Editing Photographic developer (section)

==Development==
[[File:Self portrait in the darkroom JPG01.jpg|thumb|A black and white photographic print in a tray while being processed after exposure to light under a photographic [[enlarger]]. Typically three trays are used containing either developer, [[stop bath]], or fixer, in that order. The print must then be rinsed in water to remove the fixer.]]
The developer selectively reduces [[silver halide]] crystals in the emulsion to metallic [[silver]], but only those having [[latent image]] centres created by action of light.<ref>{{cite web|last=Woodworth|first=Chuck|title=How Film Works|url=http://www.unc.edu/~jimlee/film.htm|publisher=BYG Publishing|access-date=14 March 2013|archive-date=22 December 2012|archive-url=https://web.archive.org/web/20121222050517/http://www.unc.edu/~jimlee/film.htm|url-status=dead}}</ref> The light sensitive layer or emulsion consists of silver halide crystals in a gelatin base. Two photons of light must be absorbed by one silver halide crystal to form a stable two atom silver metal crystal. The developer used generally will only reduce silver halide crystals that have an existing silver crystal. Faster exposure or lower light level films usually have larger grains because those images capture less light. Fine grain films, like Kodachrome, require more light to increase the chance that the halide crystal will absorb at least two quanta of light as they have a smaller cross sectional size. Therefore, silver halide crystal size is proportional to film speed. The metallic silver image has dark (black) appearance. Once the desired level of reduction is achieved the development process is halted by washing in a dilute acid and then the undeveloped silver halide is removed by dissolving it in a thiosulfate solution, a process called ''[[photographic fixer|fixing]]''. Most commercial film developers use a dual solution or "push" (pushes the films speed) development (compensating developer, like Diafine) procedure where the reducing agent e.g. hydroquinone solution soaks into and swells the gelatin then the film is introduced into the alkaline solution which activates (lowers reduction potential) of the developer. The areas with the most light exposure use up the tiny amount of developer in the gelatin and stop making silver crystal before the film at that point is totally opaque. The areas that received the least light continue to develop because they haven't used up their developer. There is less contrast, but time is not critical and films from several customers and different exposures will develop satisfactorily.

The time over which development takes place, and the type of developer, affect the relationship between the density of silver in the developed image and the quantity of light. This study is called [[sensitometry]] and was pioneered by F Hurter & V C Driffield in the late 19th century.<ref>{{cite book |last=Papagiannakis |first=E. E. |url=https://books.google.com/books?id=IyTYgqCIQfcC&q=F+Hurter+%26+V+C+Driffield&pg=PA397 |title=Electromagnetics and optics |last2=Krieziz |first2=D. P. |last3=Chrissoulidis |first3=A. G. |publisher=World Scientific |year=1992 |isbn=978-9810208493 |location=River Edge, New Jersey |page=397}}</ref>