Patent Publication Number: US-9848117-B2

Title: Focus control apparatus, method therefor, and storage medium

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an autofocus control technique in an image capturing apparatus. 
     Description of the Related Art 
     Conventionally known focus control methods used in digital cameras and other image capturing apparatuses include a phase-difference detection method and a contrast detection method (see Japanese Patent Laid-Open No. 09-054242 and Japanese Patent Laid-Open No. 2001-004914). Another known method is an imaging surface phase-difference detection method that takes into consideration a live-view (LV) mode in which image capture is performed while displaying captured images on, for example, a rear monitor (see Japanese Patent Laid-Open No. 2001-083407). 
     However, even with the imaging surface phase-difference detection method that takes the live-view mode into consideration, it is necessary to perform more stable focus control in conformity with the live-view mode and moving image capture. Especially with the recent trend toward higher resolution, a user easily feels a sense of discomfort if a focus state changes unexpectedly while moving images are displayed on a display unit. 
     In the imaging surface phase-difference detection method, the stability of focus control during the live-view mode and moving image capture may be improved by increasing a focus detection range. An increase in the focus detection range leads to an increase in the number of subjects that can be captured, and can alleviate an unexpected movement of a focus lens caused by movement of a temporarily-captured subject to the outside of the focus detection range. 
     However, when there is a conflict between a far point and a near point of a subject, an increase in the focus detection range could possibly make it difficult to bring the intended subject into focus. In view of this, it is considered that dividing the focus detection range into a plurality of focus detection areas is effective. However, in the case of the imaging surface phase-difference detection method, a subject image expands when a subject is significantly out of focus. This casts significant influence on the divided focus detection areas, thereby making it difficult to obtain focus detection results. Furthermore, during image capture with low light intensity, especially the signal-to-noise ratio is easily lowered in the divided focus detection areas, thereby making it difficult to obtain desired focus detection results. The same goes for the case of a low-contrast subject. 
     To address these problems, Japanese Patent Laid-Open No. 2014-32214 suggests the following technique: a phase-difference detection area is divided into a plurality of areas in a direction perpendicular to a direction of phase-difference detection, and in each of the divided areas, signals detected by pixels arranged along the direction of phase-difference detection are merged in the direction perpendicular to the direction of phase-difference detection. By thus merging the detected signals for reduction of the influence of noise, and by thus limiting a range of merger, an average pattern is yielded after merger of pixel signals, and a decrease in the precision of focus detection can be prevented. However, in Japanese Patent Laid-Open No. 2014-32214, the detected signals are merged only in the direction perpendicular to the direction of phase-difference detection, and there is a possibility that appropriate detected signals are not always obtained through merger depending on the image capturing condition. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the foregoing problems, and realizes a stable focusing operation during moving image capture while enabling focus detection in a wide area on an imaging surface in focus control based on an imaging surface phase-difference detection method. 
     According to a first aspect of the present invention, there is provided a focus control apparatus, comprising: an area setting unit that sets a plurality of divided areas by dividing an area of an imaging surface in a first direction and in a second direction that is different from the first direction, the first direction corresponding to a direction in which a focus state is detected; a focus detection unit that detects first information related to the focus state on the basis of a pair of image signals output from each of the plurality of divided areas; a calculation unit that calculates defocus information on the basis of the first information; and a control unit that performs focus control on the basis of the calculated defocus information, wherein in a first mode in which the defocus information for the focus control is calculated by combining pieces of the first information detected in at least a part of the plurality of divided areas, the calculation unit causes the part of the plurality of divided areas, in which the pieces of the first information for calculating the defocus information are combined, to vary in accordance with pieces of the first information detected in the plurality of divided areas and with an image capturing state. 
