JPH04351180A - Automatic focusing video camera - Google Patents

Abstract

PURPOSE:To prevent the mis-restart of focusing and to quicken the restart by detecting a distance between a major object and an image pickup element from a lens position just after the end of focusing and varying a passing estimate time in response to the distance. CONSTITUTION:A distance between a major object and an image pickup element is detected from a lens position just after the end of focusing and a time T2 estimated to be required for passing a focus area FA of other object in response to the distance is made variable. That is, when the distance of the both is long, since the probability of crossing of other object or entrance is high, the T2 is extended and when the distance of the both is short, since the probability of crossing or entrance is low, the T2 is decreased. Then a preset circuit 41 receives a ring location signal of a position memory 15 and the preset fed to a counter 40 is revised in response to the lens position where a focus evaluation takes a maximum value, that is, the lens position at the end of focusing.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus device for a video camera that automatically adjusts focus based on a video signal obtained from an image sensor.

0002

[Prior Art] In an autofocus device for a video camera, the method of using the video signal itself from the image sensor to evaluate the focus control state is essentially parallax-free, and when the depth of field is shallow, It has many advantages, such as being able to accurately focus on distant subjects. Moreover, there is no need for a special sensor for autofocus, and the mechanism is extremely simple.

[0003] Conventionally, an example of this autofocus method is described in "NHK Technical Report" S40, Volume 17, No. 1, Volume 86, page 21, "Automatic focus adjustment of television cameras using mountain climbing servo system" by Ishida et al. So-called mountain-climbing servo control is known. This mountain climbing servo control will be explained with reference to FIG. 2.

The image formed by the lens 1 is converted into a luminance signal by the imaging circuit 4 and is input to the focus evaluation value generation circuit 5. The configuration of the focus evaluation value generation circuit 5 is, for example, as shown in FIG. Synchronization separation circuit 5 from video signal
The vertical synchronization signal VD and horizontal synchronization signal HD separated by a are input to the gate control circuit 5b to set a focus area. In the gate control circuit 5b, a rectangular focus area FA is set at the center of the screen as shown in FIG. 13 based on the vertical synchronization signal VD, horizontal synchronization signal HD and fixed oscillator output, and only the range of this focus area is set. A gate opening/closing signal that allows passage of the luminance signal is supplied to the gate circuit 5c.

Only the luminance signals corresponding to the range of the focus area are time-divisionally extracted by the gate circuit 5c, and then passed through the HPF 5d to separate only the high-frequency components. The high-frequency component level of 20 minutes is digitally integrated and output as a focus evaluation value of the current field.

The focus evaluation value output for each field from the focus evaluation value generation circuit 5 configured as described above is first stored in the first memory 6, and when the focus evaluation value of the next field is input, the focus evaluation value is stored in the first memory 6. 2 memory 7. That is, the first
The latest evaluation value is stored in the memory 6, and the evaluation value of one field before is always updated and stored in the second memory 7. This 2
The contents of the two memories are compared by a comparator 8, and the comparison output is input to a focus motor control circuit 9.

In the focus motor control circuit 9, the output of the comparator 8 determines that if the evaluation value of the first memory is greater than the evaluation value of the second memory, the current rotation direction of the focus motor 3 is maintained, and the current rotation direction of the focus motor 3 is maintained. If the evaluation value of <the evaluation value of the second memory, the evaluation value tends to decrease, so the focus motor 3 is reversed. Due to the movement of the focus motor 3, the focus ring 2 that supports the lens 1 continues to move in the direction of increasing the focus evaluation value, and in conjunction with this, the lens 1 also moves in the optical axis direction, and Focus is achieved by changing the relative position, and after focusing, it oscillates around the maximum point of the evaluation value. In this method, as long as there is a slope of the focus evaluation value, the lens 1 is always moved in the direction of increasing the focus evaluation value even if the object changes, so the object can be tracked without stopping while remaining out of focus.

However, this method has a major drawback due to the fact that the lens is constantly moved. One of the drawbacks of this is that the lens does not stop even after focusing, so the photographic screen continues to shake even after focusing on a stationary object. Lenses currently used in television cameras change the focal length by rotating the focus ring.
For this reason, the angle of view of the photographic screen changes. For this reason, with this method, where the focus ring continues to vibrate even after focusing, the subject on the screen becomes larger or smaller at certain intervals, making the screen extremely difficult to see.

