Indicator for Proper or Improper Exposure by Automatic Electronic Flash

The invention relates to an indicator for proper or improper exposure by an automatic electronic flash, and more particularly, to such indicator which indicates the result of an actual exposure provided by an automatic electronic flash, giving an indication of an overexposure, underexposure or proper exposure.

As is well recognized, when taking a picture with the aid of an automatic electronic flash, the amount of light emission from the electronic flash can be controlled within a limited range. The control of the electronic flash tends to result in an overexposure as a distance to an object being photographed decreases or as a diaphragm aperture increases toward an open value. For example, in an automatic electronic flash of series controlled type, if an emission terminate signal is applied in synchronism with a flash emit signal, there results a finite or minimum emission of light from the electronic flash rather than resulting in zero emission. This is attributable to the nature of a forced commutation circuit including a thyristor of an automatic emission control circuit contained in the electronic flash. If the emission terminate signal causes the thyristor to conduct, it takes a given magnitude of turn-on time which is inherent to the thyristor. Recently, TTL automatic electronic flash is in practical use which is controlled in accordance with an output from an automatic exposure control circuit of a camera of TTL (through-the-lens) direct photometry type. This permits the implementation of a variety of lighting set-ups or the use of multiple electronic flashes in an automatic emission control mode, by interconnecting the camera and the electronic flash or flashes by means of electric cords. However, a transmission lag due to transmission lines running between the camera and the emission control circuit of the electronic flashes or the multiple lighting causes an increase in the minimum emission. For the sake of reference, representing the minimum emission of each individual electronic flash by G.sub.NO L, the minimum emission where N electronic flashes are used to provide a multiple lighting will be equal to G.sub.NO L.times..sqroot.N. When the minimum emission increases in this manner, an amount of exposure Ev given by actual light emission from electronic flash or flashes will be as indicated by a curve L in FIG. 1. Thus, the amount of exposure will deviate from a proper exposure level L.sub.0 toward the overexposure as the distance to an object being photographed decreases. The overexposure renders it impossible to take a picture with the aid of an electronic flash in a range of distance to an object being photographed which is located nearer than a point P toward the photographer. On the other hand, the maximum emission of the electronic flash will be exceeded with a distance to an object being photographed which is located further beyond a point D, resulting in an underexposure.

A variety of indicators for providing an indication of proper or improper exposure given by an automatic electronic flash are known, including

1. an apparatus for providing an indication of proper or improper exposure based upon the comparison against a first level corresponding to proper exposure and a second level which represents a given amount of overexposure, of an output from an integrator which integrates the amount of light incident (see Japanese Utility Model Publication No. 2,977/1982);

2. an apparatus for providing an indication of proper or improper exposure based upon a determination if an output from the integrator has reached a proper level at time which occurs a given time interval after the initiation of the emission of flashlight (see Japanese Laid-Open Patent Application No. 156,620/1977);

3. an apparatus including means to give a warning of underexposure at a given time interval after the initiation of emission of the flashlight (or closure of X-contacts), and in which the warning means is disabled if the electronic flash has ceased to emit flashlight at a proper exposure level (see Japanese Utility Model Publication No. 19,075/1981).

With the apparatus mentioned above sub-paragraph 1 above, the amount of incident light supplied from the electronic flash is monitored to determine and to indicate if an overexposure level, which is a given value above a proper exposure level, is reached. With this apparatus, while a given latitude from the proper level is considered for the overexposure, no latitude is considered in the determination of the underexposure. Consequently, if the integral of an amount of light incident when the electronic flash is allowed to provide a full emission is less than the proper level by an amount which is as little as 0.1 Ev, the apparatus provides an indication of underexposure. Accordingly, there results an unbalanced criterion of the overexposure and the underexposure. In practice, it will be desirable that a given latitude should be allowed for the underexposure in the same manner as such latitude is allowed for the overexposure. For practical purposes, a satisfactory photographing operation is achieved with a latitude on the order of -1/2 Ev to -1/3 Ev. It should also be noted that the integration of an amount of incident light during a photographing operation with the aid of an electronic flash is normally initiated in synchronism with the initiation of light emission from the electronic flash, and thus a component of an integral which is attributable to steady-state light, though of relatively low brightness, which exists until the first blind of the shutter becomes fully open, is discarded from the determination. To take this into consideration, it is reasonable to allow a given latitude for the underexposure.

