Fogging (photography)
Encyclopedia
Fogging in photography
is the deterioration in the quality of the image caused either by extraneous light or the effects of a processing chemical.
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.
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
is seen as an overall dark veil across the print or, occasionally, as unintended solarisation
.
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.
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.
Silver stain is a common problem in roller transporter processors, since the material (usually paper) is pinched between squeegee rollers to remove excess developer and brought into the fixing bath without intermediate rinse or stop. Developers for such applications usually contain an antistaining agent, usually soluble organic thiol
s that form soluble but non-reactive silver compounds with free silver ions.
With modern emulsion technology and well formulated developers, silver stain has become uncommon.
. Stannous chloride, dimethyl amine borane, and hydrazine
derivatives are used. These compounds are not very stable in contact with air, as you expect from their nature being a reducing agent. Another possible way, though less common, is to create fog centres consisting of large specks of silver sulfide
. Sodium sufide, alkaline solution of thiourea
, and other compounds having labile sulfur element can be used.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
is the deterioration in the quality of the image caused either by extraneous light or the effects of a processing chemical.
Taxonomy of fogging
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 imageLatent image
A latent image on photographic film is an invisible image produced by the exposure of the film to light. When the film is developed, the area that was exposed darkens and forms a visible 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
Propylene glycol
Propylene glycol, also called 1,2-propanediol or propane-1,2-diol, is an organic compound with formula C3H8O2 or HO-CH2-CHOH-CH3...
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
Light fogging can occur to the film in the camera because of a defect in the manufacture or use of the cameraCamera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
and is seen as dark areas in the negative which tend to occur over the full width of the film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
including the margins.
Light fogging on a print usually only occurs because of poor control of lighting in the darkroom
Darkroom
A darkroom is a room that can be made completely dark to allow the processing of light sensitive photographic materials, including photographic film and photographic paper. Darkrooms have been created and used since the inception of photography in the early 19th century...
is seen as an overall dark veil across the print or, occasionally, as unintended solarisation
Solarisation
Solarisation is a phenomenon in photography in which the image recorded on a negative or on a photographic print is wholly or partially reversed in tone. Dark areas appear light or light areas appear dark...
.
Unintentional fogging
Chemical fogging occurs at the processingPhotographic processing
Photographic processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image...
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.
Silver stains
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 sulfideSilver sulfide
Silver sulfide, Ag2S, is the sulfide of silver. This dense black solid constitutes the tarnish that forms over time on silverware and other silver objects. Silver sulfide is insoluble in all solvents, but is degraded by strong acids. Silver sulfide features a covalent bond, as it is made up of...
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.
Silver stain is a common problem in roller transporter processors, since the material (usually paper) is pinched between squeegee rollers to remove excess developer and brought into the fixing bath without intermediate rinse or stop. Developers for such applications usually contain an antistaining agent, usually soluble organic thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...
s that form soluble but non-reactive silver compounds with free silver ions.
With modern emulsion technology and well formulated developers, silver stain has become uncommon.
Intentional fogging
In reversal processing, the material is fogged before the second developer. This is often done by exposure to light, but sometimes chemical fogging is used. One common way to do this is to use a mildly strong reducing agent (much stronger than developing agents) to create fog centres consisting of a metallic silver cluster, which can be viewed as a chemically induced analogue of latent imageLatent image
A latent image on photographic film is an invisible image produced by the exposure of the film to light. When the film is developed, the area that was exposed darkens and forms a visible image...
. Stannous chloride, dimethyl amine borane, and hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
derivatives are used. These compounds are not very stable in contact with air, as you expect from their nature being a reducing agent. Another possible way, though less common, is to create fog centres consisting of large specks of silver sulfide
Silver sulfide
Silver sulfide, Ag2S, is the sulfide of silver. This dense black solid constitutes the tarnish that forms over time on silverware and other silver objects. Silver sulfide is insoluble in all solvents, but is degraded by strong acids. Silver sulfide features a covalent bond, as it is made up of...
. Sodium sufide, alkaline solution of thiourea
Thiourea
Thiourea is an organosulfur compound of with the formula SC2 . It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom, but the properties of urea and thiourea differ significantly. Thiourea is a reagent in organic synthesis. "Thioureas" refers to a broad...
, and other compounds having labile sulfur element can be used.