# Patent

## Title
Automatic focusing method and digital photographing apparatus using the same

## Abstract

Provided are an automatic focusing method in which a photograph region is brought into focus through automatic movement of a focal lens to a focal position, in response to a photographing-command signal, and a digital photographing apparatus using the method. The method includes dividing the photograph region into a plurality of sub-regions, focusing on the sub-regions and obtaining focal position values DSFOC, calculating an average focal position value ADSFOC of the focal position values DSFOC, selecting focal position values DSFOC whose differences from the average focal position value ADSFOC are equal to or less than a threshold value DTH, and moving the focal lens to a focal position corresponding to the focal position value DSFOC having the shortest focal distance among the selected focal position values DSFOC.

## Background
BACKGROUND OF THE INVENTION
This application claims the priority of Korean Patent Application No. 2004-61949, filed on Aug. 6, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an automatic focusing method and a digital photographing apparatus using the same, and more particularly, to an automatic focusing method in which a photograph region is brought into focus through automatic movement of a focal lens to a focal position, in response to a photographing-command signal, and a digital photographing apparatus using the method.
2. Description of the Related Art
A conventional digital photographing apparatus is disclosed in U.S. Patent Publication No. 119,876 entitled “Method of Notification of Inadequate Picture Quality”, filed by the present applicant in 2004. In that disclosure, a focal lens of a digital photographing apparatus is moved to a focal position in response to a photographing-command signal generated by a user, so that a photograph region is automatically brought into focus.
If a digital photographing apparatus focuses automatically on an entire photograph region, then a large amount of data that is unnecessary for focusing has the potential to cause inaccuracies in focusing. Accordingly, some conventional digital photographing apparatuses focus automatically only on the center of a photograph region. If a subject is not situated at the center of the photograph region, however, then the digital photographing apparatus can fail to focus accurately on the subject.
SUMMARY OF THE INVENTION
The present invention provides an automatic focusing method in which the position of a subject can be brought into focus and balanced focusing can be maintained in an entire photograph region, and a digital photographing apparatus using the method.
According to an aspect of the present invention, there is provided an automatic focusing method in which a photograph region is brought into focus through automatic movement of a focal lens to a focal position, in response to a photographing-command signal. The method includes dividing the photograph region into a plurality of sub-regions; focusing on the sub-regions and obtaining focal position values DSFOC; calculating an average focal position value ADSFOC of the focal position values DSFOC; selecting focal position values DSFOC whose differences from the average focal position value ADSFOC are equal to or less than a threshold value DTH; and moving the focal lens to a focal position corresponding to the focal position value DSFOC having the shortest focal distance among the selected focal position values DSFOC.
In the method used by the digital photographing apparatus, the final focal position value DSFOC selected from the focal position values DSFOC of the sub-regions is close to the average focal position value ADSFOC and has a short focal distance. Therefore, there is a high probability that the position of a subject desired by a user will be brought into focus. In addition, balanced focusing can be maintained throughout an entire photograph region.
According to another aspect of the present invention, there is provided a digital photographing apparatus using the automatic focusing method.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof, with reference to the attached drawings in which:
FIG. 1 is a perspective view illustrating the front and the top of a digital photographing apparatus according to the present invention;
FIG. 2 is a perspective view illustrating the back and the top of the digital photographing apparatus of FIG. 1;
FIG. 3 is a schematic diagram of the entire configuration of the digital photographing apparatus of FIG. 1;
FIG. 4 illustrates the structure of a portion of the digital photographing apparatus of FIG. 1 on which light is incident;
FIG. 5 is a flowchart illustrating a main algorithm of a digital camera processor (DCP) illustrated in FIG. 3;
FIG. 6 illustrates a preview mode algorithm included in FIG. 5;
FIG. 7 illustrates a photographing mode algorithm included in FIG. 5;
FIG. 8 illustrates a photograph region of a photoelectric conversion unit (OEC) of FIG. 3 divided into a plurality of sub-regions;
FIG. 9 is a graph for illustrating an automatic focusing algorithm shown in FIG. 7;
FIG. 10 is a flowchart illustrating a detailed automatic focusing algorithm shown in FIG. 7;
FIG. 11 is a flowchart illustrating S101 of FIG. 10;
FIG. 12 illustrates first and second reference characteristic curves used in S93 and S95 of FIG. 11;
FIG. 13 is a flowchart illustrating S91 of FIG. 11;
FIG. 14 is a flowchart illustrating S92 of FIG. 11;
FIG. 15 is a flowchart illustrating S93 of FIG. 11 according to an embodiment of the present invention; and
FIG. 16 is a flowchart illustrating S93 of FIG. 11 according to another embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, the front part of a digital photographing apparatus 1 according to the present invention includes a microphone MIC, a self-timer lamp 11, a flash 12, a shutter release button 13, a viewfinder 17a, a flash light intensity sensor 19, a power switch 31, a lens unit 20, and a remote receiver 41.
