Light totalizing system for photographic camera provided with a

Abstract

During the ongoing course of the exposure, the aperture area increases linearly to a maximum value and then stays at the maximum value, the instantaneous amount of exposure light therefore changing correspondingly, even if the ambient-light level remains constant during the exposure. A pulse generator includes a photodetector exposed to ambient light and generates a pulse train of light-dependent repetition frequency, the pulses of which are counted by a light-totalizing counter which eventually generates a terminate-exposure signal. Ideally, the repetition frequency should increase steplessly and linearly, for maximum accuracy, or second best increase stepwise in small steps to approximate to a stepless linear increase, but in order to use an extremely low number of stepwise frequency changes, without loss of system accuracy, no attempt is made to per se keep the light-indicating repetition frequency accurate. Instead, the number and amounts of the repetition-frequency changes are so established that, when the repetition frequency is plotted against elapsed exposure time, it is proportional to a piecewise linearization of the time integral curve of the exposure-aperture surface area, the constituent straight-line segments of the linearization being confined within a predetermined error-of-total-exposure tolerance range. In the case of a linear increase of aperture size concluding in maximum size, this corresponding to a quadratic followed by a straight-line rise of the time integral of the exposure-aperture surface area, the requisite time integral curve, despite its complexity, can, for example, be implemented using as few as only three or even as few as two stepwise changes of repetition frequency, without loss of system accuracy.