JPS626214B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an exposure control device for a camera, and more particularly to an exposure control device for a direct metering type camera.

In recent years, automatic exposure single-lens reflex cameras have been using a direct metering method. In the direct photometry method, after the shutter is released, reflected light from the front shutter curtain and the film is converted into a photocurrent by a light receiving element, and this photocurrent is integrated to calculate the amount of received light, that is, the amount of exposure. Here, when the front shutter curtain is running, the light reflected from the front shutter curtain and the film is measured, so the difference in reflectance between the two (the front shutter curtain has a lower reflectance than the film) Accurate amount of received light cannot be calculated. Conventionally, therefore, a random number pattern is formed on the leading shutter curtain to make the reflectance of the leading shutter curtain and the film the same. However, due to variations in the reflectance of the film for each frame, it is not possible to make the reflectances of both frames completely the same.

The object of this invention is to memorize the subject brightness immediately before the front shutter curtain starts running, without using a random number pattern, and to increase the difference between this stored brightness and this photocurrent by using the photocurrent from the light receiving element while the shutter front curtain is running. Then, by amplifying the photocurrent with an amplifier that has a decreasing amplification factor characteristic and keeping the photocurrent after amplification constant, the photometry error due to the difference in reflectance between the shutter front curtain and the film, which was a drawback of the conventional direct photometry method, can be corrected. An object of the present invention is to provide an exposure control device for a camera that can control the exposure of a camera.

According to this invention, the output of the light receiving element (logarithmically compressed signal) is stored in the first sample and hold circuit immediately before the front shutter curtain starts running, and the amplification factor of the amplifier logarithmically expands the output of the light receiving element while the front shutter curtain is running. is decreased as the difference signal between the stored value of the first sample and hold circuit and the output of the light receiving element increases, so that the logarithmically expanded signal becomes constant, and this difference signal is held in the second sample and hold circuit at the end of the shutter front curtain run. The amplification factor of the amplifier becomes constant after the shutter leading curtain ends running, and the logarithmically expanded signal changes as the output of the light receiving element changes.

An embodiment of a camera exposure control device according to the present invention will be described below with reference to the drawings.

First, the principle of the present invention will be explained with reference to FIGS. 1 to 3. When measuring the brightness of a subject, light incident on the camera is reflected by the front shutter curtain and film, and then received by a light receiving element, such as a silicon photocell (SPD). SPD generates a photocurrent depending on the amount of light it receives. The photocurrent is amplified by a photocurrent amplification circuit, then integrated by an integration circuit and output as a photometric signal. By the way, if the brightness of the subject is constant, the input current to the integrating circuit needs to be constant even when the front shutter curtain is running, but due to the difference in reflectance between the front shutter curtain and the film, The photocurrent of the SPD changes as shown in Figure 1 depending on the amount of travel of the curtain. In other words, the SPD receives the reflected light from only the front shutter curtain from the time the shutter is released until time t ₀ , and from time t ₀ to t ₁ it receives the reflected light from the shutter front curtain and the film surface.
After t ₁ , only reflected light from the film is received.
Therefore, the photocurrent amplification factor of the photocurrent amplification circuit is
When changed as shown in the figure, the input current to the integrating circuit (referred to as integral current) becomes constant as shown in FIG.

FIG. 4 is a circuit diagram showing an embodiment of the camera exposure control device according to the present invention. An anode and a cathode of the SPD 10 as a light receiving element are connected to an inverting terminal and a non-inverting terminal of an operational amplifier 12, respectively. The cathode of SPD10 is grounded and fixed at zero bias. operational amplifier 1
The output terminal of 2 is connected to the emitters of NPN transistors 14 and 16, and is connected to the N-channel field effect transistor as an analog switch.
It is connected to the non-inverting terminal of the operational amplifier 20 via the source-drain path of the FET 18. The base of transistor 14 is grounded, and the collector is connected to the inverting terminal of operational amplifier 12. Transistor 16 has its base and collector shorted and acts as a diode. operational amplifier 12, transistor 14,
16 constitutes the first logarithmic amplifier circuit 1.

The gate of FET 18 is connected to the source via resistor 22 and to the anode of diode 24. The cathode of the diode 24 is connected to a fixed contact of the switch 26.
The first movable contact of is connected to the positive terminal Vcc of the power supply,
Its second movable contact is connected to the negative terminal of the power supply -Vcc. A non-inverting terminal of the operational amplifier 20 is grounded via a capacitor 28. The output terminal of the operational amplifier 20 is connected to the inverting input terminal of the operational amplifier 20 and is also connected to the inverting input terminal of the operational amplifier 32 via a resistor 30 . FET18, operational amplifier 2
0, resistor 22, diode 24, switch 26,
The capacitor 28 constitutes the first sample and hold circuit 2.