     According to a second aspect of the present invention, there is provided a focus control method, comprising: setting a plurality of divided areas by dividing an area of an imaging surface in a first direction and in a second direction that is different from the first direction, the first direction corresponding to a direction in which a focus state is detected; detecting first information related to the focus state on the basis of a pair of image signals output from each of the plurality of divided areas; calculating defocus information on the basis of the first information; and performing focus control on the basis of the calculated defocus information, wherein in a first mode in which the defocus information for the focus control is calculated by combining pieces of the first information detected in at least a part of the plurality of divided areas, the calculating causes the part of the plurality of divided areas, in which the pieces of the first information for calculating the defocus information are combined, to vary in accordance with pieces of the first information detected in the plurality of divided areas and with an image capturing state. 
     According to a third aspect of the present invention, there is provided a computer-readable storage medium having stored therein a program for causing a computer to execute a focus control method, the method comprising: setting a plurality of divided areas by dividing an area of an imaging surface in a first direction and in a second direction that is different from the first direction, the first direction corresponding to a direction in which a focus state is detected; detecting first information related to the focus state on the basis of a pair of image signals output from each of the plurality of divided areas; calculating defocus information on the basis of the first information; and performing focus control on the basis of the calculated defocus information, wherein in a first mode in which the defocus information for the focus control is calculated by combining pieces of the first information detected in at least a part of the plurality of divided areas, the calculating causes the part of the plurality of divided areas, in which the pieces of the first information for calculating the defocus information are combined, to vary in accordance with pieces of the first information detected in the plurality of divided areas and with an image capturing state. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing configurations of main components of a lens apparatus and an image capturing apparatus according to an embodiment of the present invention. 
         FIGS. 2A and 2B  show a partial area of an image sensor. 
         FIG. 3  is a flowchart showing AF control processing. 
         FIG. 4  is a flowchart showing lens driving processing. 
         FIGS. 5A to 5E  show patterns of division of a focus detection range. 
         FIGS. 6A to 6E  are conceptual diagrams showing correlation amounts in a state where a subject is out of focus. 
         FIGS. 7A and 7B  are conceptual diagrams showing correlation amounts during image capture with low light intensity. 
         FIG. 8  is a flowchart showing focus detection area selection processing. 
         FIGS. 9A to 9D  show image signals obtained from a detection area targeted for detection of a defocus amount. 
         FIGS. 10A and 10B  show a correlation amount waveform, a correlation change amount waveform, and an out-of-focus amount. 
         FIGS. 11A and 11B  show a method of calculating a degree of match between two images. 
         FIG. 12  is a flowchart of phase-difference AF processing. 
         FIG. 13  is a flowchart of calculation of an effective defocus amount. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following describes an embodiment of the present invention in detail with reference to the attached drawings. Note that the following embodiment is an example of means for embodying the present invention, should be modified or changed as appropriate depending on the configuration of an apparatus to which the present invention is applied and on various conditions, and does not limit the present invention. 
     &lt;Configuration of Image Capturing Apparatus&gt; 
     A description is now given of an image capturing apparatus provided with a focus control apparatus according to an embodiment of the present invention. Although the image capturing apparatus described in the present embodiment is configured in such a manner that a lens apparatus is attachable to and detachable from the image capturing apparatus, other image capturing apparatuses, such as a digital camera with a built-in lens, may be used. 
       FIG. 1  is a block diagram showing configurations of main components of a lens apparatus and an image capturing apparatus according to the present embodiment. As shown in  FIG. 1 , in the present embodiment, a lens apparatus  10  and an image capturing apparatus  20  are provided, and information communication is performed by a lens control unit  106  that achieves coordinated control over operations of the entire lens apparatus, and by a camera control unit  207  that coordinates operations of the entire image capturing apparatus. 
     First, a configuration of the lens apparatus  10  will be described. The lens apparatus  10  includes a fixed lens  101 , a diaphragm  102 , a focus lens  103 , a diaphragm driving unit  104 , a focus lens driving unit  105 , the lens control unit  106 , and a lens operation unit  107 . An image capturing optical system is composed of the fixed first lens assembly  101 , the diaphragm  102 , and the focus lens  103 . 
