19Camera Exposure Determination
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
Determination of the optimum camera exposure is important for any photographic situation ...