The second drawback is power consumption. Currently, home video cameras are often powered by batteries for portability, and when the focus motor is constantly driven and rotates forward and reverse repeatedly, the inrush current causes the motor to rotate in a fixed direction. consumes more power than the
The available shooting time will be shorter. Additionally, constantly rotating the focus ring causes problems such as gear wear.

[0010] In order to improve these drawbacks,
As seen in Publication No. 0-135712 (G02B7/11), and as shown in FIG. A method has been proposed in which the lens 1 is returned to the position where the maximum value was taken and stopped when the focus evaluation value decreases by the threshold width L. but,
In the case of a video camera, it is necessary to keep the focus position on a subject that changes moment by moment.If the distance to the subject (subject distance) changes even after the lens has stopped at the in-focus position, the lens It is necessary to resume the mountain-climbing motion.

[0011] For this reason, the present applicant has previously applied
In No. 12112 (G02B7/11), if the evaluation value changes by more than the threshold width while the lens is stopped, it is determined that the subject has changed and the mountain climbing operation is restarted to follow the ever-changing subject. proposed a method. However, this method also has the same drawbacks as the conventional mountain climbing servo control described below.

[0012] In other words, if another object crosses between the video camera and the subject while focusing on the main subject to be imaged, the lens will try to focus on the object that has crossed as well. The main subject was blurred and the angle of view fluctuated, resulting in an unsightly image. An example of this will be explained with reference to FIGS. 4 and 5. In FIG. 4, the video camera Ca is photographing the main subject P, and when the focus is on, another subject Q moves in front of the camera as shown by an arrow. When it passes, all or part of the main subject P is blocked, and the mountain climbing operation is restarted and the lens begins to move so as to focus on the subject Q. However, since it takes some time to focus on the subject Q, the subject Q finishes passing by before the lens movement is completed. As a result, the subject P is out of focus while the subject P is passing, the subject Q is also not in focus, and immediately after the passing is completed, an unsightly screen appears in which the subject P is shown out of focus.

[0013] Conventionally, therefore, a method has been proposed in which the lens is always prohibited from driving for a certain period of time (for example, the time expected for the subject Q to finish passing) when the focus deviates from the focused state. has been done. However, with this method, when the main subject P approaches the camera as shown in the arrow as shown in Figure 5, the lens always starts driving after a certain delay time after it is determined that the focus has shifted. This results in autofocus operation with poor responsiveness.

[0014] Therefore, the applicant of the present application
As shown in Publication No. 69 (H04N5/232), there is provided a focus control means that performs a hill-climbing operation and stops the lens at a lens position that indicates the maximum point of the focus evaluation value, and the focus evaluation value when the lens is stopped is determined. It is stored as a reference value for detecting changes in the subject, and this reference value is compared with the current evaluation value, and if the evaluation value suddenly drops in level, control by the focus control means is prohibited, and the level drop continues for a predetermined period of time. If the change in the evaluation value is instantaneous, the focusing operation is not restarted, but if the level of the evaluation value falls gradually, the focusing operation is configured to restart the control by the focus control means. We have proposed an autofocus device that restarts the focusing operation after a predetermined time if the level drops for a long time after the focusing operation is performed.

This prior art will be explained below with reference to the drawings. Incidentally, the same parts as in FIGS. 2 and 3 are given the same reference numerals and explanations are omitted.

FIG. 11 is a circuit block diagram of the circuit of this embodiment. The image formed by the lens 1 is converted into a luminance signal by an imaging circuit 4 having a CCD (imaging device).
It is input to the focus evaluation value generation circuit 5. The focus evaluation value generation circuit 5 has the same configuration as that in FIG. 3 described above, and outputs the focus evaluation value for one field.

Immediately after the start of the focusing operation, the initial focus evaluation value is held in the maximum value memory 10 and the initial value memory 11. Thereafter, the focus motor control circuit 15 rotates the focus motor 3 in a predetermined direction, accordingly displaces the lens 1 in the optical axis direction, and monitors the output of the second comparator 13. The second comparator 13 compares the focus evaluation value after driving the focus motor with the initial evaluation value held in the initial value memory 11, and outputs the magnitude thereof.

