Automatic exposure control circuit

An automatic exposure control circuit of a photographing camera having a shutter blade associated with an electromagnet driven by a driving circuit is disclosed, which is featured by having a film sensitivity information detector for detecting the film sensitivity electrically to provide an output signal, means responsive to the output signal for changing a reference voltage to be applied to the driving circuit and a .gamma. value correcting means for correcting the reference voltage according to the .gamma. value of a light receiving element of the camera.

BACKGROUND OF THE INVENTION 
The present invention relates to an automatic exposure control circuit for 
use in a photographic camera and, particularly, to such control circuit to 
be supplied with a film sensitivity information as an electric signal. 
An automatic exposure control circuit for controlling an exposure time of a 
camera has been known which comprises an electromagnet for closing a 
shutter of the camera, a time constant circuit composed of a light 
receiving element and a capacitor for commencing an open motion of shutter 
blades while starting a charging of the capacitor and a driving circuit 
responsive to a charge voltage of the time constant circuit and a 
predetermined reference voltage for rendering the electromagnet to close 
the shutter when the charge voltage reaches the reference voltage. FIG. 1 
shows an example of such automatic exposure control circuit, in which, 
when a camera shutter is started to open by means of a depression of a 
shutter release button, a trigger switch 1 ganged with the shutter release 
button is opened to allow a capacitor 3 to be charged through a light 
receiving element, e.g., CdS element 2. When a voltage of the capacitor 3 
reaches a predetermined reference voltage Vt of a driving circuit 4, the 
latter blocks a current flow through an electromagnet 5 to close the 
shutter. A shutter opening time T between time instances of the turning 
off of the trigger switch 1 and of the deenergization of the electromagnet 
5 is determined by a time constant of a time constant circuit composed of 
the CdS element 2 and the capacitor 3. Since a resistance value of the CdS 
element 2 depends upon an intensity of an incident light from an object to 
be photographed, the shutter opening time T varies with a variation of the 
incident light intensity according to the following equation. 
##EQU1## 
where C is a capacitance of the capacitor 3, Ro is a resistance value of 
the CdS element 2 when the light intensity is Xo, .gamma. is a .gamma. 
value of the CdS element, x is the intensity of light fallen on the CdS 
element and corresponds to Ev value at ASA 100, Vcc is a source voltage 
and Vt is the reference voltage. 
The control circuit in FIG. 1 further includes a resister 6 for high 
intensity correction and a resistor 7 for low intensity correction, a 
winder 8 for setting the film sensitivity being provided in front of the 
CdS element 2. 
A preciseness of exposure obtained by the automatic exposure control 
circuit shown in FIG. 1 will be explained with reference to FIG. 2 in 
which various Ev-T curves and Lv-T curves are shown. 
In FIG. 2, the Ev-T curves show relations of the shutter opening time, 
i.e., the shutter speed T which is the time period between the turning off 
of the trigger switch 1 and the deenergization of the electromagnet 5 to 
the exposure value Ev (=diaphram value Av+shutter speed Tv) and are 
determined by a shutter mechanism . The Lv-T curves show relations of the 
time period T to a light value Lv (=incident light amount Bv+film 
sensitivity Sv) and are determined by the equation (1). 
The preciseness of exposure or exposure error (dEv) can be represented by a 
difference between the Ev-T curve and the Lv-T curve. 
Assuming that the control time is T when the CdS is irradiated with light 
whose intensity is Lv, the shutter mechanism provides the exposure amount 
Ev. If Lv=Ev in this case, the exposure error is 0. However, there is a 
difference between Lv and Ev, practically, and thus the exposure error dEv 
(=Lv-Ev) is produced as shown in a lower portion of FIG. 2. 
The low intensity correction or the high intensity correction is performed 
to minimize the error dEv. The term "low intensity correction" used in 
this specification means a regulation of the error due to differences in 
the capacitor 3, the resistance Ro of the CdS element and the .gamma. 
value of the same etc. between cameras by changing the reference voltage 
Vt by a regulation of the resistance of the resistor 7 so as to minimize 
the error. That is, it is assumed that a curve A in FIG. 2 is the most 
preferable curve providing the minimum exposure error. When the 
characteristics of a certain camera is shown by a curve A' which is 
deviated from the curve A by the differences in values of the constituting 
elements thereof from a camera having the characteristics curve A, the 
value of the resister 7 is reduced so that the curve A' is shifted to the 
position of the curve A. Since, in this case, the regulation of the curve 
A' to the curve A is performed as a whole, it may be enough to shift any 
one point on the curve A' to a corresponding point on the curve A. Since 
such shift is usually performed for a point corresponding to around Lv 9, 
it is referred to as the low intensity correction. 