     The diaphragm  102  is driven by the diaphragm driving unit  104 , and controls an amount of light incident on a later-described image sensor  201 . The focus lens  103  is driven by the focus lens driving unit  105 , and adjusts a focal point formed on the later-described image sensor  201 . The diaphragm driving unit  104  and the focus lens driving unit  105  are controlled by the lens control unit  106 , and determine the aperture of the diaphragm  102  and the position of the focus lens  103 . When a user operation has been performed via the lens operation unit  107 , the lens control unit  106  performs control corresponding to the user operation. The lens control unit  106  controls the diaphragm driving unit  104  and the focus lens driving unit  105  in accordance with a control instruction and control information received from the later-described camera control unit  207 , and transmits lens control information to the camera control unit  207 . 
     Next, a configuration of the image capturing apparatus  20  will be described. The image capturing apparatus  20  is capable of obtaining an image capturing signal from a beam of light that has passed through the image capturing optical system of the lens apparatus  10 . It includes the image sensor  201 , a CDS/AGC circuit  202 , a camera signal processing unit  203 , an AF signal processing unit  204 , a display unit  205 , a recording unit  206 , the camera control unit  207 , a camera operation unit  208 , and a timing generator  209 . The image sensor  201  is constituted by a CCD sensor, a CMOS sensor, or the like. A beam of light that has passed through the image capturing optical system of the lens apparatus forms an image on a light receiving surface of the image sensor  201 , and then is converted by photodiodes into signal charges corresponding to an amount of incident light. On the basis of driving pulses that are fed from the timing generator  209  in accordance with a command from the camera control unit  207 , the signal charges accumulated in the photodiodes are sequentially read out from the image sensor  201  as voltage signals corresponding to the signal charges. 
     A video signal and signals for AF that have been read out from the image sensor  201  are input to the CDS/AGC circuit  202  that performs sampling and gain adjustment; the video signal is output to the camera signal processing unit  203 , and signals for imaging surface phase-difference AF are output to the AF signal processing unit  204 . The camera signal processing unit  203  generates a video signal by applying various types of image processing to the signal output from the CDS/AGC circuit  202 . The display unit  205  is composed of, for example, an LCD, and displays the video signal output from the camera signal processing unit  203  as a captured image. The recording unit  206  records the video signal from the camera signal processing unit  203  to a recording medium, such as a magnetic tape, an optical disc, and a semiconductor memory. 
     The AF signal processing unit  204  carries out correlation computation on the basis of two image signals for AF output from the CDS/AGC circuit  202 . It also calculates correlation amounts (equivalent to later-described focus information), defocus amounts (equivalent to later-described second focus information), and reliability information (a degree of match between two images, a degree of steepness exhibited by two images, contrast information, saturation information, scratch information, and the like). Then, it outputs the calculated defocus amounts and reliability information to the camera control unit  207 . On the basis of the obtained defocus amounts and reliability information, the camera control unit  207  notifies the AF signal processing unit  204  of a change in settings related to calculation of the same. The details of correlation computation will be described later with reference to  FIGS. 9A to 11B . 
     The camera control unit  207  performs control through exchange of information within the entire camera  20 . It executes not only processing within the camera  20 , but also a wide variety of camera functions corresponding to user operations in response to input via the camera operation unit  208 ; examples of such camera functions include turning ON/OFF a power supply, changing settings, starting recording, starting AF control, and checking a recorded video. As stated earlier, it also exchanges information with the lens control unit  106  within the lens apparatus  10 , transmits the control instruction and control information for the lens apparatus, and obtains information within the lens apparatus. 
     &lt;Image Sensor&gt; 
       FIGS. 2A and 2B  show a part of the light receiving surface of the image sensor  201 . In order to enable imaging surface phase-difference AF, the image sensor  201  includes an array of pixel units that are each provided with two photodiodes, that is to say, light receiving units, corresponding to one microlens. In this way, each pixel unit can receive a beam of light that has passed through a different one of divided areas of an exit pupil of the lens apparatus  10 . 
