GB2091431A - Exposure control system for a photographic or television camera - Google Patents

Abstract

A pick up element 11 receives light passing through a lens 9 and a diaphragm 10 and converts same to an electrical signal which is amplified by an amplifier 12 and then fed to a level detector 13. A comparator 14 compares the signal level detected by the detector 13 with a reference signal set by a potentiometer, and the diaphragm 10 is adjusted in accordance with the difference between the two signals to achieve automatic exposure control. A hold circuit 16 is operable by means of a switch 17 between a first mode wherein it merely passes onto the comparator 14 the signal from the detector 13 and a second mode in which it holds the signal level from the detector 13 when the switch 17 is activated, thereby locking the exposure. When the exposure is thus locked, manual adjustment of the diaphragm 10 can be performed by means of a potentiometer. The invention is also applicable to ASC exposure systems (Figure 5, not shown). <IMAGE>

Description

SPECIFICATION Exposure control system for a camera This invention relates to an exposure control system for a camera, such as a video camera, a television camera or the like.

Some video camera and the like are provided with an automatic exposure control system so that correct exposure is obtained at all times, and in many cases this automatic exposure control system is quite useful. In some cases, however, such as when the image of a subject to be photographed is varied in size and in position, e.g.

by zooming, a correct exposure sometimes cannot be obtained because operation of the exposure control system is adversely affected by the conditions of light around the subject. More particularly, for example, if a picture of a subject such as a person is taken against the light such that the image of the subject fills the picture, and then the image is contracted by zooming, a brighter background will appear around the subject. The exposure circuitry will then react to the brighter background, as a result of which the subject will appear darker. If, in this case, zooming can be carried out while maintaining the initial exposure value, then the above-described trouble can be eliminated.

Such locking of the exposure can be achieved in the conventional exposure control system shown in Figs. 1 and 2 of the accompanying drawings. Figure 2 is a block diagram showing an automatic iris arrangement in a television camera, while Fig. 1 shows in detail an automatic diaphragm mechanism 1 0. Referring to Fig. 2 light passing through a lens 9 forms an image on a surface of an image pickup element 11 and is converted into an electrical signal thereby. The electrical signal is amplified to the necessary level by a video amplifier circuit 12. The magnitude of the signal thus amplified is detected by a level detector circuit 13 and is applied to a comparison amplifier circuit 14, where it is compared with a desired exposure value, and the difference therebetween is amplified.The output of the circuit 14 is applied through a normal/lock change-over switch 1 5 to the automatic diaphragm mechanism 10, so that the diaphragm is controlled to provide the desired exposure value.

In the automatic diaphragm mechanism 10, current applied to conductors 8 (see Fig. 1) causes a drive motor 4 to provide a torque to operate aperture blades 1 of the diaphragm. An output shaft of the motor 4 is coupled to an interlocking arm 3 and a large pulley 5. The interlocking arm 3 is coupled through a pin to a driving lever 2, so that horizontal movement of the arm 3 will open or close the aperture blades 1 to control the aperture value of the diaphragm. More specifically, a voltage detected by the level detector circuit 13 is compared with the desired value in the comparison amplifier circuit 14. When, as a result of the comparison, it is determined that the voltage is larger than the desired value, the aperture blades 1 are closed. Similarly, when the voltage is smaller than the desired value, the aperture blades 1 are opened.Automatic control is thus effected to give a correct exposure at all times.

In the conventional exposure system, the exposure can be locked in the following manner.

When a correct exposure value is obtained for an object to be photographed, a small pulley 6 is moved in the direction of the arrow along with a knob 7 until the pulley 6 is brought into contact with the large pulley 5. At the same time, the switch 1 5 is opened and the knob 7 is fixed, as a result of which the exposure is locked. The knob 7 can then be rotated to run the output shaft of the motor 4, thereby enabling the diaphragm to be operated manually.

