GB941856A - Improvements in or relating to a measuring or regulating device - Google Patents

Abstract

941,856. Photo-electric exposure control for photographic cameras; automatic control of light intensity. EUGEN BAUER G.m.b.H. Dec. 20, 1961 [Dec. 22, 1960], No. 45626/61. Headings G2A and G3R. The amount of light passing through a camera lens is detected by a photo-resistor the output of which controls an apertured diaphragm associated with the lens whereby the light intensity for exposing the film is maintained constant at some predetermined value. As shown, a cinecamera comprises an objective lens 2, a film gate 3 behind which is moved a film 4, a rotatable shutter 5 having a conical mirrored surface 7 and a photo-resistor 30 which receives light reflected from surface 7 and which is electrically in circuit with a device 20 operating a movable diaphragm 17 associated with the lens 2. The device 20 comprises a rotatable frame 19 which is connected to the diaphragm 17 and which has two parallel legs carrying superposed coils 22, 23 arranged in a magnetic field system 24, 25 so that both coils are affected by the same field strength whatever the angular position of frame 19. The coil 23 is connected together with photoresistor 30 in one branch 32 of a circuit supplied by a battery 33 which may also drive the camera motor; a second branch 31 in parallel with the first, contains the coil 22 together with resistors 44, 38 to 43 which are adjustable to suit the various operating conditions, e.g. film sensitivity, filter factors &c. When these conditions have been set and the amount of light passing through lens 2 is correct, then the currents in branches 31 and 32 are equal and device 20 is in an equilibrium position. Should there be an increase in the amount of light incident on the photo-resistor 30, and hence incident upon the film 4 when the shutter 5 frees the gate 3, then the resistance of 30 decreases, the current in branch 32 increases and the force acting on coil 23 becomes the stronger so that the frame 19 rotates about spindle 21 until the diaphragm 17 limits the amount of incident light to the original setting, the currents in branches 31 and 32 being then again equal. The opposite action takes place when the amount of light passing through lens 2 decreases. The frame 19 may carry a pointer which indicates the light strength on a scale. In a modification Figs. 5 and 6 (not shown), the diaphragm has separate apertures for the lens 2 and the photo-resistor 30.