JPH0771212B2 - Autofocus video camera - Google Patents

Description

TECHNICAL FIELD The present invention relates to an autofocus circuit for a video camera, which automatically adjusts the focus based on a video signal obtained from an image sensor.

(B) Conventional technology In an autofocus device for a video camera, the method of using the image signal itself from the image sensor for evaluating the focus control state has essentially no parallax and has a depth of field. It has many advantages such as being able to focus accurately with a shallow subject or a distant subject. Moreover, a special sensor for autofocus is unnecessary, and the mechanism is extremely simple.

An example of such an autofocus device is Japanese Patent Laid-Open No. 63-12.
An example is disclosed in No. 5910 (G02B7 / 11).

The essence of this prior art will be described below with reference to FIGS. 2 and 3.

FIG. 2 is an overall circuit block diagram of the autofocus circuit according to the above-mentioned conventional technique. The image formed by the lens (1) becomes a video signal by an image pickup circuit (4) including an image pickup element and is input to a focus evaluation value generation circuit (5).

The focus evaluation value generating circuit (5) is configured as shown in FIG. 3, for example. Synchronous separation circuit (5
The vertical synchronizing signal and the horizontal synchronizing signal separated by a) are input to the gate control circuit (5b) to set the sampling area. The gate control circuit (5b) sets a rectangular sampling area in the center of the screen based on the vertical sync signal, horizontal sync signal, and fixed oscillator output, and permits the passage of luminance signals only within this sampling area. A gate opening / closing signal is supplied to the gate circuit (5c).

The luminance signal corresponding to the area within the sampling area is converted by the gate circuit (5c) into a high-pass filter (HPF) (5
After passing through d), only the high frequency component is separated and amplitude detection is performed by the detection circuit (5e) at the next stage. This detection output is A / D
The conversion circuit (5f) converts it into a digital value, the integration circuit (5g) integrates it for each field, and the integrated value for one field is output as the focus evaluation value of the current field. Focus evaluation value generation circuit (5) configured as described above
Always outputs the focus evaluation value for one field.

Immediately after starting the focusing 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 (10) rotates the focus motor (lens position conversion means) (3) in a predetermined direction to rotate the focus ring (2) supporting the light receiving lens (1). , The light receiving lens (1) is displaced in the optical axis direction to change the distance from the image pickup device and monitor 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) includes a second comparator (9)
The focus motor (3) is rotated in the first direction until a large or small output is output, and when the output that the current focus evaluation value is larger than the initial evaluation value is made, the rotation direction is the same. When it is determined that the current evaluation value is smaller than the initial evaluation value, the rotation direction of the focus motor is reversed and the output of the first comparator is monitored.

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 stored in the maximum value memory (6). Larger than the content (first mode), preset first threshold (R
1) Two kinds of comparison signals (S1) and (S2) of (second mode) reduced above are output. 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 value is always updated. The maximum focus evaluation value up to is retained.

Reference numeral (30) is a motor position detection circuit for detecting the motor position of the focus motor (3), and specifically, an FG pulse output according to the rotation of the focus motor (3) (for example, 100 pulses are generated in one rotation). Is an UP / DOWN counter that counts, when the focus motor (3) rotates in the direction to move the lens from the near point side to the far point side, the FG pulse is added, and when the focus motor (3) rotates in the opposite direction. Subtract the FG pulse. Therefore, the count value of this counter itself becomes the motor position and is output as a motor position signal.

(13) is a motor position memory that stores a motor position by receiving a motor position signal that indicates a rotational position of a focus motor (3) that rotationally drives a focus ring (2) that supports the focus lens (1), Similar to the maximum value memory (6), it is updated based on the output of the first comparator (8) so as to always hold the motor position when the maximum evaluation value is reached. The focus ring (2) supports the light receiving lens (1), and the rotation of the ring itself causes the light receiving lens (1) to move back and forth in the optical axis support direction.
The above-mentioned motor position substantially corresponds to the lens position in the optical axis direction of the light receiving lens (1).

The focus motor control circuit (10) includes a second comparator (9)
While rotating the focus motor (3) in the direction determined based on the output, the output of the first comparator (8) is monitored, and the focus evaluation value is compared with the maximum evaluation value. The focus motor (3) is rotated in reverse at the same time when the second mode of being smaller than R1) is instructed.

Due to the reverse rotation of the focus motor (3), the moving direction of the light receiving lens (1) changes, for example, from a direction approaching the image sensor to a direction away from the image sensor, or vice versa.

After this reverse rotation, the contents of the motor position memory (13) and the current motor position signal are compared by the third comparator (14) and when they match, that is, the focus ring (2) has the maximum focus evaluation value. The focus motor control circuit (10) functions to stop the focus motor (3) when returning to the position. At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS). Incidentally, FIG. 4 is a diagram showing the relationship between the lens position and the focus evaluation value associated with the above-described focusing operation, and the point (P) shows the initial position of the lens (1).

