JPH06225199A - Automatic focus video camera - Google Patents

Abstract

PURPOSE:To always attain stable automatic focusing by changing a threshold level causing reactivation according to a zoom position by a zoom position detection signal from a zoom lens mechanism. CONSTITUTION:An output of a difference output generating circuit 16 which outputs an absolute value of the difference between an output of a focus motor control circuit 10 retained in a memory 11 and a current focus evaluation valve being an output from a focus evaluation detection circuit 5 is inputted to a comparator 18. Furthermore, the output of the memory 11 is inputted to a threshold value memory 17. The output of the memory 17 is changed by a signal from a zoom position detection circuit 19 and the comparator 18 compares the output of the circuit 16 with the output changed by the zoom position and when the difference output is larger than the output of the memory 17, the comparator 18 sends a reactivating signal to the circuit 10 and the circuit 10 drives a focus motor 3 and the focusing is restarted. As a result, the threshold value is automatically changed in a telescopic area, an intermediate area and a wide angle area thereby attaining minute automatic focus control.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus video camera which automatically adjusts a focus by using a video signal obtained from an image pickup device.

[0002]

2. Description of the Related Art A method of using a video signal itself from an image sensor in a focus control device of a video camera for evaluating a focus control state has essentially no parallax.
In addition, even if the depth of field is shallow or for distant subjects,
There are many advantages such as being able to focus accurately. Moreover, a special sensor for autofocus is unnecessary, and the mechanism is extremely simple.

An example of the conventional autofocus method is "NHK Technical Report" S40, Vol. 17, No. 1, Vol. 86, page 21, Ishida et al., "Automatic focus adjustment of television camera by mountain climbing servo system". In this hill climbing servo control, the focus ring of the lens is rotated so that the amount of high frequency components of the video signal is always maximized, so the lens remains stopped in a state where the focus is out of focus. It is a method that has very good followability.

However, this method has a major drawback in that the lens is constantly moved. One of the drawbacks is that the lens does not stop even when the subject is in focus, so that the photographing screen always shakes even after focusing on a stationary object.
A lens currently used in a television camera changes a focal length by rotating a focus ring, which changes a field angle of a captured image. For this reason, in this method, in which the focus ring continues to vibrate even after focusing, the screen image becomes very difficult to see because the subject on the screen increases or decreases in a certain cycle.

The second drawback is power consumption. Currently, home video cameras often use a battery as a power source because of their portability, and when the focus motor is constantly driven to repeat forward and reverse rotation, the motor rotates in a certain direction due to inrush current. Consume more power than if
Shooting time becomes shorter. In addition, since the focus ring is always rotated, a problem such as gear wear occurs.

[0006] The applicant is the Japanese Patent Application No. 61-273212 (H
04N 5/232), a new autofocus circuit system for eliminating these defects is proposed. The outline of the contents will be described below as a conventional example. still,
In the above-mentioned application, a very fine control method is adopted to improve the focusing accuracy, but here, only the part related to the present invention will be described.

FIG. 4 is a block diagram of the autofocus circuit according to the above application. The image formed by the lens 1 becomes a video signal by the image pickup circuit 4 including an image pickup element and is input to the focus evaluation value generation circuit 5. The focus evaluation value generation circuit 5 is configured, for example, as shown in FIG. The vertical sync signal VD and the horizontal sync signal HD separated from the video signal by the sync separation circuit 5a are input to the gate control circuit 5b to set the sampling area. In the gate control circuit 5b, the vertical synchronizing signal VD and the horizontal synchronizing signal H
Based on D and the fixed oscillator output, a rectangular sampling area is set in the central portion of the screen, and a gate opening / closing signal that permits passage of the luminance signal only in the range of this sampling area is supplied to the gate circuit 5c.

The gate circuit 5c passes only the luminance signal corresponding to the range of the sampling area through the high-pass filter (H.P.F) 5d to separate only the high-pass component, and the detection circuit 5e at the next stage. Amplitude is detected at. This detection output is integrated by the integrating circuit 5f for each field, and A /
The focus conversion value of the current field is obtained by being converted into a digital value by the D conversion circuit 5g. The focus evaluation value generation circuit 5 configured as described above always outputs the focus evaluation value for one field.

