Light measuring apparatus for camera

A light measuring apparatus for camera is disclosed. It is known that some type of light measuring apparatus for camera involves the problem of so-called "latch" which is caused by opposite charge accumulated at the high impedance location in the light measuring circuit. The light measuring apparatus is provided with means for cancelling the latch in a simple and reliable manner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light measuring apparatus for camera. 
2. Description of the Prior Art 
In the art there is known and used such type of light measuring circuit 
comprising a light receptor photo diode, a high input impedance 
operational amplifier and a logarithmic conversion diode. In this type of 
the circuit, the two ends of the photo diode is imaginarily 
short-circuited. The photo current generated in the photo diode flows into 
the logarithmic conversion diode which converts the input photo current 
into a logarithmically compressed voltage. The voltage is used as a 
photometric value for exposure control. 
The above mentioned type of light measuring apparatus has the advantage 
that the responsiveness of the output voltage to the change of light 
intensity is higher than other conventional system wherein such open 
voltage is used for exposure control which is generated at the both ends 
of a photo diode and logarithmically converted according to the intensity 
of light. 
However, the known light measuring apparatus has some difficult problems. 
When the supply voltage is applied to the light measuring circuit, there is 
generated noise in the circuit. Relative to the rise time of the supply 
voltage, a longer time is required before the operational amplifier gets 
in a stabilized state. Because of the generated noise and the unstable 
period of the operational amplifier, some amount of opposite charge is 
accumulated at the connection point of photo diode and logarithmic 
conversion diode where the input impedance is extremely high. This 
accumulation of opposite charge causes the problem of so-called "latch". 
The latched state remains not lost for a long time. This makes it 
impossible to obtain a normal measuring voltage from the circuit in a 
short time after the application of power source to the circuit. In fact, 
hitherto, it has been required a long time to obtain the normal output 
voltage from the light measuring circuit. 
To solve the problem of latch it has been proposed to provisionally 
short-circuit the two ends of the photo diode by use of a transistor only 
during the time of the operational amplifier being unstable. However, 
according to the solution, there occurs abrupt change of potential at the 
switching of the transistor. The potential change is transmitted to the 
connection point through stray capacity as noise which causes latch again. 
It has been found that the solution is not effective for cancelling the 
latch and sometimes it is rather harmful for the latch cancellation. 
As another solution to the problem of latch it is also known to use a light 
emitting element such as LED in the circuit to generate a neutralizing 
current by it only during the unstable period of the operational amplifier 
thereby cancelling the latch by the generated current. However, this 
solution has some drawbacks. It needs voluminous and expensive circuit. 
Further, the power consumption increases up. 
The above mentioned problem of latch occurs also in flash light 
photographing employing TTL flash output control. 
To electrically control the high speed shutter of camera there is generally 
used OFF-type magnet. OFF-type magnet is such type of magnet which is 
holding the closing blade of shutter during the application of current to 
the magnet coil and releases the holding of the closing shutter blade when 
the supply current to the coil is cut off. With this type of magnet it is 
possible to control a high speed shutter in a stable manner. However, when 
the supply current to the coil is rapidly cut off, a considerable amount 
of kick-back noise is produced by inverse induced voltage. In flash light 
photographing, the operation of shutter is carried out after stopping the 
flash light emission by the flash output control. This means that the 
kick-back noise is generated in the dark. In case of a camera for which a 
high density mounting is required, it is very difficult to prevent TTL 
metering circuit from being affected by the kick-back noise. In practice, 
it may be impossible. The light measuring circuit for TTL flash output 
control is necessarily latched whenever it is affected by such kick-back 
noise in the dark. When the next shooting with flash light is carried out 
immediately after one shooting, it is impossible to calculate the correct 
and proper exposure value for the next shooting unless the latch caused by 
the kick-back noise generated at the previous shooting is cancelled before 
the next shooting. The reason for this is that the output of the measuring 
circuit can not become normal unless the charge which has caused the latch 
is neutralized. So long as the output is in the abnormal state, any 
correct value of measured light can not be obtained. In the dark, the 
current available for the neutralization is substantially zero. Therefore, 
if two or more flash light exposures are carried out in succession, a 
large portion of the photo current will be consumed as the neutralizing 
current to cancel the previously caused latch. As the photo current is 
partly consumed as the neutralization current, the flash output control is 
rendered unstable and unreliable. The photo current consumed for the latch 
cancellation has an important effect on the correctness of measurement in 
particular when a film of high photosensitivity is used. The higher the 
photosensitivity of film is, the higher ability is required for the light 
measuring circuit to measure the light under the condition of less photo 
current. The error in measurement caused by the photo current consumed for 
the above neutralization, therefore, increases with increasing the film 
sensitivity. Consequently, the film which can practically be used is 
limited to the low sensitivity film only. This means that the range of 
film sensitivity usable is narrowed, which brings forth a serious problem. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to solve the problem of the 
accumulation of charge in the photo diode in the above-mentioned type of 
light measuring circuit attributable to the transient unstable state of 
the operational amplifier. 
More particularly, the object of the invention is to provide a light 
measuring apparatus which overcomes the above problem effectively and with 
a simple circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIG. 1 showing a first embodiment of the light measuring 
apparatus according to the invention, E.sub.B is a power supply source, 
SWo is a power source switch and A1 and A2 are operational amplifiers. 1 
is an exposure meter circuit. By turn-ON of the power source switch SWo, 
an electric power is supplied to the operational amplifiers A1 and A2, and 
to the exposure meter circuit 1 from the supply source E.sub.B. A photo 
diode PD is connected between two input terminals of the amplifier A1. The 
photo diode PD has photo current I.sub.L generated in the direction of 
arrow according to the illuminance on the photo diode surface. The 
operational amplifier A1 is formed as a high input impedance operational 
amplifier. A logarithmic conversion diode D1 is connected between the 
negative input terminal and the output terminal of the operational 
amplifier A1 for logarithmic conversion of the photo current I.sub.L. The 
photo current appears at the output terminal of the operational amplifier 
A1 as a logarithmically converted voltage with low impedance. The voltage 
is applied to the exposure meter circuit 1. Eo is a reference bias voltage 
source connected between the positive input terminal of the operational 
amplifier A2 and the negative line of the power source. The reference 
voltage from the bias voltage source Eo is in-phase amplified by the 
operational amplifier A2. The amplified reference voltage serves to set 
the reference potential for the output of the operational amplifier A1. 
