JP3014745B2 - Auto focus device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus device, and more particularly to an autofocus device suitable for application to a video camera to improve focusing accuracy.

2. Description of the Related Art In general, an imaging device such as a video camera is provided with an autofocus device that automatically focuses on a subject, that is, performs 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 auto-focus method, which uses the image signal to control the focusing, does not require any special optical system or sensor for auto-focus compared to other methods, the mechanism is extremely simple, and regardless of the subject depth 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. 1, No. 86, p. 26)," Automatic Focus Adjustment of Television Cameras Using a Mountain-Climbing Servo System "" It has been disclosed.

FIG. 6 is a block diagram of a conventional autofocus device, and particularly illustrates a configuration using a hill-climbing servo system. 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 a shooting angle of view together with another series of lens systems, and (16) is a camera unit thereof. A focus ring provided on an outer edge of the camera unit (1), which is linked to the focus lens so as to move the focus lens built in the camera unit (1) in the horizontal direction of the drawing by rotation, and (28) a focus for rotating the focus ring (16). motor,
(2) is an image pickup circuit that outputs an image pickup signal from a not-shown image pickup element (not shown) arranged in the image forming section of the camera section (1) as a television system video signal, and (5) is output from the image pickup circuit (2). A sync separation circuit for separating a vertical and horizontal sync signal from a brightness signal of a video signal, and (6) a brightness signal extraction area for focusing from the separated horizontal and vertical sync signals, that is, a sampling area in the imaging screen. Gate opening / closing signal GC1 for setting a relatively narrow specific area
A gate control circuit for generating the signal (3) is an imaging circuit (2) based on a gate opening / closing signal GC1 from the gate control circuit (6).
Gate circuit that allows only the luminance signal of the sampling area for the autofocus operation in the video signal output from the camera to pass through, (30) is a high-pass filter (HPF)
(7), a focus evaluation value creation unit which is composed of a detection circuit (8), an integration circuit (9), and an A / D converter (10) and outputs a focus evaluation value as a digital signal; and (31) a maximum value memory ( A focus evaluation value change detection unit that outputs a comparison signal S1 and a comparison signal S2 indicating a change in focus evaluation value from a focus evaluation value creation unit (30) composed of 11) and a comparator (12); A subject change detection unit that detects a change in a subject from a focus evaluation value from a focus evaluation value creation unit (30) composed of a memory (13) and a comparator (14) and outputs a control signal S3 and a subject change signal S4. , (32) are composed of a motor position memory (17) and a comparator (18), a focus ring position signal from a focus ring position detection sensor (29), a control signal S3 from a subject change detection unit (33), and a focus evaluation. Value change detector (31) Focus position detection unit that receives the comparison signal S1 from the control unit and outputs the control signal CL, (15)
Is the comparison signal S from the focus evaluation value change detection unit (31)
2. The subject change signal S4 from the subject change detection unit (33)
Focus which receives the control signal S3 and the control signal CL from the in-focus position detector (32), outputs a drive signal to the focus motor (28), and outputs a lens stop signal LS to the subject change detector (33). Motor control circuit,
(35) indicates a zoom motor for driving a zoom lens built in the camera unit (1), and (36) indicates a zoom switch for manually operating the zoom motor (35).

In the focus evaluation value creation unit (30), the HPF (7) has a value selected as a cutoff frequency between 200 kHz and 800 kHz, and uses the gate circuit (3) in the luminance signal from the imaging circuit (2). Only high frequency components are extracted from the signal corresponding to the passed sampling area. The obtained high frequency component is subjected to amplitude detection in the next detection circuit (8). Integrator (9)
Integrates the amplitude-detected luminance signal for each field and sends out the signal as a signal representing the level of a high-frequency component in the field of the luminance signal. The output of the integration circuit (9) is A / D
The signal is converted into a digital signal by the converter (10), is derived as a value indicating the focus state of the optical system with respect to the subject, that is, a focus evaluation value, and is output to the focus evaluation value change detection unit (31) and the subject change detection unit (33). You. The focus evaluation value change detection unit (31) that has received the focus evaluation value from the focus evaluation value creation unit (30) stores it in the maximum value memory (1).
Received in 1) and received in the comparator (12) for comparison with the value in the maximum value memory (11), and as a result, the given focus evaluation value is stored in the maximum value memory (11). The comparator (12) compares and detects whether the value is larger or smaller than the value, and outputs comparison signals S1 and S2. For the input of the maximum value, this is used as the maximum focus evaluation value at that time and the maximum value memory (11) Is controlled by the comparison signal S1.

