JPH0616134B2 - Automatic focus control method for camera - Google Patents

Description

The present invention relates to an automatic focus adjustment method for a camera, and more particularly to a sharpness detection method for an imaged screen.

(Prior Art) Generally, the focus adjustment of a camera is artificially performed while observing an image forming surface from a finder or in the case of a TV camera while observing a TV receiver. Various methods have been proposed for adjusting manually.

Conventionally, as an automatic focus adjustment method, a distance measuring device using an ultrasonic wave is used in combination with a lens moving mechanism, or the lens is moved so that two images separated by a predetermined distance on the front surface of the camera coincide with each other. There is a device using a so-called optical rangefinder.

However, in the former method using ultrasonic waves, although the distance can be measured relatively easily, since the distance between the subject and the camera is measured independently of the image formation screen, the camera has a zoom mechanism. Therefore, there is a problem in the accuracy of distance measurement when the magnification of image formation changes, and if the latter method of triangulation is not applied to automatic focus adjustment, there is a means to determine the coincident point of two image formations. It is necessary separately and complicated.

Especially when it is applied to the automatic focus adjustment of a TV camera installed in a remote place and the direction of various cameras or the position of the subject changes,
Furthermore, none of the above-mentioned conventional methods is optimal as an automatic focus adjustment method for these TV cameras when these video signals are transmitted to a required place via a communication line.

(Object of the Invention) An object of the present invention is to provide a method suitable for an automatic focus adjusting method of a television camera, in which the drawbacks of the conventional camera automatic focus adjusting method described above are compensated.

(Structure of the Invention) The present invention has the following structures to achieve this object.

That is, the video signal of the television camera is converted into an electric signal, a so-called video signal, because it is displayed on the cathode ray tube.

The present invention detects the sharpness of image formation from the video signal thus converted into an electric signal, and divides the image formation screen into required pixels, and the pixels of the adjacent pixels vertically or vertically. The light-darkness or hue is compared, the difference is extracted, and the focus adjustment mechanism of the camera is controlled so that the total sum of the differences is maximized.

(Example) Hereinafter, the present invention will be described in detail based on illustrated examples.

FIGS. 1 (a), (b) and (c) are diagrams for explaining the principle of the present invention.

First, FIG. 1 (a) is a diagram showing a circular image displayed on a cathode ray tube with proper focus.

Now, when this screen is divided into 16 × 16 pixels and the brightness of each pixel is divided into, for example, four levels from 0 to 3 and displayed, it becomes as shown in FIG.

That is, it is assumed that the density of the corner pixels located inside the imaging circle is 3 and the density of the outside is 0. When the subject moves out of focus from this state, the image on the screen becomes blurred as shown in Fig. 6 (c), and as a result, the entire image is slightly expanded and the contour image of the image becomes clear. At the same time, the change in the density becomes slow and an intermediate density portion is generated in the contour portion.

Now, when the horizontal pixel row Mn of the central arrow portion of each screen in such two states is extracted, it is respectively shown in FIGS.
Becomes

Next, when the density difference ΔM of the adjacent ones of the pixel rows Mn is obtained, it becomes as shown in FIGS. 6 (f) and (g), respectively, and further, the difference, that is, the secondary difference ΔJn is obtained (h). And as shown in (i).

The sum of the quadratic differences Kn = |
By comparing ΔJn |, it is clear from this figure that Kn = 12 in the in-focus image (b) or Kn = 4 in the out-of-focus image (c). It can be seen that the second-order difference sum Kn of

That is, according to the present invention, the automatic focus adjustment is performed by utilizing such a phenomenon that the difference in shade or hue of the image-forming contour portion changes in accordance with the focus shift, and this becomes maximum at an appropriate focus. is there.

The specific method will be described below with reference to examples.

FIG. 2 shows a slow scan television (SS
It is a block diagram which shows one Example of the automatic focus adjustment device of the camera for TV.

In the figure, reference numeral 1 is a television camera, and a picture tube 2 attached to the television camera 2 converts a formed image into an electric signal to produce a video signal A.
Is output and transmitted to an image receiving device (not shown),
A part of it is input to the A / D converter 3.

Further, after converting the video signal into a digital signal by the A / D converter, the screen is divided into a required number of pixels as described above, and the shading of each pixel is digitized and stored in the image memory 4 and the control of the next stage is performed. The lens moving motor 7 added to the television camera 1 through the motor driving unit 6 while detecting the sum S = ΣKn of the stored grayscale difference of the pixels in the entire screen or a part of the screen in the unit 5. Is controlled, and the focal length is changed.

An example of the control algorithm of the SSTV automatic focus adjustment device thus configured will be described with reference to the flow chart shown in FIG.

