All modern colour negative film works in the same basic way. Effectively they are three black and white films stuck one on top of the other, but with an important difference. The top most is only sensitive to blue light, the next to green and the last to red. Between the top blue sensitive layer and the lower two is a yellow filter, to stop the blue light, which all emulsions are sensitive to, getting through. When the film is exposed, the three layers record the varying amounts of each primary colour, as well as the intensity of each part of the image. Now comes some clever chemistry. The standard process for colour negative film is called C41. Developed by Kodak, there are variations made by different companies, such as CN16 by Fuji & Agfa's AP70, which all function in the same way, allowing any manufacturers film to go through any manufacturers process (although for the highest quality work, some photographers prefer to use film and chemistry from the same manufacturer). The first step in the process is to develop the B&W negative in all three layers. As the silver negative image forms, the chemical by-products of the reaction are used to cause dye molecules in the developer to attach to dye coupler molecules trapped in each layer of the film during manufacture. As each B&W negative forms, around it a colour negative also forms, but of the complementary colour. In the blue sensitive layer a yellow image (which is white minus blue), in the green a magenta (white minus green) and in the red a cyan (white minus red). (Note that these colours are not the same as the primary/complimentary colours in painting.) After development, the B&W negative needs to be removed, to leave just the colour negative, so the film goes through a bleach solution, which changes the silver back to silver halide. Fixing then clears all the halides, ready for the wash to remove them. Processing also removes the yellow filter layer.
Known as chromogenic processing, this indirect development method allows for some tinkering in the chemistry to give, in effect, variable development and increased latitude to the film. It is possible to slow down the formation of a dye image that is forming too quickly and so reduce the effects of over-exposure. Good use of this is made in both consumer print films, where exposure control may be poor or in some cases, such as "film in a box" cameras, non-existent, and some professional films for press photographers, where exposures are often made in far from perfect or controllable situations. However, for the best results most professional colour negative film should be exposed as carefully as B&W. The chromogenic B&W films that can be C41 processed, such as Ilford's XP2 and Kodak's T400CN, work in a similar way, but without distinguishing between colours and only needing to develop a black dye.
Next month, I shall go into the similar workings of colour transparency film. So similar that C41 colour negative (and XP2 & T400CN) can be cross-processed through the E6 colour transparency process and vice versa. But I shall talk about that another time.
© Barry Leighton FRPS