In a self-timer mode, the self-timer lamp 11 operates for a set period of time from when the shutter release button 13 is pressed to when an image starts to be captured. When the flash 12 operates, the flash light intensity sensor 19 senses the intensity of the light generated by the flash 12 and relays the sensed intensity of the light to a digital camera processor (DCP) 507 of FIG. 3 via a micro-controller 512 of FIG. 3. The remote receiver 41 receives an infrared photographing command signal and relays the infrared photographing command signal to the DCP 507 via the micro-controller 512.
The shutter release button 13 has two levels. In particular, referring to FIGS. 1 and 7, when a user lightly depresses the shutter release button 13 to a first level after operating a wide-angle zoom button 39W and a telephoto zoom button 39T, a first level signal S1 from the shutter release button 13 is turned on. When, however, a user fully depresses the shutter release button 13 to a second level, a second level signal S2 of the shutter release button 13 is turned on.
Referring to FIG. 2, the back of the digital camera 1 according to the present invention includes a mode dial 14, function buttons 15, a manual focus/delete button 36, a manual adjust/reproduce button 37, a reproduction mode button 42, a speaker SP, a monitor button 32, an automatic focusing lamp 33, a viewfinder 17b, a flash standby lamp 34, a color LCD panel 35, a wide-angle zoom button 39W, a telephoto zoom button 39T, and an external interface unit 21.
The mode dial 14 is used for selecting the operating modes of the digital camera 1, such as a synthesized photographing mode 14ML, a program photographing mode, a character photographing mode, a night view photographing mode, a manual photographing mode, a motion picture photographing mode 14MP, a user setting mode 14MY, and a recording mode 14V.
The synthesized photographing mode 14 ML is for synthesizing an input image and a supporting image. The user setting mode 14MY is an operating mode in which a user decides settings for photograph-taking for still image or motion picture photographing mode. The recording mode 14V is for recording audio only.
The function buttons 15 are used for both operation of specific functions of the digital camera 1 and as control buttons for managing the movement of an active cursor on the menu screen of the color LCD panel 35. For example, in the still or motion picture photographing mode 14MP a user may set automatic proximity focusing by pressing a macro/move-down button 15P. Also, after selecting a specific option from a menu/select-confirm button 15M, the macro/move-down button 15P can be used to move the cursor down the LCD panel 35.
When a user presses a voice-memo/move-up button 15R, a 10-second recording is possible upon consecutive photographing. Also, after selecting a specific option from the menu/select-confirm button 15M, the voice-memo/move-up button 15R can be used to move the cursor up the LCD panel 35. If the user presses the menu/select-confirm button 15M when the active cursor is on a menu item, the operation corresponding to the menu item is performed.
The manual focus/delete button 36 is used for manually focusing or deleting in a photographing mode. The manual adjust/reproduce button 37 is used for manually adjusting of specified conditions and for stopping or reproducing in a reproduction mode. The reproduction mode button 42 is used for converting into a reproducing or preview mode.
The monitor button 32 is used for controlling the operation of the color LCD panel 35. For example, in the photographing mode, when the user presses the monitor button 32, an image and photographing information are displayed on the color LCD panel 35. When the user presses the monitor button 32 again, the color LCD panel 35 is turned off. In the reproduction mode, when the user presses the monitor button 32 while an image file is being reproduced, photographing information about the image file is displayed on the color LCD panel 35: When the user presses the monitor button 32 again, only pure images are displayed.