In view of the foregoing, it is an object of the invention to provide an indicator for proper or improper exposure provided by automatic electronic flash in which an integral of an amount of incident light is compared against a pair of decision levels which correspond to a given latitude on the overexposure and the underexposure side and in which an exposure level located between these decision levels is determined to be proper while an exposure level outside the range defined by the decision levels is determined to be either an overexposure or an underexposure.

This improves the indication of the underexposure, as compared with the prior art practice.

FIG. 1 graphically illustrates the amount of exposure plotted against a distance to an object being photographed with the aid of an electronic flash;

FIG. 2 is a circuit diagram of an indicator for proper or improper exposure provided by an automatic electronic flash, which is constructed in accordance with one embodiment of the invention;

FIG. 3 graphically illustrates the operation of the indicator shown in FIG. 2, also illustrating the decision levels used to determine an overexposure and an underexposure; and

FIGS. 4(a) to 4(e) are a series of timing charts illustrating the waveform of various signals appearing in the indicator of FIG. 2.

Referring to FIG. 2, there is shown an electrical circuit of an indicator for proper or improper exposure provided by an automatic electronic flash which is constructed in accordance with one embodiment of the invention. The indicator is adapted to be incorporated into a camera of TTL direct photometry type and having a photoelectric transducer element PD.sub.1 which is used for purpose of photometry. Light from an object being photographed passes through a taking lens 1 and a diaphragm 2 of the camera to be reflected by a film surface 3 and the surface 4 of a shutter blind. The transducer element is disposed toward the bottom of the camera and is directed upward to receive light reflected by the film surface 3 and the blind surface 4. The transducer element PD.sub.1 has its anode and cathode respectively connected to the non-inverting and the inverting input terminal of an operational amplifier OP.sub.1, with the anode being connected to the ground through an integrating capacitor C.sub.1. The junction between the transducer element PD.sub.1 and capacitor C.sub.1 is connected through a trigger switch SW.sub.1 to receive a reference voltage Vref1. The switch SW.sub.1 is opened in ganged relationship with the initiation of running of the first blind of the shutter. The switch SW.sub.1 is closed in ganged relationship with the termination of a winding operation of the first blind. The output of the amplifier OP.sub.1 is connected to the inverting input terminal thereof, and is also connected through a series combination of voltage divider resistors R.sub.1, R.sub.2 to a source of reference voltage (not shown) which develops reference voltage Vref1. The output of the amplifier OP.sub.1 is also connected to the inverting input terminal of a comparator OP.sub.3 which operates to determine an underexposure, and to the non-inverting input terminal of a comparator OP.sub.4 which operates to determine an emission termination level. The junction between the resistors R.sub.1, R.sub.2 is connected to the non-inverting input terminal of a comparator OP.sub.5 which operates to determine an overexposure.