The output terminal of the operational amplifier 12 of the first logarithmic amplifier circuit 1 is connected to the non-inverting input terminal of the operational amplifier 32 via a resistor 34 . Further, a non-inverting input terminal of the operational amplifier 32 is connected to the first constant voltage terminal V _R1 via a resistor 36. The output terminal of the operational amplifier 32 is a resistor 38
It is connected to the inverting input terminal of the operational amplifier 32 via the inverting input terminal of the operational amplifier 32, and to the non-inverting input terminal of the operational amplifier 44 via the source-drain path of an N-channel FET 40 serving as an analog switch. The operational amplifier 32 and the resistors 30, 34, 36, and 38 constitute the subtraction circuit 3.

The base of FET 40 is connected to the source via resistor 44 and to the anode of diode 46. The cathode of the diode 46 is connected to a fixed contact of the switch 48.
The first movable contact of is connected to the positive terminal Vcc of the power supply,
Its second movable contact is connected to the negative terminal -Vcc of the power supply. A non-inverting input terminal of the operational amplifier 44 is grounded via a capacitor 50. The output terminal of the operational amplifier 44 is connected to the inverting input terminal of the operational amplifier 42 and also to the inverting input terminal of the operational amplifier 52. FET40, operational amplifier 42, resistor 44,
Diode 46, switch 48, capacitor 50
constitutes the second sample and hold circuit 4.

The output end of the operational amplifier 52 is connected to the cathode of a diode 54, and the anode of the diode 54 is connected to the gate of an N-channel FET 56.
The drain of the FET 56 is connected to the non-inverting input terminal of the operational amplifier 52 and to the emitter of the NPN transistor 58. The base and collector of the transistor 58 are short-circuited to function as a diode, and the collector is connected to the second constant voltage terminal V _R2 . The source of the FET 56 is connected to the base (collector) of the transistor 16 of the first logarithmic amplifier circuit 1. operational amplifier 52, diode 54,
FET 56 and transistor 58 constitute constant current circuit 5.

The base (collector) of transistor 16 is connected to the non-inverting input terminal of operational amplifier 60. The output terminal of the operational amplifier 60 is an NPN type transistor 62,
Connected to 64 emitters. transistor 62
The base and collector of are short-circuited and further connected to the inverting input terminal of the operational amplifier 60. Further, a constant current source terminal I _R is also connected to the inverting input terminal of the operational amplifier 60. The base of transistor 64 is grounded, and the collector is connected to the inverting input terminal of operational amplifier 66. 60 operational amplifiers, 6 transistors
2 and 64 constitute the second logarithmic amplifier circuit 6.

The output terminal of the operational amplifier 66 is connected to the inverting input terminal of the operational amplifier 66 via a capacitor 68, and the non-inverting input terminal thereof is grounded. A switch 70 is connected across the capacitor 68. The operational amplifier 66, the capacitor 68, and the switch 70 constitute the integrating circuit 7. The output terminal of the operational amplifier 66 is connected to the shutter control circuit 9 via the determination circuit 8.

Next, the operation of the camera exposure control device configured as described above will be explained. The output current of the constant current circuit 5 is I ₅ , and the constant current supplied from the constant current source terminal I _R is I ₆
Then, the photocurrent I ₁ of the SPD 10 is the first and second
Amplified by logarithmic amplifier circuits 1 and 6, and integrated circuit 7
The integrated current _I7 input to can be expressed as follows.

I ₇ =I ₆ /I ₅ I ₁ (1) Therefore, the integral current I ₇ can be controlled by controlling the output current I ₅ of the constant current circuit 5.

In other words, while the front shutter curtain is running, the integral current I _7
I5 is controlled to keep the value just before the front shutter curtain runs, and when the front shutter curtain finishes running completely, _I5 is kept constant so that the integrated current _I7 changes depending on the brightness of the subject. Feedback may be applied to the constant current circuit 5 so that the current is maintained.

First, in the initial state, the switches 26 and 48 are connected to the positive terminal Vcc side of the power supply as shown in Fig. 5a and b, so they function as analog switches.
FETs 18 and 40 are conductive. Further, the switch 70 is closed as shown in FIG. 5c.
Therefore, the sample and hold circuits 2 and 4 are in the sampling period, and the output voltage Vo ₁ of the operational amplifier 12 is
is the operational amplifier 20 of the first sample and hold circuit 2
Since it is supplied to the non-inverting input terminal of the operational amplifier 2
0 output voltage Vo ₂ is equal to Vo ₁ . The operational amplifier 20 is connected to the inverting input terminal of the operational amplifier 32 of the subtraction circuit 3.
Since the output voltage Vo ₂ of the operational amplifier 12 is supplied to the non-inverting input terminal of the operational amplifier 32, the resistors 34, 36, 30 _, 3
_{Assuming that the values of _ _}
(V). Since the second sample and hold circuit 4 is also in the sampling period, the input voltage Vo ₃ of the second sample and hold circuit 4 becomes the output voltage Vo ₄ as it is.
is output as Therefore, Vo ₄ = Vo ₃ = V _R
1 (V). Since the emitter potential of the transistor 58 is Vo ₄ , if its collector potential is _VR2 , the output current I ₅ of the constant current circuit 5 is expressed as follows based on diode rectification theory.