The automatic focusing lamp 33 operates when focus is well adjusted. The flash standby lamp 34 operates when the flash 12 of FIG. 1 is in a standby mode. A mode indicating lamp 14L indicates a selection mode of the mode dial 14.
FIG. 3 is a schematic diagram of the entire configuration of the digital camera 1 of FIG. 1. FIG. 4 illustrates the structure of a portion of the digital camera 1 of FIG. 1 on which light is incident. The entire configuration and operation of the digital camera 1 of FIG. 1 will be described below with reference to FIGS. 1 through 4.
An optical system (OPS) including the lens unit 20 and a filter unit 41 optically processes light. The lens unit 20 of the OPS includes a zoom lens ZL, a focal lens FL, and a compensation lens CL.
When the user presses the wide-angle zoom button 39W or the telephoto zoom button 39T included in a user input unit (INP), a signal corresponding to the wide-angle zoom button 39W or the telephoto zoom button 39T is relayed to the micro-controller 512. The micro-controller 512 controls a lens driver 510, thereby running a zoom motor MZ which in turn moves the zoom lens ZL. In other words, when the user presses the wide-angle zoom button 39W, the focal length of the zoom lens ZL becomes short, thereby widening the angle of view. When the user presses the telephoto zoom button 39T, the focal length of the zoom lens ZL becomes long, thereby narrowing the angle of view. Since the position of the focal lens FL is adjusted in a state where the position of the zoom lens ZL is set, the angle of view is hardly affected by the position of the focal lens FL.
In the automatic focusing mode, a main controller built into the DCP 507 controls the lens driver 510 through the micro controller 512, thereby driving a focus motor MF. Specifically, the position of the focal lens FL, and in this process, the position of the focal lens FL, for example, the number of driving steps of the focus motor MF having the largest high frequency component of an image signal, is set. The operation algorithm of the DCP 507 in the automatic focusing mode will be described in detail with reference to FIGS. 8 through 16.
The compensation lens CL in the lens unit 20 of the OPS is not separately operated because the compensation lens CL compensates for the entire refractive index. A reference numeral MA indicates a motor for driving an aperture (not shown).
An optical low pass filter included in the filter unit 41 of the OPS eliminates high frequency optical noise. An infrared cut filter included in the filter unit of the OPS blocks the infrared component of incident light.
A photoelectric conversion unit (OEC) of a charge coupled device or a complementary metal oxide (CMOS) semiconductor converts light from the OPS into an analog electrical signal. Here, the DCP 507 controls a timing circuit 502 to control the operations of the OEC and a correlation-double-sampler-and-analog-to-digital converter (CDS-ADC) 501. The CDS-ADC 501 processes an analog signal from the OEC, eliminates high frequency noise, adjusts amplitude, and then converts the analog signal into a digital signal.
A real time clock (RTC) 503 provides time information to the DCP 507. The DSP 507 processes the digital signal from the CDS-ADC 501 and generates a digital image composed of luminance and chromaticity values.
A light source (LAMP) is operated by the micro-controller 512 in response to a control signal generated by the DCP 507 including the main controller. Referring to FIGS. 1 and 2, the light source (LAMP) includes the self-timer lamp 11, the automatic focusing lamp 33, the mode indicating lamp 14L, and the flash standby lamp 34. The INP includes the shutter release button 13, the mode dial 14, the function buttons 15, the monitor button 32, the manual focus/delete button 36, the manual adjust/reproduce/terminate button 37, the wide-angle zoom button 39W, and the telephoto zoom button 39T.
A dynamic random access memory (DRAM) 504 temporarily stores a digital image signal from the DCP 507. An electrically erasable and programmable read only memory (EEPROM) 507 stores algorithms needed for the operation of the DCP 507. A user's memory card is inserted into or removed from a memory card interface 506.
