JPH0712151B2 - Camera exposure control circuit - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a camera including a digital-analog converter (hereinafter, referred to as a DA converter) of a so-called pulse width modulation method (hereinafter, referred to as a PWM method). It relates to an exposure control circuit.

Background of the Invention A PWM DA converter is used, for example, when converting film sensitivity information in the form of a digital value into an analog value according to the film sensitivity. FIG. 14 shows an example of an exposure control circuit of a camera having a DA converter of this kind. The film sensitivity setting circuit 15 has three switches SW1 to SW3, and eight kinds of film sensitivities are set according to the state of each switch. it can.
The film sensitivity set by each switch is converted by the microprocessor 17 into film sensitivity information of a predetermined digital value. The microprocessor 17 generates a pulse signal having a duty ratio according to the digital value and supplies it to the DA converter 5. The transistor Q1 of the DA converter 5 is turned on / off according to the pulse signal, and the DC voltage of the level corresponding to the transistor Q1 is smoothed by the filter circuit (C1, R7) and taken out as the DA conversion output. In addition, S100 is a power switch,
E indicates a battery.

To describe the DA conversion operation in detail, when the power switch S100 is turned on, the microprocessor 17 operates and a pulse train having a duty ratio according to the film sensitivity set by the film sensitivity setting circuit 15 is output to the port O3. When the output of the port O3 is "1", the transistor Q1 is turned on, so that the charge of the capacitor C1 is discharged through the resistor R7. If the output of the port O3 is "0", the transistor Q1 is turned off, and the capacitor C1 is charged through the resistors R6 and R7.
While repeating such charging and discharging, the charging voltage of the capacitor C1 converges on the analog voltage level according to the film sensitivity.

Therefore, when the power switch S100 is turned on, the capacitor
When C1 is completely discharged, it takes a considerable amount of time for the analog output to reach a voltage level that depends on the film sensitivity. Therefore, even if you want to operate the camera immediately after turning on the power switch S100, you have to wait until the voltage of the capacitor C1 has settled to a predetermined value, so you miss the shutter and operate the camera in a calm manner. And proper exposure cannot be obtained.

In short, as can be seen from Fig. 14, the PWM DA converter has a filter circuit (C
1, R7) is indispensable, and the time constant of this filter circuit has a drawback that the transient response at the time of power switch on is poor.

(Object of the Invention) An object of the present invention is to solve such problems and to provide an exposure control circuit of a camera in which the transient response of the DA converter at the time of power switch-on is improved.

(Summary of the Invention) According to the present invention, when a power switch of a camera is turned on, a DC voltage or a predetermined voltage is set so that a capacitor can be charged in a time shorter than a time required to charge the capacitor by a pulse train according to set exposure information. And an initial control means for supplying an initial signal consisting of the pulse train to the filter means.

The initial control means is such that when the initial signal is supplied to the filter means, the signal level of the modulated analog signal taken out from the digital-analog conversion means is digital when a pulse train corresponding to the set exposure information is supplied to the filter means. It is arranged to stop the supply of the initial signal to the filter means when the signal level of the modulated analog signal taken out from the analog conversion means coincides.

Alternatively, the initial control means is configured to change the time for supplying the initial signal to the filter means according to the set exposure information.

(Embodiment) -First Embodiment-FIG. 2 shows an embodiment of the present invention. The output of the absolute temperature proportional voltage generating circuit 1 as a power supply circuit is an analog-digital converter (hereinafter, AD converter) 3 And a reference signal input terminal of the digital-analog converter (DA converter) 5. The output of the photometric circuit 7 is input to the AD converter 3 via the cancel circuit 9. The output of the exposure factor setting circuit 11 is also input to the AD converter 3. Output of AD converter 3, output of initialization circuit 13 and film sensitivity setting circuit 15
The output of is input to the microprocessor 17. A control signal according to the aperture value and the exposure time calculated by the microprocessor 17 is input to the magnet drive circuit 19. The input signal from the film sensitivity setting circuit 15 is converted into a binary number by the microprocessor 17, and the corresponding pulse train is output to the DA converter 5. The output of the DA converter 5 is input to the photometric circuit 7 via the adder circuit 21 and also to the AD converter 3. Furthermore, TT
The output of the L dimming control circuit 23 is also input to the photometric circuit 7.

Each of these elements will be described in more detail with reference to FIG.