The voltage from the reference bias voltage source Eo is also applied to 
the exposure meter circuit 1 as a reference bias voltage for the necessary 
operational processings in the circuit 1. The level of the reference bias 
voltage is determined by resistors Ro and Ro' connected at the output side 
of the operational amplifier A2. The potential at the connection point 
between the positive input terminal of A1 and the anode of PD is set 
higher toward the positive side than the voltage on the negative line of 
the power source E.sub.B. The reason for this is that the higher potential 
is required for the operational processing in the exposure meter circuit 1 
on one hand and that it is intended to carry out the latch cancellation to 
some degree by a diode D2 connected parallel to the logarithmic conversion 
diode D1 on the other hand. 
The serial circuit containing diode D3, resistors R1 and R2 is connected 
parallel to the photo diode PD. A condenser C1 is connected between the 
connection point of the two resistors R1, R2 and the negative line of the 
supply source. 
Through resistors Ro and Ro' connected between the output of A2 and the 
negative line of E.sub.B, the condenser C1 is discharged during the power 
source switch SWo being open so that the condenser C1 has no charge 
thereon when the switch SWo is closed. 
The operational amplifier A1 can not instantly set in the state of normal 
operation when the power source switch SWo is closed. It has necessarily 
some finite unstable time as a transition period to the stable operation 
state. During this unstable period, it is impossible to imaginarily 
short-circuit the photo diode PD. Consequently, the output of the 
operational amplifier A1 is being unstable during the period and the 
output becomes transitionally higher than the normal output thereof. This 
produces a flow of current into the photo diode PD through the logarithmic 
conversion diode D1. Thereby, charge is stored in the junction capacitance 
of PD and positive charges are accumulated on the cathode side of PD. Such 
a storage of charge is also formed by the chattering noise of the source 
switch SWo through the stray capacity of the circuit. 
At the end of the above unstable period, the operational amplifier A1 
intends to control the photo diode PD by imaginary short-circuit. However, 
when the illuminance on the light reception surface of PD is sufficiently 
high, the accumulated charge is neutralized at once by the photo current 
I.sub.L. Therefore, in this case, the normal output is obtained instantly 
at the output terminal of the operational amplifier A1. When the 
illuminance becomes extremely low, the photo current then produced is very 
small which is in the order of several 10 PA to several PA. In this state, 
it takes a long time to restore the output to the normal level by the 
neutralization of the photo current I.sub.L only. In this case, the diode 
D2 serves to accelerate the return to the normal state up to a certain 
point. More particularly, with the accumulation of positive charge on the 
cathode side of the photo diode PD, the potential on the negative input 
side of A1 becomes more positive than the positive input side. Therefore, 
the output of the amplifier A1 drops down toward the negative line side of 
the power source and the diode D2 is biased in the forward direction. 
Consequently, the accumulated charge is discharged. In case that the power 
source for operating the operational amplifier A1 is of single system as 
in the case of the embodiment shown in FIG. 1, the minimum potential of A1 
output never drops down under the voltage of the negative line of the 
power source. To discharge the accumulated charge, at least the diode D2 
should be sufficiently biased in the forward direction when the potential 
of A1 output becomes substantially equal to the voltage of the negative 
line. To this end, a potential more positive than that on the negative 
line of the power source is given to the positive input of A1 by the 
operational amplifier A2. In this manner, the accumulated charge is 
discharged by the diode D2. After the charge is neutralized by the diode 
D2 to a certain degree, the output of the operational amplifier A1 
gradually rises up. Since the diode D2 has a logarithmic characteristic, 
the forward voltage is reduced and equivalently it has a very high 
resistance. In this state of very high resistance, no further 
neutralization can be expected. A long time of from several 100 m sec. to 
several sec. will be required to complete the neutralization and restore 
the output to its normal state. However, in the shown embodiment, as 
amplifier A2 operates after power switch SWo is turned to ON state, the 
charge current to the condenser C1 flows in the direction to draw out the 
positive charge on the cathode side of the photo diode PD through diode 
D3, resistor R2 and diode D1. Because of this current, the voltage on the 
input side of the operational amplifier A1 changes and therefore its 
output voltage also changes. Since the condenser C1 is being charged 
through resistors R2 and R1, the voltage of the condenser C1 continues to 
rise up at the time. With the rise-up of the charged voltage on the 
condenser C1, the current flowing through the diode D3 smoothly decreases 
down. When the rising charge voltage on the condenser C1 reaches the level 
about 0.2 V lower than the output voltage of the amplifier A2, the forward 
applied voltage to the diode D3 becomes insufficient to act as the forward 
bias and therefore the equivalent resistance abruptly changes very high. 
In contrast, the resistance value of the resistor R1 remains unchanged and 
therefore the resistance of R1 becomes smaller relative to the abruptly 
raised resistance of the diode D3. Consequently, at this stage, the 
condenser C1 is charged predominantly by the resistor R1 and the 
operational state of the amplifier A1 is stabilized when the voltage on 
the condenser C1 becomes equal to the potential of the output of the 
amplifier A2. 
After the voltage of the condenser C1 has reached the same potential as the 
output voltage of the operational amplifier A2, the operational amplifier 
A1 continues to operate normally. Therefore, there is produced an 
imaginary short-circuited state between the positive input and the 
negative input of the operational amplifier A1 and the potential 
difference appearing therebetween remains in the range less than several 
mv of the offset voltage of A1. In this range, the same voltage is applied 
also to the both ends of the diode D3. However, the current flowing at the 
time is very small as compared to the photo current I.sub.L so that it can 
be regarded as practically zero. Therefore, the effect of diode D3 on the 
light measurement is negligibly small even when the photo current is in 
the order of several PA. By suitably selecting the condenser C1, resistors 
R1 and R3 there can be obtained a value for obtaining the normal operation 
state in the shortest time. In the above embodiment, the resistor R2 has 
been provided to prevent overcurrent. Without the resistor there may be 
attained the same effect as above. In this case, the diode D2 also may be 
omitted. In the circuit shown above, the diode D3 for discharging the 
photo diode PD is biased by the difference between the charge voltage on 
the condenser C1 and the reference voltage which is the output of the 
operational amplifier A2. The diode D3 is independent of the output 
potential of the operational amplifier A1, that is, the discharge state of 
the photo diode PD. This assures that the unnecessary accumulated charge 
can be removed from the photo diode PD without fail in the first 
embodiment. 
FIG. 2 shows a second embodiment of the invention wherein the present 
invention is applied to an electronic shutter camera. In FIG. 2, like 
reference characters to FIG. 1 represent the same or corresponding 
elements of the circuit. 
When the power source switch SW1 is turned ON, a condenser C2 connected 
parallel to the switch SW1 is short-circuited and also a transistor Q1 is 
rendered conductive. The condenser C2 is provided for power supply timer. 
