C-41 process
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C-41 is color print film developing process. C-41, also known as CN-16 by Fuji, CNK-4 by Konica, and AP-70 by AGFA, is the most popular film process in use, with most photofinishing lab devoting at least one machine to this development process.
C-41 negatives, as with all colour materials, have an image formed of dye, and, therefore, have inherent color instability. C-41 negatives have a tendency to fade over time and do not last as long as black and white film stored under identical conditions.
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[edit] Film layers
C-41 film consists of an acetate film base on which three colour-sensitive emulsions are coated. Each emulsion is sensitive to a different colour in the optical spectrum. In Kodak films the top layer is blue sensitive. Beneath this layer is a yellow filter, made of dyes, or, more commonly, collodial silver. This filters out the blue light to which the rest of the layers are also senstive, due to the intrinsic blue sensitivity of silver halides. The second layer is green sensitive and the third is red sensitive. On some C-41 films, multiple layers of each color, each with different speeds and contrasts, are coated. This helps to increase the film's exposure latitude.
Each layer, in addition to the light-sensitive components, contain dye couplers. These couplers, located in the blue, green and red-sensitive layers will produce yellow, magenta and cyan dyes, respectively, when developed.
[edit] Process
The C-41 process is the same for all films, although different manufacturers processing chemistries vary slightly.
After exposure, the film is developed in a Paraphenylene Diamine developer known as CD-4, which creates visible dyes from the integral couplers. The developer creates both a silver and dye image in the film. C-41 developer can also develop black and white films, and has been used with Kodak's Technical Pan film by some photographers.
The control of temperature and agitation of the film in the developer is critical in obtaining consistent, accurate results. Incorrect temperature can result in colour shifts or dye density problems. Development is sometimes followed by immersion in a mildly acidic stop bath to halt the development action. A combined bleach-fix dissolves the silver generated by development and removes undeveloped silver halide. A separate bleach and fix can also be used as it is more environmentally friendly and can provide better results. The final step is a thorough washing. The process is fairly similar to other photographic processes, but the C-41 chemical composition and relative strength of the developer and fixer are unique and specific, as is the way the emulsions are laid on the base.
[edit] The negative
The resulting film is a negative, meaning that the darkest spots on the film are those areas that were brightest in the source. The negative image occurs because exposure to light and development converts the photographic silver halides into black metallic silver. Couplers during development cause dyes to be produced in the layers, the bleach then converts the silver back to the original state and the fix removes the silver from the film. The last bath is a stabiliser and wetting agent which allows the film to dry evenly without streaks and preserves the dye image. All C-41 films also include an additional orange mask to offset the optical inadequacies of the dye layers in the film. C-41 negatives always appear orange when viewed directly but this orange base is compensated for in the formulation of colour print materials.
The finished negative may then be printed using colour paper to yield a positive image.
[edit] Use with black-and-white films
While C-41 usually considered a colour process, Kodak manufactures a chromogenic black-and-white film called BW400CN that is processed using the C-41 process. Ilford, a British company noted for their loyalty towards black and white film photography, produces a chromogenic film known as XP2 Super and Fuji produce a film called Neopan 400CN. Commercially produced prints from these brands of film sometimes have a coloured hue, though many digital processors can rectify this.
Ilford's XP2 and Fuji's Neopan 400CN can be printed on black and white paper almost like any other black and white negative. Prints from XP2 tend to appear less grainy than conventional ISO 400 black and white film, though this is due to the dye image. Rather than grain, C-41 negatives have clouds of dye. The resulting appearance is different from that of silver grain.
Many C-41 films feature wide exposure latitude, meaning that they can be exposed over a range of different exposure indexes and still yield printable images. In the case of Ilford XP2, the packaging claims that exposure indexes between EI 50 and EI 800 are possible.
[edit] Cross processing
It is also possible to cross-process slide film for the E-6 process in C-41 , which yields negatives with a colour shift and stronger saturation, and vice versa, yielding slides. This process is popular in Lomography where the saturated colourful result adds to the image. Printers have to be specially set up to be able to print these very contrasty negatives.