The reference voltage generating circuit 1 includes an operational amplifier A1 and voltage dividing resistors R1 to R1.
4. It has diodes D1 and D2, which are connected so as to output a voltage signal proportional to the absolute temperature T. That is, the output voltage VA1 of the operational amplifier A1 is However, it can be expressed as K: Boltzmann's constant q: electronic charge T: absolute temperature. Where diodes D1 and D2
Have the same electrical characteristics.

The exposure factor setting circuit 11 is composed of two variable resistors VR1 and VR2. Here, the variable resistor VR1 is interlocked with the movement of the diaphragm ring of the lens, and accordingly, a voltage signal according to the amount of change of the diaphragm can be taken out. A voltage signal corresponding to the open F value of the lens is taken out from the variable resistor VR2.

The initialization circuit 13 includes a resistor R10 and a capacitor C2, and sets the microprocessor 17 to the initial state when the power is turned on. The film sensitivity setting circuit 15 is composed of three switches SW1 to SW3 in this embodiment, and eight kinds of film sensitivities can be set as shown in FIG. 3 according to the ON / OFF states of the switches SW1 to SW3. It is "0" when the switches SW1 to SW3 are on, and "1" when they are off. The microprocessor 17 is well known and has a read-on memory ROM, a random access memory RAM, and a central processing unit CPU.

Magnet drive circuit 19 consists of resistors R8 and R9, transistors Q2 and Q
3, having magnets MG1 and MG2, the base of the transistor Q2 is connected to the output terminal O1 of the microprocessor 17 via the resistor R8, and the base of the transistor Q3 is the output terminal of the microprocessor 17 via the resistor R9. It is connected to O2.
The magnet MG1 is for a diaphragm control magnet, and the diaphragm can be stopped at an arbitrary position by exciting the magnet MG1. The magnet MG2 is a magnet for controlling the rear curtain of the shutter, and by demagnetizing at any time after the front curtain of the shutter has traveled, the rear curtain of the shutter can start traveling.

The DA converter 5 is of the pulse width modulation type (hereinafter referred to as the PWM type), and the base of the transistor Q1 of the DA converter 5 is connected to the output terminal O3 of the microprocessor 17 via the resistor R5. The emitter of Q1 is grounded and the collector is connected to the output terminal of the power supply circuit 1 via the resistor R6. That is, the DA converter 5 is supplied with the temperature signal of the temperature proportional voltage generation circuit 1 as a reference voltage. The collector of the transistor Q1 is connected to one input terminal of the operational amplifier A4 of the adder circuit 21 via a filter circuit composed of a resistor R7 and a capacitor C1. Further, the analog output of the DA converter 5 is input to the channel CH4 of the AD converter 3 via the operational amplifier A4.

The photometric circuit 7 is well known and has a photo diode PD interposed between two input terminals of an operational amplifier A2 and two diodes D3 and D4 interposed in a forward direction in a feedback circuit, Photocurrent IL of photodiode PD is logarithmically converted and output. Here, the output voltage VA2 of the operational amplifier A2 is However, VA4: output voltage of adder circuit 21 IS: reverse saturation current of diodes D3, D4

The cancel cell circuit 9 for canceling the temperature-dependent characteristics of the diodes D3 and D4 has an operational amplifier A3 and two diodes D5 and D6 which are interposed in the feedback circuit in the reverse direction, and outputs the output from the photometry circuit 7. Of which, diode D3 and
Cancel the influence of the temperature characteristics of D4.

The output voltage VA3 of the cancell circuit 9 is However, it can be expressed as I2: current flowing into the cancel circuit.

The AD conversion circuit 3 is a relatively comparative type having four input terminals CH1 to CH4, and the temperature signal from the temperature proportional voltage generation circuit 1 is input to its reference voltage input terminal VREF. The temperature signal is used in common with the reference voltage of the DA converter 5 as described above. And the variable resistance VR1 is attached to the channel CH1.
However, the channel CH2 has a variable resistor VR2 and the channel CH3
Is connected to the output terminal of the cancell circuit 9, and channel CH4 is connected to the output terminal of the adder circuit 21.

In FIG. 1, switch S100 is a main switch and E is a battery.

Next, the film sensitivity (SV), photometric value (BV), aperture value apex value AV, and open F-number apex value AVO required for exposure factor control will be described in detail while explaining the operation of each element in FIG. .

1) Film Sensitivity (SV) Film sensitivity information can be input to the microprocessor 17 from the film sensitivity setting circuit 15 having three switches SW1 to SW3. When the desired film sensitivities are set as shown in FIG. 3 according to the on / off state of each switch, the microprocessor 17 converts the input data into the fourth data.
Convert to 8-bit data as shown. This 8-bit digital film sensitivity data is stored in the microprocessor.
It is converted into analog data by the 17 and the DA converter 5 as follows. In this embodiment, digital-analog conversion is performed by the PWM method.

