JPH0414964A - Automatic focus device - Google Patents

Abstract

PURPOSE:To keep focusing state even when an object is rapidly moving by suppressing the movement of a focus lens based on a 2nd focus evaluation signal when the fluctuation of the 2nd focus evaluation signal being a low frequency signal reaches a level or over. CONSTITUTION:Detection circuits 16,17 detect respectively levels of components at a lst frequency band and a 2nd frequency band lower than the lst frequency band included in a luminance signal to output lst and 2nd focus evaluation signals. Then the fluctuation of the output of the detection circuit 17 is detected and when the fluctuation is a range or over, a fluctuation detection signal is outputted by a fluctuation detection circuit 25 and even when the level of the lst focus evaluation signal is small and the focus lens is moved mainly by the 2nd focus evaluation signal, if the fluctuation of the 2nd focus evaluation signal is large, the movement of the focus lens is suppressed. Thus, even when an object is rapidly moved, the focusing state is not mis-detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an autofocus device, and particularly to an autofocus device in an imaging device such as a video camera.

[Background Art] Imaging devices such as video cameras generally include an autofocus device for automatically focusing on a subject.

One of the operating principles of an autofocus device is a method called hill-climbing control as described below.

A signal in a predetermined high frequency band is extracted from the luminance signal obtained from the image sensor, and its amplitude is detected and converted into a level signal. All of the level signals obtained in this manner are integrated over a certain period of time, such as one field or one frame. The value obtained as a result is called a focus evaluation value. That is, a focus evaluation value is calculated for each field or each frame.

When the focus lens position is close to the in-focus state, the resulting image has so-called sharp edges. A signal representing such an image contains many high frequency components. On the other hand, edges are not clear in out-of-focus images. Such a signal will have relatively few high frequency components. Therefore, the in-focus state at the position of the focus lens can be evaluated by detecting the amount of high-frequency components among the luminance signal components output from the video signal processing circuit.

FIG. 5 is a diagram showing the relationship between the position of the focus lens and the focus evaluation value. Referring to Figure 5, the horizontal axis is the focus lens position, the vertical axis is the focus evaluation value, and curve 3
4 shows the focus evaluation value of a high frequency component, and a curve 35 shows the characteristics of the focus evaluation value of a lower frequency component.

Lens position P in the figure indicates the lens position where the focus is exactly achieved. As can be seen from FIG. 5, the focus evaluation value shows a maximum value at the lens position P, and monotonically increases or decreases before and after that.

Therefore, in mountain climbing control, a focus evaluation value is obtained for each field while moving the focus lens in a fixed direction. The obtained focus evaluation value is compared with the focus evaluation value one field before. As a result, if the focus evaluation value of the current field is larger, the focus lens is further moved in the same direction as before. Conversely, if the focus evaluation value one field before is larger, the focus lens is moved in the opposite direction. This is because, as can be seen with reference to FIG. 5, it can be determined that the position of the focus lens has exceeded the in-focus position P. In this way, the focus lens is stopped when a focus evaluation value that is the same as the focus evaluation value one field before is obtained.

For example, assume that the focus lens is in the lens position Q in FIG. 5 in its initial state. By the above-mentioned hill climbing control, the focus lens initially moves in the positive direction. When the focus lens moves beyond the in-focus position P, the focus evaluation value begins to decrease.

Therefore, the focus lens returns to the negative direction and stops at the focus position P. As described above, the focus lens automatically moves to the in-focus position.

Referring to FIG. 6, in reality, the subject is generally placed at the center of the screen at 5 degrees. In order to focus on the subject, a part of the focus control area 51 at the center of the screen is usually focused, not the entire screen 5o.

However, the following problems occur in the above-described mountain climbing control.

The frequency band and level of the obtained luminance signal vary depending on the brightness, shading, etc. of the subject.

Depending on the conditions of the subject, there may be cases where almost no frequency band components to be extracted are present in the obtained luminance signal, or even if they are present, their level may not be sufficient. In such a case, even if an attempt is made to obtain a focus evaluation value from the luminance signal, an accurate value may not be obtained.

