Abstract:
An apparatus for flash photography being of the type in which for one shot a number of different flash tubes are time-serially fired and the amount of flash light emitted from each flash tube is controlled in accordance with a respective arbitrarily chosen individual value independently of the others. 
     In the above-described arrangement, one and the same comparator is sufficient for controlling the firing operation of all the flash tubes, thereby giving an advantage of simplifying the construction of the apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus for flash photography and, more particularly, to an apparatus of the type in which a plurality of electronic flash units are fired for each shot and the amounts of flash light issued from the units are adjusted to respective desired values. 
     2. Description of the Related Art 
     In the above-identified type of apparatus for flash photography, as the means for detecting whether or not the amounts of flash light issued from the individual electronic flash units have reached their respective preset levels, use has been made of an equal number of comparators to the number of units, as for example disclosed in U.S. Pat. No. 4,360,764. The use of so large a number of comparators gave rise to a problem of increasing the scale of the flash exposure control circuit. 
     SUMMARY OF THE INVENTION 
     With the foregoing in mind, one aspect of the present invention is to provide an apparatus for flash photography in which the reference values for the electronic flash units are selectively rendered cooperative with one and the same comparator in response to a light amount control operation of each flash unit. Hence, one comparator suffices for controlling the firing operation of all the units. Thus, the above-described drawback of the prior known apparatus has been eliminated. 
     Another aspect of the present invention is, under the above-described object, to provide an apparatus for flash photography in which the reference voltage for the above-identified comparator is changed over from one value to another each time the controlling of flash light amount of the corresponding one of the electronic flash units terminates. Thus, making it possible to adjust the amounts of flash light issued from the units in accordance with the respective arbitrarily preset fractions of a flash exposure value. 
    
    
     Other objects of the invention will become apparent from the following description of embodiments thereof by reference to the drawings: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical circuit diagram illustrating one embodiment of the apparatus for flash photography according to the invention. 
     FIG. 2 is a pulse timing chart illustrating the manner in which the circuit of FIG. 1 operates. 
     FIG. 3 is an electrical circuit diagram of another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the apparatus of the invention comprises a camera whose circuitry is shown by I associated with first and second electronic flash units Flash 1 and Flash 2 respectively. 
     The above-described electronic flash units are well known. So, their detailed explanation is omitted except that they include respectively trigger circuits TG 1  and TG 2  responsive to closure of the synchro switch or an actuating signal of high level coming therefrom through the interconnection terminals Ta 1  &#39; and Ta 2  &#39; for initiating a firing operation of a flash tube (not shown) and commutation circuits LC 1  and LC 2  responsive to a de-actuating signal coming through interconnection terminals Tb 1  &#39; and Tb 2  &#39; for stopping the firing operation. 
     The circuit I in the camera includes a synchro switch X turned on in response to movement of a first curtain of a shutter, a one-shot circuit MM 1  responsive to closure of the synchro switch X for producing the actuating signal which is maintained at a high level for a predetermined time, its output being connected to an interconnection terminal Ta 1  in contact with the one Ta 1  &#39;, a switch SW 1  turned off in response to change of the output of an OR gate OR to a high level as the actuating signal arrives, and maintained in the off state for the same time when the actuating signal is present, a capacitor Ci to which the switch SW 1  is connected in parallel, and a photosensitive element D connected to the capacitor Ci to constitute an integration circuit together with the latter. This photosensitive element D is positioned to receive the reflection of light entering through the photographic lens of the camera from a film surface. 
     COMP denotes a comparator whose plus input is connected to the output of the above-described integration circuit and whose minus input is connected to a selection switch SW 2  for two reference voltages Vref 1  and Vref 2  from a reference signal source Vref. Its output at which the above-described de-actuating or firing stop signal is produced is connected to the commutation circuits LC 1  and LC 2  in the electronic flash units Flash 1 and Flash 2 through an AND gate AND and a switch SW 3  respectively. 