I ₅ =Is ₁ expq (V _R2 −Vo ₄ )/kT (3) where Is ₁ is the reverse saturation current of the transistor 58. Therefore, the initial value I ₅ ' of the output current of the constant current circuit 5 becomes Is ₁ expq (V _R2 −V _R1 )/kT, which is constant.

Immediately before the shutter is released and the leading shutter curtain is run, the switch 26 of the first sample and hold circuit 2 is connected to the negative terminal of the power supply as shown in FIG. 5a.
Switched to Vcc side. This is switched, for example, in conjunction with the completion of the mirror's lifting. For this reason,
The FET 18 is made non-conductive, the first sample and hold circuit 2 enters the hold period, the initial value Vo ₁ ' of the output voltage Vo ₁ of the operational amplifier 12 is held in the capacitor 28, and the output voltage Vo ₂ of the operational amplifier 20 is
Vo ₁ ′. Then, when the front shutter curtain is run, the photocurrent _I1 of the SPD 10 gradually increases even if the brightness of the subject does not change. The initial value Vo ₁ ' of the output voltage of the operational amplifier 12 can be expressed as follows, assuming that the reverse saturation current of the transistor 14 is Is ₂ .

Vo ₁ '=-kT/qln (I ₁ /Is ₂ )...(4) Also, the output voltage Vo ₁ of the operational amplifier 12 when the photocurrent increases by ΔI ₁ can be similarly expressed as follows. .

Vo ₁ =-kT/qln (I ₁ +ΔI ₁ /Is ₂ ) (5) Therefore, while the front shutter curtain is running, the output voltage Vo ₃ of the subtraction circuit 3 is as follows.

V ₀₃ =V ₀₁ -V ₀₁ '+V _R1 =-kT/qln(I ₁ +ΔI ₁ /Is ₂ ) +kT/qln(I ₁ /Is ₂ )+V _R1 =kT/qln(I ₁ /I ₁ +ΔI ₁ ) +V _R1 ...
(6) At this time, since the second sample and hold circuit 4 is still in the sampling period, the output voltage Vo ₄ of the second sample and hold circuit 4 is also kT/qln (I ₁ /I ₁ +
ΔI ₁ )+V _R1 . Therefore, the output current of constant current circuit 5
I ₅ can be expressed as follows from equation (3).

I ₅ = I _S1 expq (V _R2 - V ₀₄ )/kt = I ₅ '・I ₁ + ΔI ₁ / I ₁ ...(7) Therefore, the integrated current I when the photocurrent I ₁ increases by ΔI _{1 7} becomes as follows by substituting equation (7) into equation (1).

That is, even when the front shutter curtain is running, the integrated current _I7 is equal to the value immediately before the front shutter curtain runs, and so-called memory photometry is performed.

Next, when the front shutter curtain has completely traveled,
The switch 48 of the second sample and hold circuit 4 is switched to the negative terminal of the power supply -Vcc as shown in FIG. 5b. Therefore, the FET 40 is rendered non-conductive, the second sample and hold circuit 4 enters a hold period, and the output voltage Vo ₃ of the subtraction circuit 3 immediately before the shutter front curtain ends running is held in the capacitor 50. Therefore, from this point on, the output current I ₅ of the constant current circuit 5 remains constant, and the integrated current I ₇ changes depending on the brightness of the subject. Since the light receiving element 10 does not receive the reflected light from the entire surface of the film, but only a certain portion of the reflected light, the switch 70 for starting integration is set as shown in FIG. 5c. It is only necessary that the front shutter curtain be opened from the time the front shutter curtain starts running until before the front shutter curtain reaches the light receiving range on the film surface. When the switch 70 is opened, the capacitor 68 starts charging.
So-called direct photometry is performed. and,
When this charging voltage, that is, the output voltage of the operational amplifier 66 reaches a predetermined voltage, the determination circuit 8 supplies a signal to the shutter control circuit 9 to release the shutter trailing curtain and terminate the shutter operation. This results in
Switches 26, 48, and 70 are reset to their initial states.

