JP3272186B2 - Auto focus video camera - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video camera having an auto-focusing device for automatically performing focus adjustment using an image signal.

[0002]

2. Description of the Related Art In an autofocus apparatus for a video camera, a method of using an imaged video signal obtained from an image sensor for evaluating a focus control state essentially has no parallax and has a shallow depth of field. There are many excellent points such as the ability to accurately focus on a subject or a distant subject.
In addition, no special sensor for autofocus is required, and the mechanism is extremely simple.

[0003] Japanese Patent Application Laid-Open No. Sho 61-105978 (H04)
N5 / 232) discloses an example of the above-described autofocus video camera.

In the prior art, as shown in FIG. 2, light incident through a lens 1 is imaged on an imaging surface of a solid-state imaging device (CCD) 2 and photoelectrically converted. In step 3, known signal processing is performed to take out the captured image signal, and the captured image signal is input to an HPF (high-pass filter) 4.

In the HPF 4, only the high frequency component of the captured video signal is extracted, and the high frequency component is extracted by the A / D converter 5.
Is converted into a digital value and input to the gate circuits 6 and 7. Here, the gate circuit 6 outputs to the digital integrator 8 at the subsequent stage only when the sampling point in the A / D converter 5 is within the small area AS set at the center of the screen as shown in FIG. When the sampling point in the A / D converter 5 is within the large area AL set on the screen as shown in FIG. 3, the gate circuit 7 outputs the signal to the digital integrator 9 at the subsequent stage.

Digital integrators 8 and 9 add digital data output from gate circuits 6 and 7 over one field period, that is, digitally integrate the digital data.

The large area AL includes the small area AS,
It is set as an area with the top of the screen removed.

With the above-described configuration, the digital integrator 8 outputs the integral value of one field of the high frequency component of the picked-up video signal in the small area AS as the first focus evaluation value V1 to the microcomputer (microcomputer) 10 at the subsequent stage. And the digital integrator 9 outputs the integrated value of one field of the high frequency component of the imaged video signal in the large area AL to the second field.
It is output to the microcomputer 10 as the focus evaluation value V2.

Normally, the level of the high-frequency component of the captured video signal has a maximum value when the lens 1 is in focus, as shown in FIG. 9, and the level decreases as the lens 1 moves away from the focal point. The microcomputer 10 pays attention to only one of the first and second focus evaluation values obtained for each field, sequentially compares the one focus evaluation value with the value one field before, and determines that the focus evaluation value is the maximum. Lens 1 to become
The focus motor 11 is driven so as to move back and forth in the optical axis direction. The focusing operation in which the lens 1 is moved forward and backward to gradually shift to the position where the focus evaluation value reaches the maximum value is usually called a hill-climbing focusing operation, and is widely and widely used.

When the hill-climbing focusing operation is performed, which of the above two types of focus evaluation values is to be used as a basis, in other words, which of the small area AS and the large area AL is selected as the focus area? In the determination, a focusing operation is usually performed based on the center-weighted, that is, the high-frequency component of the captured video signal in the small area AS, that is, the first focus evaluation value V1, and in a shooting situation as shown in FIG. Although the main subject M is focused, if the desired subject does not exist in the small area AS as shown in FIG. 4B, the focus area is expanded to the large area AL and based on the second focus evaluation value V2. A focusing operation is performed.

Here, it is determined whether or not the main subject exists in the small area AS by determining whether the first focus evaluation value is a predetermined reference value R.
It is determined by whether or not it is below EF. That is, when the first focus evaluation value V1 exceeds the reference value REF, it is determined that the subject exists in the small area AS, and the small area is selected as the focus area. When the first focus evaluation value V1 falls below the reference value REF, it is determined that the subject does not exist in the small area AS. The large area AL is selected as the focus area.

[0012]

In the prior art, as described above, the focusing operation is usually performed based on the center-weighted, that is, the high-frequency component of the imaged video signal in the small area. If the subject does not exist, by expanding the focus area to a large area, the focusing operation is completed in the out-of-focus state, or conversely, the focus lens keeps moving forward and backward without finding the maximum value of the focus evaluation value The problem has been solved.

However, when the background is dark as shown in FIG. 5A and a white or high-luminance subject exists at the end of the screen, a part of the subject M enters the end of the small area AS. A contrast between light and dark is generated between the background and the background, the focus evaluation value of the small area AS is increased to some extent, and it is erroneously determined that the subject exists in the small area AS, and the small area AS is selected as the focus area. And
In this state, the focusing operation is performed based on the focus evaluation value with low reliability. Further, when the focus is further defocused from this state, the high-luminance area is expanded as shown in FIG. 5B, and only the focus evaluation value of the small area is apparently increased as shown in FIG. There is a new problem that the out-of-focus state is erroneously determined as the in-focus state and the focusing operation ends in the out-of-focus state.

