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[MPROVEMENT IX MOTION PICTURE FILM :<7
So much from the practical experience of the cameramen. It may be of help to the new users of this new film to mention the subject of filter factors. D. R. White of the DuPont Film Company's research laboratories, explains3 that the filter factors for the DuPont fast film and the DuPont regular panchromatic are the same within very close limits. He points out that the lens stop or lighting used with the fast film should be reduced to take account of the increased speed, but, points out that the filter factors themselves are essentially unchanged.
"As all successful filter users know," says Mr. White, "the correct filter factor for a given set of conditions depends upon three things: the filter, the lighting and the sensitivity of the film. It is, of course, usual to specify a certain type or source of light and prepare a table showing the factor by which the stop should be increased to make up for the light absorbed by each of a series of filters. This is thoroughly satisfactory only when the light source is constant in quality from time to time and place to place. Daylight is not constant in either way, but must be used for so much of the work where a filter is needed that filter factors for it are of great interest."
Mr. White then gives the following table of filter factors for the Wratten filters for sunlit scenes for both the regular DuPont panchromatic and the new fast DuPont negative:
Filter Filter factor
K 1 2.2
K 2 3.1
K 3 4
G 5
F 10
A 7
B 16
C 12
These factors were obtained in sunlight and show the factor by which aperture or time should be increased to compensate for filter absorption, explains Mr. White. And he goes on to say that where the light quality is not of sunlight these factors cannot be expected to hold accurately, and a test shot should, of course, be made if the work is quite critical. Experience, he points out, has shown that for shadow shots, when there is an absence of sunlight but a predominance of light from the blue sky, the factors for blue absorbing filters should be increased somewhat over the values given; and for the blue transmitting filters, if used, somewhat decreased.
Dr. L. M. Dieterich, A. S. C, has worked out a rather comprehensive table4 governing the use of standard filters with the Eastman panchromatic Type 2 negative and the new Eastman supersensitive, or fast, film. This table which is shown on page 98 can be taken as approximately correct, and should be of considerable value to the cameraman. However, it must always be remembered that the multiplication factors for various filters are only an approximate guide and subject to sometimes radical changes when either stop values or shutter openings are selected. The color values of the object, the character of the illumination or light source, the actinic spectrum reaction of the emulsion, the desired emphasis of certain colors, special effects