     As reference,  FIG. 2A  is a schematic diagram showing a part of a surface of the image sensor with a Bayer array including red (R), blue (B), and green (Gb, Gr).  FIG. 2B  shows pixel units that are each provided with two photodiodes, that is to say, photoelectric conversion means, corresponding to one microlens; these pixel units correspond to the color filter array shown in  FIG. 2A . 
     In the image sensor configured in the foregoing manner, each pixel unit can output two signals for phase-difference AF (hereinafter also referred to as A image signal and B image signal). Each pixel unit can also output an image capturing signal for recording, which is obtained by merging signals of the two photodiodes (A image signal+B image signal). Output of a signal obtained through such merger is equivalent to output from the image sensor with the Bayer array that is schematically shown in  FIG. 2A . With the use of signals output from such an image sensor  201 , the later-described AF signal processing unit  204  carries out correlation computation for two image signals, and calculates defocus amounts and various types of information, such as reliability. 
     Note that in the present embodiment, the image sensor  201  outputs a total of three signals: a signal for image capture, and two signals for phase-difference AF. However, the present invention is not limited to this method. For example, it may output a total of two signals: a signal for image capture, and one of two image signals for phase-difference AF. In this case, after the output, the other of the two image signals for phase-difference AF is calculated using two signals output from the image sensor  201 . 
       FIGS. 2A and 2B  show an example of an array of pixel units that are each provided with two photodiodes, that is to say, photoelectric conversion means, corresponding to one microlens. Alternatively, each pixel unit in the array may be provided with three or more photodiodes, that is to say, photoelectric conversion means, corresponding to one microlens. A plurality of pixel units may be provided in such a manner that the position of an aperture for a light receiving unit relative to a microlens varies among the pixel units. That is to say, it is sufficient to ultimately obtain two signals for phase-difference AF that enable phase-difference detection, such as an A image signal and a B image signal. 
     &lt;AF Control Processing&gt; 
     A description is now given of AF control processing executed by the camera control unit  207 .  FIG. 3  is a flowchart showing AF control processing executed by the camera control unit  207  shown in  FIG. 1 . The present processing is executed in accordance with a computer program stored in the camera control unit  207 . For example, the present processing is executed at a readout cycle for an image capturing signal from the image sensor  201  for generating an image corresponding to one field (hereinafter also referred to as one frame or one screen) (in every vertical synchronization period). The present processing may be repeated multiple times within the vertical synchronization period (V rate). 
     Referring to  FIG. 3 , first, focus detection areas are set in step S 301 . As will be described later in detail, in the present embodiment, a plurality of focus detection areas are arranged at desired positions within an image capturing screen along a direction of phase-difference detection and along a direction perpendicular to the direction of phase-difference detection (area setting). For example, in a case where a focus detection range is set at the center of the image capturing screen as shown in  FIG. 5A , nine focus detection areas  501  to  509  are arranged within the focus detection range as shown in  FIG. 5B . They are referred to as first divided areas. The AF signal processing unit  204  calculates the results in the focus detection areas  501  to  509 . The focus detection range is not limited to being arranged at the center of the image capturing screen, and may be arranged at any position on the image capturing screen. 
     Next, whether AF signals have been updated in the AF signal processing unit  204  is checked (step S 302 ), and if the AF signals have been updated, the result of the update is obtained from the AF signal processing unit  204  (step S 303 ). In step S 304 , focus detection area selection processing, which will be described later, is executed. Here, a later-described effective defocus amount is calculated from defocus amounts that are obtained from the focus detection areas and indicate out-of-focus amounts, and from reliability levels of the defocus amounts. 
     Next, it is determined whether the defocus amount calculated in step S 304  is within a predetermined depth and has a reliability level higher than a predetermined level (a high reliability level), hence reliable (step S 305 ). If the defocus amount is within the predetermined depth and its reliability is higher than the predetermined level, a focus stop flag is set to ON (step S 306 ); if not, the focus stop flag is set to OFF (step S 308 ). The focus stop flag in the ON state means that a point of focus has been reached through focus control, and thus focus control should be stopped. 
