Color Photography - Simple Color Analysis
( Originally Published 1938 )
WHILE it is the purpose of this book to cover the various practical methods available for making photographs in natural colors, with as little theory as possible, it does seem wise that we should at the beginning include a simple analysis of color in photography.
Clerk Maxwell established experimentally in 1861 that all colors visible to the human eye may be matched by mixing in various proportions the light of the three primary colors, red, green and blue. By making photographic negative records of these colors it is possible to determine in what proportion they must be mixed to produce a photograph in natural color.
To obtain such balanced negative records of the color in the subject you wish to photograph you must make three exposures of the same subject, one through a red filter, one through a green filter and one through a blue filter.
ADDITIVE PROCESSES.—When these three exposures are made on one plate through a filter consisting of a balanced mixture of the primary colors, red, green and blue, we are able to produce, either directly or by means of viewing screens, positives such as Agfacolor plates, Lumiere Filmcolor, Finlay process and Dufaycolor film in color which must be viewed by transmitted light.
These transparency processes are known as additive, because the lights of the primary colors are added together to make white.
SUBTRACTIVE PROCESSES.—Three balanced exposures of the same subject made, one through a red filter, one through a green filter and one through a blue filter will record the color proportions of that subject in three black and white negatives. Positives or prints made in color from these monotone negatives, each in dyes or pigments of a color complementary to the color of its taking filter will, when superimposed in register, produce a picture in color. This method is known as subtractive because the colors used subtract from the white light falling on them.
COLOR BALANCE.—In the additive processes the proportionate amount of monotone rendering necessary for each color is taken care of by the manufacturer in the mixture of filter colors which is coated on the film or glass under the negative emulsion, or on the taking screen, as in the Finlay process, which is placed before the plate when exposing.
In the subtractive process, in which three separate negatives are made, the correct proportion of black and white negative rendering muSt be arrived at by varying the ratio of the exposures through the three separate color filters, viz., red, green and blue.
FILTERS.—Filters generally known as A (orange-red), B (green), C (blue-violet), are used to produce the most satisfactory monotone negative record for use with available dyes, pigments and printing inks.
COMPLEMENTARY PRINTING COLORS.—In making the exposure through the orange-red, commonly called the A filter, the blue and green light of the subject is absorbed (not recorded) so that the resultant negative record is printed in the complementary blue-green.
In the exposure made through the green filter, commonly called the B filter, the red and a portion of the blue light is absorbed (not recorded). This resultant negative is printed in the complementary magenta.
The plate exposed through the blue-violet (C) filter absorbs only the yellow light and is printed in the complementary yellow.
MEMORIZE THIS TABLE
The red filter negative prints blue-green.
FILTER FACTOR OR EXPOSURE BALANCE.—The approximate proportion of exposure through each filter is usually supplied by the maker with each box of plates. These are approximate and must be varied according to the light value.
In actual practice the exposures are varied until the densitieS of a scale of greys (grey scale) included in the subject will be identical when all three plates are developed. Do not attempt to find the correct factors or judge the balance of your color negatives except by use of the grey scale.
With this brief outline we will proceed to the actual making of photographs in color by both subtractive and additive methods. We refer those who wish to study further color theory to the reference book, listed at the close of this work.
TECHNIQUE.-TO be successful in color, one must develop by painstaking trial and error his particular technique. The problem is entirely one of technique, like learning to become master of the violin or piano, or, as in painting—practice, practice, and more practice. One can learn only by practice what to do and what not to do.
It is of the utmost importance that all processes be standardized to the fullest possible extent. In a procedure as complex as that involved in producing dependable results in full color, as many variables as possible should be eliminated. We consider it sound advice to urge that you stick to selected plates, developing formulas, temperatures, times, etc., until experience teaches you the results of any deviation from an established procedure.
WORK TABLES.-After careful reading of the instructions for any process which you intend to try, and before actually starting work, you will find it very helpful to make up a work table, enumerating in order the operations to be carried out and making notations alongside of each as listed of any special precautions to be taken. Paste this on a card and hang it near where you are working. A quick reference will remind you of what to do without the necessity of hunting through the book text.