Editing Photographic processing (section)

== Common processes ==
{{See also|Alternative process}}
[[File:PhotoDeveloptKeySteps.png|thumb|Key stages in production of {{Abbr|Ag|Silver}}-based photographs.  Two silver halide particles, one of which is impinged with light (''h''ν) resulting in the formation of a latent image (step 1).  The latent image is amplified using photographic developers, converting the silver halide crystal to an opaque particle of silver metal (step 2).  Finally, the remaining silver halide is removed by fixing (step 3).]]
All photographic processing use a series of chemical baths. Processing, especially the development stages, requires very close control of temperature, agitation and time.

===Black and white negative processing===

[[File:Photographic processing.jpg|thumb|center|upright=3.0|Black and white negative processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light.]]

# The film may be soaked in water to swell the [[gelatin]] layer, facilitating the action of the subsequent chemical treatments.
# The [[photographic developer|developer]] converts the latent image to macroscopic particles of metallic [[silver]].<ref>Wall, 1890, p. 30–63</ref>
# A [[stop bath]],{{efn|In modern automatic processing machines, the stop bath is replaced by mechanical squeegee or pinching rollers. These treatments remove much of the carried-over alkaline developer, and the acid, when used, neutralizes the alkalinity to reduce the contamination of the fixing bath with the developer.}} typically a dilute solution of [[acetic acid]] or [[citric acid]], halts the action of the developer. A rinse with clean [[water]] may be substituted.
# The [[photographic fixer|fixer]] makes the image permanent and light-resistant by dissolving remaining [[silver halide]]. A common fixer is ''hypo'', specifically [[ammonium thiosulfate]].<ref>Wall, 1890, p. 88–89</ref>
# Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading.<ref>{{Cite web |title=Photochem |url=http://sites.tech.uh.edu/digitalmedia/materials/3351/PHOTCHEM.pdf |archive-url=https://web.archive.org/web/20180124135711/http://sites.tech.uh.edu/digitalmedia/materials/3351/PHOTCHEM.pdf |archive-date=24 January 2018 |website=University of Houston College of Technology}}</ref>
The washing time can be reduced and the fixer more completely removed if a [[hypo clear|hypo clearing agent]] is used after the fixer.
# Film may be rinsed in a dilute solution of a [[non-ionic]] [[wetting agent]] to assist uniform drying, which eliminates drying marks caused by [[hard water]]. (In very hard water areas, a pre-rinse in [[distilled water]] may be required – otherwise the final rinse wetting agent can cause residual ionic [[calcium]] on the film to drop out of solution, causing spotting on the negative.)
# Film is then dried in a dust-free environment, cut and placed into protective sleeves.
Once the film is processed, it is then referred to as a ''negative''.

The negative may now be [[Photographic printing|printed]]; the negative is placed in an [[enlarger]] and projected onto a sheet of photographic paper. Many different techniques can be used during the enlargement process. Two examples of enlargement techniques are [[dodging and burning]].

Alternatively (or as well), the negative may be [[film scanner|scanned]] for [[digital printing]] or web viewing after adjustment, retouching, and/or [[Photograph manipulation|manipulation]].

From a chemical standpoint, conventional black and white negative film is processed by a developer that reduces silver halide to silver metal, exposed silver halide is reduced faster than unexposed silver halide, which leaves a silver metal image. It is then fixed by converting all remaining silver halide into a soluble silver complex, which is then washed away with water.<ref name="shinsaku"/> An example of a black and white developer is Kodak D-76 which has bis(4-hydroxy-N-methylanilinium) sulfate with hydroquinone and sodium sulfite.

In graphic art film, also called lithographic film which is a special type of black and white film used for converting images into [[halftone]] images for offset printing, a developer containing methol-hydroquinone and sulfite stabilizers may be used. Exposed silver halide oxidizes the hydroquinone, which then oxidizes a nucleating agent in the film, which is attacked by a hydroxide ion and converts it via hydrolysis into a nucleating agent for silver metal, which it then forms on unexposed silver halide, creating a silver image. The film is then fixed by converting all remaining silver halide into soluble silver complexes.<ref name="shinsaku"/>

===Black and white reversal processing===
This process has three additional stages:
# Following the first developer and rinse, the film is bleached to remove the developed negative image. This negative image is composed of metallic silver formed in the first developer step. The bleach used here only affects the negative, metallic silver grains, it does not affect the unexposed and therefore undeveloped silver halide. The film then contains a latent positive image formed from unexposed and undeveloped silver halide salts.
# The film is [[fogging (photography)|fogged]], either chemically or by exposure to light.
# The remaining silver halide salts are developed in the second developer, converting them into a positive image composed of metallic silver.
# Finally, the film is fixed, washed, dried and cut.<ref>Photographic Almanac, 1956, p. 149–155</ref>

=== Colour processing ===
[[Chromogenic]] materials use [[dye couplers]] to form colour images.
Modern colour negative film is developed with the [[C-41 process]] and colour negative print materials with the [[RA-4 process]]. These processes are very similar, with differences in the first chemical developer.