Furthermore, as shown in FIG. 5, when the extracted frequency band is on the high frequency side, the obtained focus evaluation value becomes small. In particular, this tendency becomes more pronounced as the position of the focus lens deviates from the in-focus position. Therefore, for example, if the position of the focus lens deviates significantly from the in-focus position in the initial state, the focus evaluation value will not be obtained sufficiently, especially if the focus evaluation value is obtained only from components in the high frequency band, and the autofocus will fail. There were times when the device did not function.

Referring to FIG. 5, the higher the frequency band is, the sharper the curve representing the change in focus evaluation value becomes. But its height is low. On the other hand, when the frequency band is low, the curve showing the change in focus evaluation value forms a gentle mountain. But its height is high.

Therefore, in order to solve the above-mentioned problem, it has been proposed to extract a plurality of different frequency band components from the luminance signal, obtain a focus evaluation value for each frequency band component, and use these depending on the subject. For this reason, the conventional autofocus device is provided with a series of circuit sections necessary for determining a focus evaluation value in the same number as the number of frequency bands to be extracted.

For example, Japanese Patent Publication No. 60-2827, Utility Model Publication No. 61-7
This type of device is disclosed in Japanese Patent No. 2967 and the like.

FIG. 4 is a schematic block diagram showing an example of a conventional autofocus device. Referring to FIG. 4, this autofocus device has a band pass filter that passes a certain frequency band, for example, around 1 MHz.
ilter: hereinafter abbreviated as rBPFJ)] and BP
BPF2, which passes a frequency band lower than FI, for example, around 100 KHz, and a detection device connected to the output of BPFI,2, respectively, detecting the output level of BPFI,2, and outputting a level signal corresponding to the focus evaluation value. circuits 16 and 17; a sync separation circuit 11 for separating sync signals Vs and Hs from the applied luminance signal; A gate circuit 12 for providing only the luminance signal obtained from the section to the BPF 1.2, a switch 19 whose input terminals are connected to the outputs of the detection circuits 16 and 17, respectively, and a switch 19 that responds to the output level of the detection circuit 16. A switching control circuit 18 for switching the switch 19, a sampling circuit 24 for sampling the output of the switch 19 according to a predetermined sampling pulse, and a focus motor is controlled according to the output of the sampling circuit 24 to perform so-called mountain climbing control. The control circuit 21 includes a control circuit 21 for controlling.

The conventional device operates as follows. Synchronous separation circuit] 1
separates the horizontal synchronization signal Hs and the vertical synchronization signal Vs from the luminance signal Y. The separated synchronization signal Is%Vs is given to the gate circuit 12.

The gate circuit 12 provides the luminance signal Y to the BPFI, 2 only when the focus control area 51 is being scanned, based on the provided synchronization signal. BPFI extracts a band component near IMHz from the luminance signal Y, and sends it to the detection circuit 16.
give to The detection circuit 16 converts the level of the output signal of the BPFI into a DC voltage level and outputs it as a level signal.

The BPF 2 extracts a band component around 100 KHz from the luminance signal Y and supplies it to the detection circuit 17 . The detection circuit 17 is B
The level of the signal output from PF2 is detected, converted to a DC level, and output as a level signal.

Referring to FIG. 5, the switching control circuit 18 is connected to the detection circuit 16.
The switch 19 is controlled so that the output of the detection circuit 16 is given to the sampling circuit 24 when the level of the level signal output from the sampling circuit 16 is higher than a certain threshold value VTH, and the output of the detection circuit 7 is given to the sampling circuit 24 at other times. The sampling circuit 24 samples the output of the switch 19 according to a predetermined sampling pulse, and supplies the sample to the control circuit 21 .

The control circuit 21 obtains a focus evaluation value based on the signal given from the sampling circuit 24 and performs hill climbing control.