On the other hand, the high intensity correnction intends to make the Lv-T 
curve close to the Ev-T curve. That is, the Ev-T curve is constituted with 
two segments, one being in a low intensity side which is stable for any 
camera and close to a corresponding portion of the Lv-T curve and the 
other being in a high intensity side which depends upon cameras and is 
substantially different from a corresponding portion of the Lv-T curve. A 
junction of the two segments corresponds substantially to a position in 
which the shutter blade is fully opened. 
In order to minimize the error dEv it is necessary to make the Lv-T curve, 
particularly, the high intensity side portion thereof, as close to the 
corresponding portion of the Ev-T curve as possible. This can be achieved 
practically by regulating the resistance value of the resister 6 connected 
in series to the CdS element 2 of the circuit shown in FIG. 1. 
Assuming the resistance value of the resister 6 as being r, the control 
time T can be represented according to the equation (1), as follow. 
##EQU2## 
Therefore, it is possible to make the Lv-T curve overlapped with the Ev-T 
curve by regulating the value of only the term C r ln Vcc/(Vcc-Vt) by 
changing the resistance value r of the resister 6. Since the portion of 
the Lv-T curve which is changed as above corresponds to the light value 
(Lv) of around 15, this regulation is referred to as the high intensity 
correction. 
In this manner, the variations of the circuit constants such as the 
capacitance of the capacitor and the .gamma. value of the CdS element etc. 
of the control circuit and/or mechanical variations of camera are 
corrected. 
On the other hand, an information of the film sensitivity which is one of 
parameters necessary to operate the control circuit appropriately is given 
as a change in a diameter of the window 8 formed in front of the CdS 
element 2 and is supplied, together with the amount of the incident light, 
to the control circuit as Lv=Bv+Sv. Such input of the film sensitivity 
information through the light receiving window 8 for the CdS element to 
the control circuit can be represented by the Lv-T curve shifted 
horizontally. 
In a case where ASA100 is selected as a reference, for example, a case of 
ASA400 can be represented by the Lv-T curve shifted leftwardly by 2Ev, as 
shown in FIG. 4. 
In any way, however, the film sensitivity information has to be input to 
the control circuit manually, i.e., by changing the size of the opening of 
the window 8 manually according to the sensitivity of a film loaded. 
In order to make an automatic input of the film sensitivity information to 
the control circuit possible, an idea has been proposed that an electric 
contact coded according to the film sensitivities is provided 
preliminarily on a patrone of a film to be loaded and, when loaded, a code 
signal is derived from the electric contact, on which the sensitivity of 
the loaded film is detected and supplied to the control circuit 
automatically according to which various parameters of the circuit are 
changed. 
This proposition seems to be very effective. However, any construction for 
practicizing this idea has not been proposed as yet. 
That is, a construction of the control circuit for practicizing the idea 
may be one shown in FIG. 3, in which a coded film sensitivity signal 
derived from the contact on the patrone of a certain film loaded is 
supplied to a decoder 9. An output of the decoder 9 which corresponds to 
one of the film sensitivities is supplied to a corresponding one of 
switching transistors 10 to turn it on to thereby connect a corresponding 
one of resisters 11 to a reference voltage input of the driving circuit 4. 
Thus, a reference voltage suitable for the sensitivity of the loaded film 
can be established at the reference voltage input of the driving circuit 
4. The switching of the reference voltage one to another in response to 
the film sensitivity can be represented by a vertical shaft of the Lv-T 
curve, as mentioned previously. 
However, a mere vertical shift of the Lv-T curve causes various problems to 
occur, which are as follows: 
Firstly, when the curve is shifted vertically by a difference dSv in the 
film sensitivity with a reference being set in the low intensity side, 
there is provided in the high intensity side a difference which in larger 
than the value dSv, causing an exposure error, as shown in FIG. 5. That 
is, when the amount of shift in the low intensity side is 2Ev, the amount 
of shift in the high intensity side which should be 2Ev as shown by a 
curve C is much larger than 2Ev. 