(11) is a fourth memory for holding the focus evaluation value at that time at the same time as the lens stop signal (LS) is issued after the focusing operation by the focus motor control circuit (10) is finished. This fourth memory (1
The content held in 1) is compared with the current focus evaluation value, and when the current focus evaluation value becomes smaller than the preset third threshold value compared with the content of the fourth memory (11), the subject is It is determined that the subject has changed, and the subject change signal is output. When the focus motor control circuit (10) receives this signal,
The focusing operation is performed again to follow the change of the subject.

(C) Problem to be Solved by the Invention The autofocus device in the above-mentioned conventional technique is excellent in focusing accuracy and adaptability to a wide range of subjects, but has the following drawbacks.

That is, during movement of the lens, as shown in FIG. 5, a large high range in which, for example, black and white stripes are alternately mixed due to panning and movement of the subject on a screen with a low high range component of the screen and a low focus evaluation value. When a subject having a component enters the sampling area, the focus evaluation value may increase despite the lens moving away from the in-focus point. Conversely, when a subject having a large high-frequency component as described above emerges from the sampling area, the focus evaluation value may decrease despite the lens approaching the in-focus point. When such a situation occurs, the lens largely moves in the wrong direction, resulting in an unsightly screen.

(D) Means for Solving the Problem The present invention performs an autofocus operation by stopping the focus lens at a position where the focus evaluation value output as the high frequency component level of the captured video signal becomes the maximum value. In doing so, two types of focus evaluation value detection means, that is, a first focus detection means that shows a sharp change in the focus evaluation value with respect to the lens position and a second focus detection means that shows a gentle change with respect to the lens position, It has a third focus evaluation value detecting means for outputting a third focus evaluation value by dividing the first focus evaluation value by the second focus evaluation value, and the lens moving means has the third focus evaluation during the lens movement. When the value decreases, the function to reverse the moving direction of the lens, and when the evaluation value increases,
It is characterized by having the function of regulating the reversal of the lens.

(E) Operation Since the present invention is configured as described above, it is possible to correctly determine whether or not the lens is moving in the focusing direction even when the focus evaluation value changes due to a subject moving in and out of the focus area. Incorrect lens movement is prevented.

(F) Embodiment One embodiment of the present invention will be described below with reference to the drawings.
In the drawings, the same parts as those in FIGS. 2 and 3 are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 is a circuit block diagram of the autofocus circuit in this embodiment. The image formed by the lens (1) becomes an image pickup video signal by the image pickup circuit (4) including an image pickup element, and the luminance signal therein is input to the gate circuit (5c) and the sync separation circuit (5a). Only the luminance signal in the sampling area set at the center of the screen by the gate circuit (5c) is the first and second filter circuits (15) (1) in the subsequent stage.
Entered in 6). Both the filter circuits (15) and (16) are high-pass filters (HPF) that extract only high-frequency components of the luminance signal, but the first filter circuit (15) is the second
It has a higher cutoff frequency than the filter circuit (16). Specifically, the cutoff frequency of the first filter circuit (15) is set to 500 KHz, and that of the second filter circuit (16) is set to 100 KHz. The outputs of the filter circuits (15) and (16) are input to the switch circuit (17) controlled by the focus motor control circuit (100) and are alternately input to the integrating circuit (18). The integrating circuit (18) is the detection circuit (5e) shown in FIG. 3 of the conventional example, the A / D conversion circuit (5
f), the integrated circuit (5g) is shown together (connected in series).

The filter circuits (15) and (16) correspond to the HPF (5d) shown in FIG. As a result, the first filter circuit (15), the switch circuit (17) and the integrating circuit (18) constitute a first focus evaluation value detecting means, and the second filter circuit (16), the switch circuit (17) and the integrating circuit ( The second focus evaluation value detecting means is constituted by 18).

The first focus evaluation value generating means has a cutoff frequency of the second
The first focus evaluation value (A), which is the output thereof, is higher than the second focus evaluation value (B) from the second focus evaluation value generating means. The shape of the mountain is steep as shown in Fig. 6.

The output of the integrating circuit (18) is input to the switch circuit (19). The switch circuit (19) switches the first and second focus evaluation values by the focus motor control circuit (10) like the switch circuit (17). Of these, the first focus evaluation value (A) is input to the focus detection circuit (20).

The focus detection circuit (20) includes the maximum value memory (6), the initial value memory (7), the first comparator (8), the second comparator (9), and the motor position shown in FIG. 2 of the conventional example. Memory (13),
The third comparator (14) is shown collectively and has a structure as shown in FIG.