     Here, the reliability level of the defocus amount is high when the precision of the calculated defocus amount can be determined to be credible. When a defocus direction indicating a direction in which a point of focus is assumed to exist is credible, the reliability level is determined to be medium. For example, when the reliability level of the defocus amount is high, an A image signal and a B image signal have high contrast and have similar shapes (a level of match between two images is high), or a main subject image is already in focus. In this case, the defocus amount is reliable, and the focus lens is driven in accordance with the defocus amount. 
     When the reliability level of the defocus amount is medium, the level of match between two images calculated by the AF signal processing unit  204  is lower than a predetermined value, but the correlation obtained by relatively shifting the A image signal and the B image signal shows a certain tendency, and the defocus direction is reliable. This determination is often made when, for example, the main subject is slightly out of focus. The reliability level is determined to be low when neither the defocus amount nor the defocus direction is reliable. This determination is made when, for example, the A image signal and the B image signal have low contrast and the level of match between images is low. This determination is often made when the subject is significantly out of focus; when this determination is made, calculation of the defocus amount is difficult. 
     If the defocus amount is within the predetermined depth and its reliability is high, lens driving for focus control is stopped (step S 307 ), and the processing proceeds to step S 308 . On the other hand, if the focus stop flag is set to OFF (step S 308 ), later-described lens driving processing is executed (step S 309 ), and the processing is ended. 
     &lt;Lens Driving Processing&gt; 
       FIG. 4  is a flowchart showing the details of the lens driving processing (step S 309 ) shown in  FIG. 3 . First, in step S 401 , the camera control unit  207  determines whether the defocus amount has been obtained and its reliability level is high. If the defocus amount has been obtained and its reliability level is high (YES of step S 401 ), a driving amount and a driving direction of the focus lens are determined on the basis of the defocus amount (step S 402 ). Then, an error count and an end count are cleared (step S 403 ), and the processing is ended. 
     If the defocus amount has not been obtained or its reliability level is not high in step S 401  (NO of step S 401 ), the camera control unit  207  determines whether the error count is larger than a first count (step S 404 ). Although the first count is not shown, it is sufficient for the first count to be a value that has been preset and prestored to a nonvolatile memory. For example, it is sufficient for the first count to be a value that is equal to or larger than the double of a later-described second count. 
     If the error count is smaller than the first count in step S 404  (NO of step S 404 ), the error count is incremented (step S 405 ), and the processing is ended. If the error count is larger than the first count (YES of step S 404 ), the camera control unit  207  determines whether a search driving flag is ON (step S 406 ). 
     If the search driving flag is OFF in step S 406  (NO of step S 406 ), a search operation has not been started yet, or the search is not currently conducted. Accordingly, the camera control unit  207  sets the search driving flag to ON (step S 407 ), and determines whether the reliability level of the defocus amount is medium (step S 408 ). 
     If the reliability is medium in step S 408 , the camera control unit  207  sets the driving direction in accordance with the defocus direction (step S 409 ), and sets a predetermined driving amount (step S 411 ). At this time, search driving is performed by driving the focus lens by the predetermined amount in the obtained defocus direction, rather than driving the defocus lens on the basis of an absolute value of the defocus amount. 
     If the reliability is not medium in step S 408  (NO of step S 408 ), the camera control unit  207  sets the driving direction of the focus lens away from a lens end (step S 410 ), and sets the predetermined driving amount (step S 411 ). It is sufficient to use a preset value in the nonvolatile memory as the predetermined driving amount in step S 411 . For example, a distance that is several times larger than the depth of focus is used as the driving amount. Alternatively, the driving amount may vary in accordance with a focal length. For example, the larger the focal length, the larger the driving amount. Note that in this case, for example, any point on a search driving direction is farther from the lens end than from the current position of the focus lens. 