In FIGS. 5 (a) and 5 (b), the subject is indicated by a solid line when it is in focus.
What is indicated by a chain line is an out-of-focus state.

[0015]

According to the present invention, there is provided an area selecting means for selecting one of a small area set at the center of a screen and a large area larger than the small area as a focus area, and an image signal in the focus area. Focus evaluation value generation means for outputting the high-frequency component level of the image as a focus evaluation value, focus control means for realizing a focused state by controlling the distance between the lens and the imaging means based on the focus evaluation value; A luminance level detecting means for detecting a luminance level of the area as a first luminance level, and detecting a luminance level of an adjacent area arranged right and left of the small area as a second luminance level; When the difference is larger than the luminance level and the difference exceeds a predetermined value, a large area is selected as a focus area.

[0016]

Since the present invention is constructed as described above, it is dark because a high-luminance main subject does not exist in a small area, and when a high-luminance main subject exists in this adjacent area, the focus area becomes a large area. Since the fixed area is fixed, the focus area is always maintained in the large area even in a shooting state where there is a possibility that the subject is erroneously determined to be present in the small area, and the focusing operation is accurately performed on the main subject. Will be.

[0017]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of the present embodiment.
The same parts as in FIG. 2 of the conventional example are denoted by the same reference numerals, and description thereof will be omitted.

The picked-up video signal is supplied to the LPF 21 at the same time as being supplied to the HPF 4, and the output of the LPF 21 is changed to A / A.
The data is converted into a digital value by the D converter 22 and
3 is input.

In the separation circuit 23, the A / D converter 22
In accordance with which of the plurality of areas set on the screen the sampling point at the time of / D conversion is supplied to the digital integrator corresponding to the area.

Here, the plurality of areas are areas obtained by subdividing the large area AL described in the conventional example, and more specifically, as shown in FIG. 6, a small area is adjacent to the left and right of the small area. The lower area A3 and the small area AS, which correspond to the left area A1 and the right area A2 and are adjacent to the lower side of the small area AS,
It can be said that the large area AL is constituted by the sum of the four areas of the left area A1 and the right area A2.

In the separation circuit 23, if the sampling point of the A / D conversion in the A / D converter 22 is in the small area AS, it is input to the digital integrator 24, and if it is in the left area A1, the digital integrator 25 is used. If it is in the right area A2, it is input to the digital integrator 26. If it is not in any of these areas, the digital integrator 24,
25, 26 are not supplied.

Each digital integrator 24, 25, 26
The input data is digitally integrated over one field period, and the integrated output for one field is output to an average calculator 2 in the subsequent stage.
7, 28 and 29.

The area of each area is stored in advance in each average calculator, and by dividing the input integral value by this area value, the average calculator 27 outputs a small area A
Data indicating the average brightness value ES of S, data indicating the average brightness value E1 of the left area A1 from the average calculator 28, and data indicating the average brightness value E2 of the right area A2 are output from the average calculator 27. Will be.

The microcomputer 30 includes digital integrators 8 and 9
Supplies the first and second focus evaluation values V1 and V2, and also supplies the average luminance values ES, E1, and E2. The microcomputer 30 executes a focus area selecting operation before executing a hill-climbing focusing operation for controlling a lens position so that a target focus evaluation value becomes a maximum value. This focus area selection operation is performed according to a routine shown in FIG.

That is, in steps S1 to S3, the small area A
S, the average luminance value ES of each of the left and right areas A1 and A2,
E1 and E2 are fetched, and small area AS is obtained in step S4.
It is determined whether or not the small area AS is sufficiently dark by determining whether or not the average luminance value ES in step S1 is smaller than the first threshold value a. Only when it is determined by this determination that the average luminance value ES is sufficiently low, the left area A is determined in step S5.
It is determined whether or not the average luminance value E1 of 1 is higher than a value obtained by adding the second threshold value b to the average luminance value ES. It is determined that there is no main subject in the small area AS, and that there is a high-luminance subject in the left area A1 on the left side of the small area AS, and that the subject slightly enters the left end of the small area AS to generate contrast. Since the focus area may be fixed to the small area AS due to erroneous determination that the main subject exists in the small area AS, the focus area is fixed to the large area AL in step S7.

