JP2756042B2 - Auto focus device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic focusing apparatus, and more particularly to an automatic focusing apparatus which is applied to a video camera and is suitable for improving the focusing accuracy.

[0002]

2. Description of the Related Art In general, an image pickup apparatus such as a video camera is provided with an auto-focus apparatus for automatically focusing on a subject, that is, for performing so-called focusing. Various methods are known for such an autofocus device, and a method of performing focusing using a video signal obtained from an imaging unit is one of them. The autofocus method that uses this imaged video signal to control the in-focus state does not require a special optical system or sensor for autofocusing compared to other methods, the mechanism is extremely simple, and regardless of the depth of field of the imaging lens. It has many advantages that focusing can be performed accurately. A hill-climbing servo system, which is an example of such an auto-focus system, is described in the document "NHK Technical Report (Vol. 17 of 1965, Vol. 86, No. 1, p. 26)," Automatic focus adjustment of television camera by hill-climbing servo system "". Is disclosed. Next, FIG. 10 shows a block diagram of a conventional autofocus device using a hill-climbing servo system.

[0003] In the figure, (1) is a camera unit which incorporates a focus lens driven in conjunction with a focusing operation by autofocus and a zoom lens for changing an imaging angle of view together with a series of other lens systems, and (2). ) Is a focus ring provided on the outer edge so as to move the focus lens incorporated in the camera unit (1) in the horizontal direction of the drawing by the rotation thereof, and (3) is the focus ring (2). A focus motor for rotating the camera, (4) an image pickup circuit for outputting an image pickup signal from a not-shown image pickup element (not shown) provided in the image forming section of the camera section (1) as a television system video signal, and (5) A synchronization separation circuit for separating vertical and horizontal synchronization signals from the luminance signal of the video signal output from the imaging circuit (4); And a gate control circuit for generating a gating signal GC1 to set a relatively narrow specific area in the imaging screen brightness signal extraction area, i.e. the sampling area for focusing from the vertical synchronization signal,
(7) a gate circuit for passing only a luminance signal of a sampling area for an autofocus operation in a video signal output from the imaging circuit (4) by a gate opening / closing signal GC1 from the gate control circuit (6); (30) is a high-pass filter (HPF) (8), a detection circuit (9),
A focus evaluation value creation unit composed of an integration circuit (10) and an A / D converter (11); and (31) a focus evaluation value creation unit (31) composed of a maximum value memory (12) and a comparator (13). 30) A focus evaluation value change detection unit that outputs a comparison signal S1 and a comparison signal S2 indicating a change in the focus evaluation value from (30), and (32) is composed of a memory (14) and a comparator (15) and generates the focus evaluation value. Department (3
A subject change detection unit that detects a change in the subject from the focus evaluation value from 0) and outputs a control signal S3 and a subject change signal S4, and (33) includes a motor position memory (17) and a comparator (18). The focus ring position signal from the focus ring position detection sensor circuit (19), the control signal S3 from the subject change detection section (32), and the comparison signal S from the focus evaluation value change detection section (31).
1 to output a control signal CL in response to 1
(16) is a comparison signal S2 from the focus evaluation value change detection section (31), a subject change signal S4 from the subject change detection section (32), a control signal S3, and a control signal CL from the in-focus position detection section (33). The focus motor control circuit outputs a drive signal to the focus motor (3) in response to the signal and outputs a lens stop signal LS to the subject change detection unit (32). (20) is built in the camera unit (1). Reference numeral 21 denotes a zoom motor for driving the zoom lens, and a zoom switch for manually operating the zoom motor 20.

Next, a focus evaluation value creating section (30),
The configurations of the focus evaluation value change detection unit (31) and the subject change detection unit (32) will be described in detail.

In the focus evaluation value creating section (30), the HPF (8) has a cutoff frequency of 200 kHz to
Only a high-frequency component is extracted from a signal having a value selected between 800 kHz and corresponding to a sampling area that has passed through the gate circuit (7) among luminance signals from the imaging circuit (4). Then, the obtained high frequency component is subjected to amplitude detection in the next detection circuit (9). An integration circuit (10) integrates the amplitude-detected luminance signal for each field, and sends out a signal representing the level of a high-frequency component in the field of the luminance signal. The output of the integration circuit (10) is supplied to the A / D converter (1
The signal is converted into a digital signal in 1), and is derived as a value indicating a focus state of the optical system with respect to the subject, that is, a focus evaluation value, and output to the focus evaluation value change detection unit (31) and the subject change detection unit (12).

The focus evaluation value change detecting section (31) receiving the focus evaluation value from the focus evaluation value creating section (30) receives the focus evaluation value in the maximum value memory (12) and compares it with the value in the maximum value memory (12). Comparator (1
5), the comparator (13) compares and detects whether the given focus evaluation value is larger or smaller than the past maximum value stored in the maximum value memory (12). , S2, and the input of the maximum value is controlled by the comparison signal S1 so as to be stored in the maximum value memory (12) as the maximum focus evaluation value at that time.