Secondly, because of a variation of the .gamma. value of the CdS element, 
the larger the difference between the sensitivity of a film to be loaded 
and the reference film sensitivity provides the larger the exposure error, 
as shown in FIG. 6. In FIG. 6, a curve A shows the Ev-T curve of a camera 
mechanism as well as the Lv-T curve adaptable thereto, where the .gamma. 
value of the CdS element is .gamma.o and the film sensitivity is ASA100, 
and a curve A' shows those for ASA400. In this case, if there is no 
variation of the .gamma. value of the CdS element, the curves A and A' are 
parallel. However, there is a variation of the CdS element camera by 
camera, practically. 
Assuming that the .gamma. value of a certain CdS element is not .gamma.o 
but .gamma.' and a curve B is obtained as shown in FIG. 6, a regulation is 
performed at a shutter speed To at which the curves A and B crosses. The 
exposure error at a certain shutter speed is e. 
When the switching of the film sensitivity to ASA400 is performed by 
changing the window size of the CdS element as in the conventional manner, 
the curve B is merely parallel-shifted resulting in a curve C. Therefore, 
the exposure error at the certain shutter speed is .DELTA.e. 
On the other hand, when the switching is performed by changing the 
reference voltage Vt of the driving circuit as mentioned, the amount of 
parallel-shift is increased, resulting in a curve B' as shown in FIG. 6. 
Thus, an additional exposure error .DELTA.E is included, which increases 
with an increase of the difference in film sensitivity. 
Finally, there must be a low intensity correction means provided 
additionally since the switching of the reference voltage Vt which is used 
for the low intensity correction in the conventional system is used as the 
film sensitivity switching. 
SUMMARY OF THE INVENTION 
An object of the present invention is the provide an automatic exposure 
control circuit for a photographic camera in which the film sensitivity is 
switched electrically, which is capable of performing both the high 
intensity correction and the low intensity correction and eliminating any 
exposure error due to a variation of the .gamma. value of a light 
receiving element of the camera. 
According to the present invention, the above object can be achieved by a 
provision of an automatic exposure control circuit including an 
electromagnet for rendering a camera shutter to be closed when energized, 
a time constant circuit composed of a light receiving element and a 
capacitor and responsive to a commencement of a shutter opening operation 
of the camera to start a charging of the capacitor and a driving circuit 
having inputs supplied with a charge voltage of the capacitor and a 
predetermined reference voltage, respectively, and responsive to the 
charge voltage reached the reference voltage to energize the electromagnet 
to thereby close the shutter blade, a film sensitivity information 
detector for detecting a film sensitivity information electrically and 
providing an output signal according to a detected film sensitivity 
information, a film sensitivity switching portion responsive to the output 
signal from the film sensitivity information detector to change the 
reference voltage to be supplied to the driving circuit and a .gamma. 
value correction responsive to the .gamma. value of the light receiving 
element to change a voltage to be applied to the film sensitivity 
switching portion to thereby correct the reference voltage to be applied 
to the driving circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 3, a time constant circuit composed of a variable resister 13 and a 
capacitor 14 is provided in addition to a time constant circuit composed 
of a CdS element 2 and a capacitor 3. A trigger switch 1 is connected in 
parallel to the capacitor 14. A charge voltage of the capacitor 14 is 
compared in a comparator 14 with a reference voltage Vt' applied thereto 
through voltage dividing resisters 16 and 17. An output of the comparator 
15 is supplied to a base of a switching transistor 18 which serves as a 
trigger switch for the capacitor 3. 
A charge voltage of the capacitor 3 is applied to a driving circuit 4 to 
which a reference voltage Vt corresponding to a resistance value of one of 
resisters 11 is also applied. The resister is selected according to a film 
sensitivity signal detected electrically through a decoder 9 and a 
transistor 10, as mentioned previously. 
The control circuit in FIG. 3 further includes an electromagnet 5 adapted 
to be driven by the driving circuit 4, a voltage dividing resister 19 
connected in series with the resisters 11 and a power supply 20. 
In operation, when the trigger switch 1 is turned off, the capacitor 14 is 
charged through the resister 13. When the charge voltage increases and 
becomes equal to the reference voltage Vt', the comparator 15 provides an 
output by which the transister 18 is turned off to charge the capacitor 3 
of the time constant circuit composed of the capacitor 3 and the Cd 
element 2. 