     Responsive to the aforesaid change of the output signal of the one-shot circuit MM 1  (to a high level), a delay circuit DL 1  changes its output to a high level in a predetermined time and holds that high level signal for a predetermined time. The output of this delay circuit DL 1  is connected to the aforesaid OR gate and switches SW 2  and SW 3  and an inverter INV. 
     The switch SW 2  moves from its &#34;a&#34; to its &#34;b&#34; position in response to the high level signal of the delay circuit DL 1 , and holds the latter position during the time when that signal is at a high level. Also, the switch SW 3  turns on in response to the high level signal of the delay circuit DL 1  and stays in the on state during the time when that signal is at a high level. 
     The time T 1  for which the one-shot circuit MM 1   is on is determined to be almost equal to the full firing time of the first electronic flash unit Flash 1, and the interval T D  from the moment at which the one-shot circuit MM 1  has started to produce the high level signal to the moment at which the delay circuit DL 1  starts to produce the high level signal is determined to be slightly longer than the aforesaid time T 1 . The duration of this high level signal of the delay circuit DL 1  is determined to be almost equal to the full firing time of the second electronic flash unit Flash 2. 
     The operation of the FIG. 1 embodiment is described by using the waveforms of FIG. 2. 
     It is now assumed that the terminals Ta 1 , Tb 1 , Ta 2  and Tb 2  and grounds G 1  and G 2  of the camera are in contact with the terminals Ta 1  &#39;, Tb 1  &#39;, Ta 2  &#39; and Tb 2  &#39; and the grounds G 1  &#39; and G 2  &#39; of the electronic flash units Flash 1 and Flash 2, respectively. 
     With this state, when the camera is released, the first curtain of the shutter runs down to turn on the synchro switch X as shown on line X in FIG. 2. Thereby the one-shot circuit MM 1  produces a pulse as shown on line A in FIG. 2. In synchronism with its rising edge, the trigger circuit TG 1  in the first flash unit Flash 1 fires the discharge tube. 
     The high level signal of the one-shot circuit MM 1  is given also to the switch SW 1  and the latter turns off. The flash light from the first flash unit Flash 1 is projected onto an object being photographed, and reflected past the lens of the camera to illuminate the film surface. The reflected light from this film surface is received by the photosensitive element D. The integrated amount of light on the element D appears in the form of increasing voltages on the capacitor Ci as shown on line Ci in FIG. 2. The output of the integrator is compared with the first reference voltage Vref 1 . When it reaches the voltage Vref 1 , the comparator COMP inverts its output to the high level as shown on line B in FIG. 2. This output is applied through the AND gate AND to the commutation circuit LC 1  in the first flash unit Flash 1, thus stopping its discharge tube from further firing. 
     Since the reference voltage Vref 1  is preset to a desired value by means of an operating member (not shown) which can change the position of the tap of the reference voltage source Vref, the predetermined light amount desired by the photographer can be obtained from the first flash unit Flash 1. After that, when the output of the one-shot circuit MM 1  inverts to the low level, the switch SW 1  turns on to discharge the charge on the capacitor Ci. Thus, the integrator is reset to the initial state. 
     In a very short time, the delay circuit DL 1  changes its output to the high level as shown on line D in FIG. 2, thereby the trigger circuit TG 2  in the second electronic flash unit Flash 2 is actuated. So, the second electronic flash unit Flash 2 starts to fire. Such change also causes the switch SW 1  to turn off again. Hence, the amount of flash light emitted from the second flash unit Flash 2 starts to be integrated on the photosensitive element D. 
     Also because the high level signal of the delay circuit DL 1  travels to the switch SW 3 , the latter turns on. The switch SW 2  also is moved to its &#34;b&#34; position. 
     When the output of the integrator reaches the second reference voltage Vref 2 , the comparator COMP produces an output signal of a high level which is applied through the switch SW 3  to the commutation circuit LC 2  in the second flash unit Flash 2, stopping the second flash unit Flash 2 from further firing. Since the second reference voltage Vref 2  also can be preset to a desired value in a similar way to that of the first one Vref 1 , the amount of flash light emitted from the flash units Flash 1 and Flash 2 can be adjusted to arbitrary values independently of each other. 