However, the conventional automatic exposure system is disadvantageous in the following respects. Mechanical parts, such as the large pulley 5, the small pulley 6 and the knob 7 for manual aperture control, are necessary for exposure locking and accordingly the position of the knob for exposure locking is limited. in addition, since use is made of the frictional force between the large and small pulley 5 and 6, the conventional system is liable to deteriorate with the passage of time.

It is an object of the present invention to obviate or mitigate the above-described problems and disadvantages.

According to the present invention, there is provided an exposure control system for a camera, comprising conversion means operative to convert light received by said camera into electrical signals, detection means operative to detect the value of said electrical signals and provide a control signal in accordance with the difference between said value and a desired value, control means operative to adjust the exposure level in accordance with the value of said control signal and thereby achieve automatic exposure correction, and holding means selectively operable by the camera operator for holding the value of said control signal and thereby interrupt said automatic exposure correction. In this way, even when the image of a subject is changed in size and position, a correct exposure can be provided for the subject at all times.

In a preferred example of the invention a voltage holding circuit is provided between the output of the level detection circuitry and the input to the comparison amplifier in a conventional exposure control system. The holding circuit is selectively operable to either pass the detector output for automatic exposure control or hold the output and thereby fix the exposure level.A manually adjustable voltage source may be used as the reference source for the comparator to enable manual adjustment of the exposure value to be performed after operation of the holding circuit The invention will now be further described, by way of example, with reference to the remaining figures of the accompanying drawing, in which: Figure 3 is a block diagram of one embodiment of an automatic exposure control system for a camera, according to the present invention; Figure 4 is a diagram of a hold circuit which forms part of the system shown in Fig. 3; and Figure 5 is a block diagram of another embodiment of an automatic exposure control system according to the invention, which is applicable to an ASC system.

Figure 3 is a block diagram of an automatic exposure control system according to the invention, wherein similar parts to those shown in Fig. 2 are designated by the same reference numerals; that is, components having the same reference numeral have the same functions in Figs. 3 and 2. However, it should be noted that the automatic diaphragm mechanism to which the invention is applied does not include the components 5, 6 and 7 shown in Fig. 1 ,.which are unnecessary. The manner in which the circuits 10 to 13 in Figure 3 process the light passing through the lens 9 is similar to that described above in relation to Fig. 2. The voltage detected by the level detector circuit 1 3 is applied to a hold circuit 1 6.

The hold circuit 1 6 has two operational modes, namely a hold mode and a simple buffer mode which are selected by means of a switch 1 7.

One example of the hold circuit 1 6 is shown in Fig. 4. The circuit 1 6 is of conventional construction and hence its operation will be only briefly described. An operational amplifier IC has a non-inverting input terminal which is used as an input terminal of the hold circuit in this case. The output of the operational amplifier IC is connected to the gate of a transistor 4 through a transistor Q1 serving as an analogue switch, the transistor 4 operating as a source foilower. The point of connection between a load resistor R3 and the source of the transistor 4 forms an output terminal of the hold circuit, and is connected to an inverting input terminal of the operational amplifier IC.

When the switch 1 7 is open, the cathode of a diode D is connected through a resistor R2 to a supply voltage B+, and therefore no current flows in the diode D; i.e. the diode D is non-conductive (off). In this case, the source and drain of the transistor Q1 will be at the same voltage, i.e. the source is conductive with the drain. Accordingly, a feed-back loop is formed from the amplifier IC through the transistors Q1, Q2 and back to the amplifier IC again. Accordingly, the input terminal and the output terminal are at the same voltage, and the hold circuit serves merely as a buffer. In the operational mode of the circuit 16, the apparatus shown in Fig. 3 operates as an ordinary automatic exposure control system.

When the switch 1 7 is closed, the cathode of the diode D is grounded. Therefore, a large reverse bias is applied to the gate of the transistor Q1, and the resistance between the source and the drain becomes considerably high. Accordingly, the charge on a capacitor C is held at the voltage level inputted immediately before the switch 1 7 was closed. The circuit 1 6 is thus in its hold mode.

