A Professional through-the-Lens CdS Exposure Control System with Optical Data Input for Motion-Picture Cameras

Abstract

A need for a precise TTL exposure measurement accompanied the development of color reversal film. This new exposure system does not electrically introduce film speed and exposure time data into the meter circuit, but instead the data input is achieved by two unique, counter-rotating precision neutral density discs which maintain the illumination level on the photoresistor at a defined reference value. This method allows the photoresistor to work at a predetermined operating point irrespective of exposure data settings and object illumination. A measuring field of about 30% of the filmed area was determined advantageous for the 16mm film. The light to be measured is diverted by a beamsplitter in the viewfinder's light path, where a relay lens projects the light onto the photoresistor. A galvanometer indicates the illumination level on the film. The reduced light during camera operation is electrically compensated by switching bridge-resistors. The power supply for the exposure meter is stabilized and derived from the camera battery. Illumination of the indicator scale is attained through a light collecting prism. This exposure system sets new standards in measuring accuracy with a wide variety of interchangeable lenses.