Kodachrome was the first modern colour film, introduced in 1935. It used *complex control of processing to produce the right colours in the right layers by diffusion of the colour forming materials, but the film itself had a less complex structure than the later films (negative and transparency) based largely on Agfa technology which incorporated the colour couplers into the emulsion layers. There were only a few labs world-wide which offered the *K-14 process. From Finland you had to send a film to Germany.
For many years, Kodachrome was the unchallenged leader in colour film. However modern films from Fuji, Agfa and Kodak's own Ektachromes began to match its qualities while being capable of being processed in any lab using the relatively simple E6 process, and now only one Kodachrome film is available - and with little future prospects.
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