II.

Photographic Film

Photographic films vary in the way they react to different wavelengths of visible light. Early black-and-white films were sensitive only to the shorter wavelengths of the visible spectrum, that is, to light perceived as blue. Later, coloured dyes were added to film emulsions to make the silver halides responsive to light of other wavelengths. These dyes absorb light of their own colour, making silver halide particles sensitive to light of that colour. Orthochromatic film, incorporating yellow dyes in the emulsion and sensitive to all light but red, was the first improvement on simple blue-sensitive film.

In panchromatic film, the next major improvement, red-toned dyes were added to the emulsion, rendering the film sensitive to all visible wavelengths. Although slightly less sensitive to green tones than the orthochromatic type, panchromatic film is better able to reproduce the entire range of colour tones. Thus, most films now used by amateur and professional photographers are panchromatic.

Two additional varieties of black-and-white film—process and chromagenic—have their special uses. Process film is used primarily for line reproduction of copy in the graphic arts. Such films have extremely high contrast, producing images with no tonal values between black and white. Chromagenic film produces dye images rather than silver images on the negative. Using dye couplers and silver halide in the emulsion, it can be developed by standard colour-negative development processes. After development, the silver is bleached out of the film, leaving a black-and-white dye image.

Special-purpose films are sensitive to wavelengths beyond the visible spectrum. In addition to visible light, infrared film also responds to the invisible, infrared portion of the spectrum (see Infrared Photography, below).

Instant film, introduced by the Polaroid Corporation in the late 1940s, provides photographs within seconds or minutes of the taking of the picture, using a camera specially designed for this purpose. In instant film the processing chemicals and emulsion are combined in a self-contained envelope or on the print itself. Exposure, development, and printing all take place inside the camera. Polaroid, the leading manufacturer of this film, uses a conventional silver-halide emulsion. After the film is exposed and a negative image produced, the negative is sandwiched with photographic paper and processing chemicals, and a fogging agent transfers the negative image to the paper, producing a print. A number of instant films are manufactured in a 35-mm format, both in black-and-white and in colour.

A.

Colour Film

Colour films are more complex than black-and-white films because they are designed to reproduce the full range of colour tones as colour, not as black, white, and grey tones. The design and composition of most colour transparency films and colour negative films are based on the principles of the subtractive colour process, in which the three primary colours, yellow, magenta, and cyan (blue-green), are combined with their complements to reproduce a full range of colours. Such films consist of three silver halide emulsions on a single layer. The top emulsion is sensitive only to blue. Beneath this is a yellow filter that blocks blues but transmits greens and reds to the second emulsion, which absorbs greens but not red. The bottom emulsion records reds.

When colour film is exposed to light by a camera, latent black-and-white images are formed on each of the three emulsions. During processing, the chemical action of the developer creates actual images in metallic silver, just as in black-and-white processing. The developer combines with dye couplers incorporated into each of the emulsions to form cyan, magenta, and yellow images. Then the film is bleached, leaving a negative image in the primary colours. In colour transparency film, unexposed silver-halide crystals not converted to metallic silver during the initial development are converted to positive images in dye and silver during a second stage of development. After the development action has been arrested, the film is bleached and the image fixed on it.

B.

Film and Camera Formats

Different types of camera require particular forms and sizes of film. Currently, the most widely used camera format is the 35 mm or small format, which produces 20, 24, or 36 images that each measure 24 x 36 mm on a roll of film. The film is wound on a spool inside a light-proof magazine or cartridge. Film for 35-mm cameras is also available in bulk, in long rolls that can be fed into individual cartridges and cut to length.

The next larger standard camera format, medium format, uses film sizes designated as either 120 or 220. Medium-format cameras produce images of various sizes, such as 6 x 6 cm or 2‚ x 2‚ in, 6 x 7 cm, and 6 x 9 cm, depending on the configuration of the camera. Larger cameras, called view cameras, use sheet film. Standard sheet film sizes correspond to standard view-camera formats: 4 x 5 in, 5 x 7 in, and 8 x 10 in. Larger special-purpose view cameras, up to 20 x 24 in, are in limited use.

C.

Film Speed

Film is classified by speed as well as by format. Film speed is defined as an emulsion's degree of sensitivity to light, and determines the amount of exposure required to photograph a subject under given lighting conditions. The manufacturer of the film assigns a standardized numerical rating in which high numbers correspond to “fast” emulsions and low numbers to “slow” ones. The standards set by the International Standards Organization (ISO) are used throughout the world, although some European manufacturers still use the German Industrial Standard, or Deutsche Industrie Norm (DIN). The ISO system evolved by combining the DIN system with the ASA (the industry standard previously used in the United States). The first number of an ISO rating, equivalent to an ASA rating, represents an arithmetic measure of film speed, whereas the second number, equivalent to a DIN rating, represents a logarithmic measure.

Low-speed films are generally rated from ISO 25/15 to ISO 100/21, but even slower films exist. Kodak's Rapid Process Copy Film, a special process film, has an ISO rating of 0.06/-12. Films in the ISO 125/22 to 200/24 range are considered medium speed, while films above ISO 200/24 are considered fast. In recent years, many major manufacturers have introduced superfast films with ISO ratings higher than 400/27. And certain films can be pushed well beyond their ratings by exposing them as though they had a higher rating and developing them for a greater length of time to compensate for the underexposure.

DX coding is a recent innovation in film and camera technology. DX-coded cartridges of 35-mm film have printed on them a characteristic panel corresponding to an electronic code that tells the camera the ISO rating of the film as well as the number of frames on the roll. Many of the newer electronic cameras are equipped with DX sensors that electronically sense this information and automatically adjust exposures accordingly.

Differences in sensitivity of a film emulsion to light depend on various chemical additives. For example, hypersensitizing compounds increase film speed without affecting the film's colour sensitivity. High-speed film can also be manufactured by increasing the concentration of large silver-halide crystals in the emulsion. In recent years, a generation of faster, more sensitive films has been created by altering the shape of crystals. Flatter silver-halide crystals offer greater surface area. Films incorporating such crystals, such as Kodak's T-grain Kodacolour films, have a correspondingly greater sensitivity to light.

The grain structure of faster films is generally heavier than that of slower films. Grain structure may give rise to a mottled pattern on prints that have been greatly enlarged. Photographs taken with slower-speed film appear less grainy when enlarged. Because of the small size of their silver-halide grains, slow-speed films generally have a higher resolution—that is, they can render fine details with greater sharpness—and can produce a broader range of tones than fast films. When tonal range and sharpness of detail are not as important as capturing a moving subject without blurring, fast films are used.

D.

Exposure Range

Each type of film has a characteristic exposure range, or latitude of exposure. Latitude is basically the margin of error in exposure within which film, when developed and printed, can render the actual colour and tonal values of the scene photographed.

The terms “overexposure” and “underexposure” are used to characterize deviations, whether intentional or not, from the optimum exposure. Film exposed to light longer than optimal will often be “blocked up” with silver in highlight areas, resulting in a loss of contrast and sharpness and an increase in graininess. Underexposure, on the other hand, produces negatives characterized as thin, a condition in which there are not enough silver crystals for accurate rendering of dark and shadowed areas.

With films that have a narrow latitude, an exposure adjusted for a shady area is likely to result in overexposure of adjacent sunny areas. The greater a film's latitude, the greater its ability to provide satisfactory prints despite over- or underexposure. Films from which negatives are made, both colour and black-and-white, generally offer enough latitude to allow the photographer a certain margin of error. Transparency films, from which colour slides are made, generally have less latitude.