Immediately after the start of the autofocus operation, the first focus evaluation value is held in the maximum value memory 6 and the initial value memory 7. After that, the focus motor control circuit (focus motor control means) 10 rotates the focus motor 3 in a predetermined direction and monitors the output of the second comparator 9. The second comparator 9 compares the focus evaluation value after driving the focus motor with the initial evaluation value stored in the initial value memory 7 and outputs the magnitude.

The focus motor control circuit 10 rotates the focus motor 3 in the first direction until the second comparator 9 outputs a large or small output, and the current focus evaluation value is set in advance more than the initial evaluation value. If the output is larger than the fluctuation range, the rotation direction is kept as it is, and the output that the current evaluation value is smaller than the fluctuation range is compared with the initial evaluation value. In this case, the rotation direction of the focus motor 3 is reversed and the first comparator 8
Monitor the output of.

The first comparator 8 compares the current maximum focus evaluation value held in the maximum value memory 6 with the current evaluation value, and the current focus evaluation value is compared with the contents of the maximum value memory 6. Two comparison signals S1 that are large (first mode) and have decreased below the preset first threshold value (second mode),
Outputs S2. Here, the maximum value memory 6 is updated based on the output of the first comparator 8 when the current evaluation value is larger than the content of the maximum value memory 6, and the maximum value of the evaluation values up to the present is always displayed. The value is retained.

Reference numeral 13 denotes a motor position memory for storing the focus ring position by receiving a focus ring position signal indicating the position of the focus ring 2 supporting the lens 1, and like the maximum value memory 6, the first comparator 8 is provided. Is updated so that the focus ring position when the maximum evaluation value is reached is always held.

The focus motor control circuit 10 monitors the output of the first comparator 8 while rotating the focus motor 3 in the direction determined based on the output of the second comparator 9, and prevents malfunction due to noise in the evaluation value. In order to do so, the first comparator 8
At the same time, the focus motor 3 is rotated in the reverse direction at the same time when the output indicates a second mode in which the current evaluation value is smaller than the preset first threshold value as compared with the maximum evaluation value. After this reverse rotation, the contents of the motor position memory 13 and the current focus ring position signal are compared by the third comparator 14, and when they match, that is, the focus ring 2 returns to the position where the focus evaluation value is maximum. Sometimes focus motor 3
The focus motor control circuit 10 functions to stop the. At the same time, the focus motor control circuit 10 outputs a lens stop signal LS.

Reference numeral 11 denotes a lens stop signal L after the autofocus operation by the focus motor control circuit 10 is completed.
It is a fourth memory which holds the focus evaluation value at that moment when S is issued, and the content held in the fourth memory 11 is compared with the current focus evaluation value by the fourth comparator 12 in the subsequent stage. When the value becomes larger than the second threshold value for restarting, it is determined that the subject has changed, and the subject change signal is output to the focus motor control circuit 10. When the focus motor control circuit 10 receives this signal, the focus motor control circuit 10 performs the autofocus operation again and follows the change of the subject.

This method has extremely high followability and high focusing accuracy, but has the following major drawbacks. Generally, a video camera often uses a zoom lens, but the shape of the focus evaluation value with respect to the lens focus position is large as shown in FIG. 6 in the telephoto area and the wide-angle area even when the same subject is photographed. Is different. That is, in the telephoto region, a sharp change such as is shown, whereas in the wide-angle region, a very slow change is shown. In the figure, the vertical axis indicates the focus evaluation value, and the horizontal axis indicates the lens position (distance from the lens to the subject).

As can be clearly inferred from the figure, since the depth of focus of the lens is shallow in the telephoto area, the focus evaluation value changes greatly even if the distance between the lens and the subject changes slightly. In addition, even if the distance between the lens and the subject does not change,
When the subject moves laterally, the focus evaluation value changes greatly.
Therefore, a slight movement of the focus ring causes a large change in the focus evaluation value, resulting in a large out-of-focus state, but in reality the subject distance does not change much in many cases. Therefore, when the hill-climbing servo is performed, it is preferable to restart for focusing after the focus evaluation value changes considerably. That is, it is desirable to set the second threshold value large. In the above-mentioned conventional example, since the second threshold value is set to be constant regardless of the zoom position, the focus ring is driven by a slight change in the focus evaluation value, so that there is a drawback that the focus is out of focus.

