Black-and-white processing and printing
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- history of photography
Negative development
Amateurs usually process films in developing tanks. In this type of development roll or miniature film is wound around a reel with a spiral groove, which keeps adjacent turns separated and allows access by the processing solutions. Once the tank is loaded (in the dark), processing takes place in normal light, the processing baths (developer, intermediate rinse, fixer) being poured into the tank at the appropriate intervals. Sheet films are similarly treated in small tanks or held in hangers and immersed sequentially in the different processing solutions. Large-scale commercial processing laboratories use machines that automatically feed the films through the solutions in proper sequence.
Developers and their characteristics
The developer consists typically of one or more developing agents, a preservative (such as sodium sulfite) to prevent oxidation by the air, an alkali (such as sodium carbonate) to activate the developer, and a restrainer or antifoggant to ensure that the developer acts only on exposed silver halide crystals. A developer’s main characteristics are activity, development speed, and effect on film gradation, graininess, and sharpness. Developers may be prepared on the basis of published formulas or bought as ready-mixed powders or concentrates for dilution with water.
The developer is allowed to act for a specific time to build up the image to the required density and contrast. This time depends on the developer, the temperature, the degree of agitation, and the film—as indicated by recommendations from film and developer manufacturers.
Fixing
The fixing bath contains a chemical (sodium or ammonium thiosulfate) that converts the silver halide into soluble, complex silver salts that dissolve in the fixer. During this process the film loses its original silver halide milkiness overlaying the image and becomes clear. The fixer also contains a weak acid (to halt the development process) and a hardening agent to reduce gelatin swelling.
Washing and drying
Washing removes all residual soluble chemicals from the emulsion and must be thorough for image permanence. Films are hung up to dry after removal from the tank.
High-speed processing
Greatly reduced processing times are possible with high-activity developers at elevated temperatures and with fast-acting fixing agents, such as ammonium thiosulfate. Such processes can cut access time to the negative down to less than a minute. One-bath (monobath) processing in a solution containing both a fast-acting developing agent and fixing chemicals also reduces processing time. In special rapid-access processing equipment, films pass through chambers spraying the processing solutions onto the film surface or run in contact with monobath-soaked webs.
Printing
The simplest printing equipment is the contact printing frame in which the negative and printing paper are held together behind a glass plate during exposure to a suitable lamp. A printing box is essentially a printing frame with a built-in light source. Contact printing gives a positive of the same size as the negative.
Enlargers
Negatives usually are enlarged to prints of the desired final size. The enlarger is a projection system on a vertical column mounted on a horizontal baseboard. It has a lens, a film holder (negative carrier), and a lighting system (typically a lamp and condenser lens) for illuminating the negative. Raising or lowering the enlarger head on the column controls the image magnification; adjustment of the lens-to-negative distance focuses the image on the enlarging paper on the baseboard. In enlargers that focus automatically these two adjustments are linked mechanically to keep the image sharp all the time. Enlargers are made in various sizes to take different maximum negative formats.
Printing papers
Papers for enlarging and contact printing are produced in grades of differing exposure range—i.e., ratios of shortest to longest exposure to produce the lightest tone and a full black, respectively. The various grades yield prints of a normal tone range from negatives of different contrasts: a soft paper grade for a high-contrast negative, a normal paper for a normal negative, a hard paper for soft negatives, and so on. Paper grades are also numbered—typically from 0 to 5—in ascending order of contrast. Variable-contrast papers use a mixture of two emulsions of a different contrast and colour sensitivity; the contribution of each is controlled by filters in the path of the exposing light.
Other characteristics of printing papers are the speed (slower for contact papers, faster for enlarging papers), image colour (blue-black to warm brown), surface texture (glossy, velvet, mat), and base thickness (single or double weight). Most printing materials use a resin-coated (plastic-laminated) paper base that absorbs no water during processing.
Printing exposures
Correct printing exposures are determined by trial and error or by test strips given a series of progressively increasing exposures. More sophisticated exposure control systems measure either the brightness of selected image portions projected on the enlarger baseboard or the average light intensity reaching the paper during the exposure. Printing papers are exposed and processed in a darkroom lit by an olive-green or orange safelight. Printing papers are sensitive to violet, blue, and sometimes green light.
Print processing
The processing of prints consists of development, an intermediate rinse or stop bath, fixing, and washing. The developer and fixer are similar in principle to those used for negative films. In the normal method, dish or tray processing, prints are immersed successively in the solutions in dishes laid out side by side. Development is checked visually, the print remaining in the developer until the image has reached its full density. For drying, the prints may be clipped to a line, placed in a heated print dryer, or squeegeed onto a mirror-finished plate for a high-gloss surface.
Stabilization processing
Certain rapid-processing papers incorporate developing agents in their emulsions and are processed on a roller processor. This processor runs the paper through an activating bath for instant development and then through a stabilizing bath, followed by a pair of squeegeeing rollers from which the print emerges merely damp. This process takes about 10 to 15 seconds; the prints, however, do not keep quite as well as conventional prints, since unexposed silver salts are not removed from the emulsion but only converted into moderately light-stable compounds. Such prints can be made more permanent by subsequent fixing and washing.
Dry processing
Processing baths can be completely eliminated by incorporating in the emulsion of the paper development and stabilization chemicals that become active on heating. One method is to disperse the processing chemicals in the emulsion in microscopic capsules containing the solution and a blowing agent. On passing the exposed paper over a heated roller, the blowing agent bursts the capsules, and the liberated processing solutions act on the silver halide immediately around each capsule. The liquid solvent instantly evaporates, leaving a dry print. Encapsulation materials are used for such purposes as making proof prints of negatives and reenlarging microfilm images. Certain non-silver processes in photocopying systems also offer dry processing.