When the focus evaluation value from the focus evaluation value creation unit (30) is smaller than the evaluation value stored in the maximum value memory (11), the comparator (1)
The comparison signal S2 is output from 2). Subject change detector (3
3) Memory (13) is focus motor control circuit (15)
It stores the focus evaluation value from the focus evaluation value creation unit (30) based on the lens stop signal LS from
This is a focus evaluation value at the time of focusing.

On the other hand, the comparator (14) compares the focus evaluation value at the time of focusing with the current focus evaluation value after the end of the autofocus operation, detects that the subject has changed when the focus evaluation value is smaller than a certain threshold value, and detects a subject change signal. S4 is output to the focus motor control circuit (15). On the other hand, the comparator (14) outputs the control signal S3 to the focus motor control circuit (15) when the focus evaluation value stored in the memory (13) is smaller than the focus evaluation value at the start of the autofocus operation. Next, when the maximum value of the focus evaluation value from the focus evaluation value creation unit (30) is detected by the focus evaluation value change detection unit (31), the focus position detection unit (32) outputs the motor position memory (17). Stores the focus ring position signal from the focus ring position detection sensor (29) based on the comparison signal S1. On the other hand, the focus ring position signal and the motor position memory (1
The comparator (18) to which the stored value from (7) is input receives the control signal CL for stopping the focus motor (28) based on the control signal S3 from the subject change detection value (33) and outputs the control signal CL to the focus motor control circuit. Output to (15).

Next, the operation of the above-described configuration will be described.

When an image of a subject is taken, the subject image is coupled onto an image sensor (not shown) by a series of lens systems including a focus lens inside the camera unit (1). The subject image is horizontally and vertically synchronized by an imaging circuit (2). Is converted into a video signal including a luminance signal having The luminance signal in the video signal is supplied to a gate circuit (3) and a synchronization separation circuit (5).
The sync separation circuit (5) separates the vertical and horizontal sync signals from the given luminance signal and supplies the separated signals to the gate control circuit (6). The gate control circuit (6) has a fixed oscillator that oscillates at a constant frequency and has a vertical synchronization signal,
Based on the horizontal synchronizing signal and the output of the fixed oscillator, a predetermined area of the luminance signal is temporally defined, and a gate opening / closing signal GC1 is generated to pass only a necessary portion of the luminance signal, and is output to the gate circuit (3). .

As a result, only the luminance signal component corresponding to the sampling area set by the gate control circuit (6) is given from the gate circuit (3) to the HPF (7), and only the high frequency component of the area is extracted. You. The high-frequency component extracted by the HPF (7) is subjected to amplitude detection by a detection circuit (8) at the next stage. The detection output of the detection circuit (8), that is, the level of the high-frequency component is given to the integration circuit (9), where it is integrated for each field and is converted into an A / D converter (1) as a focus evaluation value.
0). The A / D converter (10) outputs the focus evaluation value as a digital signal. This focus evaluation value is stored in the maximum value memory (1) of the focus evaluation value change detection unit (31).
1) and the comparator (12) as well as the memory (13) and the comparator (14) of the subject change detection unit (33).

FIG. 7 is an explanatory diagram showing the relationship between the focus evaluation value obtained as described above and the position of the focus lens.
In the figure, the horizontal axis indicates the position of the focus ring (16) which indirectly indicates the focus lens position, and 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 by moving the focus lens by rotating the focus ring (16). 16) indicates the maximum value when the focus lens is moved to a position where an arbitrary subject located at a distance of 2 m from the camera unit (1) is in focus. That is, the focus evaluation value changes while drawing a chevron centered on the position of the focus lens at the time of focusing.

