Photographic processing

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.

1. The film may be soaked in water to swell the gelatin layer, facilitating the action of the subsequent chemical treatments.
2. The developer converts the latent image to macroscopic particles of metallic silver.[3]
3. A stop bath,[lower-alpha 1] typically a dilute solution of acetic acid or citric acid, halts the action of the developer. A rinse with clean water may be substituted.
4. The fixer makes the image permanent and light-resistant by dissolving remaining silver halide. A common fixer is hypo, specifically ammonium thiosulfate.[4]
5. Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading.[5]

The washing time can be reduced and the fixer more completely removed if a hypo clearing agent is used after the fixer.

1. 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.)
2. 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 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 scanned for digital printing or web viewing after adjustment, retouching, and/or 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.[6] 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, 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.[6]

This process has three additional stages:

1. 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.
2. The film is fogged, either chemically or by exposure to light.
3. The remaining silver halide salts are developed in the second developer, converting them into a positive image composed of metallic silver.
4. Finally, the film is fixed, washed, dried and cut.[7]

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:

1. 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.[8][9]
2. A rehalogenising bleach converts the developed metallic silver into silver halide.
3. A fixer removes all silver halide by converting it into soluble silver complexes that are then washed away, leaving only the dyes.[10]
4. The film is washed, stabilised, dried and cut.[11]

In the RA-4 process, the bleach and fix are combined. This is optional, and reduces the number of processing steps.[12]

Transparency films, except Kodachrome, are developed using the E-6 process, which has the following stages:

1. A black and white developer develops the silver in each image layer.
2. Development is stopped with a rinse or a stop bath.
3. The film is fogged in the reversal step.
4. The fogged silver halides are developed and oxidized developing agents couple with the dye couplers in each layer.
5. The film is bleached, fixed, washed/rinsed, stabilised and dried as described above.[11]

The Kodachrome process is called 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.[8]

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.[6] The oxidized developer then reacts with color couplers,[6] which are molecules near the exposed silver halide crystals,[6] to create color dyes[6] 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[10] and thus doesn't spread throughout the entire layer. The developer diffuses into the film emulsion to react with its layers.[10] 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).[6] Color film has these three layers, to be able to perform subtractive color mixing and be able to replicate colors in images.

A cut-away illustration of a typical light-trap tank used in small scale developing.

In amateur processing, the film is removed from the camera and wound onto a reel in complete darkness (usually inside a darkroom with the safelight turned off or a lightproof bag with arm holes). The reel holds the film in a spiral shape, with space between each successive loop so the chemicals may flow freely across the film's surfaces. The reel is placed in a specially designed light-proof tank (called a daylight processing tank or a light-trap tank) where it is retained until final washing is complete.

Sheet films can be processed in trays, in hangers (which are used in deep tanks), or rotary processing drums. Each sheet can be developed individually for special requirements. Stand development, long development in dilute developer without agitation, is occasionally used.

In commercial, central processing, the film is removed automatically or by an operator handling the film in a light proof bag from which it is fed into the processing machine. The processing machinery is generally run on a continuous basis with films spliced together in a continuous line. All the processing steps are carried out within a single processing machine with automatically controlled time, temperature and solution replenishment rate. The film or prints emerge washed and dry and ready to be cut by hand. Some modern machines also cut films and prints automatically, sometimes resulting in negatives cut across the middle of the frame where the space between frames is very thin or the frame edge is indistinct, as in an image taken in low light. Alternatively stores may use minilabs to develop films and make prints on the spot automatically without needing to send film to a remote, central facility for processing and printing.

Some processing chemistries used in minilabs require a minimum amount of processing per given amount of time to remain stable and usable. Once rendered unstable due to low use, the chemistry needs to be completely replaced, or replenishers can be added to restore the chemistry to a usable state. Some chemistries have been designed with this in mind given the declining demand for film processing in minilabs, often requiring specific handling. Often chemistries become damaged by oxidation. Also, development chemicals need to be thoroughly agitated constantly to ensure consistent results. The effectiveness (activity) of the chemistry is determined through pre-exposed film control strips.[13]