KR101056051B1 - Focus Control and Method of Auto Focus System - Google Patents

Abstract

Disclosed are an apparatus and method for focusing an auto focus system. The image input unit receives a plurality of first image frames obtained by photographing a photographing target region at a first photographing interval set in advance according to a focal length of the focus lens. The image erosion unit generates a plurality of erosion image frames by applying an erosion mask to each first image frame. The binary image generator generates a plurality of binary image frames in which an upper value is assigned to a position of a pixel having a difference between pixel values constituting each first image frame and the corresponding eroded image frame equal to or greater than a preset reference value. The object region determiner determines a region composed of pixels of which pixel values are all higher values in a plurality of binary image frames as an object region for calculating a focus value on an image frame photographed using a focus lens. The focus setting unit sets the focal length of the focus lens having the maximum focus value calculated based on the pixel values of the pixels constituting the object area as the optimum focal length for the photographing target area. According to the present invention, when the focus value is calculated to set the focal length, the focus value is calculated only for a region where the object is assumed to exist, not the entire image frame, so that the object is accurately positioned even when the object is located in a complex background. You can make the object part in focus.

Description

Apparatus and method for focus control of auto-focusing system}

The present invention relates to an apparatus and method for adjusting a focus of an auto focusing system, and more particularly, to an apparatus and method for focusing a camera on an area in which an object is positioned on an image frame.

Auto-focusing (AF) plays an important role in image acquisition systems such as digital cameras and video cameras. Under the assumption that the main object and the background are in the same plane in the input image, the existing auto focus system estimates the proper position of the focus lens, and the focusing value calculated from the given high pass filter in each frame or field. Hill Climbing Search (HCS) algorithm based on FV) was used. At this time, various studies for focus value calculation include a Sobel operator, a sum-modified Laplacian, and a frequency selective weighted median (FSWM) filter.

However, the previous assumption that the object and the background are in the same plane is not retained when acquiring the actual scene using the 2D camera. In particular, objects that are positioned differently from the background or moving objects may result in defocusing in the existing auto focus system. To address this, high-performance Digital Single-Lens Reflected (DSLR) cameras employ AI servo-based autofocus and dynamic area selection.

Another problem with existing autofocus systems is that the background area is focused when the background area contains more high frequency components than the object area. There is a need for an autofocus system that overcomes these problems and can accurately focus on the object area.

An object of the present invention is to provide an apparatus and method for adjusting the focus of an autofocus system capable of accurately focusing on an object even when the object is located in a complex background or not at the center of the photographing target area.

Another technical problem to be solved by the present invention is a program for executing a focus control method of an auto focusing system capable of accurately focusing an object even when the object is located in a complex background or not at the center of a photographed area. To provide a computer-readable recording medium for recording the data.

In order to achieve the above technical problem, a focus adjusting apparatus of an auto focusing system according to the present invention includes a plurality of first image frames obtained by photographing a photographing target region at a first photographing interval set in advance according to a focal length of a focus lens. Image input unit for receiving a; An image erosion unit generating a plurality of erosion image frames by applying an erosion mask to each of the first image frames; Binary image generation unit for generating a plurality of binary image frame to which the upper value is given to the position of the pixel that the difference of the pixel value between each of the pixels constituting the first image frame and the erosion image frame corresponding thereto ; An object region determiner which determines an area composed of pixels of which all pixel values are higher in the plurality of binary image frames as an object area for calculating a focus value on an image frame photographed using the focus lens; And a focus setting unit configured to set a focal length of the focus lens at which a focal value calculated based on pixel values of the pixels constituting the object area is the maximum to an optimal focal length for the photographing target area.

According to another aspect of the present invention, there is provided a method of adjusting a focus of an autofocus system according to an embodiment of the present invention. Receiving an image frame; (b) generating an erosion image frame by applying an erosion mask to the first image frame; (c) generating a binary image frame in which an upper value is assigned to a position of a pixel whose difference between pixel values constituting the first image frame and the corresponding eroded image frame is greater than or equal to a preset reference value; (d) An image frame in which a region consisting of pixels whose pixel values are all higher values on a binary image frame generated from a currently input first image frame and a previously input first image frame is captured using the focus lens. Determining an object area for calculating a focus value on the image; And (e) setting a focal length of the focal lens having a maximum focal value calculated based on pixel values of the pixels constituting the object area as an optimal focal length for the photographing target area. Steps (a) to (d) are repeated whenever a plurality of first image frames obtained at a first shooting interval are sequentially input one by one.

