GB1148059A - Improvements in or relating to automatic exposure control apparatus for cameras - Google Patents

Abstract

1,148,059. Photo-electric exposure control. HONEYWELL Inc. July 20, 1966 [July 21, 1965 (2)], No. 32655/66. Heading G1A. [Also in Division G2] An automatic exposure control apparatus for a camera has one or more photo-cells 27 which produce a response dependent on the intensity of light incident thereon and a control circuit which receives a stored response when the light is interrupted to control the exposure of a film in the camera in accordance with the stored response. As applied to a single lens reflex camera, light is received by the photo-cell through objective 1 provided with a diaphragm 2, Fresnel lens 7 ground glass screen 8, mask 9 and pentaprism 10. A mirror 3 is tiltable upwards to permit exposure of a film 6 through a focal plane shutter 4. The shutter has blinds 17 and 20 controlled by a release button 14. A circuit 28A, Fig. 4, includes a battery 32 a switch 34 mechanically connected to the shutter release, transistors 35, 36, and 37, a capacitor 38 and a solenoid 29 which upon de-energization closes the blind 20 of the shutter. Switch 34 is in closed position before shutter release to connect capacitor 38 across a resistor 40. A voltage, in accordance with the resistance of photo-cell 27 and the light incident thereon, is fed to capacitor 38. Upon depression of the shutter release the mirror 3, Fig. 1 is moved upwardly such that photo-cell 27 no longer receives light. Switch 34 Fig. 4 is opened to disconnect the now charged capacitor 38 from photo-cell 27. Capacitor 38 discharges during exposure, such that when the capacitor voltage drops to the trigger level of transistors 36 and 37 solenoid 29 is de-energized to terminate the exposure. Thus, the exposure time is proportional to the charging voltage of the capacitor at the instant the switch is opened and inversely proportional to the intensity of light incident on the photo-cell. Film sensitivity is accounted for by a manually adjusted resistance 39 or the capacitance of capacitor 38 may be varied. In another embodiment, a transfluxor (44, Fig. 5 not shown) stores the response in place of capacitor 38 Fig. 4. In other embodiments the capacitor voltage is fed to a Schmidt trigger Fig.6 (not shown) which controls the energization of solenoid (29) which then closes the shutter or is fed to an amplifier (60 Fig. 7 not shown) and trigger circuit which energize and then de-energize solenoid (29). A test lamp (66) if let indicates there is sufficient light for exposure upon closure of a test button (63) which then supplies a voltage sufficient to operate trigger circuit (61) should the light falling on photo-cell (27) be sufficient. In another embodiment Fig.8 (not shown) photocell (27) is a cadmium sulphide cell the resistance of which exhibits a slow decay at low light intensities produces and stores the response. A linkage (76) partly closes diaphragm (2), upon opening the shutter and through a resistance (72) and contact (75) varies the trigger voltage to compensate for the reduction in light intensity.