Long time exposure preventing device for camera with electronic flash

Abstract

A signal indicating that the correct exposure value has been achieved and a signal indicating the completion of flash are logically summed to provide a shutter closing signal. This ensures that the shutter will be closed even when the light emitted by the flash is insufficient to achieve the proper level of exposure.

Description

BACKGROUND OF THE INVENTION

This invention relates to a long time exposure preventing system in a focal plane shutter camera equipped with a flash generating device (hereinafter referred to as "an electronic flash" when applicable). It is particularly applicable in cameras in which the electronic flash is controlled by a light measurement circuit in the camera.

If the distance between an object and a camera varies in comprising and taking a picture of that object with an electronic flash, it is necessary to adjust or change the aperture value in accordance with the varied distance. This is because the intensity of light reflected by the object will also vary and theshutter speed is usually fixed.

In order to eliminate this inconvenience, in this technology the concept of automatic electronic flash has been proposed. In an automatic electronic flash, an electronic flash is provided with a light measurement circuit. The light measurement circuit operates to measure the quantity of light reflected by the object when the electronic flash emits a flash, and to stop the flashing operation of the electronic flash when the quantity of light measured reaches a correct exposure value. Hence a picture can be taken with the correct exposure at all times.

In general, electric-eye (EE) cameras are provided with light measurement circuits. When a picture is taken with such a camera with the automatic electronic flash described above, the light measurement circuit in the camera is not used. That is, it is in idle state and the camera is used in a manual mode of operation.

Furthermore, recently a circuit has been developed in which the flashing operation of an electronic flash is controlled by using a light measurement circuit incorporated in an electronic focal plane shutter camera. In this case a correct exposure can be readily obtained in taking a picture with an electronic flash without incorporating a light measurement circuit in the electronic flash. With this type of circuit, when the shutter top curtain is allowed to run by shutter release and the light photographing optical path is opened, the "X" contact on the camera is closed to permit the electronic flash to emit a flash. When the quantity of light reflected by the object reaches a correct exposure value, a shutter closing signal is generated by the light measurement circuit, energization of the shutter bottom curtain magnet is suspended, and the shutter bottom curtain is allowed to run. The shutter closing signal is also employed as a signal for stopping the flashing operation of the electronic flash, so that a picture is taken with the correct exposure.

However, since the quantity of light emitted by the electronic flash is limited, the correct exposure is not always obtained for the aperture value set in the camera. For instance, when a film having a sensitivity of ASA 100 is used, a correct exposure cannot be obtained with an electronic flash guide number "32" and an aperture value "F4"; that is, a picture will be underexposed. In this situation, since the correct exposure cannot be obtained, no shutter closing signal is generated by the light measurement circuit in the electronic focal plane shutter camera and as a result the bottom curtain controlling magnet is maintained energized. Therefore, the bottom curtain will not run. In other words, although the electronic flash has been operated to emit a flash of maximum duration, the exposure itself runs for a longer period of time. Such action will undoubtedly lead to poor picture quality due to camera shaking, and may lead to faulty operation and trouble a that one of the magnet deenergizing signals which occurs earlier. In the case also where no magnet deenergizing signal is provided by the light measurement circuit because the object is far away from the camera and the reflected light is therefore weak, the signal indicating the flashing operation of the electronic flash is still provided and therefore the magnet is deenergized.

Therefore, when a picture is taken with a camera having the memoryless light measurement circuit the light measurement is started in synchronization with the shutter opening operation and the shutter closing signal is generated when the quantity of light measured reaches a correct exposure value. Also, with a generating device that operates in synchronization with the completion of the shutter closing operation of the camera and stops the flashing operation in response to the shutter closing signal of the light measurement circuit, the shutter does not cause a long-time-exposure. That is, it is operated at the synchronizing speed even if no shutter closing signal is generated by the light measurement circuit because the quantity of light reflected by the object is small.

Hence an improved device is obtained that overcomes a material deficiency in the prior art. It is apparent that variations of this invention may be made without departing from the essential scope of the invention.

Claims

1. In a camera system having a shutter mechanism, a light measurement system to generate a shutter closing signal in response to a correct exposure signal indicative of a correct exposure value, an electronic flash to emit a flash of light in synchronism with the opening of the camera shutter and to suspend the flashing operation in response to a shutter closing said correct exposure signal, the improvement comprising: logic means for generating a flashing operation signal indicative of the completion of the flash operation, and for generating said shutter closing signal by a logical sum operation of the corrct correct exposure signal and the flashing operation signal.

2. The camera system of claim 1 wherein said light measurement system includes first logic means for delivering a correct exposure signal output indicative of a correct exposure for a given light intensity and a given film sensitivity, and wherein said logic means to generate said shutter closing signal comprises second logic means for inverting a signal indicative of flash emission by said electronic flash and producing said flashing operation signal as an output thereof and third logic means receiving the outputs of said first and second logic means and providing an output to close the shutter mechanism.

3. The camera system of claim 2 wherein said light measurement system further comprises means for measuring light into the camera, an operational amplifier, said means for measuring being coupled across the inputs of said amplifier for zero biasing, integrating means receiving the output of said means for measuring and delivering an output with said amplifier, resistor means adjustable to film sensitivity and comparator means responsive to said resistor means and said output of said amplifier and integrating means for producing a correct exposure signal indicative of proper shutter opening time for a correct exposure.

4. The camera system of claim 2 wherein said second logic means comprises an inverter for inverting a signal level indicative of the emission of said electronic flash.

