The present invention relates to an electronic flash control device for a TTL light control type camera, and more particularly to an electronic flash control device which is free from the difficulty that light emission by the electronic flash is stopped by a noise component occurring before a suitable quantity of output light is obtained.
When it is necessary to photograph an object having a low luminance or to correct the amount of exposure, an electronic flash is generally used to increase the luminance of the object. The electronic flash is either built into the camera or is provided separately and can be connected to the camera. Whether the electronic flash is built into the camera or provided separately from the camera, it is essential to control the light emitting conditions of the electronic flash so that the amount of light applied to the object by the electronic flash has a predetermined value. In order to meet this requirement, the camera is provided with an electronic flash control device.
The electronic flash control device will be described with reference to a TTL light control type camera employing an electronic flash. FIG. 4 shows an example of a camera of this type. More specifically, FIG. 4 is a block diagram outlining the arrangement of a camera having an automatic focusing (AF) function and a built-in electronic flash proposed by the present applicant.
First, the arrangement of the camera shown in FIG. 4 will briefly be described. In FIG. 4, reference numeral 31 designates a camera body, and 11 represents a photographic lens mounted on the camera body 31.
The photographic lens 11 includes a lens system 15 including a focusing lens 13 which is movable along the optical axis and contributes to the focusing operation, and a drive force transmitting mechanism 17 for transmitting a drive force from a drive source provided for the camera body 31 to the movable lens 13. The photographic lens 11 further includes a lens ROM (read-only memory) 19 storing aperture value data of the photographic lens, position data of the movable lens 13, and data as to whether or not operating the electronic flash built into the camera body is suitable for the photographic lens, and a group of electrical contacts 21 which are connected to electrical contacts 57 on the camera body 31.
On the other hand, the camera body 31 includes an optical system having a main mirror 33, an auxiliary mirror 35, a focusing screen 37, and a pentagonal prism 39, an image pickup section 41 used for an automatic focusing operation, a drive mechanism 43 for driving the movable lens 13 in the photographic lens 11, a light detecting element 45 used for automatic exposure (AE) control for the purpose of TTL light control, a light detecting element 47 operated when the electronic flash is used, a central display section 49 for displaying camera conditions, a display unit 51 provided in the view-finder for indicating an AF or AE mode, the built-in electronic flash 53, a sequence motor 55 for winding or rewinding the film, a group of electrical contacts 57 provided on the side of the camera body which are connected to the group of electrical contacts 21 on the side of the photographic lens, a release switch 59, and a synchronizing contact 61 such as an X contact.
The camera body 31 further includes a microcomputer, namely, an IPU (indication processing unit) 71 for controlling the central display section 49, a PCU (power control unit) 73 having an E.sup.2 PROM 73a for interfacing with the image pickup section 41 and controlling the sequence motor 55 and an AF motor 43, the lens stop and the shutter release magnet, another microcomputer, namely, a DPU (data processing unit) 75 for performing photometric arithmetic operations and controlling the display unit 51 in the viewfinder, etc., and a central control microcomputer, namely, a CPU (central processing unit) 77. The CPU 77 controls the IPU 71, the PCU 73, the DPU 75, and the lens ROM 19 in the photographic lens 11.
The above-described built-in electronic flash 53 is provided above the pentagonal prism 39 and substantially at the middle of the camera body 31 so that the light flash can be readily applied to an object. When the electronic flash 53 is not in use, its light flash emitting section is retracted into the camera body. When it is in use, the light flash emitting section is protruded therefrom by operating a push button (not shown). The camera is designed so that an auxiliary electronic flash can be additionally connected thereto.
