Most photographic images are composed of two parts, the base and the light-sensitive emulsion. The base, whether metal, glass, paper or film, provides support for the thin emulsion layer. Over the years various chemical compositions have been used for the emulsion including albumen, collodion and gelatin. Embedded into the emulsion are light-sensitive chemicals, usually silver nitrate. From this the photographer can print out the image on photographic paper creating what we call a photograph.
Images have either a positive or negative polarity.
A positive image has a tonal value that corresponds with the original, in other words the original and the image are dark in the same places and light in the same places. A negative image has a reversed tonal value where the dark places of the original are white in the image and the white places of the original or dark in the image. Negatives and positives are produced by different technical processes and chemical processes. Most glass plates were developed as negative images that were used to produce positive prints on paper. Return to menu.