The focus motor control circuit 14 rotates the focus motor 3 in the initial direction until the output of the second comparator 13 outputs a large or small output, and determines that the current evaluation value is larger than the initial evaluation value. If an output is made, the rotation direction is maintained as it is, and if the current evaluation value is smaller than the initial evaluation value, the rotation direction of the focus motor 3 is reversed, and then the first comparator 12 monitor the output of

The first comparator 12 compares the current maximum evaluation value held in the maximum value memory 10 with the current evaluation value, and if the current evaluation value is larger than the contents of the maximum value memory 10 ( Two types of comparison signals are output: S1) or sufficiently small (S2). Here, the maximum value memory 10 is updated based on the output S1 of the first comparator 12 when the current evaluation value is larger than the contents of the maximum value memory 10, and the maximum value of the evaluation values up to now is always updated. Retained.

A position memory 15 receives a focus ring position signal indicating the position of the focus ring 2 supporting the lens 1 and stores the focus ring position as a lens position. Based on the output S1 of the device 12, the focus ring position is updated so as to always maintain the focus ring position when the maximum evaluation value is reached.

The focus motor control circuit 14 monitors the output of the first comparator 12 while rotating the focus motor 3 in the direction determined based on the output of the second comparator 13, thereby preventing malfunctions due to noise in the evaluation value. In order to do this, the focus motor 3 is reversed at the same time that the first comparator 12 outputs an output S2 indicating that the current evaluation value is sufficiently smaller than the maximum evaluation value. After this reversal, the contents of the position memory 15 and the current focus ring position signal are compared in the fifth comparator 16, and when they match, that is, when the lens 1 returns to the position where the focus evaluation value is maximum, the focus is focused. The focus motor control circuit 14 functions to stop the motor 3. At the same time, the focus motor control circuit 14 outputs a lens stop signal LS. This completes a series of focusing operations. Thereafter, in order to follow the change in the subject, a change in the subject is detected by a subject change detection circuit 50 composed of an OR circuit 25 from a third memory 17, which will be described later, and if the change in the subject is confirmed. , the motor 3 is started and the series of focusing operations described above are repeated.

Next, the operation of the subject change detection circuit will be explained with reference to FIGS. 7 to 9.

First, upon completion of a series of focusing operations, when the focus motor control circuit 14 issues an LS signal, the focus evaluation value at this point in time is held in the third memory 17 as a reference value. be done.

The third and fourth comparators 18 and 19 in the latter stage are as follows:
The value held in the third memory 17 is compared with the evaluation value that changes every moment thereafter. Here, the third comparator 18 constitutes a first detection circuit 60 together with first and second monomultis 20 and 21 and a first AND circuit 22, which will be described later.
has a threshold value A (25% of the content of the third memory 17) relative to the content of emanate. Further, the fourth comparator 19 constitutes a second detection circuit 61 together with a delay circuit 23 and a second AND circuit 24, which will be described later.
has a threshold value A relative to the content of the third memory 17 and another threshold value B (75% of the content of the third memory 17);
When the difference between the contents of the third memory 17 and the current evaluation value is greater than or equal to threshold value A and less than or equal to threshold value B, an output signal of H level is generated.

Reference numeral 20 denotes a first monomulti whose quasi-stable period is a fixed time T2, and outputs in synchronization with the rise of the output of the third comparator 18. Reference numeral 21 denotes a second mono multi-channel multiplier that outputs an output in synchronization with the fall of the output of the first mono multi-channel multiplier 20.

A delay circuit 23 delays the output of the fourth comparator 19 by a predetermined time T1. Here, time T1 is the time expected to be required for another subject Q in FIG. 4 to finish entering the focus area, and time T2 is set as the time expected for another subject Q to finish passing through the focus area. (Specifically, for example, T1=0.1
sec, T2=0.5sec). The output of the second monomulti 21 is input to the AND circuit 22 together with the output of the third comparator 18, and the output of the delay circuit 23 is input to the AND circuit 22 together with the output of the fourth comparator 19.
The output signal is input to the D circuit 24, and both outputs of the AND circuits 22 and 24 serve as two inputs to the OR circuit 25. OR circuit 2
5 output is supplied to the focus motor control circuit 14 as a subject change detection signal.

Next, the operation of the subject change detection circuit 50 described above will be explained. The waveform diagrams shown in FIGS. 7 to 9 are as follows:
Each corresponds to the situation shown in FIGS. 4 to 6. That is, FIG. 7 shows the case (first case) when another object Q crosses in front of the camera Ca while the main subject P is being photographed as shown in FIG. 4, and FIG. FIG. 9 shows a case where an object approaches the main subject (second case), and FIG. 9 shows a case where another object comes in and stops while the main subject is being photographed as shown in FIG. 6 (third case). 7 to 9, a shows how the focus evaluation value changes over time when the lens is fixed at the lens position where the main subject is in focus, and the vertical axis shows the focus evaluation value,
The elapsed time is shown on the horizontal axis. Note that the two-dot chain lines indicate the widths of the aforementioned threshold values A and B, respectively. Also, b is the output of the fourth comparator 19, c is the output of the third comparator 18, d is the output of the delay circuit 23, e is the output of the second monomulti 21, and f is the output of the AND circuit 22. g indicates the output of the AND circuit 24, and h indicates the output of the OR circuit 25.

