In analog photography, development is the chemical amplification ( photographic chemistry , photochemistry ) of the image, which is invisible after exposure, on plate , film or photo paper and other photographic materials to form a visible negative or positive . The chemical reactions take place in an aqueous solution of suitable chemicals.
Although the actual development process is only the first part of the entire photochemical processing process, the term is generally equated by end users with the entire processing chain: starting with film development, continuing with fixing and film cutting to the production of enlarged positive images from the negatives . The subsequent digitization of the negatives, photos and slides recorded with analog technology and the storage of the digital data obtained on data carriers are also part of the development with this approach.
For photographers and cameramen , “development” only means the photochemical development and the fixing of the recording material (negative or reversal film). The development is only part of the optical-photographic or digital post-processing.
The process was invented by Maddox (1871) and Bennet (1878) about 130 years ago and has now been largely superseded by digital cameras.
Latent image and development
During the exposure of a film, only relatively few of the silver ions (Ag + ) present in the silver salt crystals of the photo emulsion layer are reduced to metallic silver atoms by the action of light . These silver nuclei form the “latent image”, which is invisible due to the small amount and size of the silver nuclei. The developer autocatalytically enlarges the small silver crystals (Ag) present at the exposed areas . The silver ions (Ag + ) of the silver salt that are in the immediate vicinity of germs are also reduced to silver. The finely distributed metallic silver acts in transmitted light z. B. black in an enlarger . The picture is now visible. When developing orthochromatic and unsensitized materials, this process can be observed in subdued, red light with eyes that are sufficiently dark-adapted.
The processes are called chemical or physical development. In spite of the misleading, historically determined designation, physical development - like any development of photographic material - is a chemical process in which not the exposed parts of the silver halide used as the image carrier, but ultimately the unexposed parts remain on the film. The name was probably coined to differentiate it from conventional chemical development, in which the silver halide is reduced to metallic silver in the development process at the development centers created by exposure (one also speaks of the latent image or the silver nuclei). In contrast to this, in physical development the exposed film material is fixed first. The silver halide is dissolved out of the photographic layer. Only the latent image remains.
Development of a black and white film
Film developing tank
Since the exposed film has to be developed in complete darkness, a light-tight developing container is generally used in the home photo laboratory. First, the film is wound into the spiral-shaped spool of the developing box in the darkroom or in a light-tight changing bag. The film spiral provides the necessary space between the film windings and thus ensures that the photo baths have an even effect. Depending on the model, one or more of these coils fit into the light-tight development tank. After closing the container lid, which is equipped with a filling / pouring device for liquids, the following steps take place under normal lighting. As a special feature, the daylight box shown in the example photos (Agfa-Rondinax, patent in GB 1935, D unknown) enables film to be developed entirely without a darkroom, as the built-in winding device enables film to be wound in with the lid closed.
The film is processed with a developer bath, usually at 18 ° C to 20 ° C, according to the manufacturer's time and temperature specifications. The developer solution is mostly basic (pH value 8 to 9). By varying the development time, both the film and the paper image can partially correct the density and contrast.
The movement during development is necessary to prevent local differences in the concentration of the developer substances in the solution. In the case of the daylight box shown, the film spiral is rotated with the rotary control on the side , with the development boxes common today by regularly tilting the box, which is closed with an additional watertight lid.
The more intense the movement takes place during development and the higher the temperature, the shorter the development time required. However, a shorter development time leads to a coarser graininess, which is why the values recommended by the manufacturers for the rhythm of movement and for the temperature of the developer fluids should be carefully observed in order to achieve the best possible result.
The development itself already gives a visible image, which is not stable under the influence of daylight: the silver bromide (AgBr) still present in the unexposed areas would over time, solely through the action of light, become more and more silver (Ag) and bromine (Br 2 ) converted, so become black. With conventional black and white materials, the visible image consists of metallic silver.
As soon as the development time prescribed by the manufacturer has passed or as soon as the desired gradation has been reached, the development is interrupted by an interruption bath. An acidic bath (pH 4 to 5, usually two percent acetic acid or a corresponding citric acid solution ) is used to neutralize the mostly basic developer. An intermediate wash alone is not sufficient to stop the development process, especially with undiluted developer. There are developer substances that continue to work in an acidic solution. In these cases the development is immediately followed by fixing.
The fixer serves the purpose of making the developed image permanently lightfast. The fixing bath ( sodium or ammonium thiosulphate solution ) converts the sparingly water-soluble silver salts into an easily soluble form. Since the metallic silver is also attacked at the same time, the fixing takes place according to the manufacturer's instructions and depending on the exhaustion of the reusable fixing bath. Two to three times the so-called clearing time can be used as sufficient fixing time for films and plates. The clearing time is the time after which the initially cloudy image layer has become clear. With freshly prepared express fixing baths based on ammonium thiosulphate, the clearing time for 100 ASA films is around 30 seconds; for more sensitive films it can be up to 3 minutes. The fixer is considered exhausted when the clearing time has doubled for a certain type of film.
In the last step, the washing, with multiple or continuous water exchange, the complex compounds and the remnants of the fixer are washed out of the emulsion. Since the substances are removed from the emulsion by diffusion , the final washing must be long enough to achieve lasting stability of the film or the image.
Wetting agent bath
Finally, the surface tension of the water can be reduced by adding a wetting agent . This allows the water to flow off the film better and prevents the formation of drops on the film surface, which would otherwise lead to limescale stains when the film is then dried. The wetting agent basically works like the dishwashing detergent, but does not contain any coloring or fragrance additives. Wetting agent concentrates for the photo laboratory are often additionally provided with film hardening and preservation aids.
In the home laboratory, drying takes place by hanging the wet film upright with film clips in a room that is as dust-free as possible or (ideally) in a drying cabinet. In order to speed up the process, the film strip can be treated beforehand in a drying bath (usually methanol) or the coarse liquid residues can be removed from it with stripping pliers.
Archiving and shelf life
The dry film is cut into practical sections of 4 to 6 negatives per film strip (for 35mm film). These can then be archived, for example, in A4 sheets made of glassine and / or acetate film in an office folder.
Black and white films, photographs, and photographic plates are some of the most durable media known. The recording of documents on microfiches is therefore still used for permanent archiving.
- C-41 (Color Negative Film Development Process)
- E-6 (color reversal film development process)
- Magnification (photography)
- Print (photography)
- Black-and-white film
- Film speed
- Antifoggant (photo laboratory technology)
- Udo Berns: Photography and photo laboratory technology. Verlag Beruf + Schule, 1990, ISBN 3-88013-410-3 .
- Reinhard Merz: Basic course in black and white laboratory. Augustus Verlag, 1996, ISBN 3-8043-5037-2 .
- Steffen Eller: Precipitation and complex formation equilibria using the example of b / w photography ( memento of October 11, 2007 in the Internet Archive ) (article about the chemical processes involved in developing)