Negative-positive sysem

From Silvergrain Labs

Negative-positive system uses a negative material (film, dry plate, emulsion) to exposore in-camera. The material is processed to form stable, visible negative image, where highlights and shadows are reversed in tonality. This image is usually projected onto a piece of photographic paper, which also utilizes negative emulsion to reverse the tonality to yield the final positive image.

Talbot's Calotype was the first system to use this system, and it allowed to make many prints from one original camera exposure. On the other hand, Daguerreotype allowed only one final image from one camera exposure. Daguerreotype is often called "false invention" because it did not give rise to the hugely more successful negative-positive system.

Other systems

Besides the negative materials used for negative-positive system, reversal films and direct positive films are currently used. Reversal films form positive image by first forming the negative image in the first developer, and then this image is selectively bleached. The remainder of the film is developed to form positive image (Reversal Processing). Direct positive films form positive image by processing the films in a usual negative developer. Direct positive emulsions are chemically fogged in the process of emulsion manufacturing, and the emulsion is designed such that light exposure will bleach the fog center, so that the film will form positive image in usual negative developers. Direct positive emulsions are usually slow emulsions for duplicating existing slides and negatives without first making internegative or interpositive.