In the first case described above, as shown in FIG. 7, the focus evaluation value once drops by more than the threshold value B, but recovers by time T2. In addition, since the evaluation value drops extremely quickly when another object crosses in front of the camera, the pulse width of the output b of the fourth comparator 19 is shorter than the time T1, and the second AND circuit 24
No H level signal is output to the output g. Further, since the passage of another object is completed within time T2, the output of the first AND circuit 22 is also maintained at the L level, and the object change detection signal corresponding to the output h of the OR circuit 25 also becomes the L level. In other words, it will not be recognized that the subject has changed,
The focus motor control circuit 14 does not restart the focusing operation.

Next, in the second case, that is, when the main subject gradually approaches the camera, the focus evaluation value shows a relatively gradual change as shown in FIG. 8, and the output b of the fourth comparator 19
The pulse width also exceeds time T1. Therefore, the output g of the second AND circuit 24 becomes H level with a delay of time T1 from the rise of output b, and the output f of the first AND circuit 22 also becomes H level after time T2, but the output g preceding this output f is supplied to the focus motor control circuit 14 via the OR circuit 25 as a subject change detection signal. In response to this, the focus motor control circuit 14 clears the contents of the initial value memory 11 and maximum value memory 10 and restarts the series of focusing operations. Therefore, changes in the subject can be quickly confirmed with the output h before the output f, and an autofocus operation with good followability is realized.

In the third case, that is, when another object enters and stops in front of the camera, the evaluation value drops sharply, and the pulse width of the output b of the fourth comparator 19 becomes shorter than time T1. Since the length is short, the output of the second AND circuit 24 maintains the L level. On the other hand, since the output c of the third comparator 18 is synchronized with the rise of the output b and maintains the H level thereafter, the first AND
The output f of the circuit 22 becomes H level after a time T2 from the rise of the output c, and is supplied to the focus motor control circuit 14 as a subject change detection signal via the OR circuit 25, and the focusing operation is resumed. Therefore, in this third case, a time T2 after another object comes in front of the camera.
After a relatively long delay, the focusing operation will resume, but if the distance to the subject suddenly changes significantly, it will take about 2 seconds from when the focus motor 3 starts driving until it focuses. It takes time, and a time delay of about time T2 (for example, 0.5 sec) is not a problem.

[0031]

[Problem to be solved by the invention] In the above-mentioned prior art,
When determining whether the drop in focus evaluation value is due to the movement of the main subject itself or the entry of another subject into the focus area, the estimated time required for the other subject to pass through the focus area. T2
is set to a fixed value in advance. In particular, in order to avoid erroneous cross-cutting judgments, it is necessary to set this time T2 sufficiently large, at the expense of the response to restart the focusing operation when another subject enters.

By the way, if the main subject is far from the camera, there is a high possibility that another subject will cross between the main subject and the camera, so by setting the time T2 large at the expense of response, reliable judgment can be made. However, if the main subject is located close to the camera, the possibility that other subjects will cross between the two becomes extremely small. Therefore, if a long time is set for monitoring the cross-crossing of another subject, when a cross-crossing actually occurs and it is necessary to restart the focusing operation, the disadvantage is that it becomes impossible to respond quickly.

[0033]

[Means for Solving the Problems] Therefore, the present invention detects the subject distance between the main subject and the image sensor from the lens position immediately after the completion of the focusing operation, and makes the time T2 variable according to this distance. When the distance is long, there is a high probability that another subject will cross the subject, so T2 is made long, and when the distance between them is short, the probability is low, so T2 is made short.

[0034]

[Operation] Since the present invention is configured as described above, even if a subject different from the main subject crosses or enters the subject, the occurrence of erroneous restart of the focusing operation or the delay in restarting the focusing operation can be prevented depending on the probability. can be minimized.