The transistor Q1 is inserted into the positive line of the power source 
through a resistor R3. Thus, by turn-ON of the switch SW1, the circuit of 
this embodiment is powered. A certain time long after turn-OFF of the 
switch SW1, the transistor Q1 continues to be conductive owing to the 
charged current on the condenser C2. Generally, the switch SW1 is 
interlocked with the shutter release button of camera and is closed before 
the shutter button has moved over its one stroke for releasing. 
Several 10 .mu.sec. to several 100 .mu.sec. after the conduction of the 
transistor Q1, the operational amplifiers A1 and A2 get in their normal 
operation state. A resistor R5 and a Zener diode ZD1 are connected in 
series between the positive line and the negative line of the power 
supply. According to the difference between the voltage at the joint of R5 
and ZD1 and the output voltage of A2, a condenser C4 is charged positively 
on the side of resistor R5 and negatively on the side of R1. At this stage 
of operation, the voltage of condenser C3 charged through R4 is still low 
and a transistor Q3 remains non-conductive. Several m sec. after that, the 
charge voltage on the condenser C3 reaches a sufficient level enough to 
render the transistor D3 conductive. Thereby, the connection point of 
condenser C4 on the side of R5 is clamped down to the potential of the 
negative line of the power supply E.sub.B. Therefore, at this moment, the 
potential at the other connection point of the condenser C4 drops down 
toward the negative side much more than the potential on the negative line 
side. By this negative voltage, the diode D3 is forward biased through 
resistor R2 in the same manner as in the above embodiment. Thus, the 
positive charge accumulated on the cathode side of the photo diode PD 
during the unstable period of the operational amplifier A1 is completely 
and surely discharged. Thereafter, in the same manner as in the above 
embodiment, the potential of condenser C4 smoothly changes until the 
potential of the condenser C4 on the connection side of R1, R2 becomes 
equal to the potential of the output of the operational amplifier A2. In 
this manner, the operational amplifier A1 can produce at once an output 
corresponding to the photo current I.sub.L. 
SW2 is a release switch which is closed at the end of one stroke of the 
camera shutter releasing button. Even if the shutter releasing button is 
pushed down hastily so that the two switches SW1 and SW2 are closed nearly 
at the same time, the turn-ON of the switch SW2 is transmitted to the 
exposure control circuit 2 only after a certain time determined by 
resistor R6 and condenser C5 has passed. The reason for this is that owing 
to the condenser C5 the transistor Q4 continues to be non-conductive for a 
time immediately after the transistor Q1 is rendered conductive to apply 
the source voltage to the circuit. The transistor Q4 is rendered 
conductive only after the determined time by resistor R6 and condenser C5, 
that is, after the latching of the operational amplifier A1 has been 
cancelled. On the conduction of transistor Q4, the turn-ON of the switch 
SW2 is transmitted to the exposure control circuit 2 to supply current to 
the electro-magnetic release magnet Mg1. With the current supply to Mg1, 
the exposure control sequence including the known mechanical operations of 
camera is started. Thus, the shutter control magnet Mg2 starts operating 
to control the shutter. The transistor Q2 is controlled by the control 
circuit 2 in such manner that it continues to be conductive during the 
time of from camera releasing to the end of exposure operation with the 
closing of the shutter. Therefore, even when the switch SW1 is opened 
during exposure, the transistor Q1 remains conductive to prevent the 
shutter from being closed during the exposure. 
The difference between the first and second embodiments is as follows: 
In the embodiment shown in FIG. 1, at the first stage of power supply, the 
diode D3 is forward biased by the output voltage of the operational 
amplifier A2. Therefore, the output voltage of A2 is required to be higher 
than the forward voltage of D3. In contrast, according to the embodiment 
shown in FIG. 2, the potential at the connection point of condenser C4 and 
resistor R2 drops more negatively than the negative line of the power 
source. Therefore, the diode D3 can be forward biased even when the output 
voltage of A2 is lower than the forward voltage of D3. In other words, the 
diode D3 can be forward biased to instantly release the operational 
amplifier A1 from the latched state even when the output voltage of A2 is 
zero, that is, even when the connection point of the positive input side 
of A1, the anode of PD and R1 is grounded. 
In the above embodiment, the function of Zener diode ZD1 is to keep the 
charge voltage constant at the first stage of power supply even when there 
occurred any variation in the source voltage. 
FIG. 3 shows a third embodiment of the invention. In FIG. 3, like reference 
characters to FIG. 2 represent the same or corresponding elements. 
In the above second embodiment, the condenser C4 is once charged at the 
first step of power supply without biasing the diode D3. The diode D3 is 
biased after a certain determined time by resistor R4 and condenser C3. In 
contrast, the diode D3 in the third embodiment is forward biased during 
the time of the transistor Q1 being conductive. To this end, a condenser 
C6 is provided which is precharged through registors R8, R1 and resistors 
Ro and Ro' connected between the output of the operational amplifier A2 
and the negative line of the power supply. 
When the switch SW1 is closed and the transistor Q1 is rendered conductive, 
the transistor Q5 is rendered conductive in synchronism it by a bias 
voltage through a resistor R7. At the time, the potential at the terminal 
of R8 of the condenser C6 becomes equal to the potential on the negative 
supply line. The potential at the connection point of the condenser C6 and 
resistors R1, R2 drops further toward the negative side than the negative 
supply line. Thereby, the diode D3 is forward biased to perform the same 
operations as described above. To keep the degree of the forward bias of 
the diode D3 constant irrespective of possible variation in voltage of the 
power source E.sub.B there is provided a diode D4. When the potential of 
the condenser C6 on the side of connection point of R1 and R2 becomes a 
more negative value than the negative supply line, the level of the 
negative potential is clamped by the diode D4 at a certain definite value 
which is the forward voltage of the diode D4. Therefore, a constant 
forward bias is applied to the diode D3 irrespective of the possible 
variation in voltage of the power source. Other operations of the third 
embodiment correspond to those of the second embodiment and therefore need 
not be further described. 
FIG. 4 shows a fourth embodiment of the invention. 
The fourth embodiment is different from the above first to third 
embodiments in the manner of logarithmic compression of the photo current. 
In the first to third embodiments, the logarithmic conversion diode D1 has 
been connected the negative input and the output of the operational 
amplifier A1. In contrast, in the fourth embodiment, a logarithmic 
conversion diode D11 is connected between the positive input of the 
operational amplifier A1 and the negative line of the power source. In 
this case, negative charges are accumulated on the anode side of the photo 
diode PD during the above-mentioned unstable period of the operational 
amplifier A1 at the beginning of power supply thereby causing latching. 
