Editing Photographic processing (section)

=== 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.