Then, a focus evaluation value creating section (30)
Is smaller than the evaluation value stored in the maximum value memory (12), the comparator (13) outputs a comparison signal S2 as a signal indicating this. The memory (14) of the subject change detection unit (32) stores the focus evaluation value from the focus evaluation value creation unit (30) based on the lens stop signal LS from the focus motor control circuit (16). It becomes the focus evaluation value at the time of focusing.

On the other hand, the comparator (15) compares the focus evaluation value at the time of focusing with the current focus evaluation value after the end of the autofocus operation, and detects that the subject has changed when the focus evaluation value becomes smaller than a certain threshold value. The subject change signal S4 is output to the focus motor control circuit (16). On the other hand, when the focus evaluation value stored in the memory (14) is smaller than the focus evaluation value at the start of the autofocus operation, the comparator (15) controls the control signal S3.
Is output to the focus motor control circuit (16).

Next, a focus evaluation value change detecting section (3)
When the maximum value of the focus evaluation value from the focus evaluation value creation unit (30) is detected in (1), the focus ring detection unit (33) stores the focus ring in the motor position memory (17) based on the comparison signal S1. The focus ring position signal from the position detection sensor circuit (19) is stored. On the other hand, the comparator (1) to which the focus ring position signal and the stored value from the motor position memory (17) are input.
8) outputs a control signal CL for stopping the focus motor (3) to the focus motor control circuit (16) based on the control signal S3 from the subject change detection unit (32).

Next, the operation will be described.

When a subject is imaged, a subject image is formed on an image sensor (not shown) by a series of lens systems including a focus lens inside the camera unit (1). And a video signal including a luminance signal having a vertical synchronization signal. The luminance signal in the video signal is input to a gate circuit (7) and a synchronization separation circuit (5). The synchronization separation circuit (5) separates the vertical and horizontal synchronization signals from the given luminance signal. It is given to the gate control circuit (6). Gate control circuit (6)
Has a fixed oscillator that oscillates at a constant frequency, and based on the vertical synchronizing signal, the horizontal synchronizing signal, and the output of the fixed oscillator, temporally defines a predetermined area of the luminance signal and passes only a necessary part of the luminance signal. For this purpose, a gate opening / closing signal GC1 is generated and output to the gate circuit (7).

As a result, the gate circuit (7)
For (8), only the luminance signal component corresponding to the sampling area set by the gate control circuit (6) is given, and only the high frequency component of the area is extracted. HP
The high-frequency component extracted in F (8) is detected by the next-stage detection circuit (9).
The amplitude is detected by The detection output of the detection circuit (9), that is, the level of the high frequency component is given to the integration circuit (10), where it is integrated for each field, and given to the A / D converter (11) as a focus evaluation value. This A /
The D converter (11) outputs the focus evaluation value as a digital signal. The focus evaluation value is stored in the maximum value memory (12) of the focus evaluation value change detection unit (31), the comparator (13), and the memory (1) of the subject change detection unit (32).
4) and input to the comparator (15).

Next, the relationship between the focus evaluation value obtained as described above and the position of the focus lens will be described with reference to FIG. In the figure, the horizontal axis represents the position of the focus ring (2) which indirectly indicates the position of the focus lens,
The vertical axis indicates the focus evaluation value. As shown in the figure, when the subject is at a distance of 2 m from the camera unit (1), the focus evaluation value is changed to the focus ring (2) by rotating the focus ring (2) and moving the focus lens. ) Focus lens to camera part (1)
The maximum value is shown when the camera is moved to a position where an arbitrary subject located at a distance of 2 m from the object is in focus. That is, the focus evaluation value changes while drawing a mountain shape centered on the position of the focus lens at the time of focusing.

Here, the maximum value memory (12) stores the maximum value of the focus evaluation value given from the focus evaluation value creating section (30) based on the comparison signal S1 from the comparator (13), and stores the maximum value at that time. Is given to the comparator (13). The comparator (13) compares the value provided from the maximum value memory (12), that is, the maximum focus evaluation value up to the previous field with the current focus evaluation value provided from the focus evaluation value creation unit (30). The first mode when the current focus evaluation value is larger than the value stored in the maximum value memory (12) and the current focus evaluation value are stored in the maximum value memory (12). The second value when the value is smaller than a preset threshold value by a predetermined threshold value or more.
Different comparison signals S1 and S2 are output corresponding to each of the modes. The comparison signal S1 is provided to the maximum value memory (12) and the motor position memory (17), and the comparison signal S2 is provided to the focus motor control circuit (16).
Given to. Since the focus evaluation value indicates the maximum value when the focus lens reaches the in-focus position, the focus evaluation value increases as the focus lens approaches the in-focus position from the non-focus position. Hence the first
The mode indicates a state in which the focus lens is moving in a direction approaching the in-focus position, and the second mode indicates a state in which the focus lens is moving in a direction away from the in-focus position.

The maximum value memory (12) is supplied from the focus evaluation value generating section (30) in the first mode in which the focus lens has not reached the focal point in response to the output of the comparison signal S1 from the comparator (13). It is stored while updating to the latest focus evaluation value. Thus, the maximum value of the focus evaluation values up to the present is always held as the maximum focus evaluation value in the maximum value memory (12).

