Filter Factors for Camera Filters

Filter Factors for Camera Filters

  • Since filters work by subtraction, it naturally follows that the use of filters carries with it an exposure penalty. The only exception is the ultraviolet-absorbing filter: since the radiation being absorbed by a UV filter is ultraviolet, and is not being significantly measured by most light meters, little or no exposure compensation is needed when using ultraviolet-absorbing filters. This penalty, however, is not entirely due to light absorption by the filter. A portion of it comes from the reflection of light from filter surfaces. Thus, the light transmitted by a given filter is the light that is neither reflected nor absorbed.

  • The amount of light reflected from a surface of a filter is small in the center, but slightly larger at the edges, due to the different angles of incidence of the light rays involved. The difference is unimportant if only one filter is in use, but could become a problem if filters were "stacked."

  • The use of filters necessitates an increase in exposure to compensate for the light that is not transmitted by the filter. This exposure compensation is usually done by computing from a filter factor: a number related to the effective decrease in exposure caused by the particular filter. The numbering system follows the doubling plan of other exposure compensation methods, reflecting the fact that a one-stop difference in exposure either multiplies or divides it by two. Thus, a filter factor of 2, for example, requires doubling the exposure (opening the diaphragm by one stop, or slowing shutter speed by one stop); a factor of 4 requires four times as much exposure (opening up two stops, slowing the shutter two stops, or doing one of each). Or you multiply the exposure time calculated with your meter by the filter factor. There are different factors for different light sources such as daylight and tungsten lighting. Since the percentage of the various wavelengths present in the light differs, the filters do not transmit the same percentages. You may have to make tests to determine the actual factors for given filters.

  • If you use more than one filter at a time, the filter factor is a multiple of the individual factors. Thus, if one filter has a factor of 2.5 and the other a factor of 4, the combined factor will be 10. Using more than two filters, then, could require prohibitively long exposures.

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