C-41 is the name given to the process for developing a specific type of colour print film used in photography and often to the type of film itself. As of 2005, C-41 (also known as CN-16 by Fuji and AP-70 by AGFA) is the most popular film process in use, with just about every photofinishing lab having at least one machine specifically devoted to developing this type of film.

C-41 negatives, as with all colour materials, have an image formed of dye, and, therefore, are inherintly somewhat unstable. C-41 negatives have a tendency to fade over time. In addition, old, undeveloped C41 film does not last as long as black and white film stored identically.

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. 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 sensitivity, each with different speeds and contrasts, are coated.

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.

Process

The basic C-41 process is as followed. It should be noted, however, that slightly different versions exist, based on the company producing the chemistry or whether the film is hand or machine processed. After exposure, the film is developed in a colour developer. It is usually based on a Paraphenylene Diamine, which creates visible dyes from the integral couplers. The developer creates both a silver and dye image in the film. (Incidentally, C41 developer can develop black and white films, and has been used with Kodak's Tech 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 temperatre can result in colour shifts or density problems. Development is followed by immersion in a mildly acidic stop bath or rinse to halt the development action, and then by a combined bleach-fix which both dissolves the silver generated by development and removes undeveloped silver halide. In some cases, a separate bleach and fix are used, as it is more environmentally friendly and provides better results. An additive can be added to the bleach only bath to make it reusable reducing the amount of waste going to the drain.) This is followed by thorough washing. This process is fairly familiar in photography, but for C-41 the 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.

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. This result occurs because it is exposure to light and development that 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.

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 "useable" images. "Useable" is a somewhat ambiguous term in this case. In the case of Ilford XP2, the packaging claims that exposure indexes between EI 50 and EI 800 are possible.

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. It is also possible to process Kodak Ektachrome in C41, which provides a negative without the orange integral layer. Printers have to be specially set up to be able to print these very contrasty negatives.

Photographic processes

C-41 | C-41 | Процесс C-41