Thereafter, as in the same manner as that of the conventional circuit, the 
current supply to the electromagnet 5 is blocked when the charge voltage 
of the capacitor 3 becomes equal to the reference voltage Vt of the 
driving circuit 4 and thus the shutter is allowed to close. 
That is, the feature of the present invention resides in the use of the 
pair of time constant circuits. The control time T obtained by the circuit 
construction of the present invention can be represented as follow: 
##EQU3## 
As is clear from the equation (3), the time to be controlled by the first 
time constant circuit and the comparator 15 is given by a second term of 
the left side of the equation. Since this term is independent from the 
reference voltage of the driving circuit 4 which is changed according to a 
change of the film sensitivity, it is possible to perform the high 
intensity correction by using this term. The curve C in FIG. 5 shows the 
Ev-T characteristics when the high intensity correction is performed 
thereby. The degree of the high intensity correction may be performed by 
regulating the resister 13 to change the time constant. Alternatively, it 
may be possible to perform this by regulating the resisters 16 and 17 to 
change the reference voltage Vt'. 
Next, the correction of a variation of the .gamma. value of the light 
receiving element will be described. 
Assuming the film sensitivity of ASA100 being a reference, the exposure 
control time T with respect to a brightness x can be represented by the 
equation (1) or (2). However, since, in this case, a factor affecting a 
reduction of voltage is neglected, the reference voltage is given 
practically as follow: 
EQU .alpha.i=Vti/Vcc (4) 
Thus, 
EQU T=C.sub.1 R.sub.1 ln 2-C.sub.2 Ro2.sup.-.gamma.(X-Xo) ln (1-.alpha.i) (5) 
The selection of the reference voltage .alpha.i for a given film 
sensitivity is performed as follows. Assuming that the reference voltage 
for the reference film sensitivity is given by .alpha.o and the reference 
voltage for the sensitivity of a film which differs from the reference 
film sensitivity by dSv is given .alpha. and that the brightness 
corresponding to the reference voltage .alpha.o is x and that 
corresponding to the reference voltage .alpha. is (x-dSv), the following 
equation is established from the equation (5) since the exposure control 
time is common. 
EQU 2.sup.-.gamma.(X-Xo) ln (1-60 o)=2.sup.-.gamma.(X-dSv-Xo) ln (1-.alpha.) 
Thus, 
EQU .alpha.=1-(1-.alpha.o).sup.2-.gamma.dSv (6) 
As in clear from the equation (6), .alpha. is determined by not only 
.alpha.o and dSv but also the .gamma. value of the light receiving 
element. That is, if i is determined without considering the .gamma. 
value, the exposure error is increased. For example, where .gamma.=0.6, 
ASA100 and .gamma.o=0.93, the reference voltage for ASA1000, the film 
sensitivity difference dSV being given by 
EQU dSv=log 2 (1000/100)=3,322 (7) 
the value can be calculated from the equation (6) as follow: 
EQU .alpha.=0.4872 (8) 
When the .gamma. value is changed to 0.5 with the reference voltage being 
unchanged, the film sensitivity difference dSv can be calculated from the 
equation (6) as 
##EQU4## 
resulting in an exposure error of 0.665Ev due to the .gamma. value of the 
light receiving element. 
FIG. 7 shows a relation between the .gamma. value of the light receiving 
element, i.e., CdS element and the value of .alpha. with the ASA number 
being a parameter with respect to the reference of ASA100 (.alpha.o=0.9). 
As is clear from FIG. 7, since the .alpha. is deemed as being linearly 
related to the .gamma. value within a certain range, it can be represented 
as 
EQU .alpha.=a-b.gamma. (10) 
where a and b are constants (a, b, .gamma..gtoreq.0). 
FIG. 9 shows an equivalent circuit to the equation (10). The equivalent 
circuit in FIG. 9 is an adder circuit comprising an operational amplifier 
22 and resisters R1 and R2 and when voltages V1 and V2 are applied to the 
resisters R1 and R2, respectively, the amplifier 22 provides an output V2 
which is as follow: 
##EQU5## 
Therefore, 
##EQU6## 
FIG. 10 shows an example of circuit constructed according to the equations 
(12) and (13) for the ASA numbers of 100, 200, 400 and 800. In FIG. 10, 
series connected resisters R1, R2, R3, R4 and R5 constitute a voltage 
divider for dividing the source voltage Vcc into Va, Vb, Vc, and Vd one of 
which is selected by means of analog switches A, B, C and D and supplied 
to a voltage follower amplifier 24 an output of which is supplied through 
one of resisters R7, R8 and R9 selected by selectively turning on any of 
analog switches B', C' and D' to an amplifier 26. An output of the latter 
is supplied to the driving circuit of the electromagnet as the reference 
voltage V.alpha.. 