[0035]

[Embodiment] An embodiment of the present invention will be described below with reference to FIG. The difference from FIG. 11 is that an oscillator that generates a fixed frequency clock is built in instead of the first and second monomultis 20 and 21, and a DOWN counter 40 that counts down this clock is connected to the third comparator 18 and an AND circuit. 22, and a preset circuit 41 for this counter is also added.

The preset circuit 41 receives the ring position signal from the position memory 15 and changes the preset value supplied to the counter 40 according to the lens position at which the focus evaluation value takes the maximum value, that is, the lens position when the focusing operation is completed. is a circuit,
The preset value is increased as the lens position approaches the infinity point when the focusing operation is completed, and conversely decreases as the lens position approaches the near point.

The DOWN counter 40 is connected to the third comparator 18.
The oscillator clock starts counting down from the preset value set by the built-in preset circuit 41 in synchronization with the rising edge of the output, and when the count value reaches zero, the pulse width of the second monomulti 21 shown in FIG. emits the same pulse. Therefore, the period from when the counter 40 starts counting down until reaching zero and emitting a pulse is
This corresponds to the time T2 for other subjects to finish crossing the focus area, and as a result, as shown in Figure 12, this time T2 becomes a variable value depending on the subject distance to the main subject when the focusing operation is completed. That's what happened.

In this way, when another subject crosses or enters the focus area and stops at a position close to the camera, the time T2 for monitoring changes in the subject is shortened, resulting in ) and (h) are shifted forward as shown by the chain line, and the other subject enters the focus area and stops, which speeds up the resumption of the necessary focusing operation.

[0039] Also, when the other subject is located far from the camera,
When crossing the focus area, the time it takes to complete passing through the focus area is longer than at a closer position, as shown in (a) in Figure 7.
), the return of the focus evaluation value is delayed, as shown by the chain line in (b), (
The pulses in c) and (d) will also be shifted backwards as shown by the dashed line, and if time T2 is not shifted backwards, (f)
As a result, the pulse (e) is generated, and the focusing operation is restarted assuming that the subject has changed before another subject has passed. By configuring as in this embodiment, the time T2 can be lengthened. It becomes possible to respond by doing so.

[0040]

As described above, according to the present invention, the subject distance between the main subject and the image sensor is detected from the lens position immediately after the completion of the focusing operation, and the expected passing time T2 is made variable according to this distance. If the distance between the two objects is long, there is a high probability that another object will cross or enter the object, so T2 is made longer to prevent the erroneous judgment that the object has changed while crossing the object. Since the probability of another object crossing or entering is low, T2 is shortened so that the focusing operation can be quickly restarted when another object approaches and stops.

[Brief explanation of drawings]

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

FIG. 2 is a circuit block diagram of a conventional example.

FIG. 3 is a block diagram of a main circuit of a conventional example.

FIG. 4 is an explanatory diagram when another subject crosses the camera.

FIG. 5 is an explanatory diagram when the main subject approaches.

FIG. 6 is an explanatory diagram when another subject enters and stops.

FIG. 7 is an explanatory diagram of waveforms at various parts when another subject crosses the camera.

FIG. 8 is an explanatory diagram of waveforms at various parts when the main subject approaches.

FIG. 9 is an explanatory diagram of waveforms at various parts when another subject enters and stops.

FIG. 10 is a diagram showing the relationship between the lens position and focus evaluation value during a focusing operation.

FIG. 11 is a circuit block diagram of a conventional example.

FIG. 12 is a diagram illustrating the relationship between the lens position at the time of focusing and time T2.

FIG. 13 is a diagram illustrating a focus area.

[Explanation of symbols]

5 Focus evaluation value generation circuit 14 Focus control circuit 17 3rd memory 40 DOWN counter 41 Preset circuit 50 Subject change detection circuit

Claims (1)

[Claims] 1. Focus evaluation value generating means for outputting a high-frequency component level of an imaging luminance signal over a certain period of time as a focus evaluation value, and a position where the focus evaluation value takes a maximum value by displacing the relative position of the lens with respect to the image sensor. a focus control means for performing a focusing operation that causes the lens to stop at , a storage means for holding a focus evaluation value at the time when the lens stops due to the focusing operation as a reference value, and a focus evaluation value obtained while the lens is stopped. In an autofocus video camera, the autofocus video camera is provided with a detection means for detecting that a state in which the reference value continues to be reduced by more than a threshold width within a period T2, and restarts the focusing operation when the state continues. An autofocus video camera characterized in that the period T2 is variable depending on the distance between a subject and an image sensor.