Since there is no charge on condenser C10 for some time long immediately 
after the start of power application, when the switch SW1 is closed, a 
voltage determined Zener diode ZD2 is produced at the junction of 
resistors R10, R11, R12. The voltage is selected to be higher than the sum 
of the forward voltages of the diodes D10 and D11. Therefore, the diode 
D10 is forward biased and the negative charge on the anode side of D11 is 
neutralized by it. The condenser C10 is gradually charged with time and 
the potential at the junction of resistors R10, R11, R12 drops. 
Consequently, the forward current in the diode D10 decreases smoothly. On 
the completion of charging of C10, the potential at the junction of R10, 
R11, R12 becomes equal to the potential on the negative line of the power 
supply. At the time, the diode D10 is fully inverse-biased not to give any 
effect to the light measuring circuit. 
As the photo current flows in the diode D11 in the forward direction, there 
is generated a logarithmically compressed voltage at the both ends of the 
diode D11. The voltage thus generated is transmitted to other circuits at 
a low impedance by the operational amplifier A1. A certain determined time 
after the turn-ON of switch, a transistor Q11 is rendered conductive. The 
time is determined by resistor R13 and condenser. In other words, the 
transistor Q10 is rendered conductive when the charging of condenser C11 
has been substantially completed. With the conduction of the transistor 
Q10 the junction of R10, R11, R12 is short-circuited to minimize the 
change of potential at the junction of R10, R11, R12 during the operation 
of power supply even if there occurs any voltage change as caused, for 
example, by sudden change of load on the power source E.sub.B resulting 
from the control of current applied to the shutter control magnet Mg2. 
FIG. 5 shows a fifth embodiment of the invention. In FIG. 5, like reference 
characters to FIGS. 1 to 4 represent the same or corresponding elements 
which need not be further described. 
The fifth embodiment includes photo diodes PD101 to PD104 which are 
multi-metering photo sensor elements. Metering amplifiers comprising 
operational amplifiers A101 to A104 and logarithmic conversion diodes D106 
to D109 are connected to the photo diodes PD101 to PD104 respectively. 
Eo is a reference bias source which is in-phase amplified by an operational 
amplifier A2. The amplified reference bias source is used to set the 
reference potential of each the operational amplifier A101-A104. The 
reference bias source Eo is applied also to the exposure control circuit 2 
as the reference voltage for operational processings by the control 
circuit. The exposure control circuit 2 is constituted of various known 
circuits such as a control circuit for computing proper exposure value 
from the photometric outputs of the operational amplifiers A101-A104 and 
controlling the exposure, a display circuit, a release circuit etc. 
The part enclosed with a dotted line 3 in FIG. 5 indicates a latch 
cancelling circuit which includes transistor Q5, diodes D4, D101-D104, 
resistors R1, R7, R8 and condenser C6. 
The manner of operation of this embodiment is as follows: 
When the power source switch SW1 is in its opened position and the 
transistor Q1 is non-conductive state, that is, when no electric power is 
being applied to the metering circuit and the exposure control circuit 2, 
the condenser C6 is charged up to the same voltage level as E.sub.B in the 
manner described above. The charge to the condenser C6 is positive on the 
side connected to the collector of transistor Q5 and negative on the side 
connected to the cathode of diode D4. 
When the switch SW1 is closed, the transistor Q1 is rendered conductive to 
start the power supply to the circuits. In synchronism with the conduction 
of Q1, a base current flows into the transistor Q5 through resistor R7 and 
therefore the transistor Q5 is rendered conductive so that the voltage at 
the connection point of the condenser C6 to the collector of Q5 becomes 
the same level as the negative line of the power supply E.sub.B. The 
voltage of the condenser C6 at the connection point with the cathode of D4 
is shifted to a negative voltage. The voltage difference between the 
negatively shifted voltage and the output of the operational amplifier A2 
is preset to a higher value than the forward voltage of each discharging 
diode D101-D104. Therefore, at this stage of operation, all of the diodes 
D101 to D104 are forward biased and a current flows through the diodes 
D106 to D109. By the current the opposite charge is neutralized which has 
been accumulated at high impedance connection points of photo diodes 
PD101-PD104 and logarithmic conversion diodes D106-D109. 
The condenser C6 is rapidly charged with the current flowing in through the 
diodes D101-D104 and the current flowing in through the resistor R1. The 
charging stops when the negative voltage on the side connected to the 
cathode of D4 reaches the same level as the positive voltage of the output 
of A2. At this time point, the forward bias to the discharging dioded 
D101-D104 is lost. Therefore, from the time point the discharging diodes 
D101-D104 have no substantial effect on the metering circuit. In practice, 
the logarithmic conversion output begins depending on the photo current 
only even before the voltage on the cathode side of the diode D4 becomes 
equal to the output voltage of the operational amplifier A2. It begins 
when the difference between the two voltages is reduced up to the range of 
0.15 to 0.2 V. The reason for this is that when the respective bias 
voltages to diodes D101-D104 are reduced to the level of about 0.15-0.2 V, 
the current flowing through the diodes becomes very small as compared to 
the photo current. 
After the above two voltages have become equal to each other, the voltage 
applied to the both ends of diodes D101-D104 is only the offset voltage of 
every amplifier (which voltage is in the order of only several mV). 
Therefore, after the latch cancellation the measured value of light is 
never affected by the diodes D101-D104. 
After the lapse of a certain time during which the normal photometric 
voltage of the operational amplifiers A101-A104 can be transmitted to the 
control circuit 2 for controlling the proper automatic exposure, the 
transistor Q4 is rendered conductive to allow the turn-ON signal of the 
release switch SW2 to be transmitted to the cotrol circuit 2. In the 
embodiment shown in FIG. 5, it is possible to lower the cathode voltage of 
each diode D101-D104 toward the negative side beyond the negative voltage 
of the power source E.sub.B at the start of power supply. Therefore, the 
latch cancellation is possible even when the output of the operational 
amplifier A2 is set to 0 (zero) V, that is, even when the common 
connection point of the positive inputs of A101-A104 is grounded on the 
negative side of the power source E.sub.B. 
In the manner described above, the latch cancellation is performed by 
flowing a current into the diodes D101-D104. The requirement for obtaining 
the flow of current into the diodes is that at the first stage of power 
supply, the cathode voltage of the diodes D101-D104 should be set to 
negative relative to the output voltage of the operational amplifier A2, 
that is, relative to the voltage at the common connection point of the 
positive inputs of the operational amplifiers A101-A104. 
