on the lens face by the ⌀symbol. For example, a lens marking may indicate: “⌀55mm” or “55⌀” meaning it would accept a 55mm filter or lens hood.
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 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 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 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.
Graduated filters of a given width (100 mm, 67 mm, 84 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 filtersEdit
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.
Starting in the 1930s, filters were also made in a sizing system known as a series mount. The filters themselves were round pieces of glass (or occasionally other materials) with no threads. Very early filters had no rims around the glass, but the more common later production filters had the glass mounted in metal rims. To mount the filters on a camera, 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 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 became a standard of the motion picture industry and Series 9 filters are still produced and sold today, particularly for professional motion picture cinematography.
Source: Wikipeida
