Exposure Metering

(Russian, eksponometriia), the branch of photography concerned with determining the proper exposure for photographic materials in still and motion-picture photography of various objects and in photocopying so that the images obtained will be of the best possible quality (according to the quality criteria adopted for the given field of photography).

Exposure metering is based on the well-established optical relationship between the brightness B of an object whose image is being formed by an optical system having a relative aperture \\:n (where n is a positive number), and the illuminance E of the image obtained: E = gBn^-2; here g is a factor that takes into account the light losses in the camera, the illuminance distribution in the image plane, the angle at which a particular point of the image is observed, and other factors. For some exposure time t the photographic material receives an exposure H = Et; if the usable light-sensitivity of the material S = a/H is taken into account, we obtain the basic equation of exposure metering B = kn²ltS. The quantity k = alg may be termed the exposure metering constant. The equation only associates the integral characteristics of the object being photographed, the photographic material, and the imaging system. The development of all exposure meters, tables, and the like was based on the equation until the advent of small, highly sensitive photoelectric optical detectors. Optical detectors solved the problem of local exposure metering; that is, they made it possible to determine the proper exposure according to the brightness of some important area of the object being photographed. The values and the range of variation of k are established according to the field of use. They range from 10 to 17 for built-in exposure meters in general-purpose cameras and from 10 to 13.5 for separate exposure meters not in unit with a camera.

The type of functional relation between built-in exposure-metering systems and the mechanisms that establish the operating parameters of a camera during the photographic process determine to a considerable extent the automation of the photographic process and are an important characteristic of still and motion-picture cameras.