For example, if the focus evaluation value obtained from the IMHz frequency band is larger than the threshold value VTM, the control circuit 2
1 performs hill climbing control based on the focus evaluation value obtained from the IMHz frequency band component. On the other hand, if the focus evaluation value obtained from the IMHz frequency band is smaller than the threshold value VTN, the control circuit 21
Mountain climbing control is performed according to the focus evaluation value obtained from the 0KH2 frequency band component.

By doing as described above, when the focus lens is located far away from the in-focus position, hill-climbing control is performed using the focus evaluation value in the lower frequency band. In this region, the focus evaluation value on the low frequency side takes a larger value, so the focus lens can be moved correctly even if it deviates significantly from the in-focus position.

On the other hand, when the lens approaches the in-focus position, hill-climbing control is performed using the focus evaluation value in the higher frequency band. This is because near the in-focus point, the focus evaluation value in the higher frequency band has a larger rate of change, allowing more precise focusing operation. Even when the number of extracted frequency bands increases, the processing performed by the control circuit 21 is the same as described above.

[Problems to be Solved by the Invention] However, the conventional device has the following problems.

2. Description of the Related Art In imaging devices such as video cameras, the camera itself is often moved during shooting, such as so-called banging or tilting. Furthermore, there are many cases where the subject itself moves violently. In such a case, due to the limitations of the camera's dynamic resolution, even if the focus lens is at the in-focus position, the situation will be the same as if the video signal were out of focus. The focus evaluation value obtained is a very small value, and this is particularly noticeable in signals on the high frequency band side.

In such a case, the conventional device operates by switching to hill-climbing control using a signal on the low frequency side.

However, it is inevitable that the focus evaluation value on the low frequency side also decreases to some extent. Moreover, unlike the high frequency components, the low frequency components vary considerably depending on the movement of the screen. Therefore, in the above case, mountain climbing control may not be performed correctly even though the focus lens is actually at the in-focus position. That is,
Although there is no need to move the focus lens, there is a drawback in that it is determined that the focus lens is not at the in-focus position and the focus lens is moved.

Therefore, an object of the present invention is to provide an autofocus device that can maintain a focused state even when a subject is moving rapidly.

Means for Solving the Problems] An autofocus device according to the present invention includes means for capturing an image of a subject and extracting a luminance signal, and a means for detecting a level of a predetermined first frequency band component included in the luminance signal. means for outputting a first focus evaluation value signal; and means for detecting the level of a component of a second frequency band lower than the first frequency band included in the luminance signal and outputting a second focus evaluation value signal. means for controlling and moving the focus lens in response to the first and second focus evaluation value signals; and a range of variation of the second focus evaluation value signal is determined in advance. a fluctuation detection means for detecting that the threshold level is exceeded; and a focus lens movement control means based on a second focus evaluation value signal in response to the first focus evaluation value signal and the output of the fluctuation detection means. and suppressing means for suppressing the movement.

[Function] In the autofocus device according to the present invention, if the fluctuation of the second focus evaluation value signal on the low frequency side exceeds a certain level, the movement of the focus lens based on the second focus evaluation value signal is suppressed. be done. Therefore, the first
Even when the second focus evaluation value is small and the focus lens is moved mainly based on the second focus evaluation value, the focus lens will not be moved unless the second focus evaluation value is stabilized to some extent. do not have.

[Example] FIG. 1 is a block diagram showing the operating principle of an autofocus device according to the present invention. The difference between the device shown in FIG. 1 and the conventional device shown in FIG. 4 is the detection circuit]7
A fluctuation detection circuit 25 is connected to the output of the detection circuit 17 and outputs a fluctuation detection signal when the fluctuation exceeds a certain range. An operation suppression circuit connected to the output of the circuit 25 and suppressing the operation of the control circuit 21 when the output of the detection circuit 16 is below a certain level and the variation detection circuit is input from the variation detection circuit 25. 26 will be newly included.

The fluctuation detection circuit 25 detects the magnitude of fluctuation in the output of the detection circuit 17, for example, in the following manner.

