Negative Film
Many photographic processes create negative images: the chemicals involved react when exposed to light, and during developing these exposed chemicals are retained and become opaque while the unexposed chemicals are washed away. However, when a negative image is created from a negative image (just like multiplying two negative numbers in mathematics) a positive image results (see Color print film, C-41 process). This makes most chemical based photography a two step process. These are called negative films and processes. Special films and development processes have been devised such that positive images can be created directly from film; these are called positive, or slide, or (perhaps confusingly) reversal film (see Transparency, Black and white reversal film, E-6 process).
Despite the market's evolution away from film, there is still a desire and market for products which allow fine art photographers to produce negatives from digital images for their use in alternative processes such as cyanotypes, gum bichromate, platinum prints, and many others.
Read more about this topic: Negative (photography)
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