Materials and construction 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.
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
⌀ symbol. For example, a lens marking may indicate: “⌀55 mm” or “55⌀” meaning it would accept a 55 mm filter or
lens hood.
Square filters with
Cokin "A" graduated neutral density filter 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 mm and Cokin "P" sizes. warming (left) and cooling (right) light balancing gel filters for color conversion from the
Wratten 81 and 82 series, respectively 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.
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 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 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, 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==