Film

35mm film, showing sprocket holes inherited from its origins as movie film image by Voxphoto (Image rights)

Film is the general name for a light-sensitive material coated onto a flexible backing. Prior to its invention, photographers used rigid glass or metal plates. Thus, lightweight film rolls were a major breakthrough for photographers in portability and convenience.

The Science of Film

The chemical elements bromine, chlorine and iodine are collectively known as the halides, and all react with silver to form a light-sensitive compounds. Of these, silver bromide (AgBr) is especially light-sensitive, and this is what makes film photography possible. Fine silver halide grains (of controlled size) may be suspended in liquefied gelatin, and coated onto a transparent backing. This is conventionally termed an emulsion—although since it not a mixture of two liquids, this is a slight misnomer.

When light strikes one of these grains, a few bromine atoms are kicked off the surface. Gelatin has the property of immobilizing these atoms—potentially for decades—and the specks of pure metallic silver remaining on the film grains form a latent image.

When the exposed film is bathed in a suitable reducing agent, the metallic silver specks strongly catalyze the reaction converting the silver halide grains into metallic silver. By timing this chemical development correctly, unexposed areas of the film remain pale silver halide; while exposed areas develop as black, due to the fine soot-like metallic silver particles embedded in the gelatin.

Finally, another chemical bath is used which dissolves and removes the unexposed, still-light-sensitive silver halides. This fixes the image. Because exposure to light results in a dark area on the film, and vice versa, the resulting image is a negative. This image may be projected onto a similar paper-backed emulsion, for another reversal of tones, yielding a normal positive print.

It may also possible to use reversal processing on a latent film image. In this case, it is the silver of the developed negative image that is chemically dissolved. The silver halide grains remaining behind are flashed with light or chemically activated, then given a second development. This creates a dark silver image in the unexposed areas of the scene, and thus it is a positive rather than a negative.

Color

The previous description gives the essential principles behind traditional black & white films. An unmodified silver-halide emulsion has its greatest sensitivity to blue light, and little sensitivity to red. But an emulsion can include sensitizing dyes that expand the spectral sensitivity, either just into the green spectrum (giving an orthochromatic or "ortho" film) or throughout the visible spectrum (giving a panchromatic or "pan" film).

These various color-sensitizing agents make it possible to create color film, by coating multiple layers onto the same support: A topmost silver halide layer can be sensitized only to blue light; below that a yellow filter layer is coated. This blocks all blue light from reaching the subsequent layers. The next silver halide layer is an ortho emulsion, now only responding to green light. Beneath this is coated a red filter; and at the bottom a panchromatic emulsion, which now only responds to red light.

While these are essentially three black & white emulsions, they have divided the color spectrum into three, and thus can represent the color of a scene in a way that matches the eye's color vision. The image may developed into a color negative, or using reversal processing into a positive color transparency—a color slide.

Current color films incorporate color couplers—meaning, compounds that react chemically with the developed areas of the image to yield color dyes. Different types of couplers are used to create dye colors appropriate to each layer of film.

While simplifying processing, these dyes deviate considerably from the ideal in their spectral absorption. Much film manufacturer R&D has gone into managing this issue; and in practice, multiple coating layers are often used. Furthermore, film may be given anti-reflection and anti-curl layers, plus a protective surface covering. The result is that a modern film may be coated with 17 different layers. Kodak's current coating line in Rochester, New York has the capability to coat film with up to 20 layers if necessary.

History

While coating emulsions onto a flexible support was an idea that had occurred to many people, George Eastman was undoubtedly the person who brought it into the photographic mainstream. He was so certain of the promise of the idea that based on his 1884 patents he renamed his Eastman Dry Plate Co. to become the Eastman Dry Plate & Film Co. His early roll film experiments used a gelatin emulsion coated onto an opaque paper backing. For printing, the processing lab would separate the emulsion and transfer it to a transparent support. But in 1889, Eastman offered his first transparent, celluloid-base film.

Besides this technical innovation, Eastman was a shrewd businessman with an eye for promotion. He loaded 100 exposures of his paper backed roll film into an easy-to-use 1888 box camera and gave it the catchy nonsense name Kodak. He promoted it with the famous slogan "you press the button, we do the rest." While it would have been impractical to have 100 glass plates shipped to Rochester, New York and back for developing, lightweight roll film made this a viable business model. Although the $25 original price, adjusted for inflation, would equal over USD $600 today, the Kodak was an immense hit, and photography was on its way to becoming a daily part of life worldwide.

Film Types

• Traditional, cubical grained silver-halide black and white films (Examples: Ilford FP4, Kodak Tri-X)
• Engineered-grain silver-halide black and white films (Examples: Kodak TMax 400, Ilford Delta 100)
• Color negative films (print film) using development process C-41 (Examples: Fujifilm Superia 400, Kodak Ektar)
• Chromogenic black & white films using development process C-41 (Examples: Kodak BW400CN, Ilford XP2 Super)
• Color reversal film (slide film) using development process E-6 (Fujichrome Astia 100F, Kodak Elite Chrome 200)
• Kodachrome reversal film (slide film) using development process K-14 (now discontinued)

Instant-print media such as those from Fujifilm and formerly, Polaroid, are also typically known as "film" packs—although in these cases, the light-sensitive medium is simply one layer in a sandwich of components.

Film Formats

Historically there have been a plethora of film formats and image sizes used in photography. At the small end, there is the film used in the Minox camera which is only 9.2mm wide. On the larger side, view cameras may use sheet film of 8x10" or even larger. In the early 20th century, a variety paper-backed roll film sizes were in use, up to nearly 4 inches in width. Over the years, various easy-loading film formats have been pitched at the snapshooter market, including 126 and 110 cartridges, disk film and APS.

But two film formats have endured the longest: Perforated 35mm film, originally a movie stock, then later used still cameras with Kodak's 135 cassette format; and 120, a paper-backed roll film introduced for Kodak's Brownie No. 2 of 1901 (although the numbering 120 was not applied until later).

Links

• part two of History of Photography and the Camera: Improving Films and Prints, on inventors.about.com [1]