     Another embodiment of the invention is shown in FIG. 3 where the similar constituent parts to those shown in FIG. 1 are denoted by the same reference characters. 
     This embodiment differs from the FIG. 1 embodiment in the points that the photosensitive element PD is connected across the inputs of an operational amplifier OP with a diode D&#39; as the logarithmic compression element connected in its feedback loop to form a logarithmic compression circuit which is employed as the light metering circuit, that further use is made of a transistor Tr for logarithmic expansion whose base is connected to the output of the operational amplifier OP so that the capacitor Ci is charged with the collector current of the transistor Tr, and that the first and second reference voltages VC 1  and VC 2  are made selectively applicable to the emitter of the transistor Tr by means of the switch SW 2 . 
     The operation of the circuit of FIG. 3 is described below. When the synchro switch X turns on, the one-shot circuit MM 1  produces the pulse and a first electronic flash unit Flash 1 fires. At this point in time, the switch SW 1  turns off. These operations are the same as those of the FIG. 1 embodiment. The reflection of the flash light of the first flash unit Flash 1 from the object being photographed is received by the photosensitive element PD. An information representing the logarithm of the object brightness level is produced at the output of the operational amplifier OP. This output is then expanded by the transistor Tr. Therefore, the capacitor Ci is charged with the corresponding current to the output of the operational amplifier OP through the transistor Tr. For this case, since the reference voltage VC 1  is applied through the switch SW 2  to the emitter of the transistor Tr, the intensity of the collector current of the transistor Tr is determined by the value of the reference voltage VC 1  too. After a time depending on the preset value of the reference voltage VC 1  and the output of the light metering circuit, the voltage on the capacitor Ci reaches the reference voltage Vref, and the comparator COMP produces an output signal of high level, thus stopping the firing of the first flash unit Flash 1 likewise as in the FIG. 1 embodiment. 
     After that, the delay circuit DL 1  produces an output signal of a high level. Similarly to the FIG. 1 embodiment, the second electronic flash unit Flash 2 fires. Current of the corresponding intensity to the reflection of the flash light of the second flash unit Flash 2 from the object being photographed flows to and is stored on the capacitor Ci. Since, at this time, the switch SW 2  is in its &#34;b&#34; position, the charging current to the capacitor Ci goes to a value depending on the output of the operational amplifier OP and the second reference voltage VC 2 . Therefore, the time during which the voltage on the capacitor Ci reaches the reference voltage Vref is determined by the light value and the preset value of the reference voltage VC 2 . When the capacitor Ci is charged to the equal potential to the reference voltage Vref, the second flash unit Flash 2 gets stopped from further firing. 
     These reference voltages VC 1  and VC 2  can be preset to desired values. Hence, the amounts of light emitted from the flash units Flash 1 and Flash 2 can be adjusted to the desired values independently of each other. Although in the foregoing, the embodiments, the electronic flash units have been described as the separate items from each other, they may otherwise be unified in such a way that the two firing circuits are built in the same casing of electronic flash unit, and arranged to be time-serially actuated. 
     It is to be noted that another construction may be considered as a modification of the embodiment of the invention. For example, in FIGS. 1 and 3, the delay circuit DL 1  may be disconnected from the one-shot circuit MM 1  and be triggered by an output of the AND gate AND, while the trigger circuit TG 2  may be disconnected from the delay circuit DL 1  and be connected to an output of the one-shot circuit MM 1 . In such a construction, the flash light amount of each of the electronic flash units Flash 1 and Flash 2 can be controlled in a condition where both the flash units Flash 1 and Flash 2 fire simultaneously. 
     As has been described above, in the present invention, for controlling the termination of the firing operation in accordance with the received amount of light, use is made of means for changing the reference value with which the output of the integrator for the received light is compared each time the firing is transferred unti to unit. This makes it possible to simplify the structure of construction of the apparatus for flash photography of the type in which one shot is taken with multiple flashes, while the amount of flash light emitted from each flash unit is controlled in accordance with a corresponding arbitrarily chosen fraction of a flash exposure.