There is provided another source of reference voltage (not shown) which produces a reference voltage Vref2 higher than the first mentioned reference voltage Vref1, and the reference voltage Vref2 is applied to the non-inverting input terminal of an operational amplifier OP.sub.2 and to the base of each of PNP transistors Q.sub.1, Q.sub.2 and Q.sub.3. The inverting input terminal of the amplifier OP.sub.2 is connected to the collector of the transistor Q.sub.1, and its output terminal is connected to the emitter of each transistor Q.sub.1, Q.sub.2, Q.sub.3. The collector of the transistor Q.sub.1 is also connected to one end of a variable resistor Rv1 which is used to preset film speed information, the other end of the resistor Rv1 being connected to the ground. The collector of the transistor Q.sub.2 is connected through a resistor R.sub.3 to the source of reference voltage Vref1, and is also connected to the non-inverting input terminal of the amplifier OP.sub.3. The collector of the transistor Q.sub.3 is connected through a resistor R.sub.4 to the source of reference voltage Vref1, and is also connected to the inverting input terminals of the comparators OP.sub.4 and OP.sub.5. It is to be noted that the resistors R.sub.3 and R.sub.4 are chosen such that the resistance of resistor R.sub.3 is less than the resistance of resistor R.sub.4 (R.sub.3 <R.sub.4).

The output of the comparator OP.sub.3 is connected to one input of NAND circuit ND.sub.1. The output of the comparator OP.sub.4 is connected to the input of an inverter IN.sub.1. The output of the comparator OP.sub.5 is connected to one input of NAND circuit ND.sub.2. The other input to NAND circuit ND.sub.1 is fed from a first output terminal of a timing pulse controller TPC.sub.1 so as to be supplied with an underexposure examine pulse signal S.sub.1, as shown in FIG. 4(c). The other input of NAND circuit ND.sub.2 is connected to a second output terminal of the timing pulse controller TPC.sub.1, so as to be supplied with an overexposure examine pulse signal S.sub.2, as indicated in FIG. 4(b). The underexposure examine pulse signal S.sub.1 rises from its "L" level to its "H" level at a given time interval on the order of several milliseconds, which corresponds to a full emission of flashlight from the electronic flash, after the closure off X-contacts SW.sub.2 which has its one end connected to timing pulse controller TPC.sub.1 (see FIG. 4(a)). The signal S.sub.1 temporarily remains at its "H" level. The overexposure examine pulse signal S.sub.2 rises from "L" to its "H" level at an overexposure decision time, which is normally on the order of several hundreds of microseconds, after the closure of the X-contacts SW.sub.2, and temporarily remains at its "H" level.

The output of NAND circuit ND.sub.1 is connected to one input of NAND circuit ND.sub.3, which forms an R-S flipflop together with NAND circuit ND.sub.4. Specifically, the output of NAND circuit ND.sub.3 is connected to one input of NAND circuit ND.sub.4, the output of which is connected to the other input of NAND circuit ND.sub.3. Said one input of NAND circuit ND.sub.3 represents a set input while the other input of NAND circuit ND.sub.4 represents a reset input to the flipflop. As shown, the other input of NAND circuit ND.sub.4 which represents the reset input is connected to a fourth output terminal of the timing pulse controller TPC.sub.1, which produces a reset pulse signal S.sub.4 as shown in FIG. 4(e). The reset pulse signal S.sub.4 changes from its "H" to its "L" level at a given delay interval after the closure of the X-contacts SW.sub.2, and reverts to its "H" level after momentarily staying at its "L" level. The output of NAND circuit ND.sub.3, which represents the Q output of the R-S flipflop, is connected to the input of an inverter IN.sub.2, the output of which is connected to the cathode of light emitting diode LD.sub.1 which has its anode connected to receive an operating voltage Vcc in order to indicate an underexposure. The output of NAND circuit ND.sub.4, which represents the Q output of the R-S flipflop, is connected to a first input of a three input NAND circuit ND.sub.7.

The output of the inverter IN.sub.1 is connected to mating electrical contacts (not shown) of the camera and the automatic electronic flash for connection with an automatic emission control circuit (not shown) disposed within the electronic flash. An output from the inverter IN.sub.1 which changes from its "H" to its "L" level is transmitted as an emission terminate signal S.sub.5 to the electronic flash.