According to the focus adjusting apparatus and method of the auto focus system according to the present invention, when calculating the focus value to set the focus distance, the focus value is calculated only for the region where the object is assumed to exist, not the entire image frame. This ensures that the object is precisely focused even when the object is located in a complex background.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the focus control apparatus and method of the autofocus system according to the present invention.

1 is a block diagram showing the configuration of a preferred embodiment of the focus control apparatus of the autofocus system according to the present invention.

Referring to FIG. 1, the apparatus for adjusting focus according to the present invention includes an image input unit 110, an image erosion unit 120, a binary image generation unit 130, an object region determination unit 140, and a focus setting unit 150. Equipped.

The image input unit 110 receives a plurality of first image frames obtained by photographing a photographing target region at a first photographing interval set in advance according to a focal length of the focus lens.

In general, the focus lens used in the camera has a focal length of several tens to hundreds of mm. The plurality of first image frames input to the image input unit 110 are photographed at a first shooting interval set by a user from the shortest focal length of the focus lens used for capturing the photographing region to the longest focal length.

The first shooting interval is determined to be an appropriate value according to the number of first image frames necessary for determining the object area. When taking a plurality of first image frames, the first shooting interval may be input to the camera so that the camera may automatically shoot at a constant focal length interval, or the user may manually photograph the shooting target area at a predetermined focal length. If the number of first image frames is too large, the accuracy of object region determination is increased, but the amount of calculation is increased. Therefore, it is preferable to set the first shooting interval so that the first image frame within tens of frames is obtained.

The image erosion unit 120 generates a plurality of erosion image frames by applying an erosion mask to each first image frame.

An erosion of an image is one of the morphological processes of an image, which is to reduce an image by using a structuring element, which is an erosion mask of a constant size. The erosion of the image is expressed as in Equation 1 below.

That is, an eroded image is obtained by the intersection of the parallel shifted portions of the object A and the stem B shown in the image, and the outer protrusion is reduced and the inner protrusion is increased in the image. In addition, as the size of the morpheme increases, the degree of reduction of the image increases. In the present invention, a 3x3 sized morpheme is used to prevent the image from being excessively reduced and the object region is incorrectly determined.

The image erosion unit 120 sequentially inputs a plurality of first image frames photographed at a first shooting interval to the image input unit 110 and simultaneously applies an erosion image frame that is a reduced image by applying an erosion mask to the first image frame. Create As such, if an erosion image frame is generated immediately each time the first image frame is input, the processing speed can be increased compared to performing an erosion process of the image after all the first image frames are input.

The binary image generating unit 130 includes a plurality of binary image frames in which upper values are assigned to positions of pixels whose difference between pixel values constituting each first image frame and the corresponding eroded image frame is equal to or greater than a preset reference value. Create

The binary image frame is an image frame having a value of 1 or 0 of each pixel, and is generated based on a difference in pixel values between the first image frame and the eroded image frame. Therefore, the 'high value' in the present invention is determined as a value of 1 or 0. Hereinafter, an embodiment of the present invention will be described when the upper value is determined to be 1. The pixel value of each pixel constituting the binary image frame is determined by the following equation.

Here, S _i (x, y) is a pixel value at position (x, y) in a binary image frame generated corresponding to an i th first image frame among a plurality of first image frames, and g _i (x, y) is The pixel value at position (x, y) in the i-th first image frame, B is an erosion mask applied to the first image frame, and T is experimentally determined as a reference value.

Since the eroded image frame is a reduced image from the first image frame, the boundary line of the object included in the first image frame appears in the binary image frame obtained by the difference between the pixel value of the first image frame and the eroded image frame. That is, pixels having a pixel value of 1 in a binary image frame represent a boundary line of an object, that is, a high frequency component of the first image frame.

Since the defocused image frame has fewer high frequency components than the focused image frame, the binary image frame generated from the well-focused image frame includes more pixels having a pixel value of 1. In addition, when the focus lens is moved from the rear to the front, the focus position is moved from the object part located in front of the photographed area to the background part located behind. Therefore, in the binary image frame generated corresponding to each of the plurality of first image frames photographed according to the focal length of the focus lens, the pixel having 1 of the pixel value increases from the region corresponding to the object portion toward the region corresponding to the background portion. .

