Fogging (photography)

Fogging in photography is the deterioration in the quality of the image caused either by extraneous light or the effects of a processing chemical.

One kind of fog is produced from a silver halide crystal having a fog centre created by various reactions. One obvious case is when the emulsion is inadvertently exposed to light. In this case, the fog centre is created as latent image and made into visible fog by the action of the developer. Another case is when the fog centre is created chemically, with propylene glycol and white wax.

There is another kind of chemical fog, broadly classified as silver stain. This kind of fog does not require a fog centre in each silver halide crystal, since they can grow on other substrates other than photosensitive silver halide crystals.

Light fogging can occur to the film in the camera because of a defect in the manufacture or use of the camera and is seen as dark areas in the negative which tend to occur over the full width of the film including the margins.

Light fogging on a print usually only occurs because of poor control of lighting in the darkroom and is seen as an overall dark veil across the print or, occasionally, as unintended solarisation.

Chemical fogging occurs at the processing stage when old or spent chemicals are used, chemicals are used in the wrong sequence, inadequate washing between processing stages or inappropriate chemicals are used. Because of the wide range of cause, the effects can be diverse ranging from coloured streaks and blotches through to the lack of an image or a totally black image. The most common cause is the use of old or spent chemistry which often results in a lack of contrast and an undesirable background colour - usually brown.

The silver stain does not require fog centres on crystals, although they tend to occur in or near exposed areas. The silver stain can take a range of appearances, but one example is dichroic fog. Silver stains form on various hosts, such as tiny silver sulfide particles that may be present in the emulsion coating. They are very tiny and not visible, unless they are grown to larger sizes. Usual developers (that require silver ions supplied from within the silver halide crystal) do not grow these particles, but physical developers (that use silver ions from the solution to grow the development site) do. All modern practical developers function primarily as non-physical developers, but this type of fog may be seen when developed film is brought directly into exhausted fixer solution.

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