JP2675452B2 - Auto focus video camera - Google Patents

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 which automatically adjusts the focus based on a video signal obtained from an image pickup device.

[0002]

2. Description of the Related Art In an autofocus device for a video camera, a method of using a video signal itself from an image pickup device for evaluation of a focus control state has essentially no parallax and a shallow depth of field. There are many advantages such as accurate focusing even for a distant subject. In addition, no special sensor for autofocus is required, 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". A so-called hill-climbing servo control, which is described as, is known. This hill climbing servo control will be described with reference to FIG.

The image formed by the lens 1 becomes a luminance signal by the image pickup circuit 4 and is input to the focus evaluation value generation circuit 5. The focus evaluation value generation circuit 5 is configured as shown in FIG. 3, for example. Synchronous separation circuit 5 from video signal
The vertical synchronizing signal VD and the horizontal synchronizing signal HD separated by a are input to the gate control circuit 5b to set the focus area. The gate control circuit 5b sets a rectangular focus area FA in the central portion of the screen as shown in FIG. 13 based on the vertical synchronizing signal VD, the horizontal synchronizing signal HD, and the fixed oscillator output, and only this focus area range is set. The gate opening / closing signal which permits the passage of the luminance signal of the above is supplied to the gate circuit 5c.

Only the luminance signal corresponding to the range of the focus area is time-divisionally extracted by the gate circuit 5c, further passed through the HPF 5d to separate only the high frequency component, and the digital integration circuit 5e at the next stage 1 field. The high frequency component level of minutes is digitally integrated and output as the focus evaluation value of the current field.

The focus evaluation value output for each field from the focus evaluation value generating 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, 2 is transferred to the memory 7. That is, the first
The latest evaluation value is stored in the memory 6 and the evaluation value one field before is updated and stored in the second memory 7. This 2
The contents of the two memories are compared by the comparator 8, and the comparison output is input to the focus motor control circuit 9.

In the focus motor control circuit 9, when the evaluation value of the first memory> the evaluation value of the second memory, the current rotation direction of the focus motor 3 is maintained by the output of the comparator 8 and the first memory is maintained. If <the evaluation value of <the evaluation value of the second memory, the evaluation value tends to decrease, so that the focus motor 3 is reversed. The focus ring 2 that supports the lens 1 continues to move in the direction of increasing the focus evaluation value by the movement of the focus motor 3, and the lens 1 also moves in the optical axis direction in conjunction with this to move the lens to the image sensor (CCD). After changing the relative position and focusing, the object vibrates near the maximum point of the evaluation value after focusing. In this method, as long as there is an inclination in the focus evaluation value, the lens 1 is always moved in the direction in which the focus evaluation value is increased even if the subject changes, so that the focus can be followed without blurring and stopping.

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 shooting screen constantly shakes even after focusing on a stationary object. The lens currently used in a television camera changes the focal length by rotating the focus ring, which changes the angle of view of the shooting screen. 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 rotate forward and backward, the motor rotates in a certain direction due to the inrush current. Consume more power than if
Shooting time becomes shorter. In addition, since the focus ring is always rotated, problems such as wear of gears occur.

In order to remedy these drawbacks, the actual development of Sho 6
0-135712 (G02B7 / 11), and as shown in FIG. 10, the focus ring is driven in one direction to shift the evaluation value from the increasing direction to the decreasing direction. There has been proposed a method of returning the lens 1 to the position where the maximum value is taken and stopping when the focus evaluation value decreases by the threshold width L after passing through. But,
In the case of a video camera, it is necessary to make the focus position follow an ever-changing subject, and if the distance to the subject (subject distance) changes even after the lens is stopped at the in-focus position, the lens It is necessary to restart the mountain climbing operation.

For this reason, the applicant of the present invention previously disclosed in Japanese Patent Laid-Open No.
In No. 12112 (G02B7 / 11), when 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 subject that is changing every moment. Suggested a method. However, this method also has drawbacks common to the conventional hill climbing servo control described below.

