Exposure time control device for a through the lens type single reflex camera

Abstract

First and second output voltages are generated respectively in accordance with the logarithm of the intensity of light and as a function of a manually set exposure condition. A control signal to initiate shutter closing is generated when a signal which changes in proportion to the logarithm of an elapsed time period initiated by shutter release operation plus the second output voltage is substantially equal to the first output voltage. The control device includes a circuit for comparing the voltages. The device may be modified to generate a third output voltage which changes in response to the change of the signal and then comparing the first output voltage with the second plus the third output voltage.

What is claimed is: 
     
       1. An automatic shutter control device for a single lens reflex camera of the type having a photoconductive member receiving light from an object to be photographed via an objective lens, comprising: means for generating a first output voltage proportional to the logarithm of intensity of light incident on said photoconductive member;   means for storing said first output voltage;   means for separating said means for storing from said means for generating a first output voltage with the shutter release operation;   means for generating a second output voltage as a function of a manually set exposure condition;   a transistor;   means for applying constant voltage to the base of said transistor;   a first integrating means connected to the emitter of said transistor for generating a signal that changes in proportion to the logarithm of an elapsed time period initiated by the shutter release operation;   means for initiating operation of said first integrating means in response to the shutter release operation;   means for generating a control signal when said signal .[.plus.]. .Iadd.relative to .Iaddend.said .Iadd.first and .Iaddend.second output .[.voltage.]. .Iadd.voltages .Iaddend..[.are substantially equal to said first output voltage.]. .Iadd.provides a proper exposure.Iaddend.; and   means for actuating shutter closing operation in response to said control signal.   
     
     
       2. An automatic shutter control device as in claim 1, further comprising second integrating means connected to the collector of said transistor for generating a third output voltage which changes in response to the change of said signal of said first integrating means, and said means for generating a control signal includes a comparing circuit for comparing said signal plus said second output voltage with said first output voltage. 
     
     
       3. An automatic shutter control device as in claim 2, wherein said photoconductive member comprises two parallelly connected photoconductive elements and said means for generating a first output voltage includes said photoconductive elements, a compensation resistor connected to at least one of said photoconductive elements and an output resistor connected to said compensation resistor to generate said first output voltage. 
     
     
       4. An automatic shutter control device as in claim 3, wherein said comparing circuit includes a differential amplifier circuit having a first input terminal to which said first output voltage is applied and a second input terminal to which both said second and third output voltages are applied and said means for generating a second output voltage biases said second integrating means. 
     
     
       5. An automatic shutter control device as in claim 4, further comprising an electric power source and said means for applying constant voltage includes a second transistor connected between said power source by the emitter and collector thereof and having a base connected to the collector of said second transistor through a resistor, the base of said first transistor being connected to the collector of said second transistor. 
     
     
       6. An automatic shutter control device as in claim 5, wherein said means for generating a first output voltage further includes means for generating a constant current, and said means for generating a second output voltage includes a variable resistor and said constant current flows through said variable resistor. 
     
     
       7. An automatic shutter control device as in claim 6, wherein said first and second integrating means are adjustable for changing the ratio of their respective stored signals. 
     
     
       8. An automatic shutter control device as in claim 7, further comprising means for indicating shutter speed prior to shutter release and responsive to said means for generating a first output voltage. 
     
     
       9. An automatic shutter control device as in claim 8, wherein said means for indicating shutter speed comprises an ammeter having double coils and two transistors in which each collector thereof is connected to a respective one of said double coils, and the base of one of said transistors is connected to said means for generating a first output voltage and the base of the other transistor is connected to said means for generating a second output voltage. 
     
     
       10. An automatic shutter control device for a single lens reflex camera of the type having a photoconductive member receiving light from an objective to be photographed via an objective lens, comprising: an electric power source;   a detecting circuit including said photoconductive member connected to said electric power source for generating a first output voltage proportional to the logarithm of intensity of light incident on said photoconductive member;   a change over switch;   a storage means connected to said detecting circuit through said change over switch prior to exposure of the shutter for storing said first output voltage;   means including a variable resistor for generating a second output voltage as a function of an exposure condition manually set in the camera;   a transistor;   means for applying a constant voltage to the base of said transistor;   a first capacitance connected to the emitter of said transistor so that terminal voltage of said capacitor changes in proportion to the logarithm of an elapsed time period;   means for initiating the change of said terminal voltage of said first capacitor in response to opening operation of the shutter;   a second capacitor connected to the collector of said transistor and switch means for connecting said collector to said means for generating a second output voltage;   a differential amplifier circuit having two transistors, the base of one of said transistors is connected to the collector of said transistors and the base of the other transistor is connected to said storage means through said change over switch interlocked with the opening operation of the shutter for generating an output signal when the base potentials of both said transistors are substantially equal; and   an electromagnet for terminating exposure and responsive to said output signal.