Photographic filter
From Wikipedia, the free encyclopedia
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 on 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 loss of some exposure. The former is best avoided by careful use and maintenance of filters, while the latter usually will not be a problem if planned out properly.
Many filters are identified by their Wratten number.
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[edit] 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 diffractor
- Diffusion
- Sepia tone
- Spot
[edit] Clear and ultraviolet
Clear filters, also known as window glass filters or optical flats are (ideally) completely transparent, and thus not (technically) really filters at all. They are generally used to protect the front of a lens. UV (ultraviolet) filters are also often used for this purpose. While in certain cases (such as harsh environments like deserts or mud wrestling rings) this is obviously necessary, there is a debate within the photographic community as to whether or not this is a sound practice.
People in favor of the practice claim:
- 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.
Naysayers claim:
- adding another element degrades (however so slightly) image quality
- if the lens is dropped, the "protective" filter may well shatter and scar the lens it is supposed to be protecting
- it may provide a false sense of security, causing users to be less careful, or not use lens caps.
- 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.
[edit] 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 85-B 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.
[edit] 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.
[edit] 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.
[edit] 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.
There are two types of polarizing filters. A linear polarizer filter creates linearly polarized light. A circular polarizer creates circularly polarized light, by adding a second birefringent layer (typically a quarter-wave plate) to the filter after the linear polarizer. The metering and auto-focus sensors in certain cameras, notably 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.
[edit] Cross screen
A cross screen 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.
[edit] Diffusion
A diffusion filter softens subjects and generates a dreamy haze (see photon diffusion).
[edit] Materials and construction
Photo filters are commonly made from glass, resin plastics similar to those used for eyeglasses (such as CR39), polyester, or gelatin. 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 (a kind of contrast filter) 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:
[edit] Filter sizes and mountings
Manufacturers of lenses and filters have "standardized" on several different sets of sizes over the years.
[edit] Threaded round filters
For circular filters, standard sizes include 49 mm, 52 mm, 55 mm, 58 mm, 62 mm, 67 mm, 72 mm, 77 mm, and 82 mm; within this range, other sizes may be hard to find. The thread pitch is 0.5 mm or 0.75 mm, depending on the ring size.
[edit] 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. "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.
[edit] 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.
[edit] 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 VI for Rollei, and Bay 50 through Bay 93 for Hasselblad.
[edit] 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
[edit] Stuck filter removal
A stuck filter with an aluminum ring should not be removed by squeezing from both sides with your hand; aluminum deforms and this could permanently warp the filter, damaging it and also making removal even more difficult. If the filter is stuck and a filter wrench is not available, cup the whole filter with a piece of fabric and press it down against something solid. Then twist the lens barrel to unstick things.
If you have a rubber band slightly smaller than the diameter of the filter, wrapping the rubber band around the filter ring sometimes improves the grip enough to let you remove the filter by hand.
A spare shoe lace can be helpful in removing stuck filters, and it's handy to carry in your kit.