According to the above embodiment, it is unnecessary to provide separate 
latch cancelling circuits for the individual metering circuits of the 
multi-metering circuit. In the above arrangement, each one of the 
discharging diodes D101-D104 is provided connected to each one of the 
respective metering circuits. However, as the bias point for provisionally 
applying the forward bias to their discharging condenser there exists only 
one point which is the connection point of condenser C6 and resistor R1. 
All of the discharging diodes are connected, at their one end, to the 
common bias point. Even with such an arrangement of circuit, the metering 
circuits never interfere each other. The discharging diodes between every 
two neighbouring metering circuits are serially connected in the opposite 
direction, which prevents interfere between two neighbouring metering 
circuits. In other words, every discharging diode performs two functions 
at the same time, one for discharging and the other for separating two 
neighbouring metering circuits from each other. In this manner, according 
to the invention, a simple and reliable latch cancellation circuit for 
multi-metering apparatus can be realized. 
The arrangement of the latch cancelling circuit may be further simplified 
if the output voltage of the operational amplifier A5 is higher than the 
forward voltage generated when the current necessary for latch 
cancellation is flowed into the diodes D101-D104. Such a further 
simplified modification of the latch cancelling circuit will be shown in 
FIG. 6 as a sixth embodiment of the invention. 
In FIG. 6, the part enclosed with a dotted line 3' of the circuit 
corresponds to the part enclosed with the dotted line 3 previously shown 
in FIG. 5. The part indicated by 3' includes discharging diodes D111 to 
D114, condenser C1 and resistor R1. Other parts of the circuit of the 
sixth embodiment correspond to those shown in FIG. 5 and therefore they 
are omitted from FIG. 6. 
The diodes D111, D112, D113 and D114 in FIG. 6 correspond to the diodes 
D101, D102, D103 and D104 respectively. The condenser C1 in FIG. 6 
corresponds to the condenser C6 in FIG. 5. The manner of operation of this 
sixth embodiment is essentially the same as that of the first embodiment 
shown in FIG. 1 and therefore need not be further described. 
FIG. 7 shows a seventh embodiment of the invention wherein the latch is 
cancelled by a current source formed by transistor. 
In FIG. 7, the part enclosed with a dotted line 3" is a latch cancelling 
circuit which corresponds to the part enclosed with the dotted line 3 
previously shown in FIG. 5. A transistor Q1 is inserted in the current 
supply line. When the transistor Q1 is rendered conductive, the current is 
allowed to flow into a current mirror circuit composed of transistors 
Q111-Q115 through resistor R121 and condenser C121. The charged current on 
the condenser is equal to the absorption current at each the collector of 
the current mirror transistors Q112-Q115. With charging of the condenser 
C121, the absorption current gradually decreases down. Finally, it is 
reduced to zero. This collector current of Q112-Q115 flows into diodes 
D106-D109 whereby the latch is cancelled. 
Although the current flows also into a resistor R120 simultaneously with 
the conduction of the transistor Q1, the transistor Q110 remains 
non-conductive for a while after the conduction of Q1 owing to the 
condenser C120. The transistor Q110 is rendered conductive after a certain 
delay time determined by resistor R120 and condenser C120, that is, after 
the current of Q112-Q115 has decreased down to zero. With the conduction 
of the transistor Q110, the charge on the condenser C121 is discharged. 
During this discharging of C121, the collector current of Q112-Q115 
remains zero. By rendering the transistor Q110 conductive, the connection 
point of resistor R121 and condenser C121 is clamped. This clamping 
prevents error in latch cancelling operation which may be caused by 
variation in voltage of the power source E.sub.B. The source voltage may 
be changed, for example, when the current is applied to the magnet coil 
etc. for exposure control. If there occurs such change in source voltage, 
a flow of current into the condenser C121 is caused by it and the 
corresponding collector current flows in the transistors Q112-Q115. This 
may cause mal-function of the latch cancelling operation. Above clamping 
of the connection point of R121 and C121 prevents such error in latch 
cancelling operation. When the connection point is clamped, the base of 
transistor Q11-Q115 serves as the current input point. 
FIG. 8 shows an eighth embodiment of the invention. 
The eighth embodiment is different from the above seventh embodiment only 
in the point that there is provided only one output transistor (Q112) for 
the current mirror circuit and that through the associated diode D11-D114 
a forced current is applied to every junction point within each the 
metering circuit (the connection point of the negative input terminal of 
the oprational amplifier, one electrode of the photo diode and one 
electrode of the logarithmic conversion diode). By employing this 
arrangement there can be obtained the same effect as that obtainable by 
the above embodiment. 
In FIG. 8, like reference characters to FIG. 7 repesent functionally the 
same elements as those in FIG. 7. 
FIG. 9 shows a ninth embodiment of the invention wherein flash-light 
photographing according to the TTL light measuring method is possible when 
a flash light emission device is mounted on the camera. 
In FIG. 9, operational amplifier A1, photo diode PD1, logarithmic 
conversion diode D1 etc. constitute a light measuring amplifier circuit 
used for the automatic exposure control by a known exposure control 
mechanism in the camera such as shutter time control, diaphragm control 
etc. The light measuring amplifier circuit measures the illuminance of the 
object existing immediately before exposure. 
Similarly to the above embodiments, resistors R1, R2, R7, R8, condenser C6, 
transistor Q3 and diodes D3, D4 constitute a latch cancelling circuit for 
the operational amplifier A1. Designated by 10 is a known exposure control 
circuit for automatically setting proper exposure value. The exposure 
control circuit performs various known functions for automatic exposure 
control including Apex arithmetic operation, display control of set 
exposure value, control of solenoid release etc. The output of the 
operational amplifier A1 obtained by TTL photometering is introduced into 
the exposure control circuit 10. The reference voltage source Eo 
determines the output of the operational amplifier A2. The source voltage 
of Eo is also introduced into the exposure control circuit 10 for its Apex 
arithmetic operation. 
4 is a flash light emission device electrically connected to the camera 
through electric contacts P1-P4 of a known accessary shoe. When the power 
source switch (not shown) of the flash light emission device 4 is closed, 
a charge current is applied to a flash light discharging condenser for 
flash light emission. On the completion of charging of the condenser, an 
amount of current is transmitted to the camera through the contact P4 to 
light up LED provided on the camera. For example, such LED is provided 
within the viewfinder of the camera so as to let the user know the state 
of charging to the flash light emission device by means of ON-OFF of the 
LED. The anode of LED is connected also to the exposure control circuit 10 
so that when the flash light emission device 4 gets in the state 
completely charged and LED lights up, the control circuit 10 can detect 
it. When the exposure control circuit 10 detects it, the control circuit, 
when the camera's shutter is released, automatically controls the shutter 
speed to such speed at which flash light photographing is possible. At the 
same time, the control circuit 10 inhibits the automatic exposure control 
by the shutter/diaphragm control mechanism on the camera's side. Mg1, Mg3 
and Mg4 are magnets of which Mg1 is that for electromagnetic releasing, 
Mg3 is for diaphragm control and Mg4 is for shutter control. The manner of 
operation of these magnets are well known to those skilled in the art and 
therefore need not be further described. 