The analog switches are on-off controlled through a decoder according to 
the film sensitivity informations which are electrically detected 
automatically such that, when the sensitivity is ASA100, the switch A is 
turned on, when ASA200, the switches B and B' are turned on, when ASA400, 
the switches C and C' are turned on and, when ASA800, the switches D and 
D' are turned on, respectivly. 
With this construction, it becomes possible to correct the reference 
voltage of the driving circuit with respect to the respective film 
sensitivities by merely changing the voltage Ve, which, in turn, is 
determined by regulating a variable resister R10 according to a variation 
of the .gamma. value of the CdS element. Therefore, it is possible to 
eliminate the exposure error due to the variation of the .gamma. value of 
the CdS element. 
As to means for eliminating exposure error due to a variation of the 
.gamma. value of the light receiving element in the case of the electrical 
film sensitivity switching, the relations between the .gamma. value of the 
CdS element and the .alpha. value when o in the equation is 0.9 and the 
film sensitivity difference dSv in the same 1, 2 and 3, respectively, are 
as shown in FIG. 7. Since the curves each for a different film sensitivity 
difference are linear within certain ranges, respectively, as mentioned 
previously, .alpha..sub.1 and .alpha..sub.2 can be represented by simple 
equations of .alpha..sub.3 when the latter is made correspondent to the 
value, respectively, which are approximated according to the equation (6) 
by the following equations where k1 and k2 are constants: 
EQU .alpha..sub.1 =1-k.sub.1 (1-.alpha..sub.3) (14) 
EQU .alpha..sub.2 =1-k.sub.2 (1-.alpha..sub.3) (15) 
FIG. 8 shows relations between the value and 
(1-.alpha..sub.1)/(1-.alpha..sub.3) and 
(1-.alpha..sub.2)/(1-.alpha..sub.3) respectively. In a range of the 
.gamma. value of from 0.5 to 1 which is used usually, the relations are 
substantially linear and, therefore, it is possible to determine the 
constants k1 and k2, substantially. 
FIG. 11 is an example of a circuit constructed according to the concept 
shown in FIG. 8. In FIG. 11, a source voltage Vcc in divided by a variable 
resister R12 to obtain a voltage V3 which is supplied to a voltage 
follower amplifier 27. A decoder 28 which is supplied with the film 
sensitivity information in the form of electric signal, selectively turns 
on one of analog switches 0, 1, 2 and 3. When the switch 0 is turned on 
thereby, the source voltage Vcc is divided by a series connected resisters 
R and RO to provide a reference voltage Vt and, when the switch 1 is 
turned on, a voltage determined by the source voltage Vcc, the voltage V3 
and the resister R and a resister R1 is provided as the reference voltage 
Vt. The voltage Vt is determined by the voltages Vcc and V3 and resisters 
R and R2 when the switch 2 is turned on and the reference voltage Vt 
becomes V3 when the switch 3 is turned on. 
A bridge circuit is constituted with the variable resister R12 and the 
resisters R and any one of the resisters R0, R1 and R2 and the reference 
voltage Vt is derived from a center point of the bridge circuit. Ratios of 
these resisters are determined as follows: 
##EQU7## 
are obtained from which circuit constants of the circuit in FIG. 11 can be 
easily determined. 
The error correction is performed by this circuit such that, when a certain 
time To is obtained with respect to a certain brightness xo with dSv being 
0, the dSv is switched to 3 to make the CdS brightness (xo-3) Ev and the 
variable resister R12 is regulated to make the time To. 
FIG. 12 shows another example of the circuit which is similar to that in 
FIG. 11. In FIG. 12, an input voltage V3 to an amplifier 27 is derived 
from a source voltage Vcc by a variable resister R12 in the same manner as 
in the circuit in FIG. 11. The variable resister R12 and resisters R0 and 
R1 constitute a bridge circuit from a center of which a reference voltage 
Vt is derived. The voltage V3 is divided by resisters R2, R3 and R4 and 
resulting voltages are selectively added to the reference voltage by 
analog switches controlled by a decoder 28 to perform a necessary 
correction. 