On the other hand, a focus motor control circuit (16)
Rotates the focus motor (3) so that the focus lens is moved in any of the front and rear directions simultaneously with the start of imaging. Then, the focus motor control circuit (16) controls the focus lens in the second mode in which the focus lens exceeds the focal point position and in the initial state, the moving direction of the focus lens is not appropriate, that is, in the second mode in which the focus lens moves in the direction away from the focal point position. The comparison signal S2 output from the comparator (13) and the control signal S from the comparator (15)
In response to 3, the rotation direction of the focus motor (3) is reversed. Due to the reverse rotation of the focus motor (3), the direction of movement of the focus lens changes, for example, from the direction approaching the image sensor to the direction away from it, and the focus lens starts moving toward the focal point again.

For example, in FIGS. 10 and 11, when an object at a distance of 2 m from the camera unit (1) is imaged, the focus lens initially focuses on the object at a distance of 10 m from the camera unit (1). When the focus lens is located at the matching position P and starts moving in the direction a toward the in-focus position, the focus evaluation value monotonously increases until the focus lens reaches the in-focus position Q.

Therefore, the comparator (13) outputs the comparison signal S1 until the focus lens reaches the in-focus position, and the maximum value memory (12) and the motor position memory (1) are output.
The stored contents of 7) are updated one after another.

Then, the focus motor (3) is controlled by the focus motor control circuit (16) to move the focus ring (2) continuously in the direction in which the focus lens approaches the focal point. As a result, the focus lens eventually reaches the in-focus position Q and passes there.

Here, if the threshold value of the comparator (13) is the reverse reference value of the width A, when the focus ring (2) moves to the position R1 after the focus lens has moved beyond the in-focus position, the comparator ( 13) the comparison signal S indicating the second mode
2 is output. In response, the focus motor (3) is controlled by the focus motor control circuit (16) to move the focus ring (3) in the opposite direction. As a result, the focus lens starts moving in the direction toward the focal point again.

The motor position memory (17) is provided for storing the focus lens position X closest to the current focal point position. The focus ring (2)
Ring position detection sensor circuit (1
It receives the focus ring position signal output from 9) and stores the focus ring position signal in response to the comparison signal S1 output from the comparator (13). That is, in the first mode, the contents stored in the motor position memory (17) always correspond to the current focus lens position.

However, in the second mode, since the comparison signal S1 is not supplied to the motor position memory (17), the motor position memory (17) has the storage contents at the switching point from the first mode to the second mode, that is, The position signal of the focus ring (2) indicating the focus lens position at which the focus evaluation value is the maximum, that is, the focus ring position, is maintained. As a result, the motor position memory (17) always stores the focus lens position corresponding to the focus evaluation value stored as the maximum focus evaluation value in the maximum value memory (12).

The comparator (18) compares the contents stored in the motor position memory (17) with the focus ring position signal from the focus ring position detection sensor circuit (19), and responds to the coincidence. A predetermined control signal CL is output to the focus motor control circuit (16). The focus motor control circuit (16) stops the focus motor (3) in response to the control signal CL. Accordingly, the movement of the focus ring (2), that is, the movement of the focus lens is stopped. Incidentally, the content stored in the motor position memory (17) in the second mode is a signal indicating the position of the focus ring (2) at which the focus evaluation value becomes maximum.

Therefore, in the second mode, the comparator (1)
By the comparison signal S2 of 3), the focus motor (3) rotates in the opposite direction to the previous one, and when the focus lens reaches the focal point again, the comparator (18) operates and the control signal CL is output to output the focus motor. The focus motor (3) is stopped through the control circuit (16), and the focus lens is stopped at the in-focus position.

As described above, the basic autofocusing operation of this apparatus in which a predetermined area in the imaging screen is set as the focus detection area is completed.

Now, the focus motor control circuit (16)
Stops the focus motor (3) in response to the control signal CL from the comparator (18) and simultaneously stores the memory (14)
Output the lens stop signal LS. The memory (14) responds to the lens stop signal LS given at the end of the autofocus operation, and stores the focus evaluation value input from the A / D converter (11) at that time, that is, the focus evaluation value at the time of in-focus, next. Is held until a lens stop signal LS is supplied to the comparator (15).

Here, the comparator (15) is given the current focus evaluation value from the A / D converter (11). After the end of the autofocus operation, the comparator (15) compares the output of the memory (14), that is, the focus evaluation value at the time of focusing with the current focus evaluation value. The comparator (15) responds to the fact that the current focus evaluation value has become smaller than a preset threshold value as compared with the content stored in the memory (14), and the comparator (15) outputs a subject change signal S4 indicating that the subject has changed. Is output to the focus motor control circuit (16). The focus motor control circuit (16) drives the focus motor (3) again in any direction in response to the subject change signal S4, and restarts the above-described series of autofocus operations.