As described above, according to this embodiment, by controlling the amplification factor of the amplifier that amplifies the output of the light receiving element in conjunction with the running of the shutter leading curtain, it is possible to control the gain while the shutter leading curtain is running without using a random number pattern. It is possible to correct the error in the integrated current due to the difference in reflectance between the shutter leading curtain and the film. Further, since the amount of feedback to the constant current circuit 5 for generating the control current for keeping the integral current constant changes in an analog manner depending on the brightness of the subject, there is little error.
Further, since the circuit configuration is not complicated, the cost is not so high.

Note that mechanical switches may be used instead of the FETs 18 and 40. In addition, if long-term exposure is required, the capacitor becomes large, which is inconvenient, so the second sample and hold circuit 4 is used.
may be changed as shown in FIG. That is,
The output signal of the subtraction circuit 3 may be supplied to the A/D converter 72, stored as a digital quantity, and later converted into an analog signal via the D/A converter 74 and taken out. A terminal 76 is supplied with a control signal for energizing the A/D converter 72 and for holding the output signal of the subtraction circuit 3 immediately before the shutter front curtain is fully opened. Here, the resolution of the A/D and D/A converters influences the photometry error, but the output voltage of the subtraction circuit 3 is the difference between the amount of light received just before the shutter front curtain runs and the amount of light received while the shutter is running, so Since it is not large, there is no need to increase the resolution that much. For example, if the light reflected from the front curtain of the shutter and the light reflected from the film are exposed,
Even if the steps are different, the output voltage Vo ₃ of the subtraction circuit 3
is Vo ₃ = -kT/qln2 ³ I ₁ /I ₁ ≒ -54 (mV), so even if the resolution is 4 (mV), A/
The D, D/A converter only needs to have at most 4 bits.

Furthermore, another modified example of the second sample and hold circuit 4 for further reducing measurement errors due to A/D conversion is shown in FIG. A/D converter 72,
Although the use of the D/A converter 74 is the same,
The output signal of the subtraction circuit 3 is supplied to the constant current circuit 5 via the switch 78, and the output signal of the D/A converter 74 is supplied to the constant current circuit 5 via the switch 80. The switches 78 and 80 are opened and closed at timings before and after the front shutter curtain is fully opened, respectively, as shown in FIGS. 8a and 8b. That is, since it is not necessary to hold the output signal of the subtraction circuit 3 until the front shutter curtain is fully opened, the switch 78 is closed and the output signal of the subtraction circuit 3 is supplied as is to the constant current circuit 5. After the front shutter curtain is fully opened, the switch 80 is closed, and the output signal of the subtraction circuit 3 stored as a digital quantity is supplied to the constant current circuit 5.

As explained above, according to the present invention, by controlling the amplification factor of the amplifier that amplifies the output of the light-receiving element in conjunction with the running of the shutter leading curtain, the reflection of the shutter leading curtain and the film can be achieved without using a random number pattern. It is possible to provide an exposure control device for a camera that can correct the difference in rate and perform accurate direct photometry. Furthermore, since the second sample and hold circuit 4 holds the output of the subtraction circuit 3 when the leading shutter curtain has finished running, it is possible to control the light emission even for a strobe device that emits light after the leading shutter curtain has finished running.

[Brief explanation of the drawing]

1 to 3 are diagrams for explaining the principle of the invention, FIG. 4 is a circuit diagram showing an embodiment of the camera exposure control device according to the invention, and FIG. 5 is a time chart showing its operation. , FIG. 6 is a diagram showing a modification of the present invention, FIG. 7 is a diagram showing still another modification, and FIG. 8 is a time chart showing its operation. DESCRIPTION OF SYMBOLS 1... First logarithmic amplifier circuit, 2... First sample hold circuit, 3... Subtraction circuit, 4... Second sample hold circuit, 5... Constant current circuit, 6... Second logarithmic amplifier circuit, 7 ...integrator circuit.

Claims (1)

[Claims] 1: a light receiving means for receiving reflected light from the shutter front curtain and the film surface and generating a photocurrent according to the amount of received light; a logarithmic compression means for converting the photocurrent into a logarithmically compressed voltage; a first sample and hold means for storing an output voltage before the shutter front curtain starts running; and a difference between a stored value of the first sample and hold means and an output voltage value of the logarithmic compression means, and a difference voltage signal is output. a subtracting means for inputting the output of the subtracting means, and a second sample holding means for switching from a sampling operation to a holding operation at the end of running of the shutter leading curtain; 2. An amplification factor characteristic is defined such that it decreases as the output of the sample hold means increases, and includes an amplification means for amplifying the output voltage of the logarithm compression means to a logarithm expansion current by the amplification factor; and a shutter control means for starting the running of the rear shutter curtain based on the output of the integrating means, and changing the amplification factor until the running of the leading shutter curtain is completed. An exposure control device for a camera, characterized in that the logarithmic expansion current is kept constant.