Next, an electronic flash control device for the above-described camera will be described. FIG. 5 is a diagram showing the above-described electronic light flash 53 and the electronic flash control device in detail. In FIG. 5, reference numeral 81 designates the electronic flash control device. The device 81 includes as essential components the CPU 77, the DPU 75, the PCU 73, the light detecting element 47, the release switch 59 and the X contact 61. The electronic flash 53 includes a booster circuit 53a, a main capacitor 53b, a light emitting circuit 53d having a light emitting tube 53c, and a light emission stopping circuit 53e. The electronic flash 53 is connected to the electronic flash control circuit 81 through a trigger signal terminal 53f and a quench signal terminal 53g. When necessary, the aforementioned auxiliary electronic flash is connected to the electronic flash control circuit 81 through auxiliary electronic flash connecting contacts corresponding to the above-described contacts 53f and 53g which are provided at predetermined positions on the camera body 31.
The operation of the electronic flash control device 81 thus constructed will be described with reference to the timing diagrams shown in FIGS. 6(A) through 6(J).
The release switch 59 is turned on after the camera has selected the electronic flash operation mode. Then, the CPU 77, after carrying out a series of processing operations, starts detecting the variation of the voltage at the X contact. In response to the variation of the voltage at the X contact (SWX in FIG. 6(A)) which occurs when the shutter is fully opened, the CPU 77 supplies a trigger signal X.sub.TRIG shown in FIG. 6(B) to light emitting circuit 53d in the electronic flash 53 via trigger signal terminal 53f. In response to the trigger signal, the light emitting circuit 53d supplies an electrical signal S.sub.X shown in FIG. 6(C) to the light emitting tube 53c. As a result, the light emitting tube 53c emits light. (See FIG. 6(G)).
On the other hand, an integrator circuit 75a in the DPU 75 integrates the output signal of the light detecting element 47 in response to an integration start signal S.sub.S shown in FIG. 6(D) which the CPU 77 outputs in response to the trigger signal. The integration voltage S.sub.I of the integrating circuit, as shown in FIG. 6(E), gradually increases with time, and abruptly increases with the emission of light from the light emitting tube. The integration voltage S.sub.I is applied to one input terminal of a comparator 75b.
The CPU 77 applies a digital signal S.sub.D to a D/A (digital to analog) converter 75c prior to every photographing operation. The digital signal represents a suitable exposure value determined in accordance with the ISO data of the film loaded in the camera or exposure correction data when available. The digital signal S.sub.D is subjected to digital-to-analog conversion by the D/A converter 75c, the output of which is applied to the other input terminal of the comparator 75b. The output terminal of the comparator 75b is connected to one input terminal of an AND circuit in a quench signal output regulating circuit 75d in the DPU 75.
When, in the circuit thus constructed, the integration voltage S.sub.I reaches a reference value S.sub.O, the comparator 5b applies a signal to the AND circuit in the quench signal output regulating circuit 7.5d so that the quench signal is generated (in a manner to be described in more detail later). When, with an enable signal S.sub.E applied to the other input terminal of the AND circuit by the CPU 7, the integration voltage S.sub.I reaches the reference value, the regulating circuit 75d applies a quench signal Q.sub.U shown in FIG. 6(F) to the light emission stopping circuit 53e. In response to the quench signal, the emission of light of the electronic flash 53 is quickly suppressed as shown in FIG. 6(G).
The present applicant has conducted extensive research on the electronic flash control circuit described above and recognized that the output timing of the enable signal S.sub.E is very important. That is, depending on the enable signal, the control circuit may operate erroneously.
When the CPU 77 supplies the enable signal S.sub.E as shown in FIG. 6(H) to the quench signal output regulating circuit 75d before a noise pulse N.sub.T occurs within the electrical signal S.sub.X in response to the trigger signal as shown in FIG. 6(C), the circuit is operated erroneously and a false quench signal Q.sub.UF is produced as shown in FIG. 6(I), so that no light is emitted (See FIG. 6(J).) In practice, such noise pulses are often produced by the light emitting tube, and accordingly, in order to perform the emission of light correctly, it is essential to use a light emitting tube having excellent characteristics. As a result, during manufacture, it is necessary to perform an additional operation of selecting light emitting tubes suitable for the electronic flash control device.
In view of the foregoing, an object of the present invention is to provide an electronic flash control device which is free from the difficulty of the emission of the light flash being stopped by a noise component before a suitable amount of light is obtained.