     If the search driving flag is ON (YES of step S 406 ), search driving is already in execution. Therefore, the camera control unit  207  continuously performs previous focus control. Thereafter, the camera control unit  207  determines whether the lens end, which is a limit of lens driving for focus control, has been reached (step S 412 ), and if the lens end has been reached (YES of step S 412 ), increments the end count (step S 413 ). 
     If the end count is larger than one (YES of step S 414 ), it means that a credible defocus amount has not been obtained even by moving the focus lens from a near end to an infinity end. Thus, the camera control unit  207  determines that there is no subject that can be brought into focus, sets the search driving flag to OFF (step S 415 ), and stops lens driving (step S 416 ). Then, the error count and the end count are cleared (step S 417 ), and the processing is ended. If the end count is not larger than one in step S 414  (NO of step S 414 ), the camera control unit  207  sets the lens driving direction for focus control to a direction opposite to the current driving direction (step S 418 ), and sets the predetermined driving amount (step S 411 ). 
     &lt;Focus Detection Area Selection Processing&gt; 
       FIGS. 5A to 5E  show the details of focus detection areas within the image capturing screen, which are selected in the focus detection area selection processing (step S 304 ) shown in  FIG. 3 .  FIG. 5B  shows the first divided areas. The focus detection range shown in  FIG. 5A  is divided into three areas in the direction of phase-difference detection, and further divided into three areas in the direction perpendicular to the direction of phase-difference detection. A later-described correlation amount (correlation degree) indicating a degree of correlation between two images is calculated from each area, and defocus amounts are calculated from the correlation amounts. Reliability levels of the defocus amounts are also calculated. 
     Normally, an in-focus state is achieved by calculating one effective defocus amount, which will be described later, from a combination of defocus amounts obtained from the first divided areas, and by performing lens driving as focus control accordingly. Hereinafter, the term “effective defocus amount” is used to indicate the concept of one defocus amount corresponding to a plurality of focus detection areas. By dividing the focus detection range into a plurality of areas in a horizontal direction and a direction at a right angle thereto, a conflict between a far point and a near point can be eliminated, and a subject image that is assumed to be targeted by a user can be brought into focus. 
       FIG. 5C  shows second divided areas. Among the plurality of areas shown in  FIG. 5B , areas along the direction of phase-difference detection are included in each of the second divided areas. That is to say, the areas  501  to  503  shown in  FIG. 5B  are included in an area  510  shown in  FIG. 5C . Defocus amounts in the areas  510  to  512  shown in  FIG. 5C  are calculated as follows: in each area, correlation amounts in the included areas shown in  FIG. 5B  are merged into one correlation amount, and a defocus amount and its reliability level are calculated from the one correlation amount. 
     In the case of an imaging surface phase-difference detection method, the shape of a subject image that is desired to be captured varies significantly depending on the focus state (whether the subject image is significantly out of focus or is close to an in-focus state (slightly out of focus)). 
       FIGS. 6A to 6E  are conceptual diagrams showing a state in which a black subject having one vertical white line is significantly out of focus during image capture. In  FIG. 6A, 1201 to 1203  represent focus detection areas that are equivalent to, for example,  501  to  503  shown in  FIG. 5B . Also,  1204  and  1205  represent an A image signal and a B image signal, that is to say, phase-difference signals, of the subject image in the out-of-focus state, whereas  1206  and  1207  represent an A image and a B image of the subject image close to the in-focus state.  FIGS. 6B, 6C, and 6D  show correlation amounts in the areas  1201  to  1203 ; in each figure, a horizontal axis represents a shift amount, and a vertical axis represents a correlation amount.  FIG. 6E  shows a correlation amount obtained by merging the correlation amounts in the areas  1201  to  1203 . 
     As shown in  FIG. 6A , when the subject is out of focus, the A image signal and the B image signal have a shape of a gently-sloping mountain with a flaring foot, and extend across a plurality of focus detection areas. In this case, as shown in  FIGS. 6B, 6C and 6D , there is little change in the correlation amount obtained by shifting the A image signal and the B image signal. Accordingly, the later-described steepness of the correlation amount decreases, thereby lowering the precision of defocus amounts and the reliability levels of the defocus amounts. 