In the PWM method, a constant pulse output waveform is generated with a repetition period T as shown in FIG. Assuming that the minimum unit time during which the pulse waveform is "0" is t0, the relationship of t0 × 2 ⁸ = 256 ° t0 = T holds in an 8-bit DA converter. Further, when the digital data of the film sensitivity shown in FIG. 4 is represented by "D", the relation of D + = 256 is established between the digital data and the complement "". Therefore, Microprocessor 17
Using the internal timer, first turn on output port O3, load the value "D" into the timer, interrupt the "D" cycle of the counter, and turn off output port O3 when the interrupt occurs. After that, the value "" is loaded to the timer and the interrupt is generated at the "" cycle in the same manner. Repeatedly thereafter, a periodic pulse train of "D" cycle-on and "" cycle-off can be obtained.

When the pulse train thus obtained is supplied to the DA converter 5, the analog signal obtained from the DA converter 5, that is, the input voltage waveform of the operational amplifier A4, is as shown in FIG. In FIG. 6, VC indicates the average voltage and VL indicates the ripple voltage. However, if the time constant of the filter circuit (R7, C1) of the DA converter 5 is made sufficiently large, the ripple voltage VL can be ignored.
Here, the average voltage VC is Can be expressed as

The 8-bit data “D” is set so that the data value increases or decreases by 6 when the film sensitivity changes by 1 EV as shown in FIG. 4, so as is clear from equation (4), DA conversion is performed. The LSB of the circuit is a voltage equivalent to 1/6 EV. Figure 7 shows the input voltage VC corresponding to each film sensitivity at an ambient temperature of 25 ° C.

2) Aperture value AV value and open F value
AVO Using the aperture value AV of the aperture value and the aperture value AVO of the open F value when shooting with the shooting lens, the voltage VCH1 set by the variable resistor VR1 is input to the input channel CH1 of the AD converter. Is Is represented. This expression shows how many steps the aperture value deviates from the open aperture of the taking lens when taking an image. Further, the voltage VCH2 representing the open F value of the taking lens set by the variable resistor VR2 is Can be expressed as This voltage VCH2 is input to the input CH2 of the AD converter 3.

2) Photometric value (BV) Photocurrent corresponding to a certain amount of incident light from the photodiode PD.
If the incident light quantity is doubled, the photocurrent will be 2IL.
Becomes 1. Therefore, the operational amplifier A2 corresponding to each light quantity
Output voltages VA2 and VA2 ′ of Becomes Therefore, if the change in the output voltage of the operational amplifier A2 due to the change in light amount is ΔVA2, Can be expressed as That is, when the light quantity doubles, the output voltage of the operational amplifier A2 becomes Will only increase. Double the change in light intensity
Since this is nothing but an increase in the amount of light equivalent to 1EV, in the present example, at an ambient temperature of 25 ° C., the variation of factors related to exposure equivalent to 1EV is: Will change at a rate of. A resolution of 1/6 of 1EV is sufficient for camera exposure control, etc.
The voltage corresponding to LSB is It will be good if you choose it. And assuming that the resolution of the AD converter is 8 bits, the reference input voltage VA1 of the AD converter is
To Then, a resolution of 1/6 EV can be obtained. This VA1
The value of is obtained by setting R1 to R4 to appropriate values in the above formula (1).

Here, the photocurrent IL of the photodiode PD is IL = α · 2 ^(BV-AVO) …… (11) where, using the aperture value BV of the subject brightness and the aperture value AVO of the aperture value of the taking lens, α can be expressed as a constant, and by substituting the equation (11) into the equation (3), the output of the photometric circuit 7 via the cancell circuit 9, that is, the output voltage VA3 of the operational amplifier A3 is Can be expressed as Therefore, if we rewrite equation (12) using equations (1) and (4), Becomes

Let us consider what kind of data can be obtained by AD converting the output voltage VA3 expressed by equation (13) using the reference input voltage VREF proportional to temperature.

Dividing equation (13) by equation (10) gives Becomes Here, D in the first term represents film sensitivity data. The second term can be an integer with the minimum unit of 1/6 of (BV-AVO) by properly setting the constant α, and the denominator 256 represents the data length of 8 bits. Therefore, the numerator value itself can be regarded as AD converted data.