As a result, the autofocus operation is performed following the change of the subject. That is, the change in the state of the subject is performed by the subject change detection unit (32) including the memory (14) and the comparator (15). At the same time, the subject change detection unit (32) has a function of determining whether or not the rotation direction of the focus motor (3) is appropriate immediately after the start of the autofocus operation and correcting the rotation direction to the appropriate direction. That is, the memory (14) stores not only the focus evaluation value at the end of the autofocus operation but also the focus evaluation value first given from the A / D converter (11) at the start of the autofocus operation. The comparator (15) stores the focus evaluation value given from the A / D converter (11) immediately after the start of the autofocus operation and stores the focus evaluation value at the start of the autofocus operation, which is stored in the memory (14). If it is smaller than the initial value, that is, if the current moving direction of the lens is a direction that is not directed to the focal point, the control signal S3 is output to the focus motor control circuit (16). The focus motor control circuit (16) reversely drives the focus motor (3) in response to the control signal S3 from the comparator (15). At the same time, the control signal S3 is also input to the comparator (18) to stop the comparison operation. That is, the comparator (18) keeps outputting the control signal S3 until the current focus evaluation value becomes larger than the initial value stored in the memory (14). As a result, the control signal CL is not output from the comparator (18) until the current focus evaluation value becomes larger than the focus evaluation value at the start of the autofocus operation, and the focus motor (3) is driven in the direction opposite to the first driving direction. Keep moving. For this reason, even if the moving direction of the focus lens at the time of starting the autofocus operation is not appropriate, the focus lens is always driven in the direction in which the focus evaluation value becomes maximum.

Next, as shown in FIG. 11, for example, when imaging a subject at a distance of 2 m from the camera unit (1),
When the focus lens starts to move from the position P at which the object at a distance of 10 m from the camera unit (1) is in focus to the direction b away from the focus position, the focus evaluation value is the movement of the focus ring (2). It decreases with it. Here, if the threshold value of the comparator (15) is the reverse rotation reference value indicated by the width B in the figure, after the start of the autofocus operation, when the focus ring (2) moves to the position R2, the comparator (15) The control signal S3 is output. In response, the focus motor (3) starts to move the focus ring (2) in the opposite direction. Then, since the comparison operation of the comparator (15) is invalidated until the focus ring (2) exceeds the position P in the figure, the focus ring (2) is returned after returning the focus lens to the position at the start of the autofocus operation. Further, it keeps moving in the direction toward the focal point. Thereafter, the focus ring (2) is moved to the motor position memory (17) and the comparator (18).
After moving to a position slightly beyond the in-focus position Q through such operations, the operation returns to the in-focus position again and stops.

In order to make the image projected on the center of the screen in-focus, in general, not a whole area of the screen but a part of a predetermined area in the center of the screen is set as an object to be focused. Is done. This area is referred to as a focus detection area, and this area can be arbitrarily set by the gate control circuit (6).

The zoom lens system of the camera unit (1) is driven by a zoom motor (20) and moves parallel to the optical axis. As a result, the focal length of the lens system in the camera unit (1) changes, and the angle of view of the image changes. The zoom motor (20) is provided with a zoom switch (2) provided so that the user can change the angle of view as needed.
The zoom lens is moved in response to the key input from 1). That is, when the zoom switch (21) is pressed, the zoom motor (20) operates to move the zoom lens to the wide-angle side where the imaging angle of view increases or to the telephoto side where the angle of view decreases. Therefore, when the zoom switch (21) is pressed, the image picked up by the image pickup circuit (4) changes in the direction of enlargement or the direction of reduction.

[0032]

As described above, the conventional auto-focus device is based on the integrated value of the amplitude of the predetermined high-frequency component contained in the luminance signal for one field obtained by imaging the subject. The autofocus operation is performed by the hill-climbing servo system by judging the direction from the out-of-focus position to the in-focus position, but the frequency band and level of the luminance signal obtained by the brightness and contrast of the subject are On the other hand, if the state before the autofocus operation is greatly blurred even for a high-contrast subject, the integrated value of the high-frequency component becomes very small, and there is a problem that malfunction often occurs.
That is, depending on the condition of the subject, there may be a case where the frequency band to be extracted does not exist in the obtained luminance signal, or a case where the frequency band is present has an extremely low level. For this reason, depending on the subject, a predetermined high-frequency component may not be obtained, or even if it is obtained, the level may be extremely low. In such a case, a focus evaluation value cannot be obtained, or even if it is obtained, an accurate value cannot be obtained.

Accordingly, there is a problem that autofocusing based on a high-frequency component is very difficult in principle for such a subject.

In a conventional hill-climbing servo-type autofocus, a series of operations for the autofocus is performed regardless of the difference in the spectrum of the video signal depending on the subject. In other words, the autofocus operation is performed uniformly for a subject that can only obtain a luminance signal that does not include a predetermined high-frequency component and for a subject that can sufficiently obtain a predetermined high-frequency component.

Therefore, according to the conventional hill-climbing servo type auto-focusing apparatus, when an image of a subject that can only obtain a luminance signal not containing a predetermined high-frequency component is taken, the focus lens position at which the focus evaluation value becomes the maximum is determined. The focus lens continues to move unnecessarily without being output, that is, with the captured image blurred. Although a predetermined high-frequency component for deriving a focus evaluation value is included in a luminance signal obtained from a subject having a clear contrast, a subject having an extremely small contrast such as a distant mountain or sky, or a wall or ceiling. Thus, it is hardly included in a luminance signal originally obtained from a subject having no contrast. Therefore, the above-described problem often occurs when capturing an image of a subject having a low contrast.