The fluctuation detection circuit 25 integrates the output of the detection circuit 17 for a certain period of time and obtains the average value. The fluctuation detection circuit 25 also monitors the maximum and minimum values of the output of the detection circuit 17. The fluctuation detection circuit 25 detects that the fluctuation is large, for example, when the maximum value of the output of the detection circuit 17 exceeds the average value by a certain percentage, or when the minimum value of the output of the detection circuit 17 falls below the average value by a certain percentage or more. A fluctuation detection signal to that effect is given to the operation suppression circuit 26.

In the apparatus shown in FIG. 1, parts that are the same as in the apparatus shown in FIG. 4 have been given the same reference numerals and the same names. Their functions are also the same. Therefore, a detailed explanation thereof will not be repeated here.

By providing the fluctuation detection circuit 25 and the operation suppression circuit 26, the following effects are produced. When the output of the detection circuit 16 is sufficiently large, the switching control circuit 18 controls the switch 19 to provide the output of the detection circuit 16 to the sampling circuit 24. The sampling circuit 24 samples the output of the detection circuit 16 according to a predetermined sampling pulse, and supplies the sample to the control circuit 21 . The control circuit 21 performs hill climbing control based on the signal given from the sampling circuit 24. In this case, the operation suppression circuit 26 is
Since the output of is sufficiently large, the movement of the focus lens is not suppressed by the control circuit 21. That is, the movement of the focus lens is performed by hill-climbing control based on the focus evaluation value signal obtained from the IMHz frequency band component.

If the subject moves violently or the video camera moves due to an action such as banging or tilting, the following will occur. High frequency band components included in the video signal are reduced. Therefore, the output level of the detection circuit 16 becomes low. The switching control circuit 18 switches the input of the switch 19 to the detection circuit 17 side. Therefore, the control circuit 21
The mountain climbing control of the focus motor is 100KHz.
This is performed using a focus evaluation value signal obtained from the frequency band components of .

However, when the subject is moving rapidly as described above, the output of the detection circuit 17 fluctuates greatly. When the output of the detection circuit 17 is greatly fluctuating, if hill climbing control is performed based on the output value, the position of the focus lens will not be stable.

In such a case, the fluctuation detection circuit 25 monitors the level fluctuation of the output of the detection circuit 17 by operating as described above.

The fluctuation detection circuit 25 provides a fluctuation detection signal to the operation suppression circuit 26 when the level fluctuation of the output of the detection circuit 17 exceeds a certain range as described above. Operation suppression circuit 26
operates as follows when the level of the output of the detection circuit 16 is below a certain level, that is, when the output of the detection circuit 17 is supplied to the sampling circuit 24.

The operation suppression circuit 26 suppresses movement of the focus lens by the control circuit 21 when the fluctuation detection signal is applied. Therefore, in this case, the focus lens does not move. On the other hand, the operation suppressing circuit 26 does not suppress the operation of the control circuit 2 when the fluctuation detection signal is not input. Therefore, in this case, the control circuit 21 has a frequency of 100KHz.
The focus lens is moved by mountain climbing control based on the focus evaluation value signal obtained from the frequency band components.

As described above, when the focus evaluation value signal obtained from the signal of the high frequency band component decreases, hill-climbing control is normally performed in accordance with the focus evaluation value obtained from the signal of the lower frequency band component. however,
If this continues, as mentioned above, if the signal component on the low frequency band side fluctuates significantly, it will be mistakenly determined that the focus lens is out of focus, even though there is no need to move the focus lens, and the focus lens will be moved. It is possible that it may move.

However, in the autofocus device according to the present invention,
The fluctuation detection circuit 25 monitors fluctuations in the focus evaluation value obtained from the frequency component on the 1.00KH2 side. In this way, by monitoring the fluctuation of the focus evaluation value obtained from the signal on the low frequency band side,
For example, if the subject is simply moving rapidly, movement of the focus lens is suppressed. therefore,
Mountain climbing control will not cause you to lose focus.

