Filter Film Is Carefully: The finished Filter film is carefully inspected, and must come up to high standards of uniformity, clearness, and freedom from minute imperfections.
If desired, this dyed gelatin film alone can be used as a filter, but it is so delicate that it must be handled with great care when being cut to be placed between lens elements. For this reason the Filter mounted between glass is recommended for general use. Furthermore, ordinary glass is not of sufficiently high quality to be used for this purpose, since the finished Filter must compare optically with the finest lens. The finished Filter is not a mere bit of colored glass, but an accurately made optical unit.
Since a Filter absorbs some of the light which would otherwise affect the film, the exposure must be increased. The "filter factor" is a measure of the amount of light absorbed by a Filter with respect to the amount transmitted. The Filter factor represents the number times the exposure must be increased when using a Filter as compai with the exposure required without it. A Filter having a factor o with, say, Kodak Plus-X Film, will thus require twice the exposi necessary without it.
This factor depends on the filter, the color sensitivity of the fil and also the type of illumination; that is, daylight or tungsten, f example, the Filter factor of the G Filter for Kodak Super-XX Pa chromatic Roll Films and Film Packs in daylight is only 2*^. F Kodak Verichrome Film it is 5.
Below is given a Table of daylight Filter factors of the commor used filters with the different black-and-white Kodak Films.
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Photography
