JPH03226076A - Video camera automatic focus system - Google Patents

Abstract

PURPOSE:To attain focusing to a posing closer to a major object by setting a threshold level to an integration device when a high frequency component is extracted from a luminance signal of a video signal and the detected output is integrated and selecting the detected output for the integration depending on the threshold level. CONSTITUTION:A luminance signal of an image formed to an image pickup element is inputted to a high pass filter 1 and only a high frequency component of the luminance signal is extracted. The high frequency component of the luminance component is inputted to an amplifier 2 and amplified up to a proper level. The amplified high frequency component is given to a detector 3, in which the absolute value is detected. The output of the absolute value is integrated by an integration device 4 and a focus signal is outputted. The integration device 4 applies one field integration to only the absolute value output higher than the threshold level given from a threshold level generator 5. The threshold level generator 5 decreases the threshold level when the focal signal is small and increases the threshold level when the focus signal is saturated. Thus, the property of the focal signal is changed by the user in matching with an object.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a video camera autofocus system that automatically adjusts the focus of a video camera or the like.

(Prior Art) As an AF system for a video camera, a system is known that extracts a high frequency component from a luminance signal of a video signal and focuses based on the data.

(Problems to be Solved by the Invention) As one method of these systems, there is an integral evaluation method in which an integral value of high frequency components for one field is used to evaluate the focus position.

This integral evaluation method integrates everything from noise and objects with small contrast to objects with maximum contrast, so it is possible to focus on the average position of the entire screen. However, it is conceivable that the main subject may be out of focus.

Another method is a peak hold evaluation method in which the maximum value of high frequency components for one field is used to evaluate the focus position.

With this method, it is possible to focus on the area with the highest contrast, but the object with the highest contrast is not necessarily the main subject. This is because there may be objects that are located behind the main subject that is closest to the foreground and have a high contrast.

The purpose of the present invention is to introduce the idea of peak hold evaluation into the above-mentioned integral evaluation, thereby creating a video camera that can focus not on the entire screen, nor on the area with the highest contrast, but on an area closer to the main subject than the desired focus. Our goal is to provide an autofocus system for cameras.

(Means for Solving the Problems) In order to achieve the above object, a video camera autofocus system according to the present invention drives an optical lens, and detects the luminance signal of a subject image formed on an image sensor at each lens position. In a video camera autofocus system that extracts the high frequency component that is detected, integrates the detected output using an integrator, and determines the lens position where the maximum integrated value is obtained as the in-focus position, when the integrated value output becomes small, , a threshold generator is provided that generates a large threshold output when the integrated value output becomes large, and the integrator selects and integrates the detected output according to the threshold given by the threshold generator. It is composed of:

(Example) Hereinafter, the present invention will be explained in more detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a video camera autofocus system according to the present invention.

This figure shows only the parts directly related to the present invention.

A brightness signal of an image formed on an image sensor by a lens (not shown) is input to a bypass filter 1 .

A bypass filter (or bandpass filter) 1 extracts only high frequency components of the luminance signal.

The high frequency component of the luminance signal is input to the amplifier 2 and amplified to an appropriate level. The absolute value of the amplified high frequency component is detected by the detector 3.

The absolute value output is integrated by an integrator 4 and a focus signal is output.

The integrator 4 integrates only the absolute value output of the portion larger than the threshold value given from the threshold value generator 5 over one field.

The threshold value generator 5 has an automatic operation mode and a manual operation mode, and in the automatic operation mode, the generated threshold value is changed depending on the magnitude of the focus signal fed back from the integrator output. When the focus signal is small, the threshold value is made small, and when the focus signal is saturated, the threshold value is made high.

Here, the auto operation mode is a mode in which the threshold value setting of the threshold value generator 5 when focusing the lens is left to the control circuit of the camera, and can be switched to manual operation using a switch or the like.

On the other hand, when the manual operation mode is set, the darkness value can be set by external adjustment input, regardless of the integral value output. For example, when the threshold value is set high, the character of the peak hold evaluation method becomes strong, and when the threshold value is set low, the character of the integral evaluation method becomes strong.

Therefore, the user can change the characteristics of the focus signal according to the subject.

FIG. 2 is a diagram showing an example of a subject. It shows a person playing tennis against a distant view of mountains in the background.

Normally, a video camera operator expects to be able to clearly focus on the person in the foreground to about the edge of a tennis court. In such a case, it would be ideal for the center of focus to be located slightly behind the person, and for the depth of focus to extend from the person in front to the edge of the tennis court.

3 and 4 are diagrams showing the detection output and integrator output of the subject in FIG. 2, in order to compare the integral evaluation method and the method of the present invention.

When looking at the IH portion of A-A' of the subject in Figure 2, if the lens is brought near a relatively close person,
The output of the detector 3 is as shown in FIG. 3(a), and the integration result over the entire screen is as shown in FIG. 3(a). This is because the lens position is on the short distance side, so contrast is detected only from nearby people.

On the other hand, when the lens position is at infinity, the output of the detector 3 and the integrated output of the entire screen are as shown in FIGS. 4(a) and 4(b).

In this case, all the fine contrasts of the landscape at infinity are detected, so the overall integration result is larger than when the lens is located at a short distance.

Therefore, since the in-focus position of the lens is set to the maximum value of the integrated output, it is determined that the focus is at infinity,
The result is that the main subject, the person, is blurred.

The dotted lines shown in FIG. 3(C) and FIG. 4(C) are lines indicating threshold values. By setting the threshold in this way, noise and small contrasts are cut out, and only relatively large contrasts are integrated.
) and as shown in Figure 4(d). As a result, the integral output becomes larger when the lens is located at a closer distance.

Therefore, the focal point is found on the short distance side, and the vicinity of the person is in focus.

One embodiment has been described above, but two different threshold values have been described.
It is also possible to set two thresholds and integrate only the output between the two thresholds.

(Effects of the Invention) As explained above, the present invention extracts high frequency components from the luminance signal of a video signal and integrates the detected output.
A threshold value is set in the integrator, and the detection output to be integrated is selected based on the threshold value. As a result, it is possible to obtain an integral output that takes into account the peak hold evaluation method in addition to the integral evaluation method, which has the effect that it is possible to focus closer to the main subject.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of a video camera autofocus system according to the present invention, FIG. 2 is a diagram showing an example of a subject, and FIGS. 3 and 4 show an integral evaluation method and a system according to the present invention. It is a figure which shows the example of the detector output and integral output of the bargain which explains. ... Bypass filter ... Amplifier (AMP) ... Detector ... Integrator ... Threshold generator (HPF)

Claims (2)

[Claims] (1) Drive the optical lens, extract the high frequency component contained in the brightness signal of the subject image imaged on the image sensor at each lens position, integrate the detected output with an integrator, and calculate the maximum integrated value. In a video camera autofocus system that determines the obtained lens position as the in-focus position, when the integral value output becomes small, a small threshold value is generated, and when the integral value output becomes large, a large threshold value output is generated. 1. A video camera autofocus system, characterized in that a threshold generator is provided, and the integrator is configured to select and integrate a detection output based on a threshold given by the threshold generator. (2) The video camera autofocus system according to claim 1, wherein the threshold value generator is configured so that the threshold value can be changed by external input regardless of the output of the integrator.