FIG. 13 is a further example of the circuit for performing the correction 
in which a reference voltage Vt is derived from a center of a bridge 
circuit constituted with a variable resister R12 and resisters R0 and R. A 
voltage V4 which is derived from the variable resister R12 is divided by 
resisters r1, r2 and r3 and resultant voltages are supplied to amplifiers 
31, 32 and 33, respectively, so that any of these voltages is selectively 
added to the reference voltage Vt through one of the resisters R1, R2 and 
R3 according to a selected film sensitivity. 
The high intensity correction and the correction of the .gamma. value of 
the light receiving element have been described hereinbefore. As to the 
low intensity correction, the regulation of the window diameter of the 
light receiving element which has been used for the switching between film 
sensitivities is used therefore in this invention. The regulation of the 
window diameter may be performed by moving a plate having a plurality of 
holes of different diameters and disposed in front of the light receiving 
element or by changing filters. Of course other means may be used for that 
purpose. 
With a combination of the various corrections mentioned hereinbefore, a 
highly precise exposure control is obtained in the exposure control 
circuit of the type in which the film sensitivity information is 
electrically inputted. 
FIG. 14 shows an embodiment of an exposure control circuit equipped with 
all of the correction means described hereinbefore. In FIG. 14, the 
exposure control circuit comprises a film sensitivity switching portion 38 
for changing a reference voltage to be supplied to a driving circuit 4 
according to an electric signal from a decoder 28 which serves as a film 
sensitivity detector, a .gamma. value correcting portion 39 for correcting 
the reference voltage according to a variation of the .gamma. value of a 
light receiving element, a high intensity correcting portion 40 including 
a pair of time constant circuits and a low intensity correcting portion 36 
provided in front of the light receiving element. 
FIG. 15 shows another embodiment of the exposure control circuit which is 
similar to that shown in FIG. 14 except that a capacitor C4, a transistor 
4 and a low intensity detector 42 are further provided. 
The provision of the capacitor C4 and the transistor 41 are to expand a 
sensitivity range of film to be used. As can be seen from FIG. 7, the 
range of the reference voltage Vt is preferrably from 0.35 to 0.95, taking 
in a source voltage variation into consideration. The transistor 41 is 
turned on through the decoder 28 when the sensitivity of the film used in 
beyond a certain range to insert the capacitor C4 in parallel with the 
capacitor C1 which, together with the light receiving element 2, 
constitutes the time constant circuit to thereby increase the time 
constant, so that such high film intensity becomes acceptable. 
The provision of the low intensity detector 42 is to expand the low 
intensity side range in which it is possible to photograph. When the 
.gamma. value of the light receiving element is around 0.5, an 
underexposure condition is established in the low intensity side as shown 
in FIG. 16. According to this embodiment, a light intensity within a 
certain range in the low intensity side is detected by the low intensity 
detector 42 upon which the transistor 41 is turned on through the decoder 
to insert the capacitor C4 in parallel to the capacitor C1, as in the case 
of the expansion of the film sensitivity range, to thereby expand the 
control time so that the photographing range in the low intensity side is 
automatically expanded. FIG. 17 shows the effect of the provisions of the 
series circuit of the capacitor C4 and the transistor 41 and the low 
intensity detector 42. That is, the Ev-T curve shown by a dotted line has 
a steppingly changing portion in the low intensity side thereof which is 
obtained by the insertion of the capacitor C4. 
According to the present invention, in an automatic exposure control 
circuit of a photographing camera, an operation of which is automatically 
changed according to an electrically detected film sensitivity 
information, a film sensitivity information detector for electrically 
detecting the film sensitivity and providing an output signal 
corresponding to the detected film sensitivity information, a film 
sensitivity switching portion responsive to the output signal for changing 
a reference voltage to be applied to a driving circuit of the camera and a 
.gamma. value correcting portion responsive to a .gamma. value of a light 
receiving element for correcting a voltage to be applied to the film 
sensitivity switching portion to thereby correct the reference voltage are 
provided. With this construction of the present invention, not only the 
effect of the automatic switching of film sensitivity but also the effect 
of the correction of error due to a variation of the .gamma. value of the 
light receiving element of the camera can be obtained, resulting in that a 
precise exposure control can be performed.