On the other hand, if the fluctuation of the focus evaluation value obtained from the components on the low frequency band side is within a predetermined range, it is determined that the subject is not simply moving violently. Therefore, in this case, it is determined that the focus lens is really out of focus, and hill climbing control is performed in accordance with the focus evaluation value obtained from the component on the low frequency band side.

As described above, in the autofocus device according to the present invention, when the focus evaluation value obtained from the component on the high frequency band side shows a large value, the focusing operation is performed according to the focus evaluation value obtained from the high frequency band side.

On the other hand, if the focus evaluation value obtained from the components on the high frequency band side is small, hill climbing control is performed according to the focus evaluation value obtained from the components on the low frequency band side.

In this case, if the focus evaluation value obtained from the components on the low frequency band side varies greatly, such as when the subject is simply moving rapidly, the focus lens is not moved. It never goes out of focus. On the other hand, if the focus is really out of focus,
The focus evaluation value obtained from the low frequency band side falls within a certain range of fluctuation. In this case, the focusing operation is performed according to the focus evaluation value obtained from the components on the low frequency band side. Therefore, the autofocus device according to the present invention can maintain a well-focused state even when the subject moves rapidly.

Figure 2 shows the CPU (central process).
FIG. 2 is a block diagram of an example of an autofocus device according to the present invention implemented using an autofocus device. Referring to FIG. 2, this device includes a BPFI that passes a certain frequency band, for example, IMHz, a BPF 2 that passes a frequency band lower than the BPFI, for example, around 100 KHz, and the absolute value of the signal output from the BPFI, 2. A digital absolute value conversion circuit 42.43 for converting into a signal, and a digital absolute value conversion circuit 42.43.
an integrating circuit 30.31 for integrating the outputs of for a certain period of time, for example, one field; and a CPU 2 that receives the output of the integrating circuit 30.31 and performs calculations for mountain climbing control.
0, a focus motor control circuit 27 that is controlled by the CPU 20 and controls a focus motor for moving the focus lens, and a synchronization separation circuit 11 that extracts the horizontal synchronization signal Hs and the vertical synchronization signal Vs from the luminance signal. A switching gate control circuit 15 receives synchronization signals Hs and Vs and generates necessary pulses to perform focus control processing only on the video signal obtained from the focus control area 51 shown in FIG. an oscillation circuit 14 that oscillates a clock having a period of
, a gate circuit 13 connected to the switching gate control circuit 15 and the oscillation circuit 14, and a gate circuit 13 connected to the gate circuit 13,
A latch pulse generation circuit 23 that generates a latch pulse for controlling the integration circuit 30.31, and an A/D (analog-digital) connected to the gate circuit 13 and controlling the digital absolute value conversion circuit 41.42. and an A/D conversion pulse generation circuit 22 for generating conversion pulses.

The digital absolute value conversion circuit 4] detects the level of the output signal of the BPF 1, and converts the output signal of the absolute value conversion circuit 3 into a DC potential according to the level, and converts the output of the absolute value conversion circuit 3 into digital form. and an A/D converter 5.

Similarly, the digital absolute value conversion circuit 42 converts the absolute value converting circuit 4 connected to the BPF 2 and the output of the absolute value converting circuit 4 into A.
and an A/D converter 6 for A/D conversion.

Integration circuit 30 includes an adder 7 connected to the output of A/D converter 5 and a latch circuit 8 for latching the output of adder 7. The output of the latch circuit 8 is given to the CPU 20 and also to the input of the adder circuit 7. The adder circuit 7 is for adding the output of the A/D converter 5 and the output of the latch circuit 8 and providing the result to the latch circuit 8.

Integration circuit 31 similarly includes an adder 9 connected to the output of A/D converter 6 and a latch circuit 10 connected to the output of adder 9. The output of the latch circuit 10 is
and the input of the adder 9.

The A/D converter 5.6 operates by being supplied with a pulse for sampling from the A/D pulse generation circuit 22. The latch circuit 8.10 latches the output of the adder 7.9 according to the latch pulse given from the latch pulse generation circuit 23, and supplies it to the adder 7.9 and the CPU 20.

