Black-and-White Films

Film is the basic material of photography. It consists of light-sensitive emulsion—microscopic crystals of silver halide suspended in gelatin—that is coated onto a flexible support of plastic.

Virtually all black-and-white films for general use are panchromatic—equally sensitive (or approximately so) to the light of all colours. The monochromatic tonal values of a subject photographed on panchromatic film will match those of the subject quite closely.

THE LATENT IMAGE

The sensitive surface of ordinary film is a layer of gelatin carrying minute suspended silver halide crystals or grains (the emulsion)—typically silver bromide with some silver iodide. Exposure to light in a camera produces an invisible change yielding a latent image, distinguishable from unexposed silver halide only by its ability to be reduced to metallic silver by certain developing agents.

SENSITOMETRY AND SPEED

The internationally adopted scale is ISO speed, written, for example, 200/24°. The first half of this (200) is arithmetic with the value directly proportional to the sensitivity (and also identical with the still widely used ASA speed). The second half (24°) is logarithmic, increasing by 3° for every doubling of the speed (and matching the DIN speeds still used in parts of Europe). A film of 200/24° ISO is twice as fast (and for a given subject requires half as much exposure) as a film of 100/21° ISO, or half as fast as a film of 400/27° ISO.

All-around films for outdoor and some indoor photography have speeds between 80/20° and 200/24° ISO; fine-grain films for maximum image definition between 25/15° and 64/19° ISO; and high-speed and ultraspeed films for poor lighting conditions, ranged from from 400/27° to 3200/36° ISO. Typical low-light consumer film speeds tend to top out at around 800/40° ISO.

COLOUR SENSITIVITY

Initially, the silver halide emulsion is sensitive to ultraviolet radiation and to violet and blue light. Most films contain sensitizing dyes to extend their colour sensitivity through the whole visible spectrum. Such films, called panchromatic films, were introduced in 1904. They record subject colour values as gray tones largely corresponding to the visual brightness of the colours.

Infrared films, developed in 1919, are sensitized to invisible infrared wavelengths. They are used in aerial photography to cut through atmospheric haze (which scatters blue light but not infrared rays) and for special purposes in scientific and forensic photography.

FILTERS

Filters can modify the way in which a film records colours as monochrome tone values. They are disks of coloured glass or gelatin with controlled transmission characteristics. Placed in front of the camera lens, they preferentially transmit light of their own colour and hold back light of other colours. A yellow or yellow-green filter is often used in landscape photography to prevent overexposure of the blue sky and to bring out detail in cloud formations. Orange and red filters make the sky still darker and cut through haze by absorbing scattered blue light.




Yellow, green, and red filters for an SLR camera lens can improve various colours and reduce reflections in challenging lighting conditions





Yellow, green, and red filters for an SLR camera lens can improve various colours and reduce reflections in challenging lighting conditions.

Other filter types used in photography include ultraviolet, infrared, and polarizing filters. Ultraviolet-absorbing filters screen out ultraviolet rays at high altitudes (e.g., in mountain photography). Because camera lenses are not normally corrected for such rays, the rays can reduce image sharpness, even though the lenses allow only a small amount of ultraviolet to be transmitted. Infrared filters are used with infrared film to hold back visible light. Polarizing filters polarize light and can absorb polarized light if suitably oriented. Light reflected at certain angles from shiny surfaces of nonmetallic media (glass, water, varnish) is polarized; a properly oriented polarizing filter subdues such reflections in a picture.

Because a filter screens out part of the light, its use calls for extra exposure, the amount of which is indicated by a filter factor—e.g., 2×, which means the exposure time must be multiplied by 2. For cameras with an exposure-value scale, a filter may specify an exposure value reduction (such as -1 or -1 1/2; i.e., the indicated exposure value must be reduced by this amount). The factor of a given filter depends on the spectral sensitivity of the film, the colour quality of the lighting, the type of subject, the effect aimed at, and other exposure conditions.

PHOTOGRAPHIC FILTER

A photographic filter is a device used to selectively modify the component wavelengths of mixed (e.g., white) light before it strikes the film. Filters may be made of coloured glass, plastic, gelatin, or sometimes a coloured liquid in a glass cell. They are most often placed over the camera lens but can in some cases be placed over the light source with the same effect.

Black-and-white films are imperfect in their colour sensitivity, and coloured filters are used to modify the light and translate the subject into gray tones that correspond to the tones seen by the human eye. Coloured filters can also brighten or darken the reproduction of coloured objects, permitting local contrast controls at the point of exposure. In colour photography, coloured filters are used to alter the colour quality of the light to match the colour sensitivity of the film.

Since filters absorb some of the light that passes through them, an increase in the calculated exposure is usually required. This increase is known as the filter factor. Modern cameras with built-in meters measure the light after the filtration and thus take the decrease in intensity into account.

OTHER FILM CHARACTERISTICS

Of practical interest to the photographer are the graininess, resolving power, and contrast of a film. Although they are characteristics of the film itself, they are influenced by the conditions of development.

GRAIN RESOLVING POWER AND ACUTANCE

The fineness of detail that a film can resolve depends not only on its graininess but also on the light scatter or irradiation within the emulsion (which tends to spread image details) and on the contrast with which the film reproduces fine detail. These effects can be measured physically to give an acutance value, which is preferred to resolving power as a criterion of a film's sharpness performance. Fine-grain films with thin emulsions yield the highest acutance.