Similarly, regarding expression (5) regarding the aperture value of the aperture value at the time of shooting and (6) regarding the aperture value of the open F value.
By dividing the equation by the equation (10), the digital values of (AV-AVO) and AVO can be obtained.

Next, an example of the procedure of camera exposure control will be described with reference to the flowcharts of FIGS.

-Main Routine-In step S1, the initialization circuit 13 shown in FIG. 1 initializes the microprocessor 17 to start its operation. Next, at step S2, film sensitivity information is read from the on / off states of the switch groups SW1 to SW3. Step
In S3, the read film sensitivity information is converted into binary data and stored in the memory. In step S4, a subroutine is jumped to and an initial control process is performed when the power is turned on. The subroutine will be described later. At step S5, the information AV-AVO of the step difference from the open F value of the photographing lens AD-converted by the AD converter 3 to the current aperture value, the information AVO indicating the open F value and the photometric value and the film sensitivity are added. Information BV-AVO + SV Three information 17
Take in. In step S6, it is determined whether or not the camera is released, and if a negative determination is made, the process returns to step S2,
The above-described operations are repeated to wait.

If step S6 is affirmatively determined, the operation proceeds to step S7, in which EV = BV−AVO + SV + AVO = BV + SV (15) is calculated in order to obtain the EV value immediately after the release. That is, the channel CH1 of the AD converter 3
And add the information obtained from CH3. This embodiment describes a program-controlled camera,
The combination of the diaphragm AV ^* and shutter speed TV ^* for EV value is determined in advance.

Therefore, the planned aperture direct AV ^* is, in step S8, Is required as. Then in step S9, Is calculated to obtain the planned shutter speed TV ^* .

In step S10, the AD converter 3 is used to operate the operational amplifier A3.
The output VA3 of is continuously AD converted. Immediately after the release, the aperture of the taking lens is open, so analog data VA
The AD conversion output of 3 is BV-AVO + SV. Here, if the diaphragm when the photographing lens is being narrowed down is represented by AV, the AD conversion output at an arbitrary time can be represented by BV-AV + SV. Since BV and SV are constant even immediately after the release, the number of aperture steps (AV-AVO) from opening can be expressed as AV-AVO = (BV-AVO + SV)-(BV-AV + SV) (18). Since the AVO value is input to the microprocessor 17 from the channel CH2 of the AD converter 3, the aperture value AV when the aperture value is being narrowed down can be obtained from the calculation of (AV-AVO) + AVO. Next, in step S11, it is determined whether or not the aperture value AV thus calculated matches the planned aperture value AV ^*, and if a negative determination is made, the process returns to step S10,
Again, the output of the operational amplifier A3 is continuously AD converted. If a positive determination is made, the operation proceeds to step S12, and the aperture control magnet M
In order to turn on G1, port O1 is set to high level, which turns on transistor Q2. As a result, the aperture control magnet MG1 is turned on, the aperture is locked, and the aperture value is controlled to the planned aperture value AV ^* . Next, at step S13, the shutter speed AV ^* is loaded into the counter of the microprocessor 17 in response to the start of traveling of the shutter front curtain. Step S14
Then, it is determined whether or not the count value of the counter reaches the planned value AV ^* . If a negative decision is made, the counter counts until it becomes TV ^*, and if a positive decision is made, the shutter magnet MG2 is turned off and the port O2 is set to a low level in order to drive the rear curtain of the shutter. As a result, the transistor Q3 turns off and the magnet MG2 turns off, which causes the rear curtain of the shutter to start running.

—Pulse Train Generation Routine— Next, a pulse train generation routine for generating a pulse train having a duty ratio according to the film sensitivity will be described with reference to FIG.

When this program is started at a predetermined timing, step S30 turns on flag 1 of the internal timer and goes to step S30.
Proceed to 31. At step S31, the port O3 is set to high logic, and at step S32, the film sensitivity data M (D) stored in the memory is set in the timer. When the timer measures the data M (D), in other words, when the timer interrupt occurs, the flag 1 is reset in step S33 and the process proceeds to step S34. In step S34, the port O3 is set to low logic, and in step S35, the memory value M indicating the complement of D.
Set () to the timer. When the timer interrupt is applied, the process returns to step S30. Thereafter, by repeating these steps, a pulse train having a duty ratio corresponding to the film sensitivity data is obtained.

-Subroutine-Next, the subroutine of the initial control process will be described with reference to FIG.