The maximum value memory (11) stores the comparison signal S1 from the comparator (12).
The maximum focus evaluation value provided from the focus evaluation value creation unit (30) is stored based on the above, and the maximum focus evaluation value up to that point is provided to the comparator (12). The comparator (12) compares the value provided from the maximum value memory (11), 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). Then, the comparator (12) stores the current focus evaluation value in the maximum value memory (11).
In the first mode when the current focus evaluation value is larger than the value stored in the maximum value memory (11) and the second mode when the current focus evaluation value is smaller than the value stored in the maximum value memory (11) by a predetermined threshold value or more. Different comparison signals S1 and S2 are output correspondingly. Then, the comparison signal S1 is given to the maximum value memory (11) and the motor position memory (17), and the comparison signal S2 is given to the focus motor control circuit (15).

As described above, the focus evaluation value indicates the maximum value when the focus lens reaches the in-focus position. Therefore, the focus evaluation value increases as the focus lens approaches the in-focus position from the out-of-focus position. Therefore, the first mode shows a state in which the focus lens is moving in a direction approaching the focal point, and the second mode shows a state in which the focus lens is moving in a direction away from the focal point.

The maximum value memory (11) responds to the output of the comparison signal S1 of the comparator (12) in the case of the first mode in which the focus lens has not reached the in-focus position. While updating to the focus evaluation value of. 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 (11).

On the other hand, the focus motor control circuit (15) rotates the focus motor (28) so that the focus lens is moved in any of the front and rear directions at the same time as the start of imaging. Then, the focus motor control circuit (15) controls the focus lens in the second mode in which the focus lens moves beyond the in-focus 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 in-focus position. The rotation direction of the focus motor (28) is reversed in response to the comparison signal S2 output from the comparator (12) and the control signal S3 from the comparator (14). Due to the reverse rotation of the focus motor (28), 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 FIG. 7, when an object at a distance of 2 m from the camera unit (1) is imaged, the focus lens is in a position P at which an object at a distance of 10 m from the camera unit (1) is focused in an initial state. When the focus lens 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 comparison signal S1 is output from the comparator (12) until the focus lens reaches the in-focus position, and the storage contents of the maximum value memory (11) and the motor position memory (17) are updated one after another. And
The focus motor (28) is controlled by the focus motor control circuit (15) to move the focus ring (16) 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 (12) is the reverse rotation reference value of the width A, when the focus ring (16) moves to the position of R1 after the focus lens has moved beyond the in-focus position, the comparator (12) returns from the comparator (12). A comparison signal S2 indicating the second mode is output. In response, the focus motor (28) is controlled by the focus motor control circuit (15) to move the focus ring (16) in the opposite direction. Thus, the focus lens starts moving in the direction toward the focal point again.

Now, the motor position memory (17) is provided for storing the focus lens position X closest to the current focal point position, and is provided for the focus ring position detection sensor (29) coupled to the focus ring (16). It receives the focus ring position signal as an output, and stores the focus ring position signal in response to the comparison signal S1 output from the comparator (12). 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, the motor position memory (1
Since the comparison signal S1 is not given to 7), the motor position memory (17) stores the storage contents at the switching point from the first mode to the second mode, that is, the focus lens position at which the focus evaluation value becomes the maximum, that is, the focal point. The position signal of the focus ring (16) indicating the position is kept held. 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 (11).

Now, the comparator (18) compares the content stored in the motor position memory (17) with the focus ring position signal from the focus ring position detection sensor (29), and responds to the coincidence in response to the coincidence. A predetermined control signal CL is output to (15). The focus motor control circuit (15) stops the focus motor (28) in response to the control signal CL. Accordingly, the movement of the focus ring (16), that is, the movement of the focus lens stops. Incidentally, the content stored in the motor position memory (17) in the second mode is a signal indicating the position of the focus ring (16) at which the focus evaluation value becomes maximum. Therefore, in the second mode, the focus motor (28) is rotated in the opposite direction by the comparison signal S2 of the comparator (12), and when the focus lens reaches the focal point again, the comparator (18) operates. Control signal CL is output to the focus motor (28) through the focus motor control circuit (15).
To stop the focus lens at the in-focus position.

