From minilabs to huge laboratories, a variety of processing services are available to both amateur and professional photographers. Many photographers, however, prefer to develop their film and also make prints and enlargements themselves. They find it a satisfying activity that provides many possibilities for creativity and self-expression.
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.
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 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.
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.
The illumination system is made up of the light source, usually an incandescent lightbulb but sometimes another type of bulb such as fluorescent or halogen, plus an optical system for directing the light efficiently to the film. One type of optical system is the condenser, a system of lenses that focus the beam of light through the film and toward the enlarging lens. Another type is the diffuser, which scatters the light from the bulb so that it falls evenly across the film. Light sources and optical systems are chosen depending on the type of film being used and the characteristics desired on the enlarged print. Condensers are used for prints with fine detail and high contrast, whereas diffusers are used to minimize blemishes. A filter drawer between the light source and the film permits the insertion of colour filters or variable contrast filters.
Traditional film-based photographic enlarging has declined considerably with the switch to digital photography, its place being taken by printers such as the ink-jet. However, there is still a use for film-based enlargers in some commercial and art photography where the qualities of photosensitive paper are desired. In addition, some digital enlargers can work from a digital image file instead of a film negative. Some read the file and then re-create the image on a liquid-crystal display (LCD) panel that is inserted into the projection assembly in the place of the film negative. In others, a laser spot can scan directly across a photosensitive material without using an LCD.
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.
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. After development is complete, the film is rinsed in a stop bath. This bath halts the chemical action of the developer and prevents contamination of the following solution. Packaged stop baths are available, but many photographers prepare their own using a mixture of acetic acid and water. The next step in processing is to place the film in a fixing bath. This dissolves any undeveloped particles of silver halide and leaves only the developed grains of black, metallic silver. A commonly used fixer is a solution of hypo mixed with water.
Depending on the hypo's freshness, such a fixing bath may take ten minutes or longer to be fully effective. There are a number of rapid or high-speed fixers available, however, that require only a couple of minutes or less.
After fixing, the film is thoroughly washed to remove any residual chemicals and to ensure the permanence of the negative. 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.
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.
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.