The output of NAND circuit ND.sub.2 is connected to one input of NAND circuit ND.sub.5, which forms an R-S flipflop together with NAND circuit ND.sub.6 in the similar manner as the combination of NAND circuits ND.sub.3 and ND.sub.4. The other input of NAND circuit ND.sub.6, which represents a reset input to the flipflop, is connected to the fourth output terminal of the timing pulse controller TPC.sub.1 so as to be fed with the reset pulse signal S.sub.4 therefrom. The output of NAND circuit ND.sub.5, which represents the Q output of the R-S flipflop, is connected to the input of an inverter IN.sub.3, the output of which is connected to the cathode of a light emitting diode LD.sub.2 which has its anode connected to receive an operating voltage Vcc in order to display an overexposure. The output of NAND circuit ND.sub.6, which represents the Q output of the R-S flipflop, is connected to a second input of the three input NAND circuit ND.sub.7

A third input to the three input NAND circuit ND.sub.7 is connected to a third output terminal of the timing pulse controller TPC.sub.1 so as to be fed with a proper exposure display pulse signal S.sub.3 which undergoes a temporary excursion from its "L" to its "H" level in synchronism with the reversion of the underexposure examine pulse S.sub.1 to its "L" level. The output of NAND circuit ND.sub.7 is connected to one input of NAND circuit ND.sub.9, which forms an R-S flipflop together with NAND circuit ND.sub.8, in the similar manner as the combination of NAND circuits ND.sub.3 and ND.sub.4. One input of NAND circuit ND.sub.8, which represents a reset input to the R-S flipflop, is connected to the fourth output terminal of the timing pulse controller TPC.sub.1 so as to be fed with the reset pulse signal S.sub.4 therefrom. The output terminal of NAND circuit ND.sub.9, which represents the Q output of the R-S flipflop, is connected to the input of an inverter IN.sub.4, the output of which is connected to the cathode of a light emitting diode LD.sub.3 which has its anode connected to receive an operating voltage Vcc in order to display a proper exposure.

In operation, upon depression of a shutter release button of the camera, a movable reflecting mirror is resiliently driven out of a taking light path, followed by the initiation of running of the first blind of the shutter. The trigger switch SW.sub.1 is opened in interlocked relationship therewith (see FIG. 3). The incidence of light from an object being photographed which has passed through the taking lens 1 and the diaphragm 2 and reflected by the film surface 3 and the blind surface 4 onto the transducer element PD.sub.1 causes the latter to produce a photocurrent Ip, which charges the integrating capacitor C.sub.1. Accordingly, the voltage across the capacitor C.sub.1 or a potential V.sub.1 applied to the non-inverting input terminal of the amplifier OP.sub.1 increases gradually as indicated in FIG. 3. Because the capacitor C.sub.1 is previously charged to the reference voltage Vref1, the voltage V.sub.1 can be expressed as follows: ##EQU1##

The output voltage V.sub.1 of the amplifier OP.sub.1 is equal to the input voltage V.sub.1. The output voltage V.sub.1 is applied to the inverting input terminal of the comparator OP.sub.3 and to the non-inverting input terminal of the comparator OP.sub.4. The output voltage V.sub.1 is also divided by the voltage divider, whereby a voltage V.sub.2 is developed at the junction between the resistors R.sub.1, R.sub.2 which is expressed as follows: ##EQU2## The voltage V.sub.2 is applied to the non-inverting input terminal of the comparator OP.sub.5.

The transistor Q.sub.1 has its base, emitter and collector connected to the non-inverting input terminal, the output terminal and the inverting input terminal, respectively, of the amplifier OP.sub.2. The amplifier OP.sub.2 operates to maintain the collector potential equal to the reference voltage Vref2 by a collector current, which is expressed as follows:

The magnitude of the current Ic is determined in accordance with film speed which is established by an adjustment of the variable resistor Rv1. The same collector current Ic flows through the collector of each of the transistors Q.sub.2 and Q.sub.3 which have their emitters and bases connected in common with the transistor Q.sub.1, and accordingly, collector voltages V.sub.3 and V.sub.4 are developed at the collector of the transistors Q.sub.2 and Q.sub.3 by the collector current Ic which flows through the resistors R.sub.3 and R.sub.4, respectively: ##EQU3## Since resistance of the resistor R.sub.3 is less in magnitude than that of the resistor R.sub.4, the voltages V.sub.3 and V.sub.4 are related such that V.sub.3 <V.sub.4, as indicated in FIG. 3. The voltage V.sub.3 is applied to the non-inverting input terminal of the comparator OP.sub.3 as a reference voltage which is made the basis to determine an underexposure in consideration of film speed, while the voltage V.sub.4 is applied to the inverting input terminals of the comparators OP.sub.4 and OP.sub.5 as reference voltages which are made the basis to determine an emission terminate level and an overexposure, respectively, as the film speed is considered. At the initiation of running of the first blind, both V.sub.1 and V.sub.2 are less than V.sub.3 and V.sub.4, as indicated in FIG. 3, whereby the comparators OP.sub.3, OP.sub.4 and OP.sub.5 produce outputs of "H", "L" and "L" level, respectively.

When the first blind has completed running and the shutter becomes fully open, the X-contacts switch SW.sub.2 is closed. See FIGS. 3 and 4(a). Thereupon, the automatic electronic flash is activated to emit flashlight, through a signal path, not shown, and the flashlight is reflected by an object being photographed to impinge upon the camera. Accordingly, the voltage V.sub.1 which represents an integral of the amount of incident light as well as the voltage V.sub.2 which represents a division of the voltage V.sub.1 both increase rapidly, as indicated in FIG. 3.

Subsequently at a given time interval to determine an overexposure after the closure of the contacts SW.sub.2, the overexposure examine pulse signal S.sub.2 is produced by the timing pulse controller TPC.sub.1. If the voltage V.sub.2 exceeds the reference voltage V.sub.4 at the time the signal S.sub.2 is produced, this implies the existence of an overexposure if a given film latitude is taken into consideration as compared with the decision level at which the emission terminate signal S.sub.5 is produced, as indicated in FIG. 3. Accordingly, the overexposure is displayed. Specifically, at time when the output of the comparator OP.sub.5 changes to its "H" level for V.sub.2 >V.sub.4, and when the signal S.sub.2 momentarily changes to its "H" level, the output of NAND circuit ND.sub.2 momentarily assumes its "L" level, whereby the R-S flipflop formed by NAND circuits ND.sub.5 and ND.sub.6 is set, producing an "H" level output from NAND circuit ND.sub.5. This causes the inverter IN.sub.3 to produce an output of "L" level, whereby the light emitting diode LD.sub.2 is energized to display the overexposure to a photographer. If, on the contrary, the voltage V.sub.2 does not exceed the reference voltage V.sub.4 at the time the signal S.sub.2 is produced, no display of the overexposure is made. Specifically, the output of the comparator OP.sub.5 remains at its "L" level, so that a momentary change of the signal S.sub.2 to its "H" level cannot change the output of NAND circuit ND.sub.2 from its "H" level, thus failing to set the R-S flipflop to energize the diode LD.sub.2.