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

Now, the focus motor control circuit (15) uses the comparator (1
8) In response to the control signal CL from the focus motor (2
8) At the same time, the lens stop signal LS is output to the memory (13). In response to the lens stop signal LS given at the end of the autofocus operation, the memory (13) stores the focus evaluation value input from the A / D converter (10) at that time, that is, the focus evaluation value at the time of focusing, next. Is held until the lens stop signal LS is supplied to the comparator, and is output to the comparator (14) at the subsequent stage. Here, the comparator (14) is given the current focus evaluation value from the A / D converter (10). After the end of the autofocus operation, the comparator (14) compares the output of the memory (13), that is, the focus evaluation value at the time of focusing with the current focus evaluation value. In response 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 (13), the comparator (14) issues an object change signal S4 indicating that the object has changed. Is output to the focus motor control circuit (15). The focus motor control circuit (15) drives the focus motor (28) 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 detected by the subject change detection unit (33) including the memory (13) and the comparator (14).
Will be implemented. At the same time, the subject change detection section (33) has a function of determining whether or not the rotation direction of the focus motor (28) is appropriate immediately after the start of the autofocus operation and correcting the rotation direction to the appropriate direction. That is, the memory (13) stores not only the focus evaluation value at the end of the autofocus operation but also the A / D at the start of the autofocus operation.
The focus evaluation value initially given from the converter (10) is also stored. Then, the comparator (14) stores the focus evaluation value given from the A / D converter (10) immediately after the start of the autofocus operation in the memory (13). If it is smaller than the initial value, that is, if the current moving direction of the lens is a direction that does not go to the focal point, the control signal S3 is output to the focus motor control circuit (15). The focus motor control circuit (15) drives the focus motor (28) in reverse in response to the control signal S3 from the comparator (14). 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) continues to output the control signal S3 until the current focus evaluation value becomes larger than the initial value stored in the memory (13). 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 (28) 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.

As shown in FIG. 7, for example, when capturing an object at a distance of 2 m from the camera unit (1), the position P at which the focus lens focuses on the object at a distance of 10 m from the camera unit (1). When the camera starts moving in the direction b away from the focal point, the focus evaluation value decreases as the focus ring (16) moves. Here, if the threshold value of the comparator (14) is the reverse reference value indicated by the width B in the figure,
After the auto focus operation starts, focus ring (16)
Is moved from the comparator (14) to the position R2.
S3 is output. In response, the focus motor (2
8) Start moving the focus ring (16) in the opposite direction. Since the comparison operation of the comparator (14) is invalidated until the focus ring (16) exceeds the position P in the figure, the focus ring (16) returns 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 (16) moves to a position slightly beyond the focal point Q through operations of the motor position memory (17) and the comparator (18), and then returns to the focal point again and stops.

As described above, the focus motor control circuit (15) is controlled by the focus evaluation value creation unit (30), the focus evaluation value change detection unit (31), the in-focus position detection unit (32), and the subject change detection unit (33). By doing so, it is possible to realize an autofocus operation by the hill-climbing servo system.

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 to be focused. This area is referred to as a focus detection area, and this area can be arbitrarily set by the gate control circuit (6).

In addition to the configuration of the focus evaluation value creating unit (30) shown in the embodiment, a method of adding the maximum value of the high-frequency component for each scanning line in the focus detection area has also been put to practical use. Idea is the same.

On the other hand, the zoom lens system of the camera section (1) is driven by a zoom motor (35) 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. Zoom motor (3
5) moves the zoom lens in response to a key input from a zoom switch (36) provided so that the user can change the angle of view as needed. That is,
When the zoom switch (36) is pressed, the zoom motor (35)
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 (36) is pressed, the image captured by the imaging circuit (2) changes in the direction in which the image is enlarged or reduced.

[Problems to be Solved by the Invention] As described above, the conventional autofocus apparatus is based on the integrated value of the amplitude of the predetermined high-frequency component included 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, which often causes a malfunction. 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, the focus evaluation value cannot be obtained, or even if obtained, it is not an accurate value. In other words, in principle, it is very difficult to autofocus based on a high-frequency component for such a subject.

On the other hand, in the conventional hill-climbing servo type auto focus, a series of operations for auto focus is performed regardless of the difference in the spectrum of the video signal depending on the subject. That is, the autofocus operation is uniformly performed on a subject that can only obtain a luminance signal that does not include a predetermined high-frequency component and a subject that can sufficiently obtain a predetermined high-frequency component. Therefore, according to the conventional hill-climbing servo-type autofocus device, when an image of a subject that can only obtain a luminance signal not containing a predetermined high-frequency component is captured, the focus lens position at which the focus evaluation value becomes the maximum is not found. That is, the focus lens keeps moving unnecessarily while the captured image is 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 an image of a subject having low contrast is captured.