Since the depth of focus is deep in the wide-angle region and the mode of use is often pan-focus, the sharpness of the screen does not change much even if the object moves a little, except for objects near it. . If the focus ring is moved in such a situation, the focus of the subject does not change, but only the angle of view changes, which has a psychologically bad influence. Therefore, it is desirable to restart the hill-climbing servo only when the focus evaluation value changes significantly even in the wide-angle region, that is, when the state of the subject changes clearly. As a result, it is important to set a large restart threshold even in the wide-angle region because it is different from the telephoto region.

On the other hand, the change in the focus evaluation value in the intermediate area between the two is such that the focus evaluation value is in the intermediate state between the two as shown in FIG. Since the evaluation value also changes considerably, it is necessary to follow the movement of the subject in detail. Therefore, the second threshold for restarting should be smaller than that in the telephoto and wide-angle cases.

In the conventional example, since there is no means for setting the second threshold value for the zoom position, either the telephoto (wide angle) is set as the center or the intermediate region is set as the center. It was When telephoto (wide-angle) is set to the center, when shooting in the middle area, it was not restarted unless it became quite out of focus. However, there is a drawback that the image is out of focus even if there is no change, and the angle of view changes in the wide-angle region even though the sharpness of the screen does not change.

[0020]

According to the prior art, in shooting with a zoom lens, when the operation in the intermediate area is normal, the focus is not changed in the telephoto area even though the object distance does not change. However, there is a drawback that the angle of view changes even though the sharpness of the screen does not change in the wide-angle area, and if the operation in the wide-angle area or the telephoto area is normal, it will not be considerably out of focus in the middle area There was a drawback that was not done.

[0021]

SUMMARY OF THE INVENTION According to the present invention, a second threshold value for causing a restart is set at a telephoto, an intermediate, It is characterized in that it is changed according to the wide-angle region.

[0022]

Since the present invention is configured as described above, stable autofocus operation can be performed by eliminating the conventional drawbacks such as out-of-focus in the telephoto and wide-angle regions, fluctuations in the angle of view, and poor followability in the intermediate region. You can

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. Incidentally, the same parts as those of the conventional example (FIGS. 4 and 5) are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 is a circuit block diagram of this embodiment.
The image formed by the lens 1 becomes a luminance signal by the image pickup circuit 4 having an image pickup element and is input to the focus evaluation value generation circuit (focus evaluation value detection means) 5. The focus evaluation value generation circuit 5 has the same configuration as that of FIG.
The focus evaluation value for the field is output, and the focusing operation is performed in the same procedure as the conventional example.

A zoom position detection circuit 19 outputs a zoom position detection signal from a zoom ring 15 having a zoom position detection cam 53, which will be described later, and the output thereof is input to the second threshold memory 17.

Next, this zoom position detection will be described in detail. FIG. 2 is a diagram showing the structure of the zoom ring portion of the apparatus of this embodiment. A zoom ring 15 that supports a variable magnification lens (zoom lens) (not shown) is rotatably arranged in the radial direction on a lens barrel portion 51 that protrudes to the front of the video camera body 50. By being driven by a motor (not shown) to rotate, the variable power lens moves back and forth in the optical axis direction, and moves to the telephoto side and the wide-angle side.

At the front end of the outer circumference of the zoom ring 15, first to third step portions 53a, 53b, 5 having different amounts of forward projection are provided.
A cam portion 53 in which 3c is continuously provided is integrally formed, and the position detection switch 54 is fixed to the lens barrel portion 51 so that the operating piece 54a contacts the cam portion 53. The position detection switch 54 is of a three-stage switching type, and the operating piece 54a is the first
Through the contact with the third step portions 53a, 53b, and 53c, the operation is switched to three steps. That is, the zoom ring 15 is located on the wide angle side, and the operating piece 54a is attached to the first step portion 53a.
Is in the abutting state and is in the first position shown by the solid line A in FIG. 3, the terminals 55 and 56 are conductive, and "H" is applied to the input terminals 17a and 17b of the second threshold memory 17 in the subsequent stage, respectively.
The rotational position information of "L" is supplied to inform that it is in the wide-angle state. Similarly, the zoom ring is indicated by the arrow X in FIGS.
Direction and the operating piece 54a slides on the cam portion 53
It contacts the stepped portion 53b or the third stepped portion 53c. That is,
When shifting to the second position shown by the dotted line B in FIG. 3 or the third position shown by the dotted line C, the terminals 5557 and 56, 57 are electrically connected, respectively, and the rotational position of “H” “H” or “L” “H” is reached. Information is supplied to the second threshold memory 17. This "H"
The case of "H" corresponds to the intermediate state, and the cases of "L" and "H" correspond to the telephoto state.