Meanwhile, as the image erosion unit 120 sequentially inputs a plurality of first image frames to the image input unit 110 and simultaneously generates an erosion image frame corresponding to the first image frame, the binary image generation unit 130. Also, when an erosion image frame corresponding to the currently input first image frame is generated, a binary image frame corresponding to the currently input first image frame may be immediately generated from two image frames. As a result, the process of generating the plurality of binary image frames can be completed at the same time as the input of the plurality of first image frames is completed, so that the processing can be performed quickly.

The object region determiner 140 determines a region composed of pixels of which pixel values are all higher values in a plurality of binary image frames as an object region for calculating a focus value on an image frame photographed using a focus lens.

Since the object is positioned in front of the object, that is, closer to the focal lens, the high frequency component of the region corresponding to the object part in the plurality of first image frames is stronger than the background. As a result, in most binary image frames, a pixel having a higher pixel value, that is, 1, is located in a region corresponding to an object part.

The object region determiner 140 extracts a pixel in which the pixel value remains 1 over a plurality of binary image frames in this way and determines the region composed of the pixels as an object region for focusing the focus lens. That is, by applying an AND operator, which is a multiplication operator, to a plurality of binary image frames, the pixel in which the pixel value remains 1 is selected. For example, when three first image frames are input, the pixel value of each pixel constituting the object region is obtained as in Equation 3 below.

Where P (x, y) is the pixel value of the pixel corresponding to the position of (x, y) in the object region, and S _{i -1} (x, y) is a binary generated corresponding to the i-th first image frame The pixel value of the pixel corresponding to the position of (x, y) in the image frame, S _i (x, y), corresponds to the position of (x, y) in the binary image frame generated corresponding to the i th first image frame. The pixel value of the pixel and S _{i + 1} (x, y) are pixel values of the pixel corresponding to the position of (x, y) in the binary image frame generated corresponding to the i + 1 th first image frame.

From Equation 3, it can be seen that an area composed of pixels having a pixel value of 1 is determined as an object area.

As such, when a pixel in which a pixel value is continuously maintained at 1 on a plurality of binary image frames is selected, the selected pixel is likely to be a pixel corresponding to an object part, and thus a high frequency component is included in the background part and the object is an image. Even if it is not located in the center of the frame, it is possible to determine exactly where the actual object is located as the area to be focused.

When the generation of the plurality of binary image frames is completed by the binary image generating unit 130, the object region determination unit 140 selects the object region based on the pixels having all pixel values of 1 across the plurality of binary image frames. You can also decide. However, as described above, a plurality of first image frames may be sequentially input and simultaneously generated erosion image frames and binary image frames. Accordingly, the object region determiner 140 may update the object region in real time whenever binary image frames are sequentially generated, and immediately complete the determination of the final object region when the input of all the first image frames is completed.

2 is a diagram illustrating an example of determining an object area by applying an AND operator cumulatively.

In FIG. 2, an object region is determined using three first image frames g _{i -1} , g _i and g _{i +1} respectively photographed when the focus lens is positioned at i-1, i and i + 1. . In this case, the three first image frames are sequentially input to the image input unit 110 in the order of g _{i -1} , g _i, and g _{i +1} . The image erosion unit 120 generates an erosion image frame by applying an erosion mask to the first image frame whenever each first image frame is input. Next, the binary image generating unit obtains and binarizes the difference image of the first image frame and the erosion image frame generated therefrom, and then makes three binary image frames S _{i -1} , S _i, and S _i corresponding to the first image frames. ₊₁ )

Since the first image frame is sequentially input and the erosion image frame and the binary image frame are generated, the object region determiner 140 cannot determine the object region definitely until all the first image frames are input. However, when the object area is determined by waiting for all of the plurality of first image frames to be input, the processing speed is delayed, and as a result, it takes a long time to set the focal length of the camera.

Therefore, the object region determiner 140 receives the first two first image frames, that is, the first image frames of g _{i -1} and g _i , and corresponds to two binary image frames S _{i -1} and S _i . Is generated, first, an AND operator is applied to these two binary image frames to determine an object region including pixels having pixel values of 1 in both binary image frames. However, the determined object area is not deterministic, and is continuously updated whenever binary image frames are sequentially generated.

When the binary image frame S _{i +1} corresponding to the first image frame g _{i +1} is generated from the third inputted image frame g _{i +1, the} object region determiner 140 generates two binary image frames S _{i −1} , The AND operator is applied to the object region determined from S _i ) and the third generated binary image frame Si + 1. Then, the pixels having the pixel value of 1 are determined as the new object region in both the previously determined object region and the newly created binary image frame, and as a result, the object region is updated.