The digital image signal from the DCP 507 is inputted to an LCD driver 514 so that the image is displayed at a color LCD panel 35.
The digital image signal from the DCP 507 can be transmitted via a universal serial bus (USB) connector 21a or an RS232C interface 508 and an RS232C connector 21b for serial communications. The digital image signal from the DCP 507 can also be transmitted as a video signal via a video filter 509 and a video output unit 21c. 
An audio processor 513 can relay sound from the microphone MIC to the DCP 507 or the speaker SP. In addition, the audio processor 513 can output an audio signal from the DCP 507 to the speaker SP. The micro-controller 512 controls the operation of a flash controller 511 in response to a signal from the flash light intensity sensor 19, thereby driving the flash 12.
FIG. 5 is a flowchart illustrating a main algorithm of the DCP 507 illustrated in FIG. 3. The main algorithm of the DCP 507 will be described below with reference to FIGS. 1 through 5.
When power is applied to the digital photographing apparatus 1, the DCP 507 is initialized (S1). After the initialization (S1), the DCP 507 performs a preview mode (S2). In the preview mode, an image input is displayed on the display panel 35. An operation related to the preview mode will be described in detail later with reference to FIG. 6.
When the shutter release button 13 is lightly depressed, a first level signal S1 is sent to the DCP 507. Accordingly, the DCP 507 set the operating mode to the photographing mode (S3). Then, the DCP 507 performs the photographing mode (S4). The photographing mode algorithm will be described in detail with reference to FIG. 7.
When INP generated signals corresponding to a setting mode are input (S5), the setting mode, for setting an operating condition in response to the input signals from the INP, is performed (S6). When a termination signal is not generated, the DCP 507 continues to perform the following step (S7).
When a signal is generated by the reproduction mode button 42 in the INP (S8), a reproduction mode is performed (S9). In the reproduction mode, operating conditions are set in response to the input signals from the INP, and reproduction is performed. When a signal is generated by the reproduction mode button 42 again (S10), the above steps are repeated.
FIG. 6 illustrates the preview mode (S2) algorithm illustrated in FIG. 5. The preview mode (S2) algorithm will be described with reference to FIGS. 1 through 3 and FIG. 6.
The DCP 507 performs automatic white balancing (AWB) and sets parameters related to the white balance (S201). The DCP 507 performs automatic exposure and automatic focusing (S202 and S203).
The DCP 507 performs gamma correction on input image data (S204) and scales the gamma corrected image data to meet display standards (S205). The DCP 507 converts the scaled input image data from an RGB (red, green, and blue) format into a luminance-chromaticity format (S206). The DCP 507 processes the input image data depending on resolution and where the input image data is displayed, and filters the input image data (S207).
The DCP 507 temporarily stores the input image data in the DRAM 504 of FIG. 3 (S208). The DCP 507 synthesizes the data temporarily stored in the DRAM 504 of FIG. 3 and on-screen display (OSD) data (S209). The DCP 507 converts the synthesized image data from the RGB format into the luminance-chromaticity format (S210) and outputs the image data in the converted format via the LCD driver 514 of FIG. 3 (S211).
FIG. 7 illustrates the photographing mode (S4) algorithm illustrated in FIG. 5. The photographing mode (S4) algorithm will be described with reference to FIGS. 1 through 3 and FIG. 7. The photographing mode algorithm starts when the first level signal S1 from the shutter release button 13 is turned on. Here, the present position of the zoom lens ZL of FIG. 4 is already set.
The DCP 507 inspects the remaining recording space in the memory card (S4101) and determines whether the memory card has enough space for recording a digital image signal (S4102). When the memory card does not have enough recording space, the DCP 507 indicates the lack of space of the memory card and stops performing the photographing mode (S4103). When the memory card has enough recording space, the following steps are performed.
The DCP 507 sets white balance, according to a present photographing condition, and sets parameters related to the white balance (S4104). In the automatic exposure mode (S4105), the DCP 507 calculates a degree of opening of the aperture (not shown) by measuring incident luminance, outputs the calculated degree of opening to the micro-controller 512, thereby driving the aperture motor MA, and sets the exposure time (S4106). In the automatic focusing mode (S4107), the DCP 507 performs automatic focusing and drives the focal lens FL (S4108). The automatic focusing algorithm will be described in detail with reference to FIGS. 8 through 16.
