JP2847805B2 - Focus control circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contrast detection type focus control circuit suitable for application to, for example, an autofocus system of a video camera.

[Summary of the Invention]

The present invention relates to a focus control circuit of a contrast detection system suitable for application to, for example, an autofocus system of a video camera, wherein an integration of a luminance component of a predetermined first frequency is performed within a first range substantially at the center of an imaging screen. The ratio between the value and the integral value of the luminance component of the second frequency lower than the first frequency is obtained, and the integral of the luminance component of the first frequency is obtained within a second range around the first range. The ratio between the value and the integral value of the luminance component of the second frequency is determined, and the focus determination is performed based on the difference between the ratio determined within the first range and the ratio determined within the second range. In this way, accurate focus control can be performed to focus on a subject that is not affected by the background.

[Conventional technology]

The video camera's autofocus method includes a distance measurement method and a focus detection method when classified based on the principle of the focusing method. The distance measuring method measures the distance to a subject and controls the position of a lens in accordance with the distance. The focus detection method detects focus on an imaging surface and controls the position of a lens at a position where focus is achieved.

One of the focus detection methods is a contrast detection method. In the contrast detection method, for example, as shown in Japanese Patent Application No. 63-214354, focus control is performed using the fact that the edge of a captured image becomes clearer when the lens approaches a focus position. The sharpening of the edge of the captured image corresponds to an increase in the high frequency component in the video signal.

[Problems to be solved by the invention]

By the way, when performing such focus control,
2. Description of the Related Art Detection accuracy is increased by detecting an integrated value of a high-frequency component only in a predetermined range in a captured image. That is, as shown in FIG. 7, the integrated value of the high-frequency component of the luminance signal in the frame x substantially at the center in the captured image is detected, and the focus position of the lens is controlled so that the integrated value becomes maximum. That is being done. The focus control is performed in this way because the target subject to be focused is often arranged at substantially the center of the captured image.

In the case of providing the frame x in this manner, it is preferable to increase the range of the frame because the amount of information is large. However, if the range of the frame is widened in this way, for example, as shown in FIG. When a person is photographed, the background area is overwhelmingly larger than the person who is the subject, so when the background is in focus than when the subject is in focus, The integrated value of the high frequency component of the luminance signal becomes high. As a result, the subject is in focus with the background mountains, and the subject is blurred.

On the other hand, as shown in FIG. 8, if the frame y is made small only at the center, the possibility of focusing only on the subject increases, but focus control is performed using information only in the small frame y. Since the amount of information of signals used for focus control is reduced, it is impossible to consider various actual shooting conditions.

An object of the present invention is to enable a focus control circuit of a contrast detection system to always perform good focus control.

[Means for solving the problem]

The present invention is, for example, as shown in FIG. 1 - FIG. 3, the focus control circuit for contrast detection method, almost within a first range a central portion, a predetermined first brightness of the frequency f ₁ of the imaging screen and the integral value of the components, along with determining the ratio between the integral value of less than the first frequency f ₁ second luminance component of the frequency f _2, in a second range b around the first range a, and the integral value of the first luminance component of the frequency f _1, obtains the ratio of the integrated value of the second luminance component of the frequency f _2, at a ratio determined in the first range a and the second range b The focus determination is made based on the difference from the obtained ratio.

[Action]

By doing so, for example, when the ratio of the first range is larger than the ratio of the second range, it is determined that the subject is in focus, and the ratio of the second range is the first range. If the ratio is larger than the ratio of the range, it can be determined that the subject is in focus, and accurate focus control can be performed to focus on a subject without the background.

〔Example〕

Hereinafter, an embodiment of the focus control circuit of the present invention will be described as a first embodiment.
This will be described with reference to FIGS.

This example is a focus control circuit applied to an auto-focus mechanism of a video camera, and FIG. 1 shows an overall configuration. In FIG. 1, (1) denotes an imaging lens, and image light passing through the imaging lens (1) forms an image on an imaging surface of a CCD type solid-state imaging device (hereinafter, referred to as CCD) (2). . In this case, the CCD (2) is rotated by driving the focus ring (1a) of the imaging lens (1) with a motor (13) described later.
Focus adjustment of the image light to be formed on the image forming surface is performed.

An imaging signal output by the CCD (2) by imaging is supplied to an imaging signal processing circuit (3), and the imaging signal processing circuit (3) converts the imaging signal into a predetermined video signal.
This video signal is output from the output terminal (4).