As described above, in the conventional autofocus apparatus, the subject being photographed is a subject without so-called contrast that is not suitable for autofocus in principle, and as a result, the focus lens cannot be found and the focus lens moves wastefully. When the phenomenon occurs, there is a problem that the photographer can not know the cause and take appropriate measures to stop it quickly, and the captured image will be out of focus for a long time There are major issues to be addressed.

The present invention solves the above-mentioned problems of the prior art, and determines whether or not there is contrast by providing a function of determining the spectrum of a video signal of a subject. It is an object of the present invention to provide an auto-focusing device that suppresses a movement and notifies a photographer of the fact to prompt a photographer to easily focus on a subject.

Means for Solving the Problems In order to achieve the above object, the present invention provides an image pickup means for outputting a video signal of a subject imaged through an image pickup lens, and a high-frequency component of a video signal obtained from the image pickup means. A focus evaluation value creating means for extracting and creating a focus evaluation value necessary for evaluation of a focus state based on the extracted focus evaluation value; a focus control means for controlling a focus position of the imaging lens by forward / reverse driving of a focus ring; Auto focus control means for positioning the imaging lens at a position where the focus evaluation value is maximized by a hill-climbing servo system while changing the focal position of the imaging lens by the focus control means based on the focus evaluation value from the creating means, and a video signal luminance level By calculating the difference between the maximum and minimum values of And a detection unit that outputs a signal indicating that the autofocus operation is impossible due to insufficient contrast when the contrast detected by the contrast detection unit is smaller than a predetermined value. A focus device is provided.

[Operation] In the above means, the autofocus apparatus of the present invention obtains a video signal of a subject imaged through the imaging lens by the imaging means, and then extracts a high-frequency component of the video signal obtained by the focus evaluation value creating means. Based on this, a focus evaluation value necessary for evaluation of the focus state is created, and then the focus control means for controlling the focus position of the imaging lens by the forward / reverse drive of the focus ring is controlled by the auto focus control means to thereby control the imaging lens. 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 unit while changing the focus position, and positioning the imaging lens at the position where the focus evaluation value is the maximum. During this time, the maximum value of the video signal is By calculating the difference between the minimum values, the contrast of the subject is detected, and when the contrast is smaller than a predetermined value, this is detected through the detecting means, and a signal indicating that the autofocus operation is impossible due to insufficient contrast is detected. Output.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an autofocus device according to one embodiment of the present invention. In the figure, (37) is an A / D converter for digitally converting a luminance signal of a video signal obtained by the imaging circuit (2), and (38) is one field of the output of the A / D converter (37). A maximum value detection circuit for detecting and holding the maximum value from among the values; (39) a minimum value for detecting and holding the minimum value from one field of the output of the A / D converter (37) A detection circuit (40) calculates the contrast of the video signal obtained by the imaging circuit (2), that is, the contrast value of the subject by subtracting the output of the minimum value detection circuit (39) from the output of the maximum value detection circuit (38) A subtraction circuit (41) compares the contrast value with a threshold value preset in a setting device (42), and outputs a signal NC to the focus motor control circuit (15) when the contrast value is smaller than the threshold value. The comparator circuit (41), and the A / D converter 37), the maximum value detection circuit (38), the minimum value detecting circuit (39), the subtraction circuit (40), a comparator circuit (41), contrast detection circuit (43) is constituted by setter (42). Incidentally, the maximum value detection circuit (38) and the minimum value detection circuit (39) have a well-known configuration using a digital memory and a digital comparator.

The other parts in the configuration of FIG.
It has substantially the same configuration as the conventional device shown in the figure.

Next, the operation of the above configuration will be described.

The subject image obtained in the camera unit (1) is converted into a video signal by the imaging circuit (2). When the luminance signal in the image is input to the A / D converter (37), the image signal is converted into an n-bit digital signal. Is converted to Note that the luminance signal from the imaging circuit (2) is also input to the focus evaluation value creation unit (30) through the gate circuit (3) at the same time, and the focus evaluation value change detection unit (31) and the in-focus position detection unit (32) By controlling the focus motor (28) through the subject change detection unit (33) and the focus motor control circuit (15), the focus ring (16) is driven to perform an autofocus operation. The digital signal obtained by the A / D converter (37) is input to a maximum value detection circuit (38) and a minimum value detection circuit (39), and the maximum value and the minimum value of the luminance signal in the field are detected. Will be retained. The output of the maximum value detection circuit (38) and the output of the minimum value detection circuit (39) are input to a subtraction circuit (40), and a difference between them, that is, a contrast value is calculated.