As described above, in the conventional auto-focusing apparatus, the subject being photographed is a subject having no contrast, which is not suitable for auto-focusing in principle.
As a result, when the focus lens cannot be found and the focus lens moves wastefully, the photographer cannot know the cause and take appropriate measures to stop it quickly. There is a big problem to be solved, that is, a captured image is out of focus for a long time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and has a function of determining the spectrum of a video signal of a subject to determine the presence or absence of a contrast and to provide a focal point. It is an object of the present invention to provide an auto-focusing device that can suppress useless movement of a focus lens in a case where it is not possible to obtain an image and notify the photographer of the fact to encourage photographing of a subject that is easily focused. Aim.

[0038]

According to the present invention, there is provided an autofocus apparatus comprising: an image pickup device for outputting a video signal of a subject imaged through an image pickup lens; Focus evaluation value creating means for creating a focus evaluation value; focus control means for controlling the focus position of the imaging lens by forward / reverse drive of a focus ring; and the focus control based on the focus evaluation value from the focus evaluation value creating means. Auto-focus control means for positioning the imaging lens at a position where the focus evaluation value is maximized by the hill-climbing servo system while driving the means, and calculating the difference between the maximum value and the minimum value of the luminance level of the video signal obtained from the imaging means. Contrast detection means for detecting the contrast of the subject by calculating And outputting a signal indicating that a focus operation is impossible from the output of the contrast difference detecting means and the output of the contrast difference detecting means for detecting a change in the output of the contrast detecting means due to the movement of the imaging lens. It is characterized by comprising detecting means. In addition, the auto focus system
The control means is configured to receive the format from the focus evaluation value creating means.
To the scrap evaluation value and the output from the contrast detection means.
By driving the focus control means based on the
The imaging lens is positioned at the focal point.

[0039]

According to the auto-focusing device of the present invention, a video signal of a subject imaged by an imaging lens through an imaging lens is obtained, and then a high-frequency component of the video signal obtained by a focus evaluation value generating device is extracted, and based on the extracted signal, The focus evaluation value required for the evaluation of the focus state is created, and then the focus position of the imaging lens is changed by controlling the focus position of the imaging lens by the forward / reverse driving of the focus ring by the auto focus control means. A series of operations of controlling the imaging lens by the hill-climbing servo method using the focus evaluation value from the focus evaluation value creating means and positioning the imaging lens at the position where the focus evaluation value is maximum are performed. At this time, the contrast of the subject is detected by calculating the difference between the maximum value and the minimum value of the luminance level of the video signal by the contrast detection means, and calculating the amount of change in contrast by driving the lens by the contrast difference detection means. When the contrast is smaller than a predetermined value, this is detected through the detection means, and a signal indicating that the autofocus operation is impossible due to insufficient contrast is output.

[0040]

Embodiment 1 An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a block diagram of an autofocus apparatus according to one embodiment of the present invention. Note that in FIG.
The same or corresponding members as those shown in FIG. In the figure, (22) is an A / D converter for digitally converting a luminance signal of a video signal obtained by the imaging circuit (4), and (23) is for one field of the output of the A / D converter (22). A maximum value detection circuit for detecting and holding the maximum value from
A minimum value detection circuit for detecting and holding the minimum value from one field of the output of the D converter (22), and (25) a minimum value detection circuit (2) from the output of the maximum value detection circuit (23).
4) A subtraction circuit for calculating the contrast of the video signal obtained by the imaging circuit (4) by subtracting the output of the imaging circuit (4), that is, the contrast value of the subject. (26) sets the contrast value in a setting device (27) in advance. A comparator circuit that outputs a signal QC when the contrast value is smaller than the threshold value by comparing with the threshold value, and the A / D converter (2
2), maximum value detection circuit (23), minimum value detection circuit (2
4), a subtraction circuit (25), a comparator circuit (26),
The setting device (27) forms a contrast detection circuit (28). The maximum value detection circuit (23) and the minimum value detection circuit (24) have a generally well-known configuration using a digital memory and a digital comparator.

A latch circuit (29) temporarily holds the output of the subtraction circuit (25), and (34) subtracts the output of the latch circuit (29) from the output of the subtraction circuit (25). (35) compares the contrast value change amount with a threshold value preset in the setting unit (36), and outputs a signal when the contrast value change amount is smaller than the threshold value. PC
The latch circuit (29), the subtraction circuit (34), and the comparator circuit (3
5) A setter (36) is provided with a contrast difference detection circuit (37).

Next, the operation will be described. The subject image obtained in the camera section (1) is converted into a video signal by the imaging circuit (4), and the luminance signal in the image signal is converted into an A / D signal.
When input to the converter (22), it is converted into an n-bit digital signal. As in the conventional case, the luminance signal from the imaging circuit (4) is also input to the focus evaluation value creating unit (30) through the gate circuit (7) at the same time, and the focus evaluation value change detection unit (31) A focus motor (3) through a position detector (33), a subject change detector (32) and a focus motor control circuit (16);
, The focus ring (2) is driven, and the auto focus operation is performed.