In the figure, first, in step 1, the focal length L of the lens is set to the initial L _O, and in step 2, the pixel density difference sum S ₁ of the image at this time is detected by the control section 5. At the same time, in step 3, this difference is stored in the image memory 4.

Then by driving the motor 7 in step 4 with a focal length L of the camera faintly △ L by increasing the L = L O ₊ △ L, and detects the pixel shading difference sum S ₁ again in Step 5,
This is compared with S ₂ stored in step 3 to obtain the difference ΔS = S ₁ −S ₂ .

At this time, if ΔS is positive, it means that the image becomes clearer by increasing the focal length by ΔL, and the image may become clearer by making it slightly longer. On the contrary, if ΔS is negative, it means that the image is blurred by slightly increasing the focal length image formation of the lens, so that the focal length should be further shortened.

Accordingly, in step 9 as described above, the sign of ΔS is judged, and if positive, the process returns to step 4 to further lengthen the lens image plane spacing by ΔL, and if negative, proceed to step 10. On the contrary, the length is shortened by ΔL and the process returns to step 5.

The above steps are repeated to make the image clearer by controlling so as to maximize the change in the pixel density difference sum ΔS due to the minute change in the focal length.

In the above-mentioned embodiment, the case where the shades of the pixels adjacent in the horizontal direction are compared has been shown. However, when there is a possibility that a horizontal stripe figure is formed on the screen, the secondary difference is integrated not only in the horizontal direction but also in the vertical direction. Also need to do.

Also, the areas that are divided into pixels and the secondary differences of the grayscale differences are integrated can be set appropriately according to the purpose. For example, when there is a perspective difference between the subject to be imaged and the background, Since the above-mentioned difference is not obtained, it does not represent the required sharpness, so that it is generally desirable to integrate for a limited area in the center of the screen.

However, conversely, when this is applied to SSTV as in the above embodiment, the control according to the flowchart shown in FIG. 2 can be made extremely quick compared to the screen scan time, so the number of pixel divisions is set to be large over the entire screen. By adjusting the focus for each horizontal scan of the screen, it is possible to clearly capture a subject having a perspective difference over the entire screen.

That is, assuming that the upper part of the screen is a distant mountain and the subject approaches as the lower part scans, the upper part of the screen is corrected by correcting the focus adjustment for each horizontal scan as described above. In that case, if you focus on a distant mountain and move to the bottom of the screen, it will be automatically adjusted so that it will focus on a close subject, so even if there is a difference in perspective, the whole screen will be fresh. It becomes possible to form an image on.

The present invention is not limited to the embodiment described above,
As the configuration or control algorithm, various other conceivable ones may be applied. Further, it is apparent that the present invention can be applied not only to a television camera in which a video signal is interposed, but also to a normal camera, if the above-mentioned control is performed by converting a one-shot image into an electric signal.

(Effect of the invention) As described above, the present invention converts the sharpness of the image itself into an electric signal and automatically adjusts the focus while detecting the electric signal. It is extremely effective in providing an automatic focus adjustment method suitable for the above-mentioned focus adjustment, especially for SSTV cameras.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention,
(a) is a diagram showing a video screen imaged in focus, (b) is a schematic diagram in which the screen is divided into pixels and the brightness of each pixel is numerically displayed, and (c) is the same as when the focus is out of focus. , (D) is a partial pixel row in which the pixels in the horizontal row of the screen center arrow shown in (a) above are extracted, (e) is the same pixel row on the screen shown in (c) above, and (f) (g), (h) and (i) are diagrams showing the first and second differences in density of the pixel rows shown in (d) and (e), respectively, and FIG. 2 is an automatic view of the SSTV camera to which the present invention is applied. FIG. 3 is a block diagram showing an embodiment of the focus adjusting device, and FIG. 3 is a flow chart diagram showing an embodiment of a control algorithm of the device shown in FIG. 1 ... Camera, 2 ... Image tube, 3 ... A / D converter, 4 ... Image memory, 5 ... Control unit, 6 ... Motor drive unit, 7 ... Motor.

Claims (2)

[Claims] 1. An image sharpness determining means for automatically adjusting a distance between a lens of a camera and an image forming surface to perform focusing. The distance between the lens and the image plane is divided into pixels and the absolute value of the difference between the lightness and the hue of the adjacent pixels is extracted, and the absolute value of the difference between the adjacent absolute values is maximized. Alternatively, an automatic focus control method for a camera, characterized in that the focus adjusting means is controlled. 2. The distance between the lens and the image plane or other focus adjusting means is controlled for each scan in the horizontal direction of the screen on which the image is displayed. 5. An automatic focus control method according to claim 1.