SW4 is a memory switch. After ON-signal of camera's release switch SW2 has 
been transmitted to the exposure control circuit 10 and the current supply 
to the electromagnetic releasing magnet coil Mg1 has been continued for a 
determined time, a mechanical sequence of operation of the camera for 
exposure is started. During the sequence of exposure operation, the memory 
switch SW4 is closed prior to the start of stop-down of the lens aperture 
by the diaphragm control mechanism. The switch SW4 remains closed at least 
until the operation for exposure control is completed, that is, until the 
shutter is closed. When the memory switch SW4 is closed, the measured 
value of light by the output of A1 immediately before the aperture 
stop-down is electrically memorized. In other words, the memory switch SW4 
determines the timing for memorizing the measured value of light 
immediately before the aperture stop-down. The switch SW4 may be 
interlocked with the shutter to open the switch in response to the closing 
motion of the shutter at the completion of exposure. Otherwise, the switch 
may be opened in response to the film advancing operation after exposure. 
The thing necessary is that the switch SW4 is in its opened position after 
completing the shutter charge and film advancing operation for the next 
frame. 
SW3 is a trigger switch which is closed in response to the shutter opening 
motion. The function of this switch SW3 is to transmit to the control 
circuit 10 the timing for the start of counting time necessary for shutter 
control. The switch SW3 is opened interlocking with the closing motion of 
the shutter. 
Operational amplifier A204, logarithmic conversion diode D11 and photo 
diode PD2 constitute a metering circuit for TTL flash output control to 
control the light emission from the flash light emission device. The photo 
diode PD2 is disposed in the position to respond to the illuminance on the 
film plane, at least, the illuminance after the stop-down of the lens 
aperture. An operational amplifier A205, like the above amplifier A2, 
gives the operational amplifier A204 a reference bias by a in-phase 
amplifier circuit responding to the reference voltage source Eo. 
Transistor Q204, diodes D205-D210, resistors R206-R208 and condenser C203 
constitute a latch cancelling circuit of the operational amplifier A204. 
Transistor Q205 is a logarithmic expansion transistor. From the control 
circuit 10 a voltage corresponding to the light sensitivity of film is 
applied to the emitter of the transistor Q205. Applied to the base of Q205 
is the logarithmically compressed voltage of A204. In accordance with 
these applied voltages, the transistor Q205 puts out from its collector a 
logarithmically expanded current proportional to the photo current of PD2. 
The expanded current of Q205 is integrated by a condenser C204. 
The integrated voltage by C204 and the reference voltage E1 are compared 
each other by the operational amplifier A203. When the integrated voltage 
by C204 reaches the level of the reference voltage E1, the output of the 
operational amplifier A203 is inverted. The inverted output is transmitted 
to the flash light emission device 4 through the contact P2 of the 
accessary shoe 5 to terminate the emission of flash light. The timing for 
the start of integration by the condenser C204 is given by transistor 
Q206. In this embodiment, a certain delay time determined by resistor 
R210, condenser C205 and operational amplifier A206 after turn-ON of 
trigger switch SW3, the transistor Q206 is rendered conductive to start 
the integration by the condenser C204. The delay time is selected in such 
manner that the start of integration by C204 is delayed by the time of 
from the closing of SW3 to the closing of SW5. The switch SW3 is closed at 
the start of shutter opening at which time point the emission of flash 
light is not yet started. When the shutter is fully opened the switch SW5 
is closed, which is transmitted to the flash light emission device 4 to 
start the emission of flash light. 
For flash light photographing, the above embodiment operates in the 
following manner: 
On the conduction of transistor Q1 the circuit is powered. In this state, 
when the power source switch of the flash light emission device 4 is 
closed, the charging of the flash light discharging condenser is started. 
Upon the completion of the charging, LED lights up in the manner described 
above and the control circuit 10 detects it. Thereby the control mode of 
shutter and diaphragm of the camera is automatically changed over to the 
mode for flash light photographing. At the same time, the transistor Q209 
is rendered non-conductive to allow the opening/closing signal of the 
memory switch SW4 to be transmitted. By closing the release switch SW2 the 
current is supplied to the releasing magnet Mg1 to start the mechanical 
exposure control sequence of the camera. Prior to the start of stop-down 
of the lens aperture, the memory switch SW4 is closed. Simultaneously with 
the closing of SW4, the transistor Q204 is rendered conductive. As seen in 
FIG. 9, the transistor Q209 is interposed in the line extending from the 
memory switch SW4 to the base of the transistor Q204. For flash light 
photographing mode only, the transistor Q209 is rendered non-conductive by 
an output from the control circuit 10. For all other modes, the switch is 
conductive not to transmit the signal of SW4. The charged voltage on the 
condenser C203 is so determined that before the conduction of Q204, the 
charged voltage is positive on the side connected to R206 and negative on 
the side connected to R207. In other words, the charged voltage is so 
selected that the voltage generated in the serially connected diodes 
D205-D208 is higher than the output voltage of the operational amplifier 
A205. The diodes D205-D208 are not always necessary. However, the 
provision of these diodes has an effect to keep the charged voltage on the 
condenser C203 constant irrespective of possible voltage change of the 
power source E.sub.B. Therefore, the latch cancelling operation later 
described is stabilized well and a better result can be obtained by it. 
On the conduction of Q204, the potential at the terminal of the condenser 
C203 on the side connected to R206 is clamped to the potential on the 
negative supply line. Consequently, due to its charge voltage, the 
potential at the connection point of C203 with R207, R208 is further 
dropped down toward the negative side up to the level lower than the 
negative line voltage of the power source E.sub.B by the forward voltage 
of diode D10. Therefore, the diode D209 is forward biased and the current 
is forcedly flowed into the connection point of the negative input 
terminal of A204, diode D211 and photo diode PD2 through the diode D211. 
As a necessary result of it, the positive charge accumulated on the 
negative input side of A204 by which the latch is caused is surely 
discharged to the condenser C203 through diode D209 and resistor R207. 