The digital signal obtained by the A / D converter (22) is input to a maximum value detection circuit (23) and a minimum value detection circuit (24), and the maximum value and the minimum value of the luminance signal in the corresponding field are provided. Is detected and held. The output of the maximum value detection circuit (23) and the output of the minimum value detection circuit (24) are input to a subtraction circuit (25), and the difference between them, that is, the contrast value is calculated.

Generally, an image takes various levels within the range of the obtained amplitude, that is, the density (in other words, luminance), and changes every moment due to the temporal change of the luminance distribution of the subject. NTSC or P like video camera
In the case of a device that outputs a standard television signal such as AL,
Assuming that image data within one screen is viewed one pixel at a time, each has a luminance or density level f (x, y) with respect to the (x, y) coordinates of the screen. FIG. 2A is a graph showing the number of pixels having a specific luminance or density in such image data in one drawing.
It is a histogram figure shown to (b). 2A is a diagram showing a density histogram when the contrast is low, and FIG. 2B is a diagram showing a density histogram when the contrast is high. In FIG. It is getting bigger. On the other hand, the vertical axis represents the number of pixels at each luminance or density value. That is, the histogram diagram shows the distribution of brightness or density on the screen. If the entire screen has uniform brightness or density, only one bar corresponding to the brightness or density is expressed in the histogram.

Accordingly, in FIG. 2A, since the distribution of all the pixels is concentrated in a narrow range of the luminance or the density value, it can be seen that the object is a low-contrast object with a low contrast value. Therefore, in a subject such as a wall or the sky, which has a small change in shading, it is concentrated in a narrower range even if it includes a noise component.

On the other hand, in FIG. 5B, it can be seen that the distribution of all the pixels is spread over a wide range of luminance or density value, and that the object is a high-contrast subject having a large difference in luminance, that is, a large density.

Here, if the maximum value of the density value in FIG. 2A is fl _max , the minimum value is fl _min , the maximum value of the density value in FIG. 2B is fh _max , and the minimum value is fh _min , qualitatively _{fh max -fh min >> fl max -fl} min ... (1) is satisfied. Generally, when looking at the degree of shading, the variance is calculated from the histogram to determine the variation in the density distribution. However, the degree of shading can be sufficiently determined from equation (1). This can be seen from the waveform of the video signal.
(A), (b) and (c). That is, for a subject having a low contrast, only a video signal having a small difference between the maximum value and the minimum value can be obtained as shown in FIG. 10A, and for a subject having a high contrast, the maximum value and the minimum value can be obtained as shown in FIG. A video signal having a large value difference can be obtained. Each drawing corresponds to one line of a video signal, but the same holds for one screen.

On the other hand, when an image of the subject obtaining the video signal shown in FIG. 2B is picked up in a defocused state, the video signal has a waveform as shown in FIG. 3C. In this case, the high frequency component of the video signal disappears, but the difference between the maximum value and the minimum value of the signal does not become so small. Therefore, the magnitude of the contrast value can be determined by detecting the difference between the maximum value and the minimum value of the luminance level within one field screen.

For the reasons described above, the subtraction circuit (25) outputs an output corresponding to the contrast value regardless of whether the subject is at the in-focus position or the out-of-focus position with respect to the camera unit (1). Fh _max −fh _{min in} equation (1)
And a value corresponding to fl _max- fl _min can be obtained. The comparator circuit (26) to which the output of the subtraction circuit (25) is input is, for example, a commercially available IC (for example, trade name: TD3502P) that performs an operation as shown in the logic table of FIG.
Are combined). The block diagram shown in FIG. 4B shows a case where the IC is combined to form a 16-bit comparator circuit. The output of the subtraction circuit (25), that is, the contrast value obtained by the operation is input to one of the comparator circuits (26) having the above configuration, that is, the A side of the configuration of FIG. By inputting the threshold value set in the setting device (27) on the B side of the configuration, it is possible to determine whether the subject is a subject having a contrast or a subject having no contrast at the threshold. In the configuration shown in FIG. 4, "1" is output to the output X and "0" is output to the output Y in the case of a subject having contrast, and "0" is output to the output X in the case of a subject having no contrast. "1" is sent to Y.

Incidentally, the threshold level given from the set value (27) to the comparator circuit (26) may be fixed. However, the level of the video signal obtained from the imaging circuit (4) varies greatly when the illuminance changes, even for the same subject. Usually, in order to suppress this variation, the camera unit (1) includes an aperture control mechanism called an auto iris. Or an automatic gain control circuit is provided in the imaging circuit (4).
However, neither of them can completely maintain the signal level at a constant level, and cannot always cope with a change in illuminance due to a large time constant.
For this purpose, the setting circuit (27) sends the comparator circuit (2
It is preferable that the threshold value given in 6) be selected so as to sufficiently cope with a change in illuminance.

Next, FIG. 5 shows a configuration for changing the set threshold value of the setting device (27) according to the change in illuminance. In the figure, (38) is an addition circuit for adding the gate opening / closing signal GC1 to the output of the A / D converter (22) to accumulatively add the luminance level of a specific sampling area, and (39) is an addition circuit (38). This is a comparator circuit that compares the addition result with a preset value. And setting device (2
7) is a large value input data A to be used as a threshold at the time of normal illuminance based on the output of the comparator circuit (39).
Alternatively, the input data B having a small value to be used as a threshold at the time of low illuminance is selected and output to the comparator circuit (26).