The apparatus shown in FIG. 2 operates as follows.

The synchronization separation circuit 11 separates a horizontal synchronization signal Hs and a horizontal synchronization signal Vs from a luminance signal Y obtained from an image sensor (not shown). The separated synchronization signals Hs and Vs are applied to the switching gate control circuit 15. The switching gate control circuit] 5 generates a signal that opens the gate of the gate circuit 13 only when the focus control area 5] (FIG. 6) is being scanned from the synchronization signals Hs and Vs.
Give to 3.

The gate circuit 13 responds to the signal given from the switching gate control circuit 15 and changes the clock output from the oscillation circuit 14 to A/A only while the focus control area 51 is being scanned.
D conversion pulse generation circuit 22, latch pulse generation circuit 23
give to

The A/D conversion pulse generation circuit 22 includes an A/D converter 5.6.
Generates sampling pulses to operate the A/
to the D converter 5.6.

The BPFI passes only the IMHz frequency band component and supplies it to the absolute value conversion circuit 3. The absolute value conversion circuit 3 is a BPF
The signal output from I is detected, converted to a DC potential representing its level, and provided to the A/D converter 5. A/D converter 5
digitizes the output of the absolute value conversion circuit 3 in synchronization with the A/D conversion pulse and supplies it to the adder 7.

The BPF 2 extracts a 100 KHz frequency band component from the luminance signal and supplies it to the absolute value conversion circuit 4 .

The absolute value conversion circuit 4 detects the signal output from the BPF 2,
The A/D converter 6 converts the level into a DC potential representing the level.
give to The A/D converter 6 converts the output of the absolute value converting circuit 4 into a digital signal in synchronization with the A/D conversion pulse, and supplies the digital signal to the adder 9.

Latch pulse generation circuit 23 generates a latch pulse for operating latch circuit 8.10, similar to the A/D conversion pulse, and supplies it to latch circuit 8.10.

Adder 7.9 adds the output of A/D converter 5.6 and the output of latch circuit 8.10, respectively, and adds the output of latch circuit 8.10.
Give 10. The latch circuit 8.10 latches the output of the adder 7.9 in synchronization with the latch pulse, and supplies it to the CPU 20 as well as to the input of the adder 7.9.

As described above, the digital absolute value conversion circuits 41 and 42 sequentially sample the luminance signals obtained from the image of the focus control area and convert them into digital signals. The converted digital signal is sent to the integration circuit 30.31.
At , the signals are accumulated one after another in the latch circuit 8.10. The integrated value is given to the CPU 20. Latch circuit 8.
The contents of 10 are cleared in synchronization with the vertical synchronization signal.

Therefore, by the CPU 20 taking in the contents of the latch circuits 8 and 0 before they are cleared, the IMH
Focus evaluation values are obtained based on each frequency band component of z and 100 KHz.

The CPU 20 performs mountain climbing control in accordance with the focus evaluation value based on the two frequency band components obtained, and drives a focus motor (not shown) via the focus morph control circuit 27.

The feature of the autofocus device according to the present invention is that the CPU 2
This is realized by a program executed at 0.

FIG. 3 is a schematic flowchart of the program. Referring to FIG. 3, CPU 20 operates according to the following procedure.

In step S1, a sampling pulse detection operation is performed.

In step S2, it is determined whether a sampling pulse has been detected. If the answer is YES, 1Ill proceeds to step S3. Otherwise, control returns to step S1.

The sampling pulse is given to the CPU 20 from a timing pulse generation circuit (not shown) at the time when the operation of the focus control area 51 (FIG. 6) is completed.

In step S3, focus evaluation values are sampled. The latch circuit 8 provides the focus evaluation value A to the CPU 20. Latch circuit] 0 gives focus evaluation value B to the CPU 20. By sampling the output of the latch circuit 8.10 in accordance with the aforementioned sampling pulse, focus evaluation values for one field of the focus control area are obtained for two frequency bands, IMHz and 100 KHz.

