Photographic processing
From Wikipedia, the free encyclopedia
Conventional photographic films and papers need to be chemically processed after they have been exposed in order to produce the desired negative or positive image. The general process is similar whatever the make of film or paper. The only obvious exceptions are a few proprietary makes such as Polaroid and other instant self-processing techniques; Kodachrome which cannot be processed except in Kodak laboratories. There are also a small number of relatively uncommon processes using dye decomposition technologies such as Cibachrome.
Development does three things: it transforms the latent image into a visible image that can be seen, it makes the visible image permanent and resistant to deterioration with time, and it renders the film insensitive to light.
Before processing the film must be removed from the camera and from its cassette or spool in a light-proof room or container.
Contents |
[edit] Amateur processing
In amateur processing, the film is removed from the camera and wound onto a reel in complete darkness (usually inside a darkroom 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 where it is retained until final washing is complete.
[edit] Commercial processing
In commercial 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 and temperature. 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.
[edit] The process
Whether processing at an amateur or commercial level, the film is then treated in chemical baths. These baths are maintained at closely monitored temperatures and specific time periods. When making archival prints or prints of high quality, the minimum number of chemical steps are used. In particular additional stages such as hypo-eliminators or wash-aids are avoided because of uncertainty about the long term impact of such treatments on print stability and permanence.
- The developer, which makes the latent image visible.
- A stop bath, which stops the action of developer - typically a dilute solution of Acetic acid. (Sometimes plain water is used for this step.) This neutralises alkaline developer carry-over and removes excess developer reducing cross contamination of the acid fixer.
- The fixer makes the image permanent and light-resistant by dissolving any remaining silver halides. The fixer is often referred to as "hypo".
- An optional "Hypo Clearing Agent" or washing aid bath, which renders fixer more easily removable by the washing process. Before the use of a washing aid, the majority of fixer is removed in a brief rinse in water. The wash aid helps remove the remaining hypo.
- Clean water wash to remove any fixer, as it is corrosive and can dissolve the elemental silver over time leading to staining and fading. The washing process can be shorter in time if a washing aid is used after the fixer.
- Final immersion in a film conditioner is commonplace. This is a solution of alcohol, wetting agents and formaldehyde in de-ionised water. This hardens the film surface, helps drying and eliminates drying marks from hard water.
- Film is then dried in a dust free environment, and finally (if roll sized film)cut, and put into protective sleeves.
[edit] Colour processing
Colour negative processing and colour print processing is very similar. The principal difference is the inclusion of a colour developer in the development stage and the replacement of the fixer with a bleach-fix (Blix) which dissolves both the silver halides and the elemental silver leaving only a dye image.
Black and white reversal processing (to make black and white positives) has two additional stages. Following first development, the developing action is stopped, the film is then bleached to remove the elemental silver produced by development and it is then either exposed to light or chemically fogged prior to a second development. This allows the silver salts that were not exposed in the camera to be developed into silver forming a positive image. The remaining stages are as for normal processing.
In colour reversal processing the film is developed in a sophisticated black and white type developer (not a colour developer), the development is then stopped, the film is chemically fogged and is then treated in a colour developer. The remaining blix stage etc is the same as for colour negative film.
[edit] Further processing
For black and white emulsions both negative and positive, further processing may react the silver with other elements such as selenium or sulfur to produce a more permanent image. In these cases the silver in the image is changed to silverselenide or -sulfite, which are more resistant to oxidising agents in the atmosphere. The fact that a successful selenium- or sulfidetoning depends on correct fixation and rigorous washing to remove residual thiosulfate also contributes to the longetivity of thusly toned emulsions. In other cases, the silver may be chemically bleached using a Potassium dichromate solution in strong mineral acid and the result washed film then re-processed to produce unusual coloured effects. Sepia, red and blue colours may be obtained in this way. If colour negative film is processed in conventional black and white developer, and fixed and then bleached with a Hydrochloric acid and potassium dichromate solution, the resultant film, once exposed to light, can be redeveloped in colour developer to produce an unusual pastel colour effect.