Focus detection circuit (20) and focus motor control circuit (10
0) communicates with the output of each comparator and the control signal to each memory, and performs the same operation as the focus motor control circuit (10) shown in the conventional example to perform the focus lens (1).
To the in-focus position.

By the way, the switching of the switch circuits (17) and (19) is executed by the switching signal (q) from the focus motor control circuit (100), both are synchronized, and the switch circuit (17) is switched to the fixed contact (17a) side. If the switch circuit (19) is switched to the fixed contact (19a) side, and if the switch circuit (17) is switched to the fixed contact (17b) side, the switch circuit (19) is switched. ) Switches to the fixed contact (19b) side. Further, since the switching signal (q) is issued for each field, the switch circuits (17) and (19) are alternately switched for each field. Therefore, the first focus evaluation value (A) generated on the fixed contact (19a) and the second focus evaluation value (B) generated on the fixed contact (19b) are updated every two fields, that is, every one frame.

The focus detection circuit (20) executes the same focus operation as in the conventional example based on the first focus evaluation value (A) generated at the fixed contact (19a). However, since the first focus evaluation value is updated for each frame as described above, the comparison operation of each comparator forming the focus detection circuit (20) is executed for each frame. The focus motor control circuit (100) controls the drive of the focus motor (3) based on the output from the focus detection circuit (20), thereby realizing the hill climbing control to the focus point as shown in FIG. .

On the other hand, the two types of focus evaluation values output from the switch circuit (19) during the above-described focus operation in the focus detection circuit (20) are input to the arithmetic circuit (third focus evaluation value detection means) (21). , The first focus evaluation value (A) is divided by the second focus evaluation value (B), and the third focus evaluation value C (= A / B), which is the relative ratio of the two.
Which is sent to the fifth comparator (22). The third focus evaluation value (C) is calculated by using the latest two evaluation values each time either the first focus evaluation value or the second focus evaluation value is updated.

The relationship between the third focus evaluation value (C) and the degree of blurring of the subject (movement amount or shift amount from the lens position at the time of focusing) is shown in a graph, and a monotonous decrease characteristic curve as shown in FIG. 7 is obtained. Become.

This is because the third focus evaluation value (C), which is the relative ratio of the first and second focus evaluation values, is a function that can express the in-focus state (degree of blurring) of the subject, like the first and second focus evaluation values. Since it is a value and is expressed as a ratio, it is a kind of normalized state quantity and is not easily affected by the type of subject and the environment. Thus, this third focus evaluation value (C)
Can be used as a parameter of the blur degree.

The fifth comparator (22) compares this value with the previous result stored in the fifth memory (23), that is, the third focus evaluation value one field before, and when the current value becomes small, that is, the blurring occurs. If so, a decrease detection pulse (TD) is issued to increment a counter (increase / decrease determination means) (24), and when larger, an increase detection pulse (TU) is issued to reset the counter (24). The count value of the counter (24) is compared with a predetermined value (N) stored in advance in the threshold value memory (26) by the sixth comparator (increase / decrease determination means) (25), and the count value reaches N. When the third focus evaluation value (C) decreases for a predetermined N fields continuously, it is determined that the lens is moving away from the in-focus point, and the focus motor control circuit (100) causes the motor (3 ) Is output, and an inversion instruction signal (MR) is output. The number of times (N) is set to an appropriate value obtained by experiment. The result of the arithmetic circuit (21) is stored in the fifth memory (23) after the comparison by the fifth comparator (22) is completed.

When the focus motor control circuit (10) receives the reversal instruction signal (MR) from the counter (24), it immediately reverses the rotation direction of the focus motor (3) and restarts the focusing operation from the beginning.

Therefore, when the second focus evaluation value (B) increases even when the lens is actually moving away from the in-focus point, this is detected from the increase and decrease of the third focus evaluation value (C), and the lens in the wrong direction is detected. Movement can be blocked.

By the way, in the present embodiment, attention is paid only when the third focus evaluation value (C) tends to decrease, but it is also possible to limit the reversal of the focus motor (3) when the third focus evaluation value (C) tends to increase. Is. For example, as shown in FIG. 9, countdown is performed by the decrease detection pulse (TD) from the fifth comparator (22) and countup is performed by the increase detection pulse (TU).
The UP / DOWN counter (124) is substituted for the counter (24), and the count value of the counter (124) is preset in the threshold value memory (42) by the seventh comparator (increase / decrease determination means) (41). Compared with the threshold value, the result of this comparison is that the count value is +
The reversal limit command signal (MG) is input to the focus motor control circuit (100) when it becomes N or more, and the reversal command signal (MR) is input to when it becomes -N or less, so that the lens position is in focus. When approaching and the first focus evaluation value (A) falls below the maximum value by the first fighting value (R1), the focus detection circuit (20)
Focus motor control circuit (100) based on the output from
Tries to reverse the motor (3), but at this time, if the third focus evaluation value (C) tends to increase, the reverse limit command signal (MG) is input to prevent the reverse rotation of the motor (3). Then, the focusing operation is performed again. Therefore, when the first focus evaluation value (A) decreases even though the focus is actually approached, this is detected from the increase and decrease of the third focus evaluation value (C), and the lens in the wrong direction is detected. Movement can be blocked. Both the counters (24) and (124) are once reset when the focusing operation is restarted.

