JPH0628401B2 - Automatic focusing device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention detects a high frequency component of a video signal obtained from an image pickup device of a television camera and drives a lens focus matching device so as to maximize its amplitude. The present invention relates to an automatic focusing device.

Configuration of Conventional Example and Problems Thereof In recent years, television cameras have been rapidly spread along with portable video tape recorders, and an auto-focusing device is supposed to simplify the operation of the television camera and promote its further spread. Is expected.

As a conventional automatic focus adjustment device, it emits infrared rays toward a subject, receives the infrared rays reflected from the subject, measures the distance to the subject by the light receiving angle, and positions the lens focus adjustment device at a predetermined position according to the measured distance. There is a method to drive to.

Although the above method has a high response speed, the lens focus matching device requires high assembly accuracy in order to accurately measure the distance to the subject and drive the lens focus matching device in accordance with the measured distance. To be done.

There is also a method of constructing the focus matching device in a feedback loop of an autofocus system in order to make the assembly accuracy of the lens focus matching device rough. That is, the subject image formed on the light receiving surface of the image sensor is periodically and optically blurred,
This is a method in which a signal corresponding to the blur is detected and the lens focus matching device is driven so that the amplitude of the high frequency component of the video signal obtained from the image pickup element is maximized.

A conventional example in which a lens focus matching device is configured in a feedback loop of an autofocus system will be described below. FIG. 1 is a block diagram showing the configuration of the automatic focus adjusting device.
Reference numeral 1 is a lens, 2 is an image pickup tube that converts optical information from a subject into an electric signal, 3 is a preamplifier that amplifies an electric signal obtained from the image pickup tube 2, 4 is gamma correction, and blanking (BL
K) processing, processing circuit for performing addition of synchronization signal (SYNC), etc. to obtain a television signal, and 5 includes a synchronization signal (SYNC), a blanking signal (BLK),
It is a synchronization signal generation circuit for supplying a vertical drive signal (VD), a horizontal drive signal (HD), and the like. Reference numeral 6 is a deflection circuit for scanning the electron beam of the image pickup tube 2, and 7 is a circuit for detecting a high frequency component of the video signal output from the image pickup tube 2 through the preamplifier 3, for example, a bandpass filter having a center frequency of 1 MHz. Is. Reference numeral 8 denotes a gain control circuit, which equivalently controls the input dynamic signal of the detector 9 by gradually controlling the gain of the output signal of the high frequency component detection circuit 7 according to the output of the detector 9 detected by the level detection circuit 10. Expanding the range. Reference numeral 11 is a sample and hold circuit, and 14 is a reference frequency generation circuit, which generates a signal for finely changing the focus by finely moving the lens focus matching device by means of the motor 16. Reference numeral 12 is a circuit for detecting the reference frequency component from the high frequency component signal of the video signal, and the phase and amplitude of the reference frequency component is detected by the synchronous detection circuit 13 and applied to the motor drive circuit 15 and output from the image pickup tube 2. The motor 16 is driven so that the amplitude of the high frequency component of the video signal is maximized.

The operation of the conventional automatic focus matching device configured as described above will be described below with reference to the characteristic diagram of FIG.

The light from the subject that enters the lens 1 forms an image, and the image pickup tube 2
And is photoelectrically converted by and output as a video signal. This video signal is amplified by the preamplifier 3 and the high frequency component detection circuit 7
Thus, only the high frequency component is extracted. The curve in FIG. 2 shows the amplitude characteristic of the high frequency component of the video signal when the subject is at a distance D ₁ from the lens position. If the position of the focus matching device of the lens 1 matches the distance between the lens 1 and the subject, the amplitude of the high frequency component becomes maximum, and it decreases as it deviates from this position. Therefore, the focus adjustment device of the lens may be driven to a position where the amplitude of the high frequency component of the video signal becomes maximum.

The high frequency component of the video signal extracted by the high frequency component detection circuit 7 is gain-controlled by the gain control circuit 8 and
The signal is detected by the detector 9, input to the sample hold circuit 11, and sampled and held at regular time intervals. This output signal includes the change component because the focus is slightly changed. The reference frequency component detection circuit 12 extracts the change component, that is, the reference frequency component, the synchronous detection circuit 13 detects the phase and amplitude of the reference frequency component, and applies it to the motor drive circuit 15 to determine the amplitude of the high frequency component of the video signal. The motor 16 is driven so that it becomes maximum.

In FIG. 2, the reference frequency component becomes a waveform signal such as b ₁ when the lens focus matching device is displaced toward the near distance side, and a waveform signal such as b ₂ is displaced when toward the far distance side. Becomes Since the motor is driven in the c ₁ direction by the signal obtained by synchronously detecting the signal of b ₁ and the motor is driven in the c ₂ direction by the signal obtained by synchronously detecting the signal of b ₂ , the amplitude of the high frequency component of the video signal is maximum. The focus adjustment device of the lens is driven to the position where

However, in low-illuminance shooting and when shooting a low-contrast subject, the amplitude characteristics of the high-frequency component of the video signal become as shown by the curve (b) in Fig. 2, and the amplitude of the reference frequency component becomes extremely small. However, since the amount of change is also small, the circuit falls outside the dynamic range of the circuit, and a sufficient sample and hold voltage cannot be obtained for accurate and stable focusing operation. Therefore, a sufficient sample hold voltage is obtained by increasing the gain of the gain control circuit 8. However, the loop gain becomes higher and the S / N is deteriorated as compared with the case of normal shooting, so that the motor is easily shaken by noise. Therefore, the motor is shaken at the time of focusing, the image is shaken, and the stability is deteriorated.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned conventional problems and improve focusing stability at low illuminance and low contrast.