If the switch 17 is closed to hold the output voltage of the circuit 1 6, then the aperture blades 1 of the automatic diaphragm mechanism 10 will be fixed, i.e. exposure locking is obtained. In other words, since the drive motor 4 operates the aperture blades 1 according to the voltage applied thereto, the aperture blades 1 can be fixed by fixing the applied voltage. Therefore, if the camera operator subsequently wishes to change the exposure value, it is only necessary to change the reference voltage applied to the comparator circuit 14 to indicate the desired exposure value, in which case the voltage applied to the motor 4 will be changed, and the aperture value will be changed also. Thus, in this manner, the aperture value can be varied manually by operation of the potentiometer shown in Fig. 3.

The invention has been described with reference to an automatic iris; however, the same effects can be obtained for a camera utilizing an ASC exposure control system by including the same hold circuit in the exposure control circuit. In the ASC system, the target voltage of a vidicon or the like is changed to change the sensitivity of the vidicon and thereby control the exposure.

Therefore, if the output voltage of the level detector circuit is held in the manner of this invention, then the target voltage of the image pickup element can be readily locked, and therefore the exposure can be locked.

One example of such a system is shown in Fig.

5. The circuit shown in Fig. 5 differs from that shown in Fig. 3 only in that a target voltage control circuit 1 8 is additionally provided. In the circuit 18, the output of a comparison amplifier circuit 14 similar to that in Fig. 3 is inverted and a voltage provided with a suitable bias is outputted.

A target voltage VT is controlled by the voltage control circuit 1 8 with the aid of resistors 19 and 20, so that a correct exposure value is obtained.

This type of operation is known and need not be explained in further detail.

As is apparent from the above description, when compared with the conventional exposure control system wherein exposure can be locked only by using mechanical components which deteriorate with the passage of time and wherein exposure locking can be performed only for an automatic iris, the exposure control system of this invention is simple in circuit arrangement and operates in a stable manner The exposure locking device of the present system is applicable not only to automatic irises but also to ASC systems; that is, the range of application of the exposure locking device is wider. Thus, the effects in practical use of the exposure control system of the invention will be appreciated.

It should be realized that various changes to the circuitry described above could be made without departing from the scope of the invention. For example, the components R1, Q1 and D in Fig. 4 are illustrative only, and any of a variety of simple switches could be employed.

Claims (8)

1. An exposure control system for a camera, comprising conversion means operative to convert light received by said camera into electrical signals, detection means operative to detect the value of said electrical signals and provide a control signal in accordance with the difference between said value and a desired value, control means operative to adjust the exposure level in accordance with the value of said control signal and thereby achieve automatic exposure correction, and holding means selectively operable by the camera operator for holding the value of said control signal and thereby interrupt said automatic exposure correction.

2. An exposure control system as claimed in claim 1 , wherein the detection means comprises a level detector which provides an output signal representing the light level of a received image and comparison means which compares said output signal of the level detector with a reference signal representing a desired signal level and which provides said control signal in accordance with the difference therebetween, and wherein the holding means comprises a hold circuit connected between the level detector and the comparison means and selectively operable to either pass the present level detector output to the comparison means or hold a previous value of said level detector output and provide said held value to the comparison means.

3. An exposure control system as claimed in claim 2, wherein the hold circuit when activated freezes its output signal value at the value of said level detector output signal existing at the time of activating.

4. An exposure control system as claimed in claim 2 or 3, further comprising means for adjusting said reference signal while the hold circuit is providing said held value to the comparison means, thereby permitting manual adjustment of said exposure level.

5. An exposure control system as claimed in claim 4, wherein the control means comprises an adjustable diaphragm.

6. An exposure control system as claimed in claim 4, wherein the conversion means includes an electronic image detector and the control means comprises means for adjusting the response characteristics of the image detector.

7. An exposure control system as claimed in any preceding claim, further comprising manually operable means for adjusting said control signal after operation of the holding means.

8. An exposure control system for a camera, substantially as hereinbefore described with reference to Figs. 3 and 4 or Fig. 5 of the accompanying drawings.