That is, when another object crosses between the video camera and the subject while the main subject to be imaged is in focus, the lens tries to focus on the crossed object. The screen was unsightly due to displacement, blurring of the main subject, and changes in the angle of view. An example of this will be described with reference to FIGS. 4 and 5. In FIG. 4, the video camera Ca is photographing the main subject P, and when the subject is in focus, another subject Q is in front of the camera as indicated by an arrow. When it passes, all or part of the main subject P is shielded, the hill climbing operation is restarted so that the subject Q is focused, and the lens starts moving. However, since it takes some time to focus on the subject Q, the subject Q ends passing before the movement of the lens is completed. As a result, the subject P is out of focus during passage, and the subject Q is also out of focus. Immediately after the passage is completed, an unsightly screen appears in which the subject P is shown out of focus.

Therefore, conventionally, there is also proposed a method of prohibiting the lens drive for a fixed time (for example, the time expected to be required for the subject Q to finish passing) when the focus is out of focus. Has been done. However, in this method, when the main subject P itself approaches the camera as shown by the arrow as shown in FIG. 5, the lens drive is always started with a certain delay time after it is determined that the focus is out of focus. Therefore, the autofocus operation has poor responsiveness.

Therefore, the present applicant has filed Japanese Patent Application Laid-Open No. 63-2030.
As disclosed in Japanese Patent Publication No. 69 (H04N5 / 232), there is a focus control means for performing a hill climbing operation and stopping the lens at the lens position where the maximum point of the focus evaluation value is reached. It is stored as a reference value for detecting subject changes, and this reference value is compared with the current evaluation value. If the evaluation value drops sharply, the control by the focus control means is prohibited, and the level continues to drop for a predetermined period. In such a case, the control by the focus control means is restarted, and the focusing operation is not restarted when the evaluation value changes instantaneously, and quickly when the evaluation value level drops slowly. An autofocus device is proposed in which the focusing operation is restarted after a predetermined time when the focusing operation is performed and the level drop continues for a long time.

The prior art will be described below with reference to 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. 11 is a circuit block diagram of the circuit of this embodiment. The image formed by the lens 1 becomes a luminance signal by the image pickup circuit 4 having a CCD (image pickup element),
It is input to the focus evaluation value generation circuit 5. The focus evaluation value generation circuit 5 has the same configuration as that shown in FIG. 3 and outputs focus evaluation values for one field.

Immediately after starting the focusing operation, the first focus evaluation value is held in the maximum value memory 10 and the initial value memory 11. After that, the focus motor control circuit 15 causes the focus motor 3 to rotate in a predetermined direction, displaces the lens 1 in the optical axis direction accordingly, 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.

The focus motor control circuit 14 rotates the focus motor 3 in the first direction until the output of the second comparator 13 is large or small, and the current evaluation value is larger than the initial evaluation value. When the output is made, the rotation direction is maintained as it is, and when 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 maximum evaluation value held so far in the maximum value memory 10 with the current evaluation value, and the current evaluation value is larger than the contents of the maximum value memory 10 ( Two comparison signals of S1) or sufficiently small (S2) are output. 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 the present is always Retained.

A position memory 15 stores a focus ring position as a lens position in response to a focus ring position signal indicating a position of the focus ring 2 supporting the lens 1, and like the maximum value memory 10, a first comparison memory. Based on the output S1 of the container 12, the focus ring position when the maximum evaluation value is reached is updated so as to always be held.

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 to prevent malfunction due to noise in the evaluation value. First to do
At the same time as the output of the comparator 12 is output S2 that the current evaluation value is sufficiently smaller than the maximum evaluation value, the focus motor 3 is reversed. After this reverse rotation, the position memory 15
And the current focus ring position signal are compared by 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 motor 3 is stopped. The focus motor control circuit 14 functions. At the same time, the focus motor control circuit 14 outputs a lens stop signal LS. Thus, a series of focusing operations is completed. After that, in order to follow the change of the subject, if the change of the subject is detected by the subject change detection circuit 50 composed of the OR circuit 25 from the third memory 17 described later, and if the change of the subject is confirmed. , The motor 3 is started and the above-described series of focusing operations are repeated.

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

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

The third and fourth comparators 18, 19 in the subsequent stage are
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 mono-multis 20 and 21 and a first AND circuit 22 which will be described later, and the third memory 17
When the difference between the content of the third memory 17 and the current evaluation value exceeds the threshold A, an H-level output signal is output. Emit. 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,
It has a threshold value A relative to the contents of the third memory 17 and another threshold value B (75% of the contents of the third memory 17),
When the difference between the contents of the third memory 17 and the current evaluation value is not less than the threshold value A and not more than the threshold value B, an H level output signal is emitted.