When the first level signal S1 from the shutter release button 13 is on (S4109), the DCP 507 performs the following step.
The DSP 507 identifies whether the second level signal S2 is on (S4110). When the second level signal S2 is not on, it means that the user did not press the shutter release button 13 to the second level to take a photograph. Then, the DCP 507 re-performs S4105 through S4110.
When the second level signal S2 is on, it means that the user fully depressed the shutter release button 13 to the second level. Then, the DCP 507 creates an image file in the memory card (S4111). Next, the DCP 507 captures an image (S4112). In other words, the DCP 507 receives image data from the CDS-ADC 501. Then, the DCP 507 compresses the received image data (S4113). The DCP 507 stores the compressed image data in the image file (S4114).
FIG. 8 illustrates a photograph region FR of the OEC of FIG. 3 divided into sub-regions A1 through A25 during the automatic focusing (S4108 of FIG. 7) by the DCP 507. FIG. 9 is a graph illustrating the automatic focusing algorithm (S4108) illustrated in FIG. 7. FIG. 10 is a flowchart illustrating a detailed automatic focusing algorithm (S4108) illustrated in FIG. 7.
Referring to FIG. 9, DS is the number of driving steps of the focal lens FL illustrated in FIG. 4, and FV is a focus value that is proportional to the amount of high frequency contained in an image signal. DSI is the number of driving steps of the focal lens FL corresponding to a set maximum distance. DSFOC is the number of driving steps of the focal lens FL corresponding to a maximum focus value FVMAX. DSS is the number of driving steps corresponding to a set minimum distance.
The automatic focusing algorithm (S4108 of FIG. 7) will now be described with reference to FIGS. 8 through 10. The DCP 507 scans sub-regions A1 through A25 within a scanning region between DSI and DSS and obtains the maximum focus values FVMAX and focal position values DSFOC for A1 through A25 (i.e., the number of driving steps of the focal lens FL) corresponding to the maximum focus values FVMAX (S101). S101 will be described in detail later with reference to FIGS. 11 through 16.
The DCP 507 obtains an average focal position value ADSFOC as a result of averaging the focal position values DSFOC of sub-regions A1 through A25 (S102). The DSP 507 obtains an average maximum focus value AFVMAX as a result of averaging the maximum focus values FVMAX of sub-regions A1 through A25 (S103).
The DCP 507 selects sub-regions having focal position values DSFOC whose differences from the average focal position value ADSFOC are equal to or less than a threshold value DTH (S 104). From the selected sub-regions, the DCP 507 selects sub-regions having maximum focus values FVMAX, which correspond to focal position values DSFOC, that are equal to or greater than the average maximum focus value AFVMAX (S 105).
The DCP 507 selects a particular sub-region having the shortest focal distance from the selected sub-regions and outputs the focal position value DSFOC of the particular sub-region to the micro-controller 512 (S106). Accordingly, the micro-controller 512 controls the lens driver 510 to drive the focus motor MF. Therefore, the focal lens FL is moved to a final focal position.
According to the automatic focusing method, the final focal position value DSFOC selected from the focal position values DSFOC of sub-regions A1 through A25 is close to the average focal position value ADSFOC, and the focal distance is short. Therefore, there is a high probability that the position of a subject desired by a user will be brought into focus. In addition, balanced focusing can be maintained in the entire photograph region FR.
FIG. 11 is a flowchart illustrating an algorithm for obtaining the maximum focus value FVMAX and the focal position value DSFOC of one of sub-regions A1 through A25 into which the photograph region FR is divided (S101 of FIG. 10). FIG. 12 illustrates first and second reference characteristic curves used in S93 and S95 of FIG. 11.
