Exposure multiple setting device for programming electric shutter

#The present invention relates to programming electric shutters for cameras, and particularly to an exposure multiple setting device for programming shutters.

#When a subject to be photographed is bright, the opening process of the cmaera shutter until the opening blade is fully released will vary together with an elapse of time. The aperture diameter and the time will be determined according to the brightness of the subject. When the subject to be photographed is dark, only the time will vary after the opening blade is fully opened. Thus, exposure is determined automatically according to the brightness of the subject. The .gamma. characteristic of the photoconductive element of the above mentioned programming shutter is not linear as will be seen from curves I and II of FIG. 3. When the subject to be photographed is bright, that is within the range in which the opening blade is not fully opened, and an aperture diameter and a time are determined simultaneously, the value of .gamma. must be smaller than 1 while within the range when the subject to be photographed is dark and only the time varies, after the opening balde is fully opened, the value of .gamma. must be approximately 1.

#The introduction of an exposure multiple in a programming shutter requiring the above mentioned characteristics of a photoconductive element cannot be achieved by the conventional methods of changing-over the time-limiting capacitors of the shutter circuit or changing the switching levels in switching circuits. The relationship between brightness and the resistance value of a photoconductive element for different film sensitivities is illustrated in FIG. 3. Assuming that curve I represents the relationship between brightness and the resistance value of the photoconductive element for film having a ASA rating of 100, the relationship in the case of ASA 400 is represented by curve II. The curves show the resistance values of the photoconductive elements at points where the EV value in both cases is identical. There is the following relationship between the curves of ASA 100 and ASA 400:

#The nature, objects and advantages of the invention will be more fully understood from the following description in conjunction with the accompanying drawing, in which:

#FIG. 1 is an electric shutter circuit having an exposure multiple setting device in accordance with the present invention;

#FIG. 2 is an enlarged schematic view showing the light receiving surface of a photoconductive element for use in the embodiment of FIG. 1;

#An electric shutter control circuit in accordance with the invention as illustrated by way of example in FIG. 1, comprises a power source 1, a power switch 2, a photoconductive element 4, an adjustable resistance 6 for matching the characteristics of the photoconductive element to that of the shutter, a time-limiting capacitor 7 and a trigger switch 3 which normally short circuits the capacitor 7 and is opened in synchronization with the opening movement of the shutter blade. The time-limiting capacitor 7 is connected in series with the photoconductive element 4 and the adjustable resistance 6. The input of a switching circuit 8 is fed with the voltage across the time-limiting capacitor 7 and the output of the switching circuit is reversed when the charging voltage reaches a predetermined level. The switching circuit 8 is of known construction and may, for example, comprise a Schmidt circuit. The switching circuit 8 controls an electromagnet 9 for controlling the closing movement of the shutter blade.

#In accordance with the invention, the photoconductive element 4 is a triode element comprising two photoelectric bodies CdS.sub.1 and CdS.sub.2. As shown schematically in FIG. 2, the photoconductive element 4 has three leads a, b and c connected respectively to electrode sections A, B and C. The photoconductive body CdS.sub.1 is disposed between the electrode sections A and B while the photoconductive body CdS.sub.2 is disposed between electrode sections B and C. A change-over switch 5 is connected between the leads b and c of the photoconductive element 4. When the change-over switch is opened as shown in solid lines in FIG. 1, the photoconductive bodies CdS.sub.1 and CdS.sub.2 are in series with one another so that the photoconductive element has the characteristics shown by the curve I in FIG. 3. When the change-over switch 5 is closed as shown in dotted lines in FIG. 1, the photoconductive body CdS.sub.2 is short circuited so that only the photoconductive body CdS.sub.1 is effective. In this condition, the photoconductive element 4 has the characteristics represented by the curve II in FIG. 3. The characteristics of the photoconductive bodies CdS.sub.1 and CdS.sub.2 are selected so as to provide the resistance characteristics shown respectively in curves I and II in FIG. 3 according to the position of the change-over switch 5.

#The operation of the circuit shown in FIG. 1 using a photoconductive element of the kind illustrated in FIG. 2 will now be described. When the release button the camera is depressed, the power switch 2 is turned on to supply current to the electromagnet 9 for controlling the blade closing operation of the shutter under control of the switching circuit 8. Upon further depression of the release button, the shutter blade is mechanically released and simultaneously the trigger switch 3 is opened so that the time-limiting capacitor 7 is no longer short circuited. Consequently, the time-limiting capacitor 7 will begin to be charged through the photoconductive element 4 and the adjustable resistance 6. In this instance, when the change-over switch 5 for selecting film sensitivity is set for ASA 100 as shown in full lines in FIG. 1, both photoconductive bodies CdS.sub.1 and CdS.sub.2 of the photoconductive element 4 will be connected in series with the adjustable resistance 6 and time-limiting capacitor 7. The resistance of the RC timing circuit is thus the values of CdS.sub.1 and CdS.sub.2 plus the adjustable resistance 6. As the adjustable resistance 6 remains constant during operation of the shutter, the rate of charging of the capacitor 7 will depend on the resistance values of CdS.sub.1 and CdS.sub.2 and hence on the brightness of the subject being photographed.