When this process is repeatedly performed until the last first image frame is input, the finally updated object region is determined as an object region whose focus value is calculated to set a focal length when the corresponding target region is photographed with a focus lens. will be. In addition, as shown in FIG. 2, whenever the first image frame is input, the speed of setting the focal length is increased by updating the object region.

When the object region for calculating the focus value, which is the basis of the focal length setting of the focus lens, is determined on the image frame by capturing the photographing target region by the above-described process, the focus value is calculated from the actually determined object region. The process of setting the focal length is performed.

The focus setting unit 150 sets the focal length of the focus lens having the maximum focus value calculated based on the pixel values of the pixels constituting the object area as the optimum focal length for the photographing target area. To this end, the focus setting unit 150 includes a focus value calculating unit 152 and a focal length setting unit 154.

The focus value calculator 152 calculates a focus value based on pixel values of pixels constituting a region corresponding to the object region on the plurality of second image frames input to the image input unit 110.

The plurality of second image frames are obtained by photographing a photographing target area at a preset second shooting interval according to the focal length of the focus lens, and the first shooting interval from the shortest focal length to the longest focal length to determine the object area. It is obtained by the same method as the plurality of first image frames obtained by photographing with the. The second shooting interval may be set to be the same as the first shooting interval, or may be set at an appropriate interval to obtain a maximum focus value from the plurality of image frames.

The focus value calculation is performed in a region corresponding to the object region determined by the object region determiner 140 on the plurality of second image frames. In this case, for convenience of calculation, a rectangular area having a minimum size including an object area in each second image frame may be determined as an area for calculating a focus value. As described above, the object region is composed of pixels having a pixel value of 1 in all binary image frames, and its shape does not always appear as a rectangle.

When calculating the focus value, conventionally known focus value calculation techniques may be used, and a sum modified Laplacian using a Laplacian filter, which is a kind of a high pass filter, may be used. That is, the absolute value may be taken as the second derivative in the horizontal and vertical directions for each pixel constituting the object region or the focus value calculation region, and then the sum of the absolute values may be determined as the focus value of the corresponding region.

The focal length setting unit 154 may determine the focal length of the focus lens when the second image frame having the maximum focus value of the region corresponding to the object area among the plurality of second image frames is photographed. Set to.

The focus value indicates the intensity of the high frequency component of the image, and a portion where the intensity of the high frequency component in the image is large indicates the portion where the object is located. Accordingly, the second image frame having the maximum focus value calculated for the region corresponding to the object region among the plurality of second image frames corresponds to the image frame focused on the object region. Since the focal length of the focus lens when each second image frame is photographed is already known information, the focal length setting unit 154 compares the calculated focus values with respect to the plurality of second image frames, respectively, to maximize the focus value. Find the second video frame.

When the second image frame having the maximum focus value is selected, the focus values for each second image frame may be compared after all the second image frames are input. However, it is preferable for the speed of processing to select the second image frame having the maximum focus value at the same time that the input of all the second image frames is completed by updating the maximum focus value whenever each second image frame is sequentially input. Do. When the second image frame having the maximum focus value is photographed, the focal length of the focus lens is set to an optimal focal length for capturing a photographing target region, and is then generated by capturing a photographing subject region using the focal lens set as the optimal focal length. Will print the image.

In the conventional methods for setting the focal length, the focal length is set by calculating a focal value for the pixels constituting the entire image, so that when the background part contains a high frequency component that is stronger than the object part, the focal point is adjusted to the background part. It happens that the focal length is set to lose. However, as in the present invention, by determining an area for calculating a focus value in advance and setting only an optimum focal length by comparing only the calculated focus value with respect to the determined area, an image in which the object is accurately focused can be obtained. .

3 is a flowchart illustrating a preferred embodiment of the method for adjusting the focus of the auto focus system according to the present invention.

Referring to FIG. 3, the image input unit 110 receives a first image frame obtained by photographing a photographing target region at a first photographing interval set in advance according to a focal length of the focus lens (S310). The image erosion unit 120 generates an erosion image frame by applying an erosion mask to each first image frame (S320). Next, the binary image generating unit 130 generates a binary image frame in which an upper value is given to a position of a pixel whose difference between pixel values of each pixel constituting the first image frame and the erosion image frame corresponding thereto is equal to or greater than a preset reference value. (S330).