Referring to FIG. 12, DS is the number of driving steps of the focal lens FL of FIG. 4, and FV is a focus value. C1 is the first reference characteristic curve, and C2 is the second reference characteristic curve. BDS is a region around the maximum focus value FVMAX to be finally set where the second reference characteristic curve C2 is used. ADS and CDS are regions where the first reference characteristic curve C1 is used. The algorithm for obtaining the maximum focus value FVMAX and the focal position value DSFOC of a region will now be described with reference to FIGS. 11 and 12.
The DCP 507 is initialized for automatic focusing (S91). The DCP 507 performs scanning without using a supporting light source, regardless of the intensity of illumination around a subject (S92).
If a macro-mode for a case where the subject is within a first distance, for example, 30 through 80 centimeters, from the focal lens FL is set by a user, the DCP 507 scans a region of the focal lens FL corresponding to the first distance. Conversely, if a normal mode for a case where the subject is beyond the first distance, for example, 80 centimeters to infinity, from the focal lens FL is set by the user, the DCP 507 scans a region of the focal lens FL corresponding to the distance beyond the first distance.
During macro-mode scanning or normal-mode scanning of the scanning step (S92), the DCP 507 outputs a focus value FV proportional to the amount of high frequency contained in an image signal for every first number of driving steps of the focus motor MF (FIG. 3), for example, 8 driving steps. Whenever a focus value FV is output, the maximum focus value FVMAX is updated.
Whenever a focus value FV is acquired (S92), it is determined, from a first reference characteristic curve C1 of FIG. 12, whether the output focus value FV has increased or decreased (S93). More specifically, if a percentage of decrease ratio of the current focus value FV to the maximum focus value FVMAX is greater than a first reference percentage based on the first reference characteristic curve C1, the DCP 507 determines that the current focus value FV has decreased. If not, the DCP 507 determines that the output focus value FV has increased.
The first reference percentage of the first reference characteristic curve C1 is low as 10 to 20 percent because there is a high probability that the position of a current focus value FV will not be around the position of the maximum focus value FVMAX to be set finally, and that if not be around the position of the maximum focus value FVMAX to be set, the difference between focus values of adjacent positions is low.
If the output focus value FV is determined to have decreased (S94), the position of a current maximum focus value FVMAX is regarded as the position of the maximum focus value FVMAX in an entire range of motion of the focal lens FL. Accordingly, the DCP 507 checks the position of the maximum focus value FVMAX with reference to the second reference characteristic curve C2 of FIG. 12 (S95). In this case, the macro-mode scanning or the normal-mode scanning of the scanning process (520) are stopped, and a region around the position of the maximum focus value FVMAX is scanned for every second number of driving steps, for example 1 driving step, which is smaller than the first number of driving steps. Thus, the final position of the focal lens FL is set.
Specifically, the DCP 507 outputs a focus value FV, which is proportional to the amount of high frequency contained in an image signal, for every driving step of the focus motor MF. Whenever a focus value FV is output, the maximum focus value FVMAx is updated.
Whenever a focus value FV is acquired (S92), it is determined, from a second reference characteristic curve C2 of FIG. 12, whether the output focus value FV has increased or decreased (S93). More specifically, if a percentage of decrease ratio of the current focus value FV to the maximum focus value FVMAX is greater than a second reference percentage from the second reference characteristic curve C2, the DCP 507 determines that the current focus value FV has decreased. If not, the DCP 507 determines that the current focus value FV has increased (See FIG. 16 or 17).
The second reference percentage of the second characteristic curve C2 is larger than the first reference percentage because the difference between focus values of adjacent positions around the position of the final maximum focus value FVMAX is large. If the current focus value FV is determined to have decreased (S94), the position of a current maximum focus value FVMAX is regarded as the position of the maximum focus value FVMAX in the entire range of motion of the focal lens FL.
If the current focus value FV is not determined to have decreased (S94), the position of a current maximum focus value FVMAX is not regarded as the position of the maximum focus value FVMAX in the entire range of motion of the focal lens FL. Accordingly, the scanning step (S92) and its following steps are performed.