CONTRAST

High-contrast films reproduce tone differences in the subject as great density differences in the image; low-contrast films translate tone differences into small density differences. The characteristic curve of a film obtained by plotting the density against the logarithm of the exposure can be used to express a film's contrast. The slope of the straight-line section of the curve (sometimes called the gamma, actually the tangent of the angle α) indicates contrast: the steeper the slope, the higher the contrast rendering. General-purpose films yield medium contrast (gamma 0.7 to 1). High-contrast films (gamma 1.5 to 10) are used for copying line originals and other specialized purposes; low-contrast films for continuous-tone reproduction. Gamma is also used to indicate degree of development, since increased development generally results in a higher gamma.

FILM STRUCTURE AND FORMS

Film consists of a number of layers and components: (1) A supercoat of gelatin, a few micrometres (one micrometre is 0.001 millimetre) thick, protects the emulsion from scratches and abrasion marks. (Pressure and rubbing can produce developable silver densities.) (2) The emulsion layer (silver halide suspended in gelatin) is usually nine to 12 micrometres (up to 1/2,000 inch) thick but may sometimes reach 25 micrometres. (3) A substrate or subbing layer promotes adhesion of the emulsion to the film base. (4) The film base, or support, is usually cellulose triacetate or a related polymer. The thickness may range from 0.08 to 0.2 millimetre (0.003 to 0.008 inch). Films for graphic arts and scientific purposes are often coated on a polyethylene terephthalate or other polyester support of high dimensional stability. Glass plates—once the most common support for negative materials—are now used only for applications requiring extreme emulsion flatness. (5) A backing layer on the rear of the film base counteracts curling. Usually it contains also a nearly opaque dye to suppress light reflection on the rear support surface. Such reflection (halation) reduces definition by causing halolike effects around very bright image points. Some film bases (especially in 35-mm films) are tinted gray to absorb light that has passed through the emulsion layer.

SHEET FILM




An opened back of a 35-mm camera reveals a loaded roll of film that is perforated and uses a transport sprocket to draw the film from a cartridge onto another spool as it advances from picture to picture





An opened back of a 35-mm camera reveals a loaded roll of film that is perforated and uses a transport sprocket to draw the film from a cartridge onto another spool as it advances from picture to picture.
ROLL FILM PERFORATED FILM

Some film is perforated along its edges and rolled up on its own inside a light-tight cartridge, which can be loaded into the camera in daylight. Once the camera is closed, a transport sprocket engaging the edge perforations draws the film from the cartridge onto a spool and advances it from picture to picture. The most common film width is 35 mm (for 35-mm miniature cameras), and its cartridge typically holds enough film for up to 36 (sometimes 72) exposures. A 70-mm film for larger cameras and 16-mm strips for ultraminiatures are packed and used in a similar way.

In March 1983 the Eastman Kodak Company announced the development of a new coding system for 35-mm film and cartridges. The DX film system employs optical, electrical, and mechanical encoding to transmit to appropriately equipped cameras such information as film type, film speed, and number of exposures. The system also supplies data that enable automatic photofinishing equipment to identify and sort film quickly, simplifying processing and printing. In the interest of uniformity, Kodak freely offered the DX system to all film and camera manufacturers, and within two years it was generally adopted.

EASTMAN KODAK COMPANY

Eastman Kodak Company, also called Kodak, is an American manufacturer of film and photographic supplies and provider of digital imaging services and products. The company's headquarters are in Rochester, New York.

The company was incorporated in 1901 as the successor to a business established in Rochester in 1880 by George Eastman, who perfected the newly developed method of making photographic dry plates. Before 1880, photographers had to coat a plate with fresh, wet chemicals each time they wanted to take a picture. Eastman developed a machine that mechanically produced dry, precoated plates, and he set up the company that later would become Eastman Kodak to sell his plates.

Four years later Eastman introduced roll film, and in 1888 he introduced the Kodak camera, the first camera that was simple and portable enough to be used by large numbers of amateur photographers. The camera was sold with film sealed inside, and the whole unit was mailed back to Rochester for film processing and replacement. In 1900 Eastman introduced the less-expensive Brownie, a simple box camera with a removable film container, so that the whole unit no longer needed to be sent back to the plant.

In following years the company continued to produce innovations for the amateur photographer. Kodak was the first to make home-movie equipment and an easy-to-use colour slide film, Kodachrome. In the 1960s the company introduced the Instamatic series of still and movie cameras, all of which used cartridge-loaded film. The first digital camera was invented in 1975 by a Kodak employee, but the technology was not developed at that time. The Disc camera series, introduced in 1982, was a technologically advanced group of cameras that could automatically activate the use of the flash, if necessary, in addition to adjusting the focus and advancing the film. In 1984 the company entered the video market by introducing an 8-millimetre video camera system and videocassette tapes.

By the early 21st century, facing significant declines in the consumer market for cameras and other products related to film photography, Kodak placed new emphasis on digital photography products. The company remained a leading supplier of photographic films, papers, and chemicals for professional photographers and also manufactured motion-picture film, single-use and other consumer digital cameras, aerial surveillance films, and equipment for the graphic communications industry into the 21st century. Kodak filed for bankruptcy in January 2012 and announced soon after that it would no longer manufacture digital cameras and some other digital imaging products.

DISK FILM

Some compact mass-market cameras took circular disks of film, 65 millimetres in diameter, in light-tight cartridges and coated on a 0.18-mm polyester base. In the camera the disk rotated as up to 15 exposures (frame size 8 × 10 millimetres) were recorded around the disk circumference. The disk laid flatter in the camera than rolled-up film and was suitable for more automated photofinishing; the high printing magnification required, however, limited the image quality.