The baseband luminance signal component extracted from the imaging signal by the imaging signal processing circuit (3) is sent to the first and second bandpass filters (6) and (7) via the detection range switching circuit (5). Supply. In this case, the detection range switching circuit (5) switches the detection range under the control of the central control device (12) described later, and as shown in FIG.
Selection of a signal within a first range a near the center in the captured image, selection of a signal within a second range b around the first range a, and selection of a signal within the first range a. Selection of both signals with the signal within the range b of 2. Then, a signal of a frequency band of 1 MHz is extracted from an output signal of the detection range switching circuit (5) by a first band pass filter (6), and a signal of a frequency band of 100 KHz is extracted by a second band pass filter (7). Extract the signal. The output signals of the band-pass filters (6) and (7) are converted into digital signals by analog-to-digital converters (8) and (9), and then supplied to the integration circuits (10) and (11). The data obtained by the integration are supplied to a central control unit (CPU) (12). In this case, the integration circuits (10) and (11) are digital integration circuits and are provided for one screen supplied for each field (however, limited to a predetermined range for one screen by the detection range switching circuit (5)). ) Is a circuit for integrating the luminance level.

The central controller (12) determines the integrated value data from the integration circuits (10) and (11) based on a determination operation described later, and outputs the result to the focus ring (1a) rotating motor (13). A predetermined drive signal is supplied to perform focus adjustment.

Next, the focus control operation performed by the central control device (12) will be described with reference to the flowchart of FIG. 2. When the focus control circuit is started, the central control device (12) sets the first range a and the second range a The detection range switching circuit (5) is controlled so as to output signals in both ranges of the range b. In this state, the first band-pass filter (6) converts the baseband luminance signal extracted by the imaging signal processing circuit (3) from the imaging signal output by the CCD (2) to 1 MHz.
The central control unit (12) determines the data for each screen integrated by extracting the signal of the frequency band of
The focus ring (1a) is rotated so that the integrated value becomes maximum. That is, for example, from the imaging surface of this video camera
Assuming that there is a subject at a distance of 2 m and the distance to the focal point is 2 m, as shown in FIG. 4, when the focus ring (1a) of the imaging lens (1) is at a position at a distance of 2 m, The characteristic W of the curve in which the integral value increases is obtained. Here, if the rotational position of the focus ring when starting the focus control circuit (1a) is distance 1 m, the central control unit (12) has the integral value W ₁ is obtained. At this time, the central control unit (12) is pivoted in both directions slightly the focus ring is supplied (1a) a control signal to the motor (13), a focus ring (1a from the integrated value W ₁ in either direction )
The central controller (12) determines whether or not the integral value increases when is rotated. When the determination operation is completed, for example, by rotating the focus ring in the direction in which the integration value is increased by the control of (12), detects that the integrated value becomes the maximum value W ₀ when the distance 2m. In this case, since the focus ring (1a) detects the integral value W ₂ which is slightly reduced from the maximum value W ₀ exceeds the distance 2m, the integral value W ₀ when the distance 2m is to be the maximum value the central control The device (12) determines and stops the focus ring (1a) at a position of 2m.

Then, in this example, after detecting the maximum integral value W ₀ , focus determination is performed to determine whether or not the integral value W ₀ is a focal point. That is, at the time of this focus determination, first, the central control device (1
2) controls the detection range switching circuit (5) so as to output only the signal in the first range a. In this state, one screen data obtained by integrating the luminance signal component of the frequency band of ₁ MHz (hereinafter referred to as frequency f1) through the first band-pass filter (6) by the integration circuit (10) is centrally controlled. with the device (12) determines, 100 KHz (hereinafter referred to as frequency f ₂₎ of one screen every was integrated by the integrating circuit the luminance signal component of the frequency band (11) of which through the second band-pass filter (7) The data is determined by the central controller (12). And the frequency f ₁
Integrated value Va ₁ and a ratio R ₁ between the integrated value Va ₂ frequency _{f 2 (R 1 = Va 1} /
Va ₂ ) is detected by the central controller (12). When the judgment is made based on the signal in the first range a, the central control unit (12) controls the detection range switching circuit (5) so as to output the signal in the second range b. In this state, the data for each screen obtained by integrating the luminance signal component of the frequency band of the frequency f ₁ (1 MHz) through the first band-pass filter (6) by the integration circuit (10) is transmitted to the central controller (12). ) And the frequency through the second bandpass filter (7)
integrating circuits the luminance signal component of the frequency band of f ₂ (100KHz) (1
The central control unit (12) determines the data for each screen integrated by 1). Then, the ratio _{R 2 (R 2 = Va 1} / Va 2) of the integral value Vb ₂ integrated value Vb ₁ and frequency f ₂ of the frequency f _1, detected by the central control unit (12).