Generally, an image takes various levels within a predetermined amplitude, that is, a range of density or luminance, and changes every moment due to a temporal change of the luminance distribution of the subject. In the case of a device that outputs a standard television signal such as NTSC or PAL, such as a video camera, image data within one screen is one by one.
When viewed in units of one pixel, the screen has a luminance or density level f (x, y) for each (x, y) coordinate. The histograms shown in FIGS. 2A and 2B are graphs in which the number of pixels having a specific luminance or density in such image data is counted in one screen and graphed. In FIG. 2, the brightness level is higher as the horizontal axis is closer to the right in density, that is, the density value is larger. 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. Therefore, FIG. 2 (a)
In, 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 having a low contrast value. Therefore, a subject such as a wall or the sky, which has a small change in shading, concentrates on a narrower range even if a noise component is included. On the other hand, in FIG. 3B, the distribution of all the pixels is spread over a wide range of the luminance or the density value, and it can be seen that the luminance difference, that is, the density of the subject is large and the contrast is high.

Here, the maximum value of the density value in FIG.
ax, the minimum value is Flmin, the maximum value of the density value in FIG. 2 (b) is fhmax, and the minimum value is fhmin, qualitatively fhmax-fhmin >> flmax-flmin ... (1) holds. Incidentally, normally, when the degree of shading is to be checked, the variance is calculated from the histogram and the variation of the density distribution is determined. This is shown in FIGS. 3 (a), (b), and (c) when viewed from the waveform of the video signal. 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, FIG.
When the subject obtaining the video signal is captured in a defocused state, the video signal has a waveform as shown in FIG. In this case, the high frequency component of the video signal is eliminated, 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 above reasons, the output corresponding to the contrast value can be obtained from the subtraction circuit (40) regardless of whether the subject is at the in-focus position or the out-of-focus position with respect to the camera unit (1). And f hmax−f hmin in equation (1) and
A value equivalent to flmax-flmin can be obtained. The output of the subtraction circuit (40) is input to a comparator circuit (41). The comparator circuit (41) operates on the basis of, for example, a logic table shown in FIG. TD3502P). The block diagram in FIG. 3B illustrates a case where the IC is combined to form a 16-bit comparator circuit. The output of the subtraction circuit (40), that is, the contrast value obtained by the operation is input to one of the comparator circuits (41) having the above configuration, that is, the A side in the configuration of FIG. By inputting the threshold value set in the setting device (42) to the B side of the configuration in the figure, 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 of FIG. 4, "1" is output to the output X and "0" is output to the output Y in the case of a subject having a contrast, and "0" is output to the output X in the case of a subject having no contrast. "1" is sent to the output Y. Then, the output signal NC of the comparator circuit (41)
Is connected to the focus motor control circuit (15), the movement of the focus motor (28) can be prohibited at the time of low contrast such that the focusing position cannot be reached even when the focusing operation by the hill-climbing servo method is performed. . As a result, it is possible to prevent the focus motor (28) from continuing to move unnecessarily at the time of a low-contrast subject.

By the way, from the setting device (42) to the comparator circuit (41)
May be fixed. However, the level of the video signal obtained from the image pickup circuit (2) 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 (2). However, neither of them can completely maintain the signal level at a constant level, nor can they sufficiently follow the change in illuminance with a large time constant. For this reason, it is preferable that the threshold value given from the setting device (42) to the comparator circuit (41) is selected to be a value that can sufficiently cope with a change in illuminance.

FIG. 5 is a block diagram showing an example of a configuration for changing the setting threshold value of the setting device (42) according to a change in illuminance. In the figure, (44) is an addition circuit for adding the gate opening / closing signal GC1 to the output of the A / D converter (37) and accumulatively adding the luminance level of a specific sampling area, and (45) is an addition circuit (44). Is a comparator circuit that compares the result of addition with a preset value. And setting device (4
2) Based on the output of the comparator circuit (45), selects the input data A having a large value to be used as a threshold at the time of normal illuminance or the input data B having a small value to be used as a threshold at the time of low illuminance. ).