A difference output 16 outputs the absolute value of the difference between the output of the focus motor control circuit 10 held in the fourth memory 11 and the current focus evaluation value output from the focus evaluation value detection circuit 5. The output of the generation circuit is input to the fifth comparator 18.

The output of the fourth memory 11 is also input to the second threshold memory 17. The second threshold value memory 17 receives the output of the fourth memory 11 and changes its output according to the signal from the zoom position detection circuit 19, and the comparison circuit 18 changes it according to the difference output generation circuit 16 and the zoom position. If the difference output is larger than the output of the second threshold value memory 17, a restart signal is sent to the focus motor control circuit 10, and the focus motor control circuit 1
At 0, the focus motor 3 is driven and the focusing operation is restarted. The above operation will be specifically described below.

Now, let us say that the maximum value of the focus evaluation value held in the fourth memory 11 by the signal from the focus motor control circuit 10 is A. This value is taken as the absolute value | AA ′ | of the difference from the current evaluation value A ′ output from the focus evaluation value generation circuit 5 by the difference output generation circuit 16. On the other hand, the second threshold memory 17 outputs A / 4 when the zoom position signal from the zoom position detection circuit 19 is telephoto and wide angle, and outputs A / 2 when the zoom position signal is in the intermediate region. These values are compared with the absolute value output | A-A '| by the comparison circuit 18, and for each zoom ring position in the telephoto region: | A-A'|> A / 4 in the intermediate region Case: | A-A '|> A / 2 In the case of wide-angle region: | A-A'|> A / 4 In the case of restarting is performed.

As a result, according to the present invention, assuming that the subject has changed during autofocusing, the second threshold value for causing restarting is automatically changed in the telephoto, middle, and wide-angle regions,
Fine-tuned autofocus control can be performed.

In the above embodiment, the zoom area is divided into three areas of telephoto, wide angle, and intermediate for convenience.
It is apparent that this area division is affected by the resolution of the image pickup optical system, the depth of focus, and the S / N ratio of the image pickup circuit, and is not necessarily limited to the above embodiment. In addition, the fourth memory 11, the differential output generation circuit 16, the second
Although the threshold memory 17 and the comparison circuit 18 are shown in hardware for the sake of understanding, it is obvious that all of these operations can be processed by software using a microprocessor.

[0033]

As described above, according to the present invention, when the hill climbing servo control is performed, the illuminance of the subject is largely changed during the hill climbing servo or when the subject is in the in-focus state. It is possible to realize an autofocus video camera that provides a stable image without a malfunction such as performing a focusing operation on or a wrong restart.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing an embodiment of the present invention.

FIG. 2 is a view showing a mechanical portion of a zoom position detecting means of the above embodiment.

FIG. 3 is an operation explanatory diagram of a zoom position detection unit of the above embodiment.

FIG. 4 is a block diagram of a main part of a conventional autofocus video camera.

FIG. 5 is a block diagram of the focus evaluation value generation circuit.

FIG. 6 is a characteristic diagram showing a change in focus evaluation value with respect to a subject distance.

[Explanation of symbols]

 3 focus motor, 4 image pickup circuit, 5 focus evaluation value generation circuit (evaluation value detection means) 10 focus motor control circuit 15 zoom ring 17 second threshold value memory 19 zoom position detection circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location G02B 7/36

Claims (1)

[Claims] 1. An evaluation value detecting means for detecting a high frequency component level of a video signal obtained from an image pickup device as focus evaluation, and a zoom position detecting means for detecting a zoom position of an image pickup zoom lens changing from a telephoto to a wide angle. A focus motor that moves a focus lens based on a focus evaluation value from the evaluation value detection means; and stop the focus motor at a position where the focus evaluation value is a maximum value, and from that state, perform the focus evaluation. A focus motor control circuit that restarts the focus motor when the value changes by a preset threshold value or more, and a unit that switches the threshold value according to the zoom position detected by the zoom detection unit. Autofocus video camera.