According to the present invention, a subject image formed on a light receiving surface of an image sensor is periodically and optically blurred, and the amount of blur is detected to maximize the high frequency component of a video signal. In a system for driving a focus matching device, the gain increased due to the limitation of the dynamic range of the circuit during low-illuminance shooting and when shooting a low-contrast object is equivalent to the entire system by reducing the detection gain of the reference frequency component. This is an automatic focusing device that has improved stability by preventing the loop gain from becoming extremely high.

Description of Embodiments FIG. 3 is a block diagram showing the configuration of an automatic focusing device in one embodiment of the present invention. 1 to 16 in FIG.
1 performs the same operation as in FIG. 1, but the first gain control circuit 8 controls the gain at both the detection level of the luminance component and the detection level of the high frequency component. The luminance component detection circuit 17 is an LPF for detecting the luminance component of the video signal. Reference numeral 18 is a detection circuit, 19 is a second level detection circuit, and 20 is a second gain control circuit, which perform the same operations as the detection circuit 9, the first level detection circuit 10, and the first gain control circuit 8, respectively. The control circuit 21 controls the second gain control circuit 20.

The operation of the automatic focusing device of this embodiment constructed as described above will be described below.

When photographing a low-illuminance object and a low-contrast object, the high-frequency component of the video signal decreases as shown by the curve (b) in FIG. Utilizing this, it is detected that the illuminance is low and the contrast is low. That is, the output level of the detection circuit 9 is detected by the first level detection circuit 10, the output signal controls the gain of the first gain control circuit 8, and when the gain exceeds a certain value, the control circuit 21 is operated. Second
By controlling the gain of the gain control circuit 20, it is possible to prevent the loop gain of the system from becoming extremely high. Also,
When the illuminance is low, the luminance component of the video signal also decreases, so that it can be detected that the illuminance is low.
That is, the luminance component detection circuit 17 extracts the luminance component of the video signal, and the detector 18 detects it. If the detected output is below a certain value, the gain of the second gain control circuit 20 is controlled through the control circuit 21. This suppresses an extreme increase in the system loop gain and improves stability. In this case, the response speed will be slower, but when shooting in low-illuminance and when shooting a low-contrast subject, the degree of blurring will not appear more clearly than in normal shooting, so even if the response speed is a little slow, it will not be unpleasant, and noise will cause Is more problematic. In the embodiment of FIG. 3, the gain control is performed by the amplitude of the reference frequency component, but the motor drive voltage or the width of the motor drive pulse may be directly controlled.

As described above, the automatic focus matching device of the present invention is provided with the luminance component detection circuit, detects the level of the high frequency component and the level of the luminance component of the video signal, and when these levels are below a certain value. By suppressing an extreme increase in the loop gain of the system, it is possible to improve stability during low-illuminance shooting and during focusing when shooting a low-contrast subject.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a conventional example of an automatic focusing device, FIG. 2 is a diagram for explaining a principle of detecting a motor driving direction, and FIG. 3 is an automatic focusing in one embodiment of the present invention. It is a block diagram which shows the structure of a matching device. 1 ... Lens, 2 ... Image pickup tube, 7 ... High frequency component detection circuit, 8 ... First gain control circuit, 9 ... Detection circuit (high frequency component), 10 ... First level detection circuit (high frequency component) ,
11 ... Sample hold circuit, 12 ... Reference frequency component detection circuit, 13 ... Synchronous detection circuit, 14 ... Reference frequency generation circuit, 16 ... Motor, 17 ... Luminance component detection circuit, 18 ... Detection circuit ( Luminance component), 19 ... Second level detection circuit (luminance component), 20 ... Second gain control circuit,
21 ... Control circuit.

Claims (1)

[Claims] 1. An optical path length varying means for periodically varying an optical path length from a lens to an image pickup device according to a reference frequency, an image pickup device for converting a subject image formed on a light receiving surface into an electric signal, and the image pickup device. A high frequency component detection circuit for detecting a high frequency component from an electric signal output from the element, and a reference frequency component detection circuit for extracting a reference frequency component serving as a reference in the moving direction of the lens from the output of the high frequency component detection circuit. A first level detection circuit that detects the amplitude of the high frequency component, a first gain control circuit that controls the gain of the high frequency component, and a second gain control circuit that controls the gain of the reference frequency component.
Gain control circuit, a control circuit for sending a control signal to the second gain control circuit, a brightness component detection circuit for detecting a brightness component from an electric signal output from the image sensor, and an amplitude of the brightness component. A second level detection circuit for detecting, a reference frequency generation circuit for generating a reference frequency signal, a synchronous detection circuit for detecting the amplitude of the reference frequency component from the reference frequency signal and the reference frequency component, and the synchronous detection circuit Moving means for moving the position of the lens based on the output of the first level control circuit, and when the output of the first or second level detection circuit decreases, the first gain control circuit controls the first level Automatically characterized by increasing the gain of the detection circuit and reducing the gain of the system loop by the second gain control circuit to reduce the operation speed of the moving means. Point matching device.