Reference numeral 20 denotes a first mono-multi having a metastable period of a constant time T2, which is output in synchronization with the rising of the output of the third comparator 18. Reference numeral 21 is a second mono-multi that outputs an output in synchronization with the fall of the output of the first mono-multi 20.

A delay circuit 23 delays the output of the fourth comparator 19 by a fixed time T1. Here, the time T1 is an estimated time required for another subject Q in FIG. 4 to finish entering the focus area, and the time T2 is set as an expected time for another subject Q to finish passing through the focus area. (Specifically, for example, T1 = 0.1
sec, T2 = 0.5 sec). The output of the second mono-multi 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 together with the output of the fourth comparator 19 is AN.
It is input to the D circuit 24, and both outputs of the AND circuits 22 and 24 are two inputs of the OR circuit 25. OR circuit 2
The five outputs are supplied to the focus motor control circuit 14 as subject change detection signals.

Next, the operation of the subject change detection circuit 50 will be described. The waveform diagrams shown in FIGS. 7 to 9 are:
Each corresponds to the situation shown in FIGS. 4 to 6. That is, FIG. 7 shows a case where another object Q passes in front of the camera Ca (first case) while photographing the main subject P as shown in FIG. 4, and FIG. 8 shows the main subject as the camera as shown in FIG. FIG. 9 shows a case where the main object is approaching (second case), and FIG. 9 shows a case (third case) where another object comes in and stops while photographing the main subject. In FIGS. 7 to 9, “a” shows a state of temporal change of the focus evaluation value 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 horizontal axis shows elapsed time. The two-dot chain lines indicate the widths of the above-mentioned 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. Output, g is AND circuit 24
, And h represents the output of the OR circuit 25, respectively.

In the above-mentioned first case, the focus evaluation value once decreases by the threshold value B or more as shown in FIG. 7, but recovers by the time T2. In addition, since the evaluation value of another object passes extremely quickly across the 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 of the. Since the passage of other objects is completed within the time T2, the output of the first AND circuit 22 is also maintained at L level, and the subject change detection signal corresponding to the output h of the OR circuit 25 also becomes L level. That is, it is not 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 is shown.
Also exceeds the 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 the output b, and the output f of the first AND circuit 22 also becomes H level after the time T2, but the output g preceding this output f Is supplied to the focus motor control circuit 14 as an object change detection signal via the OR circuit 25. In response to this, the focus motor control circuit 14 clears the contents of the initial value memory 11 and the maximum value memory 10 and restarts a series of focusing operations. Therefore, the change of the subject can be confirmed quickly by the output h before the output f, so that the autofocus operation with good followability is realized.

In the third case, that is, when another object comes in front of the camera and stops, the evaluation value drops sharply, and the pulse width of the output b of the fourth comparator 19 is longer than the time T1. Since it 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 maintains the H level after that in synchronization with the rising of the output b, the first AND
The output f of the circuit 22 becomes H level after a time T2 from the rise of the output c, is supplied to the focus motor control circuit 14 as an object change detection signal via the OR circuit 25, and restarts the focusing operation. Therefore, in this third case, the time T2 has passed since another object came in front of the camera.
After a relatively long time delay, the focusing operation is restarted. However, when the distance to the subject changes drastically, it takes about 2 seconds from the start of driving of the focus motor 3 to the focusing. It takes time, and a time delay of time T2 (for example, 0.5 sec) does not matter.

[0031]

In the above-mentioned prior art,
Estimated time required for another subject to pass through the focus area 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. T2
Is set to a fixed value in advance. In particular, in order to avoid erroneous cross-cutting determination, it is necessary to set the time T2 sufficiently large at the expense of the response that the focusing operation is restarted when another subject enters.

By the way, if the main subject is located far from the camera, there is a high possibility that another subject will cross between the main subject and the camera. Therefore, a reliable judgment is made by setting the time T2 large at the expense of the response. However, when the main subject is close to the camera, the possibility of another subject crossing the two is extremely small. Therefore, if a long time is set for cross-cutting monitoring of another subject and the cross-cutting actually occurs and the focusing operation needs to be restarted, the disadvantage that it cannot respond promptly becomes a problem.

[0033]

Therefore, 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 time T2 is made variable in accordance with this distance, and When the distance is long, the probability that another subject will cross is high, so T2 is lengthened, and conversely, when the distance between the two is short, the probability is low, so T2 is shortened.