In this manner, the ratio R ₁ by the signal at the first range a is sought after ratio and R ₂ by the signal at the second range b, or R ₁
R ₂ is used to determine the magnitude of the value. Here, when the value of the ratio R ₁ by the signal in the first range a is larger than the value of the ratio R ₂ by the signal in the second range b, or R ₁ and R ₂ are almost the same. When the value is a value, it is determined that the object is in focus. Further, towards the signal ratio of R ₂ values due in the second range b is, when greater than the signal ratio value of R ₁ by in the first range a, it is defocused by the background run over occurs Judge.

Explaining the reason why the focus and the out-of-focus are determined, the integrated value of the luminance signal component is low-frequency component of about 100 KHz.
Although there is almost no change in any of the out-of-focus conditions, the high-frequency component around 1 MHz is high during in-focus and low during out-of-focus. That is, if focusing is performed in the first range a, the fifth
As shown in FIG. A, the integrated value of the luminance signal component hardly changes at each frequency. If focusing is not performed in the second range b, the integral value decreases as the frequency becomes higher as shown in FIG. 5B. Here, considering an actual shooting state, as shown in FIG. 3, there are many subjects in a first range a in a central portion, and a background is captured in a second range b around the center. Often. Therefore, when the integral value changes as shown in FIG. 5 and it is detected that the first range a is in focus and the second range b is out of focus, the subject at the center is in focus. It is considered that the surrounding background is out of focus, and it is determined that the focus position is good.

Conversely, as shown in FIG. 6A, the integrated value of the first range a decreases as the frequency becomes higher, and as shown in FIG.
When the integrated value of the range b hardly changes, it is considered that the subject in the center is out of focus and the surrounding background is in focus, and it is determined that the background is depressed (the state shown in FIG. 7). Find another focal point.

When a change as shown in FIG. 5A is detected in the first range a and a change as shown in FIG. 6B is detected in the second range b, the in-focus state is detected in both ranges a and b. It is considered that the size of the subject is large enough to cover the entire second range b even if the subject is only a distant view such as a mountain or a close subject, and good focus is obtained. Judge that it is the position.

As described above, according to the focus control circuit of the present embodiment, the switching of the image frame in the captured image (the switching between the first range a and the second range b) causes the focus to be focused on the subject or the background blurring. It is possible to determine whether or not the focus has occurred, thereby improving the accuracy of focus control.

In the above-described embodiment, switching between the first range a and the second range b is performed only at the time of in-focus determination, and when searching for the maximum integrated value, signals in both ranges are simply added and integrated. However, the signal in the first range a at the center may be weighted, added to the signal in the second range b, and integrated. The frequency f ₁ and f ₂ of the high-frequency components and low-frequency components to detect the in-focus state determination has been with 1MHz and 100 KHz, other frequencies if there is a similar correlation high-frequency component is increased during focusing For example, the frequency f ₁ is ₁ M to 2.5 MHz,
Frequency f ₂ may be detected in a range of 100K～1MHz. Furthermore, the present invention is not limited to the above-described embodiment, but may take various other configurations.

[Effects of the Invention] According to the present invention, a frame of a first range and a second range is provided in an imaging screen, and a ratio of a high-frequency component to a low-frequency component is detected for each frame, so that a subject is detected. It is possible to determine whether the subject is focused on the subject or the subject is focused on the background, and it is possible to perform accurate focus control focused on a subject that is not affected by the background.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a focus control circuit of the present invention, FIG. 2 is a flowchart for explaining the example of FIG. 1, and FIGS. 3, 7, and 8 are examples of a photographing state. , FIG. 4, FIG. 5, and FIG. 6 are characteristic diagrams for explaining the example of FIG. (1) is an imaging lens, (1a) is a focus ring,
(3) is an imaging signal processing circuit, (5) is a detection range switching circuit, (6) is a first bandpass filter, and (7) is a second bandpass filter.
, (10) and (11) are integrating circuits,
(12) is a central control unit, and (13) is a motor.

Claims (1)

(57) [Claims] A focus control circuit for detecting a luminance component in an image signal and performing focus control, wherein integration of the luminance component at a predetermined first frequency is performed within a first range substantially at the center of an image screen. Calculating a ratio between the value and an integral value of the luminance component at a second frequency lower than the first frequency. Within the second range around the first range, the ratio of the first frequency is calculated. The ratio between the integral value of the luminance component and the integral value of the luminance component at the second frequency is determined, and the difference between the ratio determined within the first range and the ratio determined within the second range is calculated. A focus control circuit for performing a focus determination.