Title: Electric shutter operating circuits

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Abstract: In an electric shutter operating circuit of a photographic camera wherein the shutter is operated in accordance with the photoelectric current produced by a photoelectric element, there are provided a D. C. source and an operational amplifier having two inputs connected across the photoelectric element, a first impedance element with one terminal connected to one input of the operational amplifier for passing the photoelectric current, a second impedance element connected between a negative pole of the D. C. Source and the other input of the operational amplifier, a feedback circuit extending between the input and output of the operational amplifier for passing the feedback current through the second impedance element, an integrating capacitor which begins to integrate the feedback current when the shutter release button is depressed, a shutter trigger circuit connected to the output of the operational amplifier for closing the shutter when the voltage across the integrating capacitor reaches a predetermined value, and a constant current circuit connected to the output of the operational amplifier. The constant current from said constant current circuit is small and the sum of said constant current and the output of the operational amplifier is equal substantially to the feedback current which is proportional to the photoelectric current. Hence in the normal brightness range the shutter time is determined by said feedback current which is proportional to the photoelectric current whereas under low brightness conditions such that the output current from the operational amplifier is reduced to zero, the shutter time is determined by small constant current from constant current circuit.


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