Photographic film

Photographic film is a sheet of plastic (polyester, PET, nitrocellulose or cellulose acetate) coated with an emulsion containing light-sensitive silver halide salts (bonded by gelatin) with variable crystal sizes that determine the sensitivity, contrast and resolution of the film. When the emulsion is sufficiently exposed to light (or other forms of electromagnetic radiation such as X-rays), it forms a latent (invisible) image. Chemical processes can then be applied to the film to create a visible image, called a photograph, in a process called film developing. In black-and-white photographic film there is usually one layer of silver salts. When the exposed grains are developed, the silver salts are converted to metallic silver, which blocks light and appears as the black part of the film negative. Color film uses at least three layers. Dyes, which adsorb to the surface of the silver salts, make the crystals sensitive to different colors. Typically the blue-sensitive layer is on top, followed by the green and red layers. During development, the exposed silver salts are converted to metallic silver, just as with black-and-white film. But in a color film, the by-products of the development reaction simultaneously combine with chemicals known as color couplers that are included either in the film itself or in the developer solution to form colored dyes. Because the by-products are created in direct proportion to the amount of exposure and development, the dye clouds formed are also in proportion to the exposure and development. Following development, the silver is converted back to silver salts in the bleach step. It is removed from the film in the fix step. This leaves behind only the formed color dyes, which combine to make up the colored visible image. Newer color films, like Kodacolor II, have as many as 12 emulsion layers,[citation needed] with upwards of 20 different chemicals in each layer. Due to film photography's long history of widespread use, there are now around one trillion pictures on photographic film or photographic paper in the world,[1] enough to cover an area of around ten thousand square kilometres (4000 square miles), about half the size of Wales. There are several types of photographic film, including: Print film, when developed, turns into a negative with the colors (or black and white values, in black-and-white film) inverted. This type of film must be "printed"that is either enlarged by projecting through a lens, or placed in direct contact as light shines through itonto photographic paper (which in turn is itself developed) in order to be viewed as intended. Print films are available in both black-and-white and color. Color print films use an orange color correction mask to correct for unwanted dye absorptions and improve color accuracy. Although color processing is more complex and temperature-sensitive than black-and-white processing, the great popularity of color and minimal use of black-and-white prompted the design of black-and-white film which is processed in exactly the same way as a standard color film. Color reversal film after development is called a transparency and can be viewed directly using a loupe or projector. Reversal film mounted with plastic or cardboard for projection is often called a slide. It is also often marketed as "slide" film. This type of film is often used to produce digital scans or color separations for mass-market printing. Photographic prints can be produced from reversal film, but the process is expensive and not as simple as that for print film. Black-and-white reversal film exists, but is uncommon. Conventional black-and-white negative stock can be reversal-processed, to give black & white slides, as by dr5 Chrome.[3] Some kits were available to enable B&W reversal processing to be done by home-processors, but most are discontinued. B&W transparencies can be produced from almost all B&W films.[4] In order to produce a usable image, the film needs to be exposed properly. The amount of exposure variation that a given film can tolerate while still producing an acceptable level of quality is called its exposure latitude. Color print film generally has greater exposure latitude than other types of film. Additionally, because print film must be printed to be viewed, after-the-fact corrections for imperfect exposure are possible during the printing process.