5. The camera of claim 2 wherein said third logic means comprises an OR gate receiving outputs of said first and second logic means to close said shutter mechanisn mechanism and wherein said shutter mechanism comprises magnet means operable with a bottom curtain of said shutter mechanism and said OR gate generates an output to selectively deenergize said magnet means.

6. The camera system of claim 5 wherein said second logic means comprises an inverter for inverting a signal level indicative of the emission of said electronic flash and delivering an output to one input of said OR gate.

7. The camera system of claim 6 wherein said first logic means comprises an operational amplifier, means for measuring light into the camera and delivering an output in response thereto, integrating means receiving the output of said measuring means, an adjustable resistor providing an output indicative of film sensitivity, and comparator means receiving as one input the amplifier and integrating means signals and as a second input the signal from said resistor, said comparator producing an output signal to said OR gate indicative of proper shutter opening time for a correct exposure.

8. The camera system of claim 7 wherein the output of said first logic means is delivered to said flash unit to terminate the emission of light therefrom.

9. The camera system of claim 2 wherein the output of said first logic means is provided to said flash unit to terminate the emission of light therefrom.

10. The camera system of claim 1 further comprises switch means to initiate operation of said first logic means.

11. A camera system, comprising; means for generating a correct exposure signal indicative of a correct exposure value, flash means for emitting a flash of light, and including flash triggering means, flash stopping means, and an output, said correct exposure signal being applied as an input to said flash stopping means, means connected to said output for producing a signal indicative of the completion of a flash operation, and logic means for forming a logical sum of said flash operation completion signal and said correct exposure signal to generate a shutter closing signal.

12. A camera system, comprising; means for generating a correct exposure signal indicative of a correct exposure value, flash means for emitting a flash of light, and including flash triggering means, flash stopping means, and an output, said correct exposure signal being applied as an input to said flash stopping means, means connected to said output for producing a signal indicative of the completion of a flash operation, and means for generating a shutter closing signal in response to said correct exposure signal and said flash operation completion signal.

13. A camera system, comprising:

a shutter mechanism which closes in accordance with a shutter closing signal;

means for generating a correct exposure signal;

flash means for performing a flashing operation to generate a flash of light;

means for generating a flash operation signal indicative of the completion of said flashing operation; and

means for forming a logical sum of said correct exposure signal and said flash operation signal to form said shutter closing signal.

14. A camera system as claimed in claim 13, further comprising means for terminating said flashing operation in response to said correct exposure signal.

15. A camera system as claimed in claim 13, wherein said logic means comprises OR gate means, said shutter mechanism including magnet means, and said shutter closing signal being applied to said magnet means to effectuate shutter closing.

16. A camera system as claimed in claim 13, wherein said correct exposure signal generating means comprises an operational amplifier having inputs and light measuring means coupled across said inputs of said amplifier, integrating means receiving an output of said light measuring means and delivering an output with an output of said amplifier, resistor means adjustable to film sensitivity, and comparator means responsive to said resistor means and a combined output of said amplifier and said integrating means, said comparator means producing said correct exposure signal at an output thereof.

17. A camera system as claimed in claim 13, wherein said flash means includes an output terminal for providing a signal in accordance with the charge on said main capacitor, said means for generating said flash operation signal comprising inverter means for receiving and inverting the signal at said output terminal.

18. A camera system as claimed in claim 17, wherein said flash means comprises flash triggering means for initiating said flashing operation in response to the operation of a synchro switch, and flash stopping means for terminating said flashing operation in response to said correct exposure signal. 19. A camera as claimed in claim 18, wherein said flash means further comprises a main capacitor and a power supply for charging said main capacitor, said flash stopping means and said flash triggering means being arranged in parallel with said main capacitor.

20. A camera system as claimed in claim 18, wherein said flash triggering means comprises a flash discharge tube for discharging the charge on said main capacitor to emit light, a transformer for triggering said flash discharge tube and a capacitor coupled between said transformer and said synchro switch.

21. A camera as claimed in claim 18, wherein said flash triggering means includes a flash discharge tube for discharging the charge on said main capacitor to emit light, said flash stopping means comprising means for receiving said correct exposure signal, bypass means connected in parallel with said flash discharge tube, and means for rendering said bypass means conductive in response to said correct exposure signal.

22. A camera system, comprising, in combination, a camera having a light measurement system, and an electronic flash for emitting a flash of light; said camera including a shutter mechanism, said light measurement system generating a shutter closing signal in response to a correct exposure signal indicative of a correct exposure value, said electronic flash including an input terminal for receiving said correct exposure signal and means for terminating said flash of light in response thereto, said electronic flash further including an output terminal for delivering an output signal indicative of the completion of a flash operation, and said light measurement system further including means for generating said shutter closing signal in response to said electronic flash output signal.

23. A camera system, comprising, in combination, a camera and an electronic flash;

said camera including: means for generating a correct exposure signal indicative of a correct exposure value, an output terminal for delivering said correct exposure signal to said flash, and means for generating a shutter closing signal in response to either said correct exposure signal or a flash output signal indicative of the completion of a flash emission;

said electronic flash including: an input terminal for receiving said correct exposure signal, means for terminating a flash emission in response to said correct exposure signal, means for providing said flash output signal indicative of the completion of a flash emission, and an output terminal for delivering said flash output signal to said camera.

24. A camera system comprising the combination of a camera and a flash unit, said camera including means for generating a correct exposure signal, and means for generating a shutter closing signal in response to one of said correct exposure signal and a flash output signal; said flash unit including an input for receiving said correct exposure signal, means for terminating a flash emission in response to said correct exposure signal and means for providing said flash output signal, said output signal being indicative of the completion of a flashing operation.