This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
While any color filter will give effects of a kind, yet for choice results the filter must be adjusted to the plate; it naturally follows that the plate maker is in the best position to determine this point. It however, often happens that owing to movement or other causes, it is not possible to give any extended lengthening of the exposure, and retain a sharp picture, and in such cases it is decidedly desirable to sacrifice some of the exactitude of color rendering, and yet obtain superior results. In such cases it would be better to have the view only, say, half corrected, or even less. Again: in the the photographing of a landscape illuminated by the setting sun, it is obviously not necessary to use so deeply colored a ray-filter as at midday, because the view is already colored yellow.
For these reasons the Iso filters are made in three different depths of color requiring respectively double, three times, and five times the exposure necessary for an un-screen plate. They are made of two plates of the very highest grade of optical glass, which have been reground and polished plane, and cemented between them a thin film of dyed gelatine which has been spectrographically adjusted. These filters are made of such exactitude that each or all of the separate shades test precisely identical. They are intended for use behind the lens, fittings being provided with each.
The reason why a ray-filter requires to be adjusted to the plate with which it is intended to be used, is because various plate manufacturers make "iso" or "orthochromatic" plates which are as variously color sensitive as are the number of "brands," consequently the blue and violet require filtering out (or holding back) to a greater or less extent, depending upon the ability of the manufacturer to increase his color sensitiveness in proportion to the action in the blue-violet. The higher the sensitiveness to other colors the quicker the photographic action, and the less dense is the ray-filter.
The Cramer Instantaneous Isochromatic plates have
Special Instruction for Cramer Plates.
251d the highest action in the yellow green of any "iso" or "orthochromatic" plate manufactured; therefore our ray-filters require less time than any other.
While the Isos filters are specially adjusted for the Cramer plates, they will perform well with any other "Iso" or "orthochromatic" plate, as they are fitted for work with the highest type of "iso" plate.
Isos Film Filters. - For the convenience of those who desire to test for themselves the value of the Isos Ray-filters at a minimum expenditure, we have also made them up in thin gelatine film. This film corresponds to a definite measured amount of dye stuffs dissolved in a definite weighed amount of gelatin, flowed over a definite measured area. These films are guaranteed to be exact spectroscopic duplicates of the permanent cemented filters and are intended for use in front of or behind the diaphragm between the lenses. If an iris diaphragm, simply unscrew the front or back lens and drop in the film (cut to size with a scissors) or if the diaphragms are of the Waterhouse type, then slip it in with the stop. Of course these film-filters are much more easily injured than the permanent glass cemented filters and consequently require to be handled with greater care.
The Isos ray-filters are the most rapid adjusted filters manufactured: the Isos III gives correction which is not obtained by any other filter under 9 times exposure.
Isos I (Light) for objects in motion, studio portrait work, etc., and where but one-half correction is desired. Exposure equals double the time necessary for unscreened plate. Isos II (Medium) for objects and views where more time is allowable, giving about two-thirds full correction. Exposure equals three times. Isos III (Dark) for copying paintings, blue prints, etc., and for use in landscape, portrait and genre photography. Exposure equals five times.
With the Trichromatic plate we do not recommend Isos III for general work: Isos II gives full correction. For the copying of blue prints however, the Trichromatic and Isos III gives positive results.
Ray-Filters for Photomicroscopists and Commercial Photographers. - Color screens of this type form a class by themselves. For the commercial photographer who is engaged in the copying of numberless colored articles, furniture, etc., or for the copying of colored labels, writings, and the thousand and one things which go to make up the ordinary line of work, their use is simply invaluable. This class of ray-filters are known as "Contrast Filters" and by their aid the worker can subdue or eccentuate whatsoever color or line he desires. Objects of any color can be represented either with full detail, or full contrast, (either as black or white) according as the filters are used. The Cramer Company stand ready to supply any information on this point which may be required.
Photomicrographic ray-filters are a still different type of article, and represent a color screen which can not be made use of as in ordinary photography. They are simply intended for use between the illuminant and the object and for such purpose do not require to be made on optical glass. Their colors are spectroscopically adjusted to the transmission (or absorption) of the stains most commonly in use by micrographic workers. By their use the microscopist is enabled to obtain results in the photography of stained sections, and other preparations which would be impossible except by making use of complicated and expensive spectral light apparatus.
Monochromatic Color Filters for the use of spectro-scopists, physicists, etc., are also manufactured by the Cramer Company, who have installed a laboratory equipment solely for that purpose, and who are prepared to furnish color filters of any absorption or transmission for any purpose, and upon surfaces of any degree of accuracy required up to a measured error of 1/80,000 of an inch.
Special Instruction for Cramer Plates.
Tri-color Filters. - There is perhaps no class of color filters in which so great a variance prevails as in filters intended for three-color work. Almost every manufacturer of photographic specialties has at some time or another during the course of his business, produced a set of so-called "adjusted tri-color filters" which, however, are absolutely worthless when it comes to fulfilling theoretical or practical conditions. There are two causes to blame for this condition: First, plate errors, viz., insensitive color regions; and second, imperfect filter absorption, viz., lack of theoretical knowledge. The first of these imperfections has been overcome by the even sensitiveness of the Cramer Paniso plate and the second by placing such work under the direction of an authoritative theoretical and practical worker. The Cramer tri-color filters are not only designed to transmit regions which the concensus of opinion of the world's best worker is however the hues of the three-color printing inks upon which they are coated is tested not only for parallelism, but also for thickness and planeity. This exhaustive testing in the laboratory results in a combination of filters and plate in which the images are of absolutely the same size in all three plates, and the gradation of the copy is faithfully preserved: the optical adjustment of the filter dyes assures clean and correct "cut outs."
The chief source of trouble to the present day American worker is however the hues of the three-color printing inks supplied him by the ink manufacturer which are not nearly correct.
The G. Cramer Dry Plate Company have recently established a Research Laboratory for this and kindred work, under the direction of R. James Wallace, formerly Head of the Department of Photophysics of the University of Chicago, and the highest authority in the United States upon all matters connected with the photography of color. This Laboratory is equipped with spectroscopic and general physical apparatus of the most refined, modern type, and its establishment constitutes an innovation in the dry plate industry of America, which is not even approached by any other manufacturer. Consultation upon all matters connected with the photography of color will receive courteous consideration.