If no high-luminance subject exists in the left area A1 in step S5, step S6
It is determined whether or not the average luminance value E2 of the right area A2 is greater than a value obtained by adding the second threshold value b to the average luminance value ES. It is assumed that there is no main subject in the small area AS, it is determined that there is a high-luminance subject in the right area A2 on the right side of the small area AS, and the subject slightly enters the right end of the small area AS and focuses. Area is small area A
Since there is a possibility that the focus area is fixed to S, the focus area is fixed to the large area AL in step S7.

The first threshold value a, which is a threshold value for determining whether or not the small area is dark, and a criterion for determining whether or not the average luminance value of the left and right areas is significantly higher than the small area so as to cause contrast. The second threshold value b is set to an optimum value by an experiment in advance.

If no high-luminance subject exists in the right area A2 in step S6, there is no high-luminance subject in both the left and right areas. In this case, the focus area determination routine of step S8 is executed. Is done.
Note that this step S8 is also executed when it is determined in step S4 that the small area AS is not dark.

In the focus area determination routine,
When the first focus evaluation value V1 in the small area AS exceeds the reference value REF, the focus area is fixed to the small area AS on the assumption that the subject exists in the small area AS. Sets the focus area to the large area AL assuming that there is no subject.

Thus, step S7 or step S8
When the selection of the focus area is completed, the hill-climbing focusing operation is executed in step S9 based on the focus evaluation value in the selected focus area.

In the above-described embodiment, the focusing operation is realized by moving the lens 1 with respect to the CCD 2. However, the lens 1 is fixed instead, and the CCD 2 itself is moved in the optical axis direction by the focus motor. Needless to say, a piezoelectric element can be used instead of the focus motor.

In the above-described embodiment, when the focus area is determined in step S8, an area is selected in accordance with the magnitude of the first focus evaluation value. It is also possible to select an area based on the difference.

In the above embodiment, when the gate circuits 8 and 9 and the separation circuit 23 determine which digital integrator the output of the A / D converters 5 and 22 is supplied to for each area, To determine where the sampling point of the D converter is on the screen, use the CCD
This is performed by counting two drive pulses.

Further, in the above embodiment, the comparison of the average luminance value with the small area is limited to the left and right adjacent areas. However, the upper and lower adjacent areas can be added, and a more accurate object can be compared. Position detection becomes possible.

[0035]

As described above, according to the present invention, since the main subject does not exist in the small area at the center of the screen and the subject exists in the adjacent area, a part of the subject enters the end of the small area. Therefore, even in a shooting situation in which the subject may be erroneously detected as being mainly in a small area, the focus area is always maintained in the large area, and the focusing operation is accurately performed on the main subject. become.

[Brief description of the drawings]

FIG. 1 is an overall block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional example of the present invention.

FIG. 3 is an explanatory diagram of two types of focus areas.

FIG. 4 is an explanatory diagram when a main subject exists at the center and the periphery of a screen.

FIG. 5 is an explanatory diagram when a main subject around the screen enters a small area end;

FIG. 6 is an explanatory diagram of area setting according to an embodiment of the present invention.

FIG. 7 is a flowchart of an autofocus process in the microcomputer according to the embodiment of the present invention.

FIG. 8 is a diagram illustrating a change in a focus evaluation value in the state of FIG. 4;

FIG. 9 is a diagram illustrating a relationship between a lens position and a focus evaluation value.

[Explanation of symbols]

 Reference Signs List 1 lens 2 CCD 4 HPF 6 gate circuit 7 gate circuit 8 digital integrator 9 digital integrator 27 average calculator 28 average calculator 29 average calculator 30 microcomputer

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/232 G02B 7/28

Claims (1)

(57) [Claims] 1. A small area set at the center of a screen, and the small area and an adjacent area arranged adjacent to the small area.
Area selecting means for selecting any one of the large areas including as a focus area, imaging means for photoelectrically converting the light incident through the lens and outputting it as an image signal, and setting the high-frequency component level of the image signal in the focus area. Focus evaluation value generation means for outputting as a focus evaluation value; focus control means for realizing a focused state by controlling the distance between the lens and the imaging means based on the focus evaluation value; A brightness level detecting means for detecting a brightness level as a first brightness level and detecting a brightness level of an adjacent area arranged adjacent to the small area as a second brightness level; and wherein the second brightness level is the first brightness level. When the level is equal to or more than a reference value, the large area is selected as a focus area by the area selecting means. Auto focus video camera.