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. 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)
Read more about this topic: Photographic Filter
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