FIG. 13 is a flowchart illustrating S91 of FIG. 11. Referring to FIG. 13, if the macro-mode is set by a user (S911), the number of position steps of the focus motor MF corresponding to a start position at which the focal lens FL starts to move is set to the number of position steps corresponding to 30 centimeters from a subject. The number of position steps of the focus motor MF corresponding to an end position at which the focal lens FL stops moving is set to the number of position steps corresponding to 80 centimeters from the subject. The number of driving steps of the focus motor MF is set to 8, and the number of position steps of the focus motor MF corresponding to the boundary position of the focal lens FL is set to the number of position steps of the focus motor MF corresponding to the end position of the focal lens FL plus the number of driving steps (8) times 2 (S912). Alternatively, the boundary position may not be used.
If the normal mode is set by the user (S911), the number of position steps of the focus motor MF corresponding to the start position of the focal lens FL is set to the number of position steps corresponding to infinite distance from the subject. The number of position steps of the focus motor MF corresponding to the end position of the focal lens FL is set to the number of position steps corresponding to 80 centimeters from the subject. The number of driving steps of the focus motor MF is set to 8, and the number of position steps of the focus motor MF corresponding to the boundary position of the focal lens FL is set to the number of position steps of the focus motor MF corresponding to the end position of the focal lens FL minus the number of driving steps (8) times 2 (S913). The boundary position may not be used.
The DCP 507 drives the focus motor MF using the micro-controller 512 and moves the focal lens FL to the start position (S914).
FIG. 14 is a flowchart illustrating S92 of FIG. 11. Referring to FIG. 14, the DCP 507 drives the focus motor MF to a predetermined number of driving steps using the micro-controller 512, thereby moving the focal lens FL (S921).
The DCP 507 drives the aperture motor MA using the micro-controller 512 and exposes the OEC of FIG. 4 (S922). The DCP 507 processes frame data received from the CDS-ADC 501 and outputs a focus value FV that is proportional to the amount of high frequency contained in the frame data (S923).
The DCP 507 updates a current focus value FV to the output focus value FV (S924). If the updated current focus value FV is greater than the maximum focus value FVMAX (S925), the DCP 507 updates the maximum focus value FVMAX to the updated current focus value FV and, accordingly, the maximum focus-value position is updated to the current focus-value position (S926).
FIG. 15 is a flowchart illustrating S93 of FIG. 11 according to an embodiment of the present invention. Referring to FIG. 15, the DCP 507 calculates a decrease ratio between a maximum focus value FVMAX and a current focus value FV (S931) using




  
    
      
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If the percentage difference, obtained by multiplying the difference by 100, is greater than the first reference percentage RTH of the first reference characteristic curve C1, the DCP 507 determines that the current focus value FV has decreased (S932 and S934). If not, the DCP 507 determines otherwise (S932 and S933).

FIG. 16 is a flowchart illustrating S93 of FIG. 11 according to another embodiment of the present invention. S93 will now be described with reference to FIG. 16. The embodiment of S93 illustrated in FIG. 16 enables more accurate determination about whether the current focus value FV has increased or decreased than the embodiment of S93 illustrated in FIG. 15.
If the current focus value FV is greater than a previous focus value, the DCP 507 determines that the current focus value FV has increased and ends S93 (S121 and S124). If the current focus value FV is less than the previous value, the DCP 507 performs the following step.
The DCP 507 calculates a decrease ratio using Equation 1 (S122). If the decrease ratio expressed as a percentage is greater than the first reference percentage RTH from the first reference characteristic curve C1, the DCP 507 determines that the current focus value FV has decreased (S123 and S125). If not, the DCP 507 determines otherwise (S123 and S124).
As described above, according to an automatic focusing method and a digital photographing apparatus using the same, a final focal position value DSFOC selected from focal position values DSFOC of sub-regions A1 through A25 is close to an average focal position value ADSFOC and a focal distance is short. Therefore, there is a high probability that the position of a subject desired by a user will be brought into focus. In addition, balanced focusing can be maintained throughout the entire photograph region.