Subsequently, when a time interval corresponding to a full emission from the electronic flash has passed since the closure of the contacts SW.sub.2, the timing pulse controller TPC.sub.1 produces the underexposure examine pulse signal S.sub.1, as indicated in FIG. 4(c). If the voltage V.sub.1 does not reach the level of the reference voltage V.sub.3 at the time the signal S.sub.1 is produced, this represents an underexposure if a given film latitude is taken into consideration as compared with the decision level where the emission terminate signal S.sub.5 is produced, as shown in FIG. 3. Accordingly, the underexposure is displayed. Specifically, when the signal S.sub.1 momentarily changes to its "H" level while the output of the comparator OP.sub.3 remains at its "H" level, the output of NAND circuit ND.sub.1 momentarily changes to its "L" level, thus setting the R-S flipflop formed by NAND circuits ND.sub.3 and ND.sub.4. Consequently, the output of NAND circuit ND.sub.3 assumes its "H" level while the output of the inverter IN.sub.2 assumes its "L" level, energizing the light emitting diode LD.sub.1 to display the underexposure to a photographer. If, on the contrary, the voltage V.sub.1 exceeds the reference voltage V.sub.3 at the time the signal S.sub.1 is produced, no display of the underexposure is made. Specifically, if the signal S.sub.1 momentarily changes to its "H" level when the output of the comparator OP.sub.3 reverts to its "L" level, the output of NAND circuit ND.sub.1 does not change from its "H" level, whereby the R-S flipflop formed by NAND circuits ND.sub.3 and ND.sub.4 cannot be set to energize the diode LD.sub.1.

Immediately after the underexposure examine pulse signal S.sub.1 is produced, the proper exposure display pulse signal S.sub.3 is produced by the timing pulse controller TPC.sub.1, as shown in FIG. 4(d). It is to be noted that at the time when the signal S.sub.3 is produced, the examination of both the overexposure and the underexposure has already been completed, and the R-S flipflop formed by NAND circuits ND.sub.3 and ND.sub.4 and the R-S flipflop formed by NAND circuits ND.sub.5 and ND.sub.6 are in either their set or reset condition. Assuming that the examination of the overexposure and the underexposure failed to detect either condition, both flipflops are reset, feeding two inputs of "H" level to the three input NAND circuit ND.sub.7. When the third input to the circuit ND.sub.7 momentarily changes to its "H" level in response to the application of the signal S.sub.3 while other two inputs assume the "H" level, the output of NAND circuit ND.sub.7 momentarily changes to its "L" level, thus setting the R-S flipflop formed by NAND circuits ND.sub.8 and ND.sub.9. Thus the output of NAND circuit ND.sub.9 assumes its "H" level and hence the output of the inverter IN.sub.4 assumes its "L" level to energize the light emitting diode LD.sub.3 to display the proper exposure to a photographer. On the other hand, if the overexposure or the underexposure has already been detected when the signal S.sub.3 is produced, either R-S flipflop formed by NAND circuits ND.sub.3 and ND.sub.4 or ND.sub.5 and ND.sub.6 would be set, whereby one of the first and the second input to NAND circuit ND.sub.7 assumes its "L" level. Hence, the application of the signal S.sub.3 to change the third input to its "H" level momentarily does not result in changing the output of NAND circuit ND.sub.7 to "L" level. Thus, the R-S flipflop formed by NAND circuits ND.sub.8 and ND.sub.9 cannot be set to energize the diode LD.sub.3.

Thus it will be seen that by the time the proper exposure display pulse signal S.sub.3 is produced, a determination is given indicating whether the photographing operation performed with the aid of the electronic flash has resulted in an overexposure, an underexposure or a proper exposure, by selectively energizing either one of the diodes LD.sub.1, LD.sub.2 or LD.sub.3. At a given time interval for display after the closure of the X-contacts SW.sub.2, the timing pulse controller TPC.sub.1 produces the reset pulse signal S.sub.4, as indicated in FIG. 4(e), which signal is applied to the other input of NAND circuit ND.sub.4, the other input of NAND circuit ND.sub.6 and one input of NAND circuit ND.sub.8, thus resetting all of the R-S flipflops. Hence, the output of NAND circuits ND.sub.3, ND.sub.5 and ND.sub.9 assume "L" level, deenergizing the light emitting diode LD.sub.1, LD.sub.2 or LD.sub.3 which has previously been energized to terminate the indication of proper or improper exposure provided by the electronic flash.

In the described embodiment, the indicator for proper or improper exposure has been assumed to be disposed within the camera, but it should be understood that it may alternatively be disposed within the automatic electronic flash.