ABSTRACT

Other chapters have explained image formation by a lens and the response to light of various photosensors such as film. After due consideration of subject composition, perspective and sharpness, the photographer judges the best moment to make the exposure. An optical image of appropriate intensity is transmitted by the lens and reaches the photoplane for an appropriate length of time. This camera exposure is controlled by the combination of lens aperture (N) and shutter speed (t). More generally, the photographic result of exposure (H) is the product of image illuminance (E) and exposure duration (t), so that H = Et (1)

Determination of the optimum camera exposure is important for any photographic situation in terms of the resulting quality of the photographic image. Any subject contains areas of different luminances giving the overall subject luminance ratio (or range), so that image capture in turn gives a range of exposures at the photoplane, which on processing produces a range of monochrome or colour densities or values that form the image. With photographic materials, the range of image densities produced is predicted by the

characteristic curve of the film, which depends on the type of film and the development conditions used, given the positioning of the range of log exposures on the curve, which depends on camera exposure.