Electronic exposure control arrangement in photographic apparatus

Claim 1

In a photographic camera having a shutter, an arrangement for timing the duration of shutter opening, comprising in combination, a shutter release activatable for effecting opening of the shutter; a difference amplifier having two inputs and an output; a shutter-control electromagnet connected to the output of said difference amplifier and operative for closing the shutter when equal voltages are applied to the two inputs of said difference amplifier; a voltage source; a voltage divider connected across said voltage source and comprising a lightsensitive element responsive to scene brightness and a first resistor, the voltage divider having a middle tap at the junction between said element and said resistor; a first energy-storing timing capacitor having one terminal connected to one of said inputs to apply to the same a voltage dependent upon the voltage across said first resistor; a second resistor; first switch means coupled to said shutter release and operative in response to activation of said release for effecting a change in the voltage across said first capacitor by so connecting said first capacitor to said second resistor that capacitor current flows through said second resistor, with said change in voltage occurring at a rate dependent upon the RC time constant of said first capacitor and said second resistor; a second timing capacitor connected to the other input of said difference amplifier for applying to the same voltage dependent upon the voltage across said second capacitor; a third resistor constituting a discharge resistor provided for the specific purpose of discharging said third capacitor with an RC time constant dependent upon said second capacitor and said third resistor; and additional switch means coupled to said release and operative in response to activation of said release for connecting said second capacitor in parallel with said first resistor with one terminal of said second capacitor connected to said middle tap, and thereafter operative for disconnecting said one terminal of said second capacitor from said middle tap and connecting said second capacitor in parallel with said third resistor to cause said second capacitor to discharge through said third resistor during the time the shutter is open with an RC time constant determined by said second capacitor and said discharge resistor, with the voltage applied to said other input of said difference amplifier constituting a scene-brightnessdependent threshold signal which changes with time while said shutter is open and changes with time at a rate dependent upon the value of said second capacitor and said discharge resistor, and wherein the RC time constant of said second capacitor and said third resistor is greater than the RC time constant of said first capacitor and said second resistor, the RC time constants being so chosen that the exposure time is approximately proportional to the value of said light-sensitive element over a predetermined range of scene-brightness values.