     However, by merging the correlation amounts in areas along the direction of phase-difference detection, the steepness of the resultant correlation amount can be increased as shown in  FIG. 6E . That is to say, merging the correlation amounts in areas along the direction of phase-difference detection makes it possible to capture the subject image over a wide range, improve the precision of defocus amounts when the subject is out of focus, and increase the reliability levels of the defocus amounts. Furthermore, as division is performed in the direction perpendicular to the direction of phase-difference detection, a conflict between a far point and a near point can be eliminated as well. 
       FIG. 5D  shows third divided areas. Among the plurality of areas shown in  FIG. 5B , areas along the direction perpendicular to the direction of phase-difference detection are included in each of the third divided areas. That is to say, the areas  501 ,  504 , and  507  shown in  FIG. 5B  are included in an area  513  shown in  FIG. 5D . Defocus amounts in the areas  513  to  515  shown in  FIG. 5D  are calculated as follows: in each area, correlation amounts in the included areas shown in  FIG. 5B  are merged into one correlation amount, and a defocus amount and its reliability level are calculated from the one correlation amount. 
     During image capture of a subject with low light intensity, levels of an A image signal and a B image signal are low. If the ISO film speed is increased in the image capture, more noise is included, and hence the signal-to-noise ratio is lowered. As a result, the precision of defocus amounts and the reliability levels of the defocus amounts are lowered. 
       FIGS. 7A and 7B  are conceptual diagrams showing a state of image capture of a black subject having one vertical white line with low light intensity.  FIG. 7A  shows a correlation amount in one area; in the figure, a horizontal axis represents a shift amount, and a vertical axis represents a correlation amount.  FIG. 7B  shows a correlation amount obtained by merging the correlation amount in one area shown in  FIG. 7A  with the correlation amounts in areas along the direction at a right angle to the direction of phase-difference detection. As shown in  FIG. 7A , when image capture is performed with low light intensity, the signal-to-noise ratio is lowered, and thus the correlation amount fluctuates significantly. As a result, the steepness of the correlation amount decreases, and the precision of a defocus amount and the reliability level of the defocus amount are lowered. 
     However, as shown in  FIG. 7B , merging the correlation amounts in areas along the direction perpendicular to the direction of phase-difference detection can improve the signal-to-noise ratio and increase the steepness of the resultant correlation amount. As a result, the precision of defocus amounts can be improved during image capture with low light intensity, and the reliability levels of the defocus amounts can be increased. Furthermore, as division is performed in a direction parallel to the direction of phase-difference detection, a conflict between a far point and a near point can be eliminated as well. The foregoing description applies not only to the case of low light intensity, but also to the case of a low-contrast subject, and the same effects can be achieved in the latter case. 
       FIG. 5E  shows a fourth divided area. All of the plurality of areas shown in  FIG. 5B , along the direction of phase-difference detection and the direction at a right angle thereto, are included in the fourth divided area. That is to say, the areas  501  to  509  shown in  FIG. 5B  are included in an area  516  shown in  FIG. 5E . A defocus amount in the area  516  shown in  FIG. 5E  is calculated as follows: correlation amounts in the included areas shown in  FIG. 5B  are merged into one correlation amount, and a defocus amount and its reliability level are calculated from the one correlation amount. 
     In this case, although it is difficult to eliminate a conflict between a far point and a near point, the aforementioned effects of  FIG. 5C  and effects of  FIG. 5D  can both be achieved, the precision of the defocus amount can be improved, and the reliability level of the defocus amount can be increased, even if the following conditions are satisfied: light intensity is low, or a subject has low contrast; and the subject is in an out-of-focus state. 
     As described above, by changing a method of merging correlation amounts obtained from different areas depending on the condition of a subject and the image capturing condition, the precision of defocus amounts can be improved and their reliability levels can be increased. This facilitates capturing of a subject image, and makes it possible to stably bring the subject into focus. 