In the above-described embodiment, the number of consecutive changes in the same direction is used to detect the change in the third focus evaluation value (C), but instead, the motor (3) rotates in the same direction as the present. A method of taking the magnitude of the difference from the reference value with the third focus evaluation value (C) at the start point as a reference is also conceivable.

Although the first focus evaluation value (A) is used for the focusing operation in the focus detection circuit (20) in the above embodiment, the second focus evaluation value (B) may be used instead. Is possible,
Further, the first focus evaluation value (A) is used in the vicinity of the in-focus point where the change of the evaluation value is remarkable, and the second focus evaluation value (B) or the sum of the first and second focus evaluation values is used at the position apart from the in-focus point. It is also possible to use.

Further, it goes without saying that the circuit operation of FIG. 1 can be processed by software using a microcomputer. The flowchart at this time is shown in FIG.

(G) Effect of the Invention As described above, according to the present invention, since the degree of blurring of a subject is detected as a relative ratio in a normalized state, the focus evaluation value changes due to a change in the subject such as a new subject entering. Even if occurs, it is possible to correctly determine whether or not the direction of change in the relative position of the lens with respect to the image pickup element is in the in-focus direction, and the movement in the out-of-focus direction linked to the fluctuation of the focus evaluation value is blocked. Unstable focusing operation is prevented.

[Brief description of drawings]

1, FIG. 6 to FIG. 8 and FIG. 10 relate to an embodiment of the present invention. FIG. 1 is a circuit block diagram, FIG. 8 is a main circuit block diagram, FIG. 6 and FIG. FIG. 11 is a diagram showing a relationship between a lens position and a degree of blurring and a third focus evaluation value, and a flowchart of FIG. FIG. 9 is a circuit block diagram of the essential parts of another embodiment. 2 and 3 are circuit block diagrams of a conventional example, FIG. 4 is a diagram showing a change in focus evaluation value due to lens movement during focusing operation, and FIG. 5 is a subject sample containing a lot of high frequency components. It is a figure when entering an area. (A) ... First focus evaluation value, (B) ... Second focus evaluation value, (C) ... Third focus evaluation value, (1) ... Lens,
(3) ... Focus motor (lens position changing means),
(21) ... Calculation circuit (third focus evaluation value detection means), (2
4) (124) …… Counter (increase / decrease determination means), (25) ……
Sixth comparator (increase / decrease determination means), (41) ... Seventh comparator (increase / decrease determination means).

Claims (2)

[Claims] 1. A first focus evaluation value detection means for outputting a first high-frequency component level of a video signal obtained from an imaging means having an image sensor as a first focus evaluation value, and the first focus evaluation value of the video signal. Second focus evaluation value detection means for outputting as a second focus evaluation value a second high frequency component level that also includes components in the low frequency region higher than the high frequency component level; and the first or second focus evaluation value increases. Position changing means for changing the relative position of the lens with respect to the image pickup element in the direction to reach the in-focus position where the first or second focus evaluation value becomes the maximum value, and the first focus evaluation value. Relative ratio detection means for outputting a ratio to the second focus evaluation value as a relative ratio, and increase / decrease determination means for determining whether the relative ratio increases or decreases with a change in the relative position, are provided. Near infinity or infinity When the relative ratio increases with respect to the change in the direction, it is possible to prevent reversal of the relative position changing direction by the position changing unit regardless of the change of the first or second focus evaluation value. A featured autofocus video camera. 2. A first focus evaluation value detecting means for outputting a first high frequency component level of a video signal obtained from an imaging means having an image sensor as a first focus evaluation value, and the first focus evaluation value of the video signal. Second focus evaluation value detection means for outputting as a second focus evaluation value a second high frequency component level that also includes components in the low frequency region higher than the high frequency component level; and the first or second focus evaluation value increases. Position changing means for changing the relative position of the lens with respect to the image pickup element in the direction to reach the in-focus position where the first or second focus evaluation value becomes the maximum value, and the first focus evaluation value. Relative ratio detection means for outputting a ratio to the second focus evaluation value as a relative ratio, and increase / decrease determination means for determining whether the relative ratio increases or decreases with a change in the relative position, are provided. Near infinity or infinity When the relative ratio decreases with respect to the change in the direction, the position changing means reverses the relative direction change direction regardless of the change in the first or second focus evaluation value. Autofocus video camera to.