A negative is the primary photographic image produced on a light sensitive emulsion - coated on a transparent film base or, in earlier photographic processes, on glass - as the direct result of exposure to light followed by chemical development. In a negative, the tonal values of the original subject are reversed relative to those of the real world scene: areas of the subject that were brightly lit and reflect the most light during the exposure produce the greatest density in the developed image and therefore appear darkest in the negative, while areas that were in deep shadow and reflected little light during exposure produce the least density and therefore appear lightest or clearest in the negative. This tonal reversal - in which highlights become dark and shadows become light - is the defining characteristic that gives the negative its name and distinguishes it from a positive image such as a print or transparency in which tonal values correspond directly to those of the original subject.
The negative is formed through the photochemical action of developer on the exposed silver halide crystals within the emulsion. During exposure, silver halide crystals that receive light absorb photons and form microscopic clusters of metallic silver atoms known as the latent image. When immersed in developer solution, these exposed crystals are chemically reduced to metallic silver at a rate proportional to the amount of exposure they received, building up a deposit of dense, opaque metallic silver in the most heavily exposed areas and a thinner, more translucent silver deposit in the less exposed areas. The subsequent fixation stage dissolves away the unexposed silver halides that remain after development, leaving a clear film base in the areas that received no exposure, completing the characteristic tonal reversal of the negative image.
The negative serves as the master image from which positive prints are made. By shining light through the negative onto light sensitive photographic paper in the darkroom enlarger, the tonal reversal of the negative is reversed once more to produce a positive print in which the tonal values correspond to those of the original subject - bright areas appear light, dark areas appear dark, and the full gradation of tones between is reproduced in their correct relative relationships. This two stage process of negative followed by positive print is one of the foundational principles of traditional photography, and its great practical advantage over the direct positive processes - such as the daguerreotype and ambrotype - that preceded it lies in the ability to produce multiple positive prints from a single negative, making photography a reproducible and widely distributable medium rather than a process yielding only unique individual images.
Negatives are available in colour as well as black and white. In colour negative film, the tonal reversal of the black and white negative is accompanied by a complementary colour reversal, in which the colours of the original subject are represented in their complementary opposites - reds appear cyan, greens appear magenta, and blues appear yellow - in addition to the overall orange mask that characterises developed colour negative film and is incorporated into the film's design to improve colour accuracy in the printing process. As with black and white negatives, the colour reversal is corrected in the printing or scanning stage to produce a positive image with the correct colour rendering of the original scene.
The quality of a negative - its density, contrast, colour balance, sharpness, and grain structure - is determined by a combination of factors including the exposure given at the time of shooting, the film stock used, the development process and chemistry applied, and the care taken in handling and storage. A well exposed and correctly developed negative contains the maximum amount of tonal and detail information that the film is capable of recording, providing the most complete and flexible starting point for producing high quality prints or digital scans. Underexposed or overdeveloped negatives suffer from blocked shadows, excessive contrast, or a combination of both, while overexposed or underdeveloped negatives lack density and contrast, with thin, flat tonal rendering that limits print quality. Understanding and controlling the negative making process to consistently produce well balanced negatives of predictable quality is one of the most fundamental skills in traditional film photography.