In this configuration, when the input level of the A / D converter (22) drops at low illuminance, the output of the adder circuit (38) also drops, but at this time, the setting value of the comparator circuit (39) becomes lower. Determine a level that can be determined as illuminance. As a result, when the output of the adding circuit (38) is larger than the set value of the comparator circuit (39), the setting unit (27)
The input data A is supplied to the comparator circuit (26) by the select input given to the comparator circuit (26). Conversely, when the output of the adder circuit (38) is smaller than the set value of the comparator circuit (39), the input to the setter (27) is made. The applied select input is inverted and the input data B is converted to a comparator circuit (2
6). As a result, different threshold values for determining the contrast value are given to the comparator circuit (26) in accordance with the change in the illuminance of the subject, and the reliability of the contrast value determination can be increased. The setting value in the setting device (27) is not limited to two, and may be selected from more values according to the illuminance level. This has the effect of increasing the reliability of the determination.

As described above, it is possible to cope with a change in illuminance by adaptively changing the threshold value. However, with the above-described configuration alone, there is a case where the determination of the presence or absence of the contrast is erroneous. For example, when illumination light is projected onto a white wall (assuming no contrast) from above or from the side, the illuminance (luminance) on the surface of the wall changes continuously depending on the angle between the light source and the wall. That is, the brightness is higher near the light source, and lower near the light source. This is represented by a density histogram as shown in FIG. 6A. Even when a subject having no contrast is actually picked up, the distribution of the density histogram is so wide that there is no difference from the case of the subject having contrast. In such a case, it is determined that "there is a contrast" only with the above-described configuration, and it is impossible to prevent the focus motor (3) from continuing to move wastefully as in the conventional case. In order to avoid such an error, a contrast difference detection circuit (37) is provided.

FIG. 6 shows a histogram in a situation where the variance of the density values becomes wider than it actually is due to the way of illumination in an object having no contrast as described above.
The width (contrast value) between the maximum value and the minimum value hardly changes at the in-focus point in FIG. 6A and at the out-of-focus point in FIG. 6B. When the image is out of focus, the number of pixels near the average density value of the image increases and the number of pixels whose density value is farther than the average decreases, as compared with the time of focusing. The minimum does not change significantly. On the other hand, in a subject having contrast, the histogram at the focal point is widely dispersed as shown in FIG. However, when the position of the focus lens changes and the image is out of focus, the width of the histogram becomes narrow as shown in FIG. 7B.

Accordingly, the difference between the contrast detection output before driving the lens and the output after driving the lens is calculated,
By determining whether the value is larger or smaller than a certain value, it is possible to determine the presence or absence of the contrast of the subject.

The above operation will be described with reference to FIG. The output (corresponding to the contrast value) of the subtraction circuit (25) in the contrast detection circuit (28) is once held by the latch circuit (29) in the contrast difference detection circuit (37), and the subtraction circuit (25) after driving the lens. ) Is calculated by the subtraction circuit (34). An output of the subtraction circuit (34) is connected to one input terminal of the comparator circuit (35), and a setting device (36) is connected to the other input terminal.
Is given, and the values of both inputs are compared.
The comparator (35) outputs "1" as an output signal PC when it is larger than the threshold value and outputs "0" when it is smaller than the threshold value. The signal PC has the same configuration as that shown in FIG. 4 so that the signal PC outputs "1" when the change due to the lens drive is large and outputs "0" when the change is small. By taking the logical product of this output signal PC and the output signal QC of the above-mentioned contrast detection circuit (26) by an AND gate (40), a focus motor control signal NC is created.

FIG. 8 shows the logic of the NC and the corresponding judgment and control contents. That is, if the output signal QC is "0", it is determined that there is no contrast, and the focus motor is not driven (a in the figure). If the output signal PC is "0" even if the output signal QC is "1",
Also, it is determined that there is no contrast, and the drive of the focus motor is stopped (FIG. 2B). Only when both the output signals QC and PC are "1", it is determined that there is a contrast, and the hill-climbing is performed by driving the lens (FIG. 4D). Since the case where the output signal QC is “0” and the output signal PC is “1” is impossible, the column “c” in FIG. 8 is blank.

As described above, the contrast detection circuit (2
8), FIG. 9 is a flowchart showing a series of operations of the contrast difference detection circuit (37).

In the above configuration, the presence or absence of contrast can be accurately determined only by the signal PC.
Since the focus lens must be driven to obtain the signal PC, it is meaningful to use the signal QC. That is, by using the contrast detection circuit (28) for obtaining the signal AC together, there is an advantage that a subject having no contrast can be determined without driving the lens.

Embodiment 2 In the above-described embodiment, the configuration in which the video signal from the image pickup circuit (4) is directly input to the A / D converter (22) has been described, but the brightness in the sampling area for focusing is shown. The output of the gate circuit (7) extracting the signal is A / D
The signal may be input to the converter (22) to determine whether or not the autofocus operation is possible based on the contrast value in the sampling area.