In the above configuration, the A / D converter (3
When the input level of 7) decreases, the output of the addition circuit (44) also decreases. At this time, the set value of the comparator circuit (45) is determined to a level at which it can be determined that the illuminance is low. As a result, the adder circuit (4
When the output of (4) is large, the input data A is input to the comparator circuit (4) by the select input given to the setter (42).
1) will be given to the contrary, the comparator circuit (45)
When the output of the adder circuit (44) is smaller than the set value of (4), the select input given to the setter (42) is inverted and the input data B is given to the comparator circuit (41).
As a result, different threshold values for determining the contrast value are given to the comparator circuit (41) 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 (42) is not limited to two, and may be selected from more values according to the illuminance level, which has the effect of further improving the reliability of the determination.

In the above embodiment, the configuration is described in which the video signal from the imaging circuit (2) is directly input to the A / D converter (37), but the luminance signal in the sampling area for focusing is extracted. The output of the gate circuit (3) may be input to the A / D converter (37) to determine whether or not the autofocus operation is possible based on the contrast value in the sampling area. On the other hand, in the above-described embodiment, the contrast detection circuit (43) is exemplified as a circuit that operates digitally. However, the contrast detection circuit (43) may be configured by an analog circuit to obtain the same function and effect.

Further, in the above embodiment, the comparator circuit (41)
The output signal NC of the focus motor control circuit (15)
The auto-macro function automatically controls the zoom to the wide-angle side in order to focus when the closest object is taken on the telephoto side. May be configured to prohibit the full-range autofocus function.

On the other hand, instead of providing the output of the comparator circuit (41) to the focus control system, the output may be used as a warning signal to the photographer that autofocus is not possible, so as to prompt the photographer to change to a subject having contrast.

[Effects of the Invention] As described above, according to the present invention, the contrast of an object is detected from the maximum value and the minimum value of a video signal, and the detected contrast is compared with a predetermined threshold value, whereby the contrast value is set to an autofocus operation. It is possible to obtain an excellent photographed image by suppressing unnecessary operation of the lens.

[Brief description of the drawings]

FIG. 1 is a block diagram of the auto-focusing device of the present invention, FIG. 2 is a histogram diagram for explaining the operation principle of the configuration of FIG. 1, and FIG. 3 is a waveform diagram showing the relationship between the contrast of a subject and a video signal. FIG. 4 is an explanatory view showing a configuration example of a digital comparator, FIG. 5 is a block diagram showing an example of a configuration for changing a set threshold value for contrast value determination according to a change in illuminance, and FIG. FIG. 7 is a block diagram of a conventional autofocus device, and FIG. 7 is an explanatory diagram showing a relationship between a focus evaluation value and a position of a focus lens. (1) is a camera unit, (2) is an imaging circuit, (3) is a gate circuit, (5) is a synchronization separation circuit, (6) is a gate control circuit, (7) is an HPF, (8) is a detection circuit, (9) is an integration circuit, (10) is an A / D converter, (11) is a maximum value memory, (1)
2) is a comparator, (13) is a memory, (14) is a comparator, (1
5) focus motor control circuit, (16) focus ring, (17) motor position memory, (18) comparator,
(28) is a focus motor, (29) is a focus ring position detection sensor, (30) is a focus evaluation value creation unit,
(31) is a focus evaluation value change detection unit, (32) is a focal point position detection unit, (33) is a subject change detection unit, and (37) is A / D.
A converter, (38) is a maximum value detection circuit, (39) is a minimum value detection circuit, (40) is a subtraction circuit, (41) is a comparator circuit,
(42) is a setting device, and (43) is a contrast detection circuit. In the drawings, the same reference numerals indicate the same or corresponding parts.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/232 G02B ⁷ /28-7/38

Claims (1)

(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 focal position of the lens, and the imaging lens is positioned at a position where the focus evaluation value is maximized by a hill-climbing servo system while driving the focus control means based on the focus evaluation value from the focus evaluation value creation means. Auto-focus control means for performing the operation, a contrast detection means for calculating a difference between the maximum value and the minimum value of the video signal luminance level, and an auto-focus operation when the contrast detected by the contrast detection means is smaller than a predetermined value. Detection means for outputting a signal indicating that the Autofocus device, characterized in that.