( Originally Published 1963 )
Film.—Process film consists of an acetate or other clear plastic base, coated on one side with a light-sensitive solution and on the other side with a gelatin backing to keep film from curling as it dries. Most all films have what is called an antihalation backing, which is a dye coating placed between the emulsion and the film base to prevent internal reflections.
Types.—There are many kinds of film, and each type has its own particular purpose. Film of wide latitude and contrast for the production of line and top-quality halftone negatives is required for use in the Government Printing Office.
Photographic emulsions.—The emulsion on sensitized materials consists of a colloid such as gelatin, silver halides, and other dye additives to produce certain effects. The silver salts are color sensitive to the blue portions of the spectrum only, and to increase the sensitivity to red and green, color sensitizing dyes are added to the photographic emulsion. It can thus be seen that emulsions differ in their response to various colors of light. This and other variables make it possible to choose a film for any type of photographic work. Normal or color-blind film is sensitive only to blue and violet colors. Orthochromatic film is sensitive to all colors except orange and red. Panchromatic film is sensitive to all colors but least of all to green.
Dimensional stability.—The dimensional stability of film is of concern in a number of cases, such as reproduction of mechanical drawings, maps, color separations, and other graphic-arts uses. Photographic film is essentially a laminate of two chemically different materials, which are affected differently by environment and age, causing change of size. The factors influencing dimensional change are humidity expansion or contraction caused by gain or loss of moisture from the air, temperature changes, and processing shrinkage. When stable size is of importance, polystyrene or other plastic base film is used. This film is manufactured to hold dimensional changes to an absolute minimum.
Chemistry.—The chemistry of photography will he treated here in an elementary manner.
Film.—As has been stated previously, the emulsions of film contain silver salts such as silver bromide, silver iodide, and silver chloride, mixed with a colloidal substance, such as gelatin, which holds the particles in suspension. When the emulsion is exposed to light, a latent image is formed. This latent image is the actual conversion of minute particles of silver halide to metallic silver. The latent image is converted to a visible image when the film is developed, by the chemical reduction of the silver halides to black metallic silver.
Developer.—Developing solutions consist of (I) solvent, such as water; (2) reducing agent, metal hydroquinone; (3) preservative (or antioxidant), sodium sulphite; (4.) restrainer, potassium bromide; and (5) accelerator, paraformaldehyde. Developers also contain other additives to achieve various effects.
Fixing.—When development has been completed, the areas of silver not affected by exposure must be removed to make the image permanent. The unexposed areas are removed from the film in the fixing bath. The fixing bath contains chemicals which act on the unexposed silver image. The composition of a fixing bath is (1) water, (2) a silver halide solvent such as sodium thiosulphate or ammonium thiosulphate, and (3) a hardener such as potassium alum.
Drying.—During the course of processing in the various solutions and washing, the base of the film has absorbed considerable moisture and this must be removed before the film can be further processed. Small films can be dried in room air without difficulty, but the most practical method of drying is to place it in a chamber where moisture is mechanically removed from air and continuously recirculated and heated to an optimum temperature for drying. Heated air will hold much more moisture than cooler air. In recent years, plastic bases have been formulated that are virtually impervious to moisture, and these bases permit very rapid drying of processed film, since virtually all of the absorbed moisture is contained in the very thin layer of the light-sensitive coating.
AIDS TO NEGATIVE PRODUCTION
Filters.—There are two basic uses for filters in negative making. In one, filters are used to emphasize tonal areas in making reproductions from colored or soiled copy. The other use is in making color reproductions from color copy. A filter is a device which transmits a specific amount of light of certain color or colors and absorbs light of other colors. Each filter has a factor which will require additional exposure. This must be considered when determining the proper exposure time.
Densitometers.—The densitometer is used to measure density by the use of transmitted light, which is defined as a standard means of ex-pressing tonal value in the form of a standard series of numbers. These numbers are logarithmic values of the amount of transmitted light.
Gray scales.—Gray scales are a valuable aid in classifying copy. A gray scale is a strip of photographic paper containing tones ranging from white through intermediate grays to black. By comparing tone steps of the scale with the copy, a basis for exposure can be determined.
There are many other aids to negative production such as magnifiers, thermometers, timers, slide rules, and numerous others, all serving a useful purpose in enabling the photographer to produce quality negatives.