In step S4, it is determined whether the focus evaluation value A is greater than or equal to a predetermined constant value. If the answer is YES, control proceeds to step S5. Otherwise, control proceeds to step S7.

If the control proceeds to step S5, this means that the focus evaluation value obtained from the components on the high frequency band side has a sufficiently large value. Mountain climbing control is performed according to the focus evaluation value A. To this end, in step S5, the contents of the counter C prepared for calculating the average value of the focus evaluation values obtained from the components on the low frequency band side are cleared.

Subsequently, in step S6, 0 is set to a flag indicating which frequency band component the focus evaluation value obtained is to be used for mountain climbing control. Control then proceeds to step 513.

On the other hand, if the control proceeds to step S7 as a result of the determination in step S4, the following operations are performed. What should be noted is that in this case, since the focus evaluation value A is smaller than a certain value, hill climbing control is basically performed in accordance with the focus evaluation value B obtained from the component on the low frequency band side.

In step S7, the contents of counter C are incremented by one. The contents of counter C indicate the number of samples of focus evaluation value B.

In step S8, it is determined whether the content of counter C is smaller than a predetermined constant n.

The constant n is selected to be larger than a certain number so that fluctuations in the focus evaluation value B can be calculated with high reliability. If the answer to the judgment in step S8 is No,
Control proceeds to step S9, otherwise control returns to step S1.

In step S9, since the number of samples of the focus evaluation value B is sufficiently large, the recent n
Based on the individual focus evaluation value B, its maximum value MA
X, the minimum value MIN, and the average value AVE are calculated.

In step 510, the maximum value MAX is the average value AV
It is determined whether or not it is greater than 9/8 times H. If the answer is No, control proceeds to step S11. In this case, the range of fluctuation of the focus evaluation value B is at most the average value A
This means that the upper side of VH is within 78 times the average value AVH. If the answer to the determination in step 510 is YES, this means that the maximum value MAX of the focus evaluation value B is fluctuating in a range larger than 1/8 times the average value AVH. Therefore, since the focus evaluation value B on the low frequency band side is not sufficiently stable, processing such as driving the focus lens is not performed, and the control returns to step S1.

Subsequently, in step Sll, the minimum value M IN is
It is determined whether it is smaller than 7/8 times the average value AVH. If the answer is no, control continues to step 512.

In this case, the focus evaluation value B of the past n times is the average value A
This means that it is within the lower 1/8 of VE. Since the focus evaluation value B is determined to be sufficiently stable, mountain climbing control is performed in accordance with the focus evaluation value B on the low frequency band side.

Therefore, in step 512, the flag is set to 1.

If the answer to the determination in step 51.1 is YES, this means that the minimum value of the focus evaluation value B does not fall within the region of 1/5 AVE below the average value AVE. Since it is determined that the focus evaluation value B on the low frequency band side is not sufficiently stable, control returns to step S1.

After the flag is set to 1 in step S12, control proceeds to step 51B.

In step 813, the direction and amount of movement of the focus lens are determined by mountain climbing control according to the contents of the flag. That is, the hill climbing control is performed according to the focus evaluation value A if the content of the flag is O, and according to the focus evaluation value B if the content of the flag is 1.

In step 51.4, the focus motor is driven by a predetermined amount in a predetermined direction according to the value determined in step 3131::. After that, the control is at step S]
Return to

The processing performed in steps S9, S10, and Sll is such that the focus evaluation value B is centered around the average value AVE from the past 0 samplings, and is set at 1/5 AVE above and below
This is a judgment as to whether or not it falls within the range of . If the focus evaluation value B falls within this range of variation, it is determined that the focus evaluation value on the low frequency band side is stable. Therefore, as described above, hill climbing control is performed using the focus evaluation value B on the low frequency band side. On the other hand, step S1
If the answer to the judgment in 0.51.1 is YES, this indicates that the focus evaluation value B is not sufficiently stable. In this case, the control directly returns to step S1 from step 5IOSSll, so step S13 and step S
Hill climbing control in step 14 is not performed. That is, the answer to the determination in step S4 is No, and the answer to the determination in step SIO or step S11 is YE.
As long as S, the focus lens will not move. Therefore, even when the focus evaluation value A obtained from the components on the high frequency band side is small, as long as the focus evaluation value B obtained from the components on the low frequency band side fluctuates greatly, the focus lens No movement takes place. If the subject is moving rapidly, the above-mentioned conditions will occur, or even in such a case, the focus lens will not be moved by mistake. As a result, there is no risk of losing focus due to rapid movement of the subject.

