This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917.
Color filters used with ortho-chromatic or panchromatic plates necessarily increase the exposure that must be given, because the filters cut out a portion of the light to which the plates are most sensitive. It is not possible to make a plate that is not much more sensitive to violet and blue than to the other colors of the spectrum and ordinary plates are not appreciably sensitive to any other colors than violet and blue.
Orthochromatic plates, however, are sensitive to yellows and greens, but they are so much more sensitive to violets and blues that filters must be used to cut out some of these rays of light, otherwise the plate would be fully exposed before the light from yellow or green objects could be recorded, and the value of the plate's color sensitiveness would be lost.
The amount of light the filler absorbs or cuts out determines the additional amount of exposure that must be given when the filter is used. Thus, if the proper exposure without a filter is one second and the filter cuts out three-fourths of the effective light the exposure with the filter must be four seconds. This would be known as a four-times filter or a filter having a factor of 4. If the filter gave greater correction by cutting out nine-tenths of the effective light, the filter would increase exposure ten times and would have a factor of 10.
Filter factors are misleading unless the basis for determining the factors is known. A filter that is recommended for securing cloud effects may have a low factor, because it is based on sky rather than foreground exposures. Such factors must be doubled or trebled to give proper results when detail in the foreground of landscapes is desired.
The factors for Wratten Filters are based on shadow detail and may seem high, but in every case where cloud effects are desired in preference to detail in the foreground of the subject, the K filter factors of 3 or 15 may be reduced to 1 or 5 and the best results secured.
It must also be understood that a Wratten Filter, say the K 2, when used with ordinary orthochromatic plates, has a factor of 15 and gives five times as much correction as the K 1 filter with a factor of 3, because it cuts out five times as much of the excess of blue light and is five times as efficient in recording the yellows and greens to which the orthochromatic plate is sensitive.
It is not possible for any filter, giving the same amount of correction as a Wratten Filter, to be used with less exposure and produce the same results on the same plate or film. It is possible, however, for a filter to be darker and require more exposure than a Wratten Filter without giving more correction. In such cases the filter is not of the correct color to absorb the maximum amount of blue light. The color correction a filter gives is entirely a matter of its correctness of color and not its darkness of color.
To make this point plain, it may be cited as an example, that some of the so-called ruby glass is not safe in color for developing lights because it is not pure in color and transmits a considerable amount of blue light, even though it is darker than other ruby glass that is much safer to use.
Filter factors also vary with the color sensitiveness of different plate emulsions. This is entirely a matter of the quality of the plate emulsion, as the filter does not change. With Wratten Panchromatic Plates the K 2 filter has a factor of 3, and the K 1 filter has a factor of 1/4. These factors are very low considering the fact that they are based on full exposure for detail in shadows, but this is due to the fact that these plates are extremely sensitive to other colors than blue and violet, and blue and violet are the only colors which the Kl and K2 filters retard.
The K 3 filter is not recommended for use with orthochro-matic plates because it is especially made to fit the wider color sensitive range of the panchromatic. It may be used with the orthochromatic plates, but does not materially improve the results that may be secured with the K2 filter, which requires much less exposure.
It is well to remember in using orthochromatic plates that a filter that will give the best rendering of yellows and greens that can be secured must necessarily have a factor of about 15 for shadow detail, but that this factor may be reduced to 5 when shadow detail may be sacrificed for clouds or sky. This amounts to the same thing as a shorter exposure for such subjects.
It is also well to remember that factors of from 3 to 5 recommended for filters are either based on sky and clouds, and must be increased to 10 or 15 for shadow detail to avoid underexposure, or the filters with such factors are not of a color that will give the best color rendering that can be secured on a good orthochromatic plate.
From An Artura Iris Print By H. Mishkin New York.
From An Artura Iris Print By H. Mishkin New York.