Photographic Techniques

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.

III

The Camera and Its Accessories

Modern cameras operate on the basic principle of the camera obscura. Light passing through a tiny hole, or aperture, into an otherwise light-proof box casts an image on the surface opposite the aperture. The addition of a lens sharpens the image, and film makes possible a fixed, reproducible image. The camera is the mechanism by which film can be exposed in a controlled manner. Although they differ in structural details, modern cameras consist of four basic components: body, shutter, diaphragm, and lens. Located in the body is a light-proof chamber (Latin, camera) in which film is held and exposed. Also in the body, located opposite the film and behind the lens, are the diaphragm and shutter. The lens, which is attached to the front of the body, is actually a grouping of optical glass lenses. Housed in a metal ring or cylinder, it allows the photographer to focus an image on the film. The lens may be fixed in place or set in a movable mount. Objects located at various distances from the camera can be brought into sharp focus by adjusting the distance between the lens and the film.

The diaphragm, a circular aperture behind the lens, operates in conjunction with the shutter to admit light into the light-proof chamber. This opening may be fixed, as in many amateur cameras, or it may be adjustable. Adjustable diaphragms are composed of overlapping strips of metal or plastic that, when spread apart, form an opening of the same diameter as the lens; when meshed together, they form a small opening behind the centre of the lens. The aperture openings correspond to numerical settings, called f-stops, on the camera or the lens.

The shutter, a spring-activated mechanical device, keeps light from entering the camera except during the interval of exposure. Most modern cameras have focal-plane or leaf shutters. Some older amateur cameras use a drop-blade shutter, consisting of a hinged piece that, when released, pulls across the diaphragm opening and exposes the film for about 1/30th of a second.

In the leaf shutter, at the moment of exposure, a cluster of meshed blades springs apart to uncover the full lens aperture and then springs shut. The focal-plane shutter consists of a black shade with a variable-size slit across its width. When released, the shade moves quickly across the film, exposing it progressively as the slit moves.

Most modern cameras also have some sort of viewing system or viewfinder to enable the photographer to see, through the lens of the camera, the scene being photographed. Single-lens reflex cameras (SLRs) all incorporate this design feature, and almost all general-use cameras have some form of focusing system as well as a film-advance mechanism.

A

Exposure Control

By adjusting shutter speed and diaphragm aperture, the photographer obtains just enough light to ensure a proper exposure. Shutter speed and aperture setting are directly proportional: a one-increment change in shutter speed is equal to a change of one f-stop. A “one-stop” adjustment in exposure can refer to a change in either shutter speed or aperture setting; the resulting change in the amount of light reaching the film will be the same. Thus, if the shutter speed is increased, a compensatory increase must be made in aperture size to allow the same amount of light to reach the film. Fast shutter speeds, 1/125th of a second or less, can capture objects in motion.

In addition to regulating the intensity of the light that reaches the film, the diaphragm aperture is also used to control the depth of field. Also called the zone of focus, depth of field refers to the area in which objects recorded in the picture will be sharply focused. Decreasing the size of the aperture increases the overall depth of field; widening the aperture decreases it. When great depth of field is desired—maximum sharpness of all points in the scene, foreground to background—a small aperture and slow shutter speed are used. Since the faster shutter speeds needed to capture motion require, in compensation, larger apertures, the depth of field is reduced. On many cameras, the lens ring contains a depth-of-field scale that shows the approximate sharp-focus zone for the different aperture settings.

B

Camera Designs

Cameras come in a variety of configurations and sizes. The first cameras, “pinhole” cameras, had no lens. The flow of light was controlled simply by blocking the pinhole. The first camera in general use, the box camera, consists of a wooden or plastic box with a simple lens and a drop-blade shutter at one end and a holder for roll film at the other. The box camera is equipped with a simple viewfinder that shows the extent of the picture area; some models have, in addition, one or two diaphragm apertures and a simple focusing device.

The view camera, used primarily by professionals, is the camera closest in design to early cameras that is still in widespread use. Despite the unique capability of the view camera, however, other camera types, because of their greater versatility, are more commonly used by both amateurs and professionals. Chief among these are the single-lens reflex, twin-lens reflex (TLR), and rangefinder. Most SLR and rangefinder cameras use the 35-mm film format, while most TLR as well as some SLR and rangefinder cameras use medium-format film, that is, size 120 or 220.