In a case where the subject no longer moves, if the lens is at the in-focus position, the focus evaluation value obtained from the component on the high frequency band side increases again. Therefore, the answer to the determination in step S4 becomes YES, and the hill climbing control is again performed based on the focus evaluation value A.

On the other hand, if the focus lens is not at the in-focus position after the movement of the subject has subsided, step 813 and S14
In this step, hill-climbing control is performed using the focus evaluation value B obtained from the components on the low frequency band side. Therefore, as described above, the lens gradually moves to the in-focus position, and finally the focus evaluation value A on the high frequency band side becomes a sufficiently large value. As a result, normal mountain climbing control will be performed.

The present invention has been described above according to embodiments. However, the above-described embodiments are merely illustrative, and it goes without saying that the present invention can be implemented with various modifications.

[Effects of the Invention] As described above, according to the present invention, even when the level of the first focus evaluation value signal is small and the focus lens is moved mainly based on the second focus evaluation value signal, the first focus evaluation value signal is If the fluctuation of the focus evaluation value signal No. 2 is large, the movement of the focus lens is suppressed. The above-mentioned condition occurs when the focus evaluation value drops significantly due to rapid movement of the subject or movement of the imaging device. The focus state will not be incorrectly determined. Therefore, the focus lens will not be moved unnecessarily to determine focus.

That is, it is possible to provide an autofocus device that can maintain a well-focused state even when the subject is moving rapidly.

[Brief explanation of the drawing]

FIG. 1 is a principle block diagram of an autofocus device according to the present invention, FIG. 2 is a circuit block diagram when the autofocus device according to the present invention is realized using a CPU, and FIG. 2 is a schematic flowchart of a program executed by the CPU 20 in FIG. 2, FIG. 4 is a schematic block diagram of a conventional autofocus device, and FIG. 5 is a diagram showing the relationship between focus lens position and focus evaluation value. , FIG. 6 is a schematic diagram showing the relationship between the screen and the focus control area. In the figure, 1.2 is a band pass filter, 16.17 is a detection circuit, 18 is a switching control circuit, 19 is a switch, and 20 is a C
PU, 24 is a sampling circuit, 25 is a fluctuation detection circuit,
26 is an operation suppression circuit, 30.31 is an integration circuit, 34.3
5 represents the first and second focus evaluation value characteristic lines, and 41 and 42 represent digital absolute value conversion circuits, respectively. Note that the same reference numerals in the figures indicate the same or equivalent parts. Applicant: Sharp Co., Ltd. Figure 7 19 Nis 4・Niko 3 Figure 5 Fut-1) Suren (Izu y1)

Claims (1)

[Claims] (1) An autofocus device that is used in an imaging device that includes a focus lens and performs a focusing operation by moving the focus lens, comprising means for capturing an image of a subject and extracting a luminance signal, and a means for capturing an image of a subject and extracting a luminance signal; means for detecting the level of a predetermined first frequency band component to output a first focus evaluation value signal; and a second frequency lower than the first frequency band included in the luminance signal. means for detecting the level of a band component and outputting a second focus evaluation value signal; and means for controlling and moving the focus lens in response to the first and second focus evaluation value signals. focus lens movement control means; fluctuation detection means for detecting that the range of fluctuation of the second focus evaluation value signal is equal to or higher than a predetermined threshold level; the first focus evaluation value signal; and suppressing means for suppressing the operation of the focus lens movement control means based on the second focus evaluation value signal in response to the output of the fluctuation detection means.