When jumping from step S4 of the main routine to this subroutine, at step S21, a fast path flag 2 (hereinafter referred to as flag 2) for identifying whether or not the pass of this routine is the first time is set. In step S22, the state of flag 2 is detected, and if it is the first pass, step S23 is executed, and if it is the second time or later, it is jumped to step S29. In step S23, microprocessor 1
The port O3 of 7 is set to "L", that is, a low logic state.
The transistor Q1 in the figure is turned off. That is, a low level signal is supplied to the DA converter 5 as an initial signal. In step S24, the channel CH4 of the AD converter 3 is selected, and the capacitor C1 of FIG. 1 is output via the output of the operational amplifier A4.
Set the charging voltage of to the state that can be monitored. In step 25, the analog voltage VF preset according to the film sensitivity input from the switches SW1 to SW3 is read and stored in a predetermined area. Next, in step S26, the AD conversion operation is started and the reading of the charging voltage of the capacitor C1 is started. In step S27, the analog voltage VF corresponding to the film sensitivity read in step S25 is compared with the AD conversion value of the DA conversion output, and if they match, the process proceeds to step S28 to stop the AD conversion operation. Then, in step S29, flag 2 is turned off and the process returns to the main routine.

Next, the transient response characteristic when the power switch is turned on will be described with reference to FIG.

FIG. 11 shows the relationship between the charging voltage of the capacitor C1 and the time in FIG. 1, and the curve A shows the microprocessor 17
When the port O3 of the device is "L", that is, in the low logic state (transistor Q1 is off), the capacitor C1 is connected to the resistors R6 and R6.
7 shows the charging characteristic of being charged to the reference voltage VREF of the AD converter 5. A curve B shows the charging characteristics of the capacitor when the capacitor C1 is charged with a pulse train having a duty ratio determined by a certain film sensitivity.

In curve B, for example, the voltage equivalent to film sensitivity ISO100
It takes time to reach 1.342V. In other words, when the power switch S100 is turned on and a pulse with a duty ratio according to the set film sensitivity ISO100 is output from the port O3, it takes time TO until the charging voltage of the capacitor C1 reaches 1.342V. Will be costly.

On the other hand, looking at curve A, the time until the charging voltage reaches the voltage corresponding to the same film sensitivity ISO 100 is TX,
<It becomes a relationship of TO. This depends on the charging time constant that time TX is determined by resistors R6 + R7 and capacitor C1, while time TO is the charging time constant determined only by resistors R6 + R7 and capacitor C1 and resistor R7 when transistor Q1 is on. And it is nothing but because it is determined by the balance with the discharge time constant determined by the capacitor C1.

-Second embodiment-The same as the first embodiment except that the subroutine shown in FIG. 10, the input channel CH4 of the microprocessor 17 and its connection are not provided. Therefore, the second embodiment will be described below. Only the subroutine will be described.

In the first embodiment, the charging voltage of the capacitor C1 is converted into the AD converter 3
However, in the second embodiment, the curve A in FIG. Paying attention to what is represented by, the capacitor C1 and the resistors R6 and R7
If the value of is constant, the charging voltage VC1 of the capacitor C1 depends only on the time t. Therefore, the charging voltage of the capacitor C1 is indirectly monitored depending on the time. As shown in FIG. 7, an operational amplifier compatible with film sensitivity
Since the input voltage of A4 is fixed, instead of monitoring the charging voltage of the capacitor C1 shown by the curve A in FIG. 11 with the AD converter, the time elapsed after the microprocessor 17 started operating The charging voltage of the capacitor C1 can be indirectly checked by monitoring with a timer.

Here, when C1 = 0.22 μF, R6 = 4.7 KΩ, and R7 = 75 KΩ, the relationship between the film sensitivity and the charging time is obtained from the equation (19), and is as shown in FIG. In Fig. 12, ISO3
The theoretical value for a film sensitivity of 200 is 1.520V,
In this case, according to the equation (19), it takes an infinite time to converge to 1.520V, so here, the charging time is calculated using 1.517V. That is, 0.003V (1.520-1.517)
Is a voltage equivalent to 1 / 12EV with respect to ISO3200, but in practice, an error of about 1 / 12EV has a small effect on exposure and can be ignored.

The flow chart of the second embodiment will be described with reference to FIG. FIG. 13 corresponds to the subroutine of FIG.