If there is no object region determined previously (S320), the object region determiner 140 includes pixels having all upper values, that is, 1s, on the plurality of binary image frames up to the currently generated binary image frame. The region thus formed is determined as an object region (S350), so that a focus value is calculated based on pixels constituting the object region on an image frame photographed using the focus lens. In addition, when the object region determined by the object region determiner 140 based on the previously generated binary image frames already exists (S320), the object region and the object region are applied by applying an AND operator to the corresponding object region and the currently generated binary image frame. The existing object region is updated by determining a new object region including pixels having pixel values of all 1 in the currently generated binary image frame (S340). This has been described in detail above.

When all of the plurality of first image frames are input (S360), a process of setting an optimal focal length through a focus value calculation is then performed. In operation S370, the image input unit 110 sequentially inputs a plurality of second image frames obtained by photographing a photographing target region at a preset second photographing interval according to the focal length of the focus lens. The focus value calculator 152 calculates a focus value of the second image frame based on pixel values of pixels constituting a region corresponding to the object area on each second image frame (S380). Finally, the focal length setting unit 154 optimizes the focal length of the focus lens when the second image frame in which the focal value of the region corresponding to the object area is the maximum among the plurality of second image frames is captured. The focal length is set (S390).

Experiments were conducted to evaluate the performance of the present invention. In order to evaluate the accuracy of object region decision, we used image frame with complex background. 4 is a diagram illustrating a test image frame used in an experiment. Both video frames of FIG. 4 have a size of 640 × 426 pixels. The video frame of FIG. 4 (a) includes windows and trees of the building in the background, and the background of the video frame of FIG. 4 (b). Contains only the windows of the building.

FIG. 5 is a diagram illustrating an object area determined from each image frame of FIG. 4. In FIGS. 5A and 5B, the portions indicated by the rectangles are portions in which the object region is located. Although both image frames contain complex backgrounds, it can be seen that the object area is correctly determined to be located at the part where the actual object is located.

In addition, the performance of the focal length setting of the present invention was compared with the Sobel method and the modified Laplacian sum (SML) method, which are methods of calculating the focus value.

FIG. 6A illustrates a region focused on the image frame of FIG. 4A compared to a conventional method, and FIG. 6B illustrates a region focused on the image frame of FIG. 4B. It is shown in comparison with the method.

First, referring to FIG. 6A, since the focal length is set based on the object area determined using the present invention, (a) is precisely focused on the object located in front. However, (b) and (c) are image frames photographed with the focal length set by the existing Sobel and SML techniques, and are focused on the background part including a lot of high frequency components. In addition, referring to FIG. 6B, (a) is focused on the object located in front of the focal length is set according to the present invention, but in the image frame of (b) and (c) is focused on the background portion .

6A and 6B, the present invention may set a focal length to accurately focus on an area where an object is located even when a complex background is included in the photographing target area or when the object is not located at the center of the image frame. Therefore, the performance is improved compared to the existing techniques.

FIG. 7 is a graph illustrating focus values calculated for a plurality of image frames obtained by photographing a photographing target region included in the image frame of FIG. 4 according to a focal length. Referring to FIG. 7, since a lot of high-frequency components are included in the background part, when using the Sobel and SML methods, which are conventional methods, the focus value is maximum when the background part is focused. Since the focus value is calculated only for the area corresponding to the part, the maximum focus value can be obtained when the object part is focused.

8 is a graph illustrating focus values calculated for a plurality of image frames obtained by photographing a photographing target region included in the image frame of FIG. 4B according to a focal length. Referring to FIG. 8, as in the case of FIG. 7, the focus values are maximum when the background part is focused in the Sobel method and the SML method, but the focus value is maximum when the object part is focused in the present invention. You can see that.

The present invention can also be embodied as computer-readable codes on a computer-readable recording medium. The computer-readable recording medium includes all kinds of recording devices in which data that can be read by a computer system is stored. Examples of the computer-readable recording medium include a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, an optical data storage device, and the like, and may be implemented in the form of a carrier wave (for example, transmission via the Internet) . The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion.

Although the preferred embodiments of the present invention have been shown and described above, the present invention is not limited to the specific preferred embodiments described above, and the present invention belongs to the present invention without departing from the gist of the present invention as claimed in the claims. Various modifications can be made by those skilled in the art, and such changes are within the scope of the claims.