Further, in the above embodiment, the contrast detection circuit (28) or the contrast difference detection circuit (37)
Has been described as a circuit that operates digitally, but the same operation and effect can be obtained by using an analog circuit.

In addition, in the above embodiment, the signals QC and P, which are the outputs of the comparator circuits (26) and (35),
The logical product output NC of C is connected to the focus motor control circuit (1
Although the configuration in which the movement of the focus motor (3) is restricted by applying to (6) has been described, the automatic zoom control to the wide angle side is performed in order to focus when a close object is photographed on the telephoto side. The configuration may be such that the macro function and the full-range autofocus function are prohibited. In short, the configuration may be such that the autofocus operation is prohibited based on the detection result of the detection unit.

Further, instead of giving the output of the AND gate (40) to the focus control system, the output may be used as a warning signal indicating that the auto-focus cannot be performed for the image of the photographer, thereby urging the photographer to change to a subject having contrast. .

Further, in the above description, since the output QC of the contrast detecting section (28) is large at the time of focusing and becomes small at the time of defocusing, the signal QC is used as an auxiliary (evaluation value creating section (30) It can also be used as information for ()). The circuit configuration in this case is composed of a maximum value memory and a comparator similarly to the focus evaluation value change detection section (31), and the output is similarly input to the focus motor control circuit (16). By doing so, the autofocus operation can be reliably performed even at low illuminance or subject movement, which is difficult to focus with the output of only the focus evaluation value creation unit (30).

[0066]

According to the present invention, the contrast of an object is detected from the difference between the maximum value and the minimum value of the video signal and the amount of change in the difference, and these are compared with a predetermined threshold value to thereby obtain the contrast value. Is determined whether or not it is sufficient for the autofocus operation, and there is an effect that a good captured image can be obtained by suppressing useless operation of the lens.

[Brief description of the drawings]

FIG. 1 is a block diagram of an autofocus apparatus showing one embodiment of the present invention.

FIG. 2 is a histogram diagram for explaining the operation principle of the configuration of the contrast detection circuit in FIG. 1;

FIG. 3 is a waveform chart showing a relationship between a contrast of a subject and a video signal.

FIG. 4 is an explanatory diagram showing a configuration of a digital comparator.

FIG. 5 is a block diagram showing a configuration for changing a set threshold value for contrast value determination according to a change in illuminance.

FIG. 6 is a histogram diagram when an object having no contrast is illuminated from one side;

FIG. 7 is a histogram diagram of an object having high contrast.

FIG. 8 is a diagram showing a logical product of the output of the contrast detection circuit and the output of the contrast difference detection circuit, and the judgment and control based on the logical product.

FIG. 9 is a flowchart illustrating an algorithm when the contrast detection circuit and the contrast difference detection circuit are implemented as software.

FIG. 10 is a block diagram showing a conventional autofocus device.

FIG. 11 is an explanatory diagram showing a relationship between a focus evaluation value and a position of a focus lens.

[Explanation of symbols]

 (1) Camera section (2) Focus ring (3) Focus motor (4) Imaging circuit (5) Synchronous separation circuit (6) Gate control circuit (7) Gate circuit (8) HPF (9) Detection circuit (10) Integration Circuit (11) A / D converter (12) Maximum value memory (13) Comparator (14) Memory (15) Comparator (16) Focus motor control circuit (17) Motor position memory (18) Comparator (19) Focus ring position detection sensor circuit (22) A / D converter (23) Maximum value detection circuit (24) Minimum value detection circuit (25) Subtraction circuit (26) Comparator circuit (27) Setting device (28) Contrast detection circuit ( 29) Latch circuit (30) Focus evaluation value creation unit (31) Focus evaluation value change detection unit (32) Subject change detection unit (33) Focus Point position detector (34) Subtraction circuit (35) Comparator circuit (36) Setting device (37) Contrast difference detection circuit (40) AND (logical product) gate circuit

Claims (2)

(57) [Claims] An imaging unit that outputs a video signal of a subject imaged through an imaging lens; a focus evaluation value generation unit that generates a focus evaluation value based on a high-frequency component of a video signal obtained from the imaging unit; Focus control means for controlling the focus position of the imaging lens by forward / reverse rotation of a focus ring; and a focus evaluation value which is maximum by a hill-climbing servo system while driving the focus control means based on a focus evaluation value from the focus evaluation value creation means. Auto-focus control means for positioning an imaging lens at a position, and contrast detection means for detecting a contrast of a subject by calculating a difference between a maximum value and a minimum value of a luminance level of a video signal obtained from the imaging means. Changes in the contrast detection means due to movement of the imaging lens An auto-focusing device comprising: a contrast difference detecting means for detecting the difference; and a detecting means for outputting a signal indicating that an auto-focus operation is impossible from the output of the contrast detecting means and the output of the contrast difference detecting means. 2. The auto-focusing device according to claim 1,
Oite, the autofocus control unit, the focus evaluation
Focus evaluation value from the value creation means and the contrast
The focus control means based on an output from the
The imaging lens at the focal point by driving
An autofocus device characterized by the above.