While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

## Claims

1. An automatic focusing method in which a photograph region is brought into focus through automatic movement of a focal lens to a focal position in response to a photographing-command signal, the method comprising:
dividing the photograph region into a plurality of sub-regions;
obtaining focal position values DSFOC and maximum focus values FVMAX for the sub-regions;
calculating an average focal position value ADSFOC for the focal position values DSFOC of the sub-regions;
calculating an average maximum focus value AFVMAX for the maximum focus values FVMAX of the sub-regions;
selecting a first set of sub-regions having focal position values DSFOC that differ from the average focal position value ADSFOC by a value that is equal to or less than a threshold value DTH;
selecting a second set of sub-regions from the first set of sub-regions, sub-regions of the second set having maximum focus values FVMAX that are greater than or equal to an average maximum focus value AFVMAX;
selecting from the second set of sub-regions, a particular sub-region having the shortest focal distance; and
moving the focal lens to a focal position corresponding to the focal position value DSFOC of the particular sub-region.

2. The method of claim 1, wherein the photographing command signal is generated when a shutter release button is depressed.

3. The method of claim 1 wherein the focal position value DSFOC is the number of driving steps of a focus motor that moves the focal lens.

4. An automatic focusing method in which a photograph region is brought into focus through automatic movement of a focal lens to a focal position, in response to a photographing-command signal, the method comprising:
dividing the photograph region into a plurality of sub-regions;
obtaining maximum focus values FVMAX for the sub-regions;
obtaining focal position values DSFOC for the sub-regions which correspond to the maximum focus values FVMAX;
calculating an average focal position value ADSFOC for the focal position values DSFOC of the sub-regions;
calculating an average maximum focus value AFVMAX for the maximum focus values FVMAX of the sub-regions;
selecting a first set of sub-regions having focal position values DSFOC that differ from the average focal position value ADSFOC by a value that is equal to or less than a threshold value DTH;
selecting from the first set of selected sub-regions, a second set of sub-regions having maximum focus values FVMAX that are equal to or greater than the average maximum focus value AFVMAX;
selecting from the second set of sub-regions, a particular sub-region having the shortest focal distance; and
moving the focal lens to a focal position corresponding to the focal position value DSFOC of the particular sub-region.

5. A digital photographing apparatus using an automatic focusing method in which a photograph region is brought into focus through automatic movement of a focal lens to a focal position, in response to a photographing-command signal, the method comprising:
dividing the photograph region into a plurality of sub-regions;
obtaining focal position values DSFOC and maximum focus values FVMAX for the sub-regions;
calculating an average focal position value ADSFOC for the focal position values DSFOC of the sub-regions;
calculating an average maximum focus value AFVMAX for the maximum focus values FVMAX of the sub-regions;
selecting a first set of sub-regions having focal position values DSFOC that differ from the average focal position value ADSFOC by a value that is equal to or less than a threshold value DTH;
selecting a second set of sub-regions from the first set of sub-regions, sub-regions of the second set having maximum focus values FVMAx that are greater than or equal to an average maximum focus value AFVMAX;
selecting from the second set of sub-regions, a particular sub-region having the shortest focal distance; and
moving the focal lens to a focal position corresponding to the focal position value DSFOC of the particular sub-region.

6. A digital photographing apparatus using an automatic focusing method in which a photograph region is brought into focus through automatic movement of a focal lens to a focal position, in response to a photographing-command signal, the method comprising:
dividing the photograph region into a plurality of sub-regions;
obtaining maximum focus values FVMAX for the sub-regions;
obtaining focal position values DSFOC for the sub-regions which correspond to the maximum focus values FVMAX;
calculating an average focal position value ADSFOC for the focal position values DSFOC of the sub-regions;
calculating an average maximum focus value AFVMAX for the maximum focus values FVMAX of the sub-regions;
selecting a first set of sub-regions having focal position values DSFOC that differ from the average focal position value ADSFOC by a value that is equal to or less than a threshold value DTH;
selecting from the first set of selected sub-regions, a second set of sub-regions having maximum focus values FVMAX that are equal to or greater than the average maximum focus value AFVMAX;
selecting from the second set of sub-regions, a particular sub-region having the shortest focal distance; and
moving the focal lens to a focal position corresponding to the focal position value DSFOC of the particular sub-region.