As described above, the potential at the connection point of C203, R207 and 
R208 provisionally becomes a negative potential. However, some time after 
the condenser C203 is again charged by the current flowing through 
resistors R207 and R208. But, this time, the condenser is charged in the 
opposite direction to that of the charge voltage prior to the conduction 
of Q204. Therefore, finally the voltage becomes equal to the output 
voltage of A205. At this time, the operational amplifier A204 is normally 
operating and the difference in voltage between the positive input and the 
negative input is in the state of imaginary short. There is no current 
flowing to diode D209 at this time. 
The above operation is completed prior to the closing of the emission start 
switch SW5. In this manner, the latch of the operational amplifier A204 is 
cancelled immediately before the stop-down of lens aperture and the output 
of A204 is restored to its normal state. 
On the closing of the switch SW5, an emission of flash light is started. 
The transistor Q206 is rendered non-conductive and the condenser C204 
starts the integration of voltage. The integrated voltage on the condenser 
C204 is compared with the voltage of the reference power source E1. When 
the former reaches the level of the latter, the output of the operational 
amplifier A203 is turned to High from Low, which is transmitted to the 
flash light emission device 4 through the contact P2. The device 4 detects 
it and stops the emission of flash light. Thus, there is performed an 
automatic flash output control according to TTL metering. Thereafter, the 
current supply to the magnet Mg4 is cut off to close the shutter. At the 
cut-off of the supply current to Mg4, there is generated a kick-back noise 
when the flash light photographing is carried out in the dark as described 
above. Such kick-back noise is transmitted to the negative input terminal 
of the operational amplifier A204 and the latter is latched by it. In the 
dark there is no photo current of the photo diode PD2 available for 
cancelling the latch. Therefore, the latch cancellation by photo current 
can not be expected in this case. However, in this embodiment of the 
invention, the cancellation of latch of A204 has previously been completed 
before the emission of flash light. As described above, the latch 
cancellation has been effected by transistor Q204, diodes D205-D210, 
resistors R206-R208 and condenser C203 at the shutter release of the 
camera. Therefore, according to the invention, always stable TTL flash 
output control can be attained. 
The function of diode D210 is, like diodes D205-D208, to further improve 
the stability of latch cancellation against the voltage variation of the 
power source E.sub.B. The resistor R207 is not always necessary. 
FIG. 10 shows a tenth embodiment of the invention. 
This tenth embodiment is different from the above ninth embodiment in the 
point that the metering circuit for automatic exposure control by 
controlling the camera's shutter and diaphragm and metering circuit for 
TTL flash output control use a common light measuring circuit. The 
integration circuit for TTL flash output control is provided on the flash 
light emission device. In FIG. 10, like reference characters to FIG. 9 
represent the same and functionally corresponding elements. 
The photo diode PD10 is disposed in the position to put out a photo current 
directly corresponding to the quantity of light passed through the lens 
diaphragm before releasing the shutter and to put out a photo current 
corresponding the quantity of light reflected upon the film plane after 
releasing the shutter. The quantity of the reflected light corresponds to 
the illuminance on the film plane at that time. 
Similarly to the ninth embodiment, operational amplifier A210 and 
logarithmic compression diode D221 constitute a metering circuit which 
produces out a voltage corresonding to the value of generated photo 
current and arithmetically compressed. Like A2, A205 in the above 
embodiment, the operational amplifier A211 in this embodiment produces an 
output voltage corresponding to the voltage of the reference voltage 
source Eo as a reference bias to the operational amplifier A210. To 
perform the latch cancellation as will be described later, the output of 
A211 is higher than the forward voltage of diode D220. 
Before the power source switch of the flash light emission device 40 is 
closed, there is no current flowing into the camera from the flash device 
40 through the contact P4. In this position, therefore, the transistor 
Q212 is non-conductive. The exposure control circuit 100 is in the 
position for automatic exposure control because there is generated no 
voltage on the anode side of LED. 
The power supply to the circuit is started when the transistor Q1 is 
rendered conductive by closing the switch SW1. For a time long immediately 
after the start of power supply to the circuit, the voltage on the 
condenser C210 remains zero and the output voltage of A211 continues to 
have the above relation. Therefore, the diode D220 is forward biased to 
cancel the latch caused by the unstable state of A210 at the beginning of 
the power supply. Thereafter, the release magnet Mg1 in the control 
circuit 100 starts operating provided that the transistor Q4 has 
previously been rendered conductive by closing the release switch SW3. At 
the same timing as that in the above embodiment, the memory switch SW4 is 
closed to memorize the output of the operational amplifier A210. Then an 
automatic exposure is carried out when the trigger switch SW2 is closed. 
When the power source switch of the flash light emission device 40 is 
closed, at first a small current flows into LED through the contact P4 and 
the transistor Q212 is rendered conductive. However, the current flowing 
into LED at the time is too weak to light it up to the extent visually 
observable. The voltage generated at the anode of LED is detected by the 
exposure control circuit 100. Therefore, in the manner as previously 
described, the mode of operation is changed over to the mode for flash 
light photographing. Upon the completion of charging the flash light 
discharging condenser in the flash device 40, a sufficiently large current 
enough to clearly lighten LED flows into LED from the flash device 40 
through the contact P4. Thus, the LED in the camera lights up brightly, 
which is visible by the user of the camera. However, there occurs no 
change in the state of the transistor Q212 and in the mode of the control 
circuit 100 by this large current. Since Q212 is conductive, the condenser 
C210 is being discharged and therefore the diode D220 is forward biased. 
There is produced a flow of current through diode D220 and therefore also 
through D221. The operational amplifier A210 produces a voltage 
logarithmically compressed by the current flowing through the diode D220. 
A voltage corresponding to the sensitivity of film is being applied to the 
emitter of transistor Q210 from the control circuit 100. Based on the 
emitter voltage and the above output of the operational amplifier A210 
determined by the current flowing through D220 there is produced at the 
collector of Q210 an expanded current which flows into the flash device 40 
through the contact P2. However, the operation for integrating the current 
transmitted through P2 is not carried out before starting the flash light 
emission by closing the switch SW5. 
When the camera's releasing mechanism is brought into operation by closing 
the switch SW2, at first the switch SW4 is closed in the same manner as in 
the above embodiment. Therefore, the transistor Q211 is rendered 
conductive and Q212 is rendered non-conductive. Since the transistor Q212 
is non-conductive, the condenser C210 is charged through R220 and R221 up 
to the same voltage level as the operational amplifier A211. At this time, 
the operational amplifier A210 is in the state of normal output and 
waiting. The output depends on the photo current of photo diode PD10. 
There is no latch operation as caused by the accumulation of positive 
charge on the cathode side of PD10. Thereafter, in response to the opening 
motion of the shutter, the switch SW3 is closed to start counting the 
shutter time. When the shutter is fully opened, the switch SW5 is closed 
to start the emission of flash light. At the same time, on the side of the 
flash device, the integration of the collector current of Q210 starts. 