[0034]

Since the present invention is configured as described above, even if a subject other than the main subject is crossed or approached, the focusing operation may be erroneously restarted or the restart may be delayed depending on the probability. Can be kept to a minimum.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. 11 is different from the first and second monomultis 20 and 21 in that it includes an oscillator that generates a fixed frequency clock, and a DOWN counter 40 that counts down the clock is combined with a third comparator 18 and an AND circuit. This is that the counter is inserted between 22 and a preset circuit 41 of this counter is further added.

The preset circuit 41 receives the ring position signal of the position memory 15 and changes the preset value supplied to the counter 40 in accordance with the lens position where the focus evaluation value has the maximum value, that is, the lens position when the focusing operation is completed. Circuit,
The preset value is increased as the lens position at the completion of the focusing operation approaches the infinity point, and conversely decreases as it approaches the near point.

The DOWN counter 40 includes a third comparator 18
In synchronization with the rising edge of the output, the oscillator clock countdown from the preset value set by the built-in preset circuit 41 is started, and when the count value reaches zero, the pulse width of the second mono-multi 21 in FIG. Emits the same pulse as. Therefore, the period until the counter 40 starts counting down, reaches zero, and emits a pulse is
This corresponds to the time T2 when another object finishes passing the focus area, and as a result, as shown in FIG. 12, this time T2 becomes a variable value according to the object distance to the main object when the focusing operation is completed. It has become.

In this way, when another subject crosses the focus area or approaches and stops at a position close to the camera, the time T2 for monitoring the change of the subject becomes short, and (e) and (f) of FIG. ), (H) pulse is shifted forward as shown by a chain line, and another subject enters the focus area and is stopped, so that the necessary focusing operation is restarted earlier.

Further, when another subject is far from the camera,
When crossing the focus area, the time to finish passing through the focus area is longer than that at a near position, and the recovery of the focus evaluation value is delayed as shown by the chain line in (a) of FIG.
The pulses of (b), (c), and (d) are also shifted backward like a chain line. Unless the time T2 is shifted backward, the pulse of (e) is generated as (f), and Since the subject has changed before the subject has finished passing through, the focusing operation is restarted, and the configuration of the present embodiment makes it possible to extend the time T2 and cope with it.

[0040]

As described above, according to the present invention, the subject distance between the main subject and the image pickup device is detected from the lens position immediately after the focusing operation is completed, and the estimated passage time T2 is made variable according to this distance, When the distance is long, the probability of crossing or approaching another subject is high, so T2 is lengthened to prevent erroneous determination that the subject has changed during the crossing, and conversely when the distance between the two is short. Since the probability of crossing or approaching another subject is low, T2 is shortened to quickly restart the focusing operation by approaching or stopping another subject.

[Brief description of the drawings]

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

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

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

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

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

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

FIG. 7 is an explanatory diagram of waveforms of each part when another subject crosses.

FIG. 8 is an explanatory diagram of waveforms of each part when a main subject approaches.

FIG. 9 is a waveform explanatory diagram of each part when another subject enters and stops.

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

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

FIG. 12 is a diagram illustrating a relationship between a lens position and time T2 when focusing is performed.

FIG. 13 is a diagram illustrating a focus area.

[Explanation of symbols]

 5 Focus Evaluation Value Generation Circuit 14 Focus Control Circuit 17 Third Memory 40 DOWN Counter 41 Preset Circuit 50 Subject Change Detection Circuit

Claims (1)

(57) [Claims] 1. A focus evaluation value generating means for outputting a high frequency component level of an imaged video signal for a certain period as a focus evaluation value, and a relative position of a lens with respect to an image pickup element is changed so that a photographer desires to take an image. Focus control means for performing a focusing operation to stop the main subject positioned at the center of the screen at a position where the focus evaluation value takes the maximum value, and focus evaluation at the time when the lens is stopped by the focusing operation. A storage unit that holds the value as a reference value, and a state in which a focus evaluation value obtained while the lens is stopped after completion of the focusing operation is reduced to a threshold value width or more with respect to the reference value continues for a period T2. An autofocus video camera that restarts the focusing operation when the state is maintained for a period T2. Immediately after, the subject distance detecting means for detecting the distance between the main subject and the image sensor from the position of the lens, the period T2 is lengthened when the subject distance is long, and the period T2 is shortened when the subject distance is short. An autofocus video camera, characterized in that it comprises a period setting means.