GB1303715A - - Google Patents

Abstract

1303715 Photographic exposure time control NIPPON KOGAKU KK 22 Oct 1970 [28 Oct 1969] 50221/70 Heading G2A A camera has an exposure time control circuit in which a first voltage is produced which varies arithmetically with geometric variations in the brightness of a subject to be photographed, and a second voltage is produced which varies in proportion with the logorithm of the time elapsed from the opening of the shutter, a switching circuit being actuated to close the shutter when the difference between the voltages reaches a predetermined value. The first voltage is stored on a capacitor 9, switch 8 being opened upon the movement of a mirror in a single lens reflex camera, and is applied to the gate of a first FET T 1 which with a second FET T 2 forms a differential amplifier, the second voltage applied to the gate of T 2 . A switch 5 short circuiting a timing capacitor 4 is opened upon initiation of a shutter opening operation. Transistor T 4 is normally conducting, the current through coil 11 maintaining the shutter open. T 4 is switched to the non-conducting state by transistor T 3 when the difference voltage has reached its predetermined value. Potentiometer 14 is adjusted to take account of exposure factors, e.g. film speed, diaphragm aperture. Resistor 13 is adjusted to take account of the γ of the photo-conductive element 1. A second embodiment is described, Fig. 2 (not shown) which uses three differential amplifiers and has an indicator (25) giving the controlled exposure time.