When the integrated current reaches a determined value, the emission of 
flash light from the flash device 40 is automatically terminated and TTL 
flash output control is carried out. After the termination of flash light 
emission by the flash output control operation, the shutter is closed and 
switch SW5 is opened. Also, the switch SW4 is opened. In this manner, the 
latch cancelling operation is carried out always immediately before 
shooting. Therefore, even when the light measuring operation is performed 
in the dark for TTL flash output control, always stable TTL flash output 
control operation is assured. 
FIG. 11 shows an eleventh embodiment of the invention. 
In the above tenth embodiment, the latch cancellation of the light 
measuring amplifier has been carried out only for the operation of TTL 
flash output control and the timing of the latch cancellation has been 
selected to be a time point immediately before the start of flash light 
emission. In contrast, in this eleventh embodiment, the latch cancellation 
is carried out not only for TTL flash output control mode but also for 
ordinary photographing mode of the camera. The timing of the latch 
cancellation is selected to be a time point immediately after the 
completion of every photographing operation. 
In FIG. 11, like reference characters to FIG. 10 represent the same or 
corresponding elements. The manner of operation of the eleventh embodiment 
is as follows: 
300 is an exposure control circuit adapted to control the shutter of the 
camera. Mg4 is a magnet for shutter control. By closing the switch SW4 the 
transistor Q220 is rendered conductive through resistor R230. When the 
switch SW3 is closed, a timer in the control circuit 300 starts counting 
the shutter time. At the end of the count, the transistor Q220 is rendered 
non-conductive to initiate the closing motion of the shutter. The 
transistor Q220 remains non-conductive during the operations of film 
advancing and shutter charging subsequent to the exposure. It is rendered 
again conductive when the above described operation is repeated by 
releasing the camera again. 
Transistor Q221 is controlled through resistor R231 in synchronism with the 
transistor Q220. When the power supply to the circuit is started by 
closing the switch SW1 to render the transistor Q1 conductive, the charged 
voltage on the condensers C210 and C220 is zero, and the transistor Q221 
is non-conductive. Therefore, transistor Q222 is rendered conductive by 
the charge current to C220 flowing through R232. Like that in the tenth 
embodiment, the output voltage of operational amplifier A211 is so preset 
to the higher than the forward voltage of diode D220. Consequently, the 
diode D220 is forward biased. On the completion of charging the condenser 
C220 through R232 (the charge time may be in the range of several 100 
.mu.sec. to several m sec.), the transistor Q222 is rendered 
non-conductive and the charging of the condenser C210 is started. The 
condenser C210 is charged with the current flowing through D220 and the 
current flowing through R221 up to the level equal to the operational 
amplifier A211 (the charging time of C210 may be in the range of several m 
sec. to several 10 m sec.). By the above operation the latch caused by 
unstable state in operation of the operational amplifier A210 at the 
beginning of power supply is completely cancelled. When the release switch 
SW2 is closed, the camera's shutter is released in the same manner as 
above. 
After the closing of SW2 and before the closing of trigger switch SW3, the 
transistor Q220 is rendered conductive by the control circuit 300 to apply 
current to the shutter control magnet coil Mg4. At the same time, 
transistor Q221 is also rendered conductive to discharge the condenser 
C220. The timing for rendering Q220 and Q221 conductive may be at the same 
time as the closing of memory switch SW4. With the start of shutter 
opening motion, the trigger switch SW3 is closed to start counting the 
shutter time. At the completion of counting the shutter time, transistors 
Q220 and Q221 are rendered conductive to cut off the power supply to the 
magnet Mg4. Simultaneously with the cut-off of the power supply to Mg4, 
charging of condenser C220 through R232 is started. When the charging of 
C220 is completed, the transistor Q222 is rendered conductive and 
continues to be conductive for a certain determined time long as 
previously described. By this conduction of Q222 for a determined time, 
the condenser C210 is discharged. As a result of it, the diode D220 is 
forward biased and the latch cancelling operation is performed in the same 
manner as above. In a certain determined time after rendering the 
transistor Q222 non-conductive, the voltage of condenser C210 reaches the 
same level as the output voltage of the operational amplifier A211. At 
this time point, the latch cancelling operation comes to end. 
In this embodiment, the above latch cancelling operation is executed not 
only for TTL flash output control mode but also for automatic exposure 
control mode without any distinction therebetween. Herein, the term 
"automatic exposure control mode" means the mode where the camera's 
shutter and/or diaphragm are automatically controlled to obtain the 
optimum exposure value. 
The latching of the light measuring amplifier occurs at the beginning 
period of power supply to the circuit and at the time of the power supply 
to the shutter magnet coil being instantly cut off. In the latter case, 
the latching is mainly caused by the kick-back noise due to the inverse 
induced voltage generated when the power supply is abruptly cut off. There 
is generated no kick-back noise when the current is applied to the coils 
of magnets Mg1, Mg3, Mg4. Since the magnets Mg1 and Mg3 are not required 
to respond at a high speed, they may be of the type which does the work 
when the current is being applied to its coil. However, the magnet Mg4 is 
used to control a high speed shutter. Therefore, it is preferred that Mg4 
be such type of magnet which does the work when the application of current 
to it is cut off as in the case of FIG. 11. According to the eleventh 
embodiment, the latch cancellation is surely executed every time of the 
power supply to the magnet. 
In the above eleventh embodiment, the cut-off of Mg4 and the latch 
cancellation have been carried out at the same time. However, it is to be 
understood that it is not always necessary to carry out the two operations 
at the same time. The latch cancellation may be carried out at any time 
point between the closing of the shutter and the time until which the 
operational amplifier A210 is required to have been restored to its normal 
output state for the next shutter releasing. For example, the same effect 
as above may be attained when the latch cancellation is carried out at a 
time some time later than the closing of the shutter or at the completion 
of one frame film advancing after exposure. 
In the ninth and tenth embodiments, the latch cancellation has been carried 
out at the same time as the closing of memory switch SW4. However, it is 
not always necessary to carry out the latch cancellation simultaneously 
with the closing of SW4. The latch cancellation may be carried out 
simultaneously with the closing of release switch SW2'. In case of the 
ninth and tenth embodiments, the thing necessary is that the latch 
cancellation should be started and completed between the releasing of the 
camera's shutter and the time point until which the light measuring 
amplifier is required to have been restored to its normal output state, 
namely, before the start of flash light emission. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in that art that various changes and modifications may be made 
therein without departing from the spirit and scope of the invention.