First, in step S41, the first pass flag 2 for identifying whether or not this routine is the first time is set. In step S42, the state of flag 2 is detected, and if it is the first pass, the process proceeds to step S43, and if it is the second time or later, it jumps to step S47 to reset the flag 2. At step S43, the port O3 of the microprocessor 17 is set to "L" to turn off the transistor Q1 shown in FIG. In step S44, the timer in the microprocessor 17 is set to the time TS corresponding to the set film sensitivity. In step S45, the timer is started, and in step S46, it is determined whether or not the timer time T reaches the set time TS, and if a positive determination is made, step S47
Then, the flag 2 is turned off and the process returns to step S5 of the main routine of FIG.

In the second embodiment, the channel of the microprocessor 17 is used.
CH4 is unnecessary, and DA converter 5 and its channel C
The connection with H4 is also unnecessary, and the circuit is simplified as compared with the first embodiment.

In the above explanation, the low-level initial signal was supplied to the base of the transistor Q1 when the power was turned on, but a pulse train with an off time t1 longer than the off time tO in the pulse train with the duty ratio corresponding to the set film sensitivity is used. It may be supplied to the transistor Q1 as an initial signal. Also,
The information for DA conversion is not limited to the film sensitivity information,
Other exposure information, such as the open F value, aperture value, aperture value, etc., is also included.

According to the present invention, when the power switch of the camera is turned on, the PW
The initial signal that shortens the capacitor charging time of the filter means provided in the M type DA converter is temporarily supplied to the DA converter instead of the pulse train, so the capacitor charging time is shortened and the power switch is turned on. Time transient response is improved.

[Brief description of drawings]

FIG. 1 is an exposure control circuit diagram showing an embodiment of the present invention, and FIG.
FIG. 4 is a block diagram thereof, FIG. 3 is a diagram showing a relationship between a switch state of the film sensitivity setting circuit and film sensitivity, FIG. 4 is a diagram showing 8 bit data corresponding to film sensitivity, and FIG.
Fig. 6 is a waveform diagram for explaining the operation of the PWM DA converter, Fig. 6 is a waveform diagram showing an example of DA conversion output, Fig. 7 is a diagram showing analog voltage corresponding to film sensitivity, and Fig. 8 is A flow chart showing an example of a main routine of the exposure control program, FIG. 9 is a flow chart showing an example of a pulse train generation routine, FIG. 10 is a flow chart showing an example of an initial control routine, and FIG. 11 is charging of the capacitor C1. Figure showing characteristics, Figure 12 shows charging time against film sensitivity, Figure 13 is a flow chart showing another example of the initial control routine, and Figure 14 is a circuit diagram showing an example of a PWM DA converter. Is. 1 ... Temperature proportional voltage generation circuit, 3 ... AD converter, 5 ...
DA converter, 7 ... Photometric circuit, 11 ... Exposure factor setting circuit,
15 …… Film sensitivity setting circuit, 17 …… Microprocessor, 19 …… Magnet drive circuit, A1 ～ A4 …… Operational amplifier, R1 ～ R10 …… Resistor, C1 ～ C2 …… Capacitor, PD ……
Photodiodes, VR1, VR2 ... Variable resistors, D1-D6 ...
… Diodes, Q1 to Q3 …… Transistors, MG1, MG2 ……
Magnet, SW1 to SW3 ... Switch, S100 ... Power switch.

Claims (3)

[Claims] 1. An exposure control circuit of a camera for converting exposure information in the form of a digital value into an analog value and performing exposure control using at least the analog value, comprising: (a) a pulse train having a duty ratio corresponding to the exposure information. And (b) a filter means composed of a capacitor and a resistor, which modulates the supplied reference voltage into an analog value according to the exposure information according to the duty ratio of the pulse train, And a digital-analog conversion means for extracting a modulated analog signal, and (c) when the power switch of the camera is turned on, the time is shorter than the time required to charge the capacitor with the pulse train according to the set exposure information. In order to charge the capacitor, an initial signal consisting of a DC voltage or a specified pulse train is used for the filter operation. Exposure control circuit of the camera, characterized by comprising the initial control means for supplying, to the. 2. A signal level of a modulated analog signal taken out from the digital-analog conversion means when the initial signal is supplied to the filter means, wherein the initial control means has the signal level corresponding to the set exposure information. Claim: When the pulse train is supplied to the filter means, if the signal level of the modulated analog signal taken out from the digital-analog conversion means matches, the supply of the initial signal to the filter means is stopped. The exposure control circuit of the camera described in the section 1 of the above. 3. The camera according to claim 1, wherein the initial control means changes the time for supplying the initial signal to the filter means in accordance with the set exposure information. Exposure control circuit.