1 is a block diagram showing the configuration of a preferred embodiment of the focus control apparatus of the autofocus system according to the present invention;

2 is a diagram illustrating an example of determining an object area by applying an AND operator cumulatively;

3 is a flowchart illustrating a process of performing a preferred embodiment of the method for adjusting the focus of the auto focus system according to the present invention;

4 is a view showing a test image frame used in the experiment,

5 is a diagram illustrating an object region determined from each image frame of FIG. 4;

6A and 6B illustrate the areas focused on the image frame of FIG. 4A in comparison with the conventional method, and the areas focused on the image frame of FIG. 4B, respectively. Drawing shown in comparison with the method,

FIG. 7 is a graph illustrating a focus value calculated for a plurality of image frames obtained by photographing a photographing target region included in the image frame of FIG. 4 according to a focal length, and

FIG. 8 is a graph illustrating focus values calculated for a plurality of image frames obtained by photographing a photographing target region included in the image frame of FIG. 4B according to a focal length.

Claims (9)

An image input unit configured to receive a plurality of first image frames obtained by photographing a photographing target region at a first photographing interval set in advance according to a focal length of the focus lens; An image erosion unit generating a plurality of erosion image frames by applying an erosion mask to each of the first image frames; Binary image generation unit for generating a plurality of binary image frame to which the upper value is given to the position of the pixel that the difference of the pixel value between each of the pixels constituting the first image frame and the erosion image frame corresponding thereto ; An object region determiner which determines an area composed of pixels of which all pixel values are higher in the plurality of binary image frames as an object area for calculating a focus value on an image frame photographed using the focus lens; And And a focus setting unit configured to set a focal length of the focus lens at which a focal value calculated based on pixel values of the pixels constituting the object area is the maximum as an optimal focal length for the photographing target area. Focusing device. The method of claim 1, The image input unit receives a plurality of second image frames obtained by photographing the photographing target region at a second photographing interval set in advance according to a focal length of the focus lens. The focus setting unit, A focus value calculator configured to calculate a focus value based on pixel values of pixels constituting an area corresponding to the object area on the plurality of second image frames; And A focal length setting unit configured to set a focal length of the focus lens when the second image frame having the maximum focal value of the region corresponding to the object area is photographed among the plurality of second image frames as the optimum focal length; Focus control device comprising a. 3. The method of claim 2, The focus value calculator is configured to add a sum of absolute values of second and second derivatives in the horizontal and vertical directions calculated for each pixel constituting a region corresponding to the object region in each of the second image frames. Focusing apparatus, characterized in that determined by the focus value. The method according to any one of claims 1 to 3, The image input unit sequentially receives the plurality of first image frames, The object region determiner determines that the pixel values of the object region determined from the plurality of first image frames previously input and the binary image frames generated from the currently input first image frame are higher than each other when the first image frame is input. And updating the object area based on the pixels as values. (a) receiving a first image frame obtained by photographing a photographing target region at a first photographing interval set in advance according to a focal length of the focus lens; (b) generating an erosion image frame by applying an erosion mask to the first image frame; (c) generating a binary image frame in which an upper value is assigned to a position of a pixel whose difference between pixel values constituting the first image frame and the corresponding eroded image frame is greater than or equal to a preset reference value; (d) An image frame in which a region consisting of pixels whose pixel values are all higher values on a binary image frame generated from a currently input first image frame and a previously input first image frame is captured using the focus lens. Determining an object area for calculating a focus value on the image; And (e) setting a focal length of the focal lens that maximizes a focal value calculated based on pixel values of pixels constituting the object region as an optimal focal length for the photographing target area; And repeating steps (a) to (d) each time a plurality of first image frames obtained at the first shooting interval are sequentially input one by one. The method of claim 5, In step (e), (e1) receiving a second image frame obtained by photographing the photographing target region at a second photographing interval set in advance according to a focal length of the focus lens; (e2) calculating a focus value based on pixel values of pixels constituting an area corresponding to the object area on the second image frame; And (e3) The focus lens when a second image frame having a maximum focus value of a region corresponding to the object region is photographed among a plurality of second image frames obtained by repeating steps (e1) and (e2). And setting a focal length of the focal length to the optimum focal length. The method of claim 6, In the step (e2), the sum of the absolute values of the second and second derivatives in the horizontal and vertical directions calculated for each pixel constituting the region corresponding to the object region in the second image frame is calculated. Focusing method characterized in that determined by the focus value. The method according to any one of claims 5 to 7, In the step (d), every time the first image frame is input, the pixel value of both the object region determined from the plurality of first image frames previously input and the binary image frame generated from the first input image frame are all. And updating the object area based on the upper pixel. A computer-readable recording medium having recorded thereon a program for executing the focus adjusting method according to any one of claims 5 to 7.

Images