The C-41 and RA-4 processes consist of the following steps:
# The colour developer develops the silver negative image by reducing the silver halide crystals that have been exposed to light to metallic silver, this consists of the developer donating electrons to the silver halide, turning it into metallic silver; the donation oxidizes the developer which then activates the dye couplers to form the colour dyes in each emulsion layer, but only does so in the dye couplers that are around unexposed silver halide.<ref name="kodak.com">{{Cite web |title=Kodak Color Films |url=https://www.kodak.com/uploadedfiles/motion/US_plugins_acrobat_en_motion_education_kodak_color_films.pdf |url-status=dead |archive-url=https://web.archive.org/web/20200325121449/https://www.kodak.com/uploadedfiles/motion/US_plugins_acrobat_en_motion_education_kodak_color_films.pdf |archive-date=2020-03-25 |access-date=2020-08-15}}</ref><ref>{{Cite web |title=Film Structure |url=https://www.kodak.com/uploadedfiles/motion/US_plugins_acrobat_en_motion_newsletters_filmEss_04_How-film-makes-image.pdf |url-status=dead |archive-url=https://web.archive.org/web/20200325120116/https://www.kodak.com/uploadedfiles/motion/US_plugins_acrobat_en_motion_newsletters_filmEss_04_How-film-makes-image.pdf |archive-date=2020-03-25 |access-date=2020-08-15}}</ref>
# A rehalogenising bleach converts the developed metallic silver into silver [[halide]].
# A fixer removes all silver halide by converting it into soluble silver complexes that are then washed away, leaving only the dyes.<ref name="kodakalaris">{{Cite web |title=Chemicals and Chemical Handling |url=https://imaging.kodakalaris.com/sites/default/files/wysiwyg/pro/chemistry/z101.pdf |url-status=live |archive-url=https://web.archive.org/web/20230322143904/https://imaging.kodakalaris.com/sites/default/files/wysiwyg/pro/chemistry/z101.pdf |archive-date=2023-03-22 |access-date=2023-03-01}}</ref>
# The film is washed, stabilised, dried and cut.<ref name ="Langford">{{cite book |title=Basic Photography |last=Langford |first=Michael |year=2000 |publisher=Focal Press |location=Oxford |isbn=0-240-51592-7 |pages=[https://archive.org/details/basicphotography00lang/page/210 210; 215–216] |url-access=registration |url=https://archive.org/details/basicphotography00lang/page/210 }}</ref>

In the RA-4 process, the bleach and fix are combined. This is optional, and reduces the number of processing steps.<ref>Photographic Almanac, 1956, p. 429–423</ref>

Transparency films, except [[Kodachrome]], are developed using the [[E-6 process]], which has the following stages:
# A black and white developer develops the silver in each image layer.
# Development is stopped with a rinse or a stop bath.
# The film is fogged in the reversal step.
# The fogged silver halides are developed and [[oxidation state|oxidized]] developing agents couple with the [[dye couplers]] in each layer.
# The film is bleached, fixed, washed/rinsed, stabilised and dried as described above.<ref name="Langford" />

The Kodachrome process is called [[K-14 process|K-14]]. It is very involved, requiring 4 separate developers, one for black and white and 3 for color, reexposing the film in between development stages, 8 or more tanks of processing chemicals, each with precise concentration, temperature and agitation, resulting in very complex processing equipment with precise chemical control.<ref name="kodak.com"/>

In some old processes, the film emulsion was hardened during the process, typically before the bleach. Such a hardening bath often used aldehydes, such as [[formaldehyde]] and [[glutaraldehyde]]. In modern processing, these hardening steps are unnecessary because the film emulsion is sufficiently hardened to withstand the processing chemicals.

A typical chromogenic color film development process can be described from a chemical standpoint as follows: Exposed silver halide oxidizes the developer.<ref name="shinsaku">Organic Chemistry of Photography. Shinsaku Fujita. doi: 
https://doi.org/10.1007/978-3-662-09130-2. 
Springer-Verlag Berlin Heidelberg, 2004.
ISBN 978-3-540-20988-1.</ref> The oxidized developer then reacts with color couplers,<ref name="shinsaku"/> which are molecules near the exposed silver halide crystals,<ref name="shinsaku"/> to create color dyes<ref name="shinsaku"/> which ultimately create a negative image, after this the film is bleached, fixed, washed, stabilized and dried. The dye is only created where the couplers are. Thus the development chemical must travel a short distance from the exposed silver halide to the coupler and create a dye there. The amount of dye created is small and the reaction only occurs near the exposed silver halide<ref name="kodakalaris"/> and thus doesn't spread throughout the entire layer. The developer diffuses into the film emulsion to react with its layers.<ref name="kodakalaris"/> This process happens simultaneously for all three colors of couplers in the film: cyan (in the red-sensitive layer in the film), magenta(for the green-sensitive layer), and yellow (for the blue-sensitive layer).<ref name="shinsaku"/> Color film has these three layers, to be able to perform subtractive color mixing and be able to replicate colors in images.