       FIG. 8  is a flowchart showing the details of the focus detection area selection processing (step S 304 ) shown in  FIG. 3 . First, in step S 601 , the camera control unit  207  calculates defocus amounts in areas in which correlation amounts have been obtained. Specifically, it calculates correlation amounts and defocus amounts in the first to fourth divided areas. It also calculates the reliability levels of the defocus amounts. 
     Next, it is determined whether the defocus amounts have been obtained in the first divided areas and their reliability levels are high (step S 602 ). If the defocus amounts have been obtained in the first divided areas and their reliability levels are high (YES of step S 602 ), the camera control unit  207  calculates a later-described effective defocus amount (step S 603 ), and ends the processing. On the other hand, if the determination in step S 602  results in NO, the processing moves to step S 604 . 
     In step S 604 , the camera control unit  207  determines whether image capture is performed with low light intensity (low light intensity determination). If the image capture is not performed with low light intensity (NO of step S 604 ), it is determined whether the defocus amounts have been obtained in the second divided areas and their reliability levels are high (step S 605 ). If the defocus amounts have been obtained in the second divided areas and their reliability levels are high (YES of step S 605 ), the camera control unit  207  calculates an effective defocus amount from the results obtained in the second divided areas (step S 606 ), and ends the processing. If the determination in step S 605  results in NO, the processing moves to step S 609 . 
     If it is determined in step S 604  that the image capture is performed with low light intensity (YES of step S 604 ), the camera control unit  207  determines whether the defocus amounts have been obtained in the third divided areas and their reliability levels are high (step S 607 ). If the defocus amounts have been obtained in the third divided areas and their reliability levels are high (YES of step S 607 ), the camera control unit  207  calculates an effective defocus amount from the results obtained in the third divided areas (step S 608 ), and ends the processing. On the other hand, if the determination in step S 607  results in NO, the processing moves to step S 609 . 
     In step S 609 , the camera control unit  207  determines whether the defocus amount has been obtained in the fourth divided area and its reliability level is high. If the defocus amount has been obtained in the fourth divided area and its reliability level is high (YES of step S 609 ), an effective defocus amount is calculated from the result obtained in the fourth divided area (step S 610 ). On the other hand, if the determination in step S 609  results in NO, the processing is ended. 
     In the present embodiment, in step S 601 , the defocus amounts are calculated from the correlation amounts in all divided areas, and the reliability levels of the defocus amounts are calculated; however, the present invention is not limited in this way. For example, in step S 601 , the correlation amounts and defocus amounts in the first divided areas may be calculated. Then, the reliability levels of the defocus amounts may be calculated. Thereafter, before step S 605 , the correlation amounts and defocus amounts in the second divided areas may be calculated, and the reliability levels of the defocus amounts may be calculated; that is to say, computations may be carried out in sequence. 
     Alternatively, computations may be carried out in multiple sequential batches while switching between determinations for the first to fourth divided areas. For example, determination for the first divided areas is made in the first field, and if the determination in step S 602  results in NO, step S 604  is executed in the second field, followed by determination for the second or third divided areas. 
     In this case, whether the image capture is performed with low light intensity may be determined in step S 604  using a first determination method of determining whether the ISO film speed is equal to or larger than a predetermined value. If the ISO film speed is equal to or larger than the predetermined value, it is determined that the image capture is performed with low light intensity; if not, it is determined that the image capture is not performed with low light intensity. Alternatively, the determination may be made using a second determination method of determining whether a luminance peak value of a video signal is equal to or larger than a predetermined value. If the luminance peak value of the video signal is equal to or larger than the predetermined value, it is determined that the image capture is not performed with low light intensity; if not, it is determined that the image capture is performed with low light intensity. Alternatively, the determination may be made using a third determination method of determining whether an exposure value is equal to or smaller than a predetermined value. If the exposure value is equal to or smaller than the predetermined value, it is determined that the image capture is performed with low light intensity; if not, it is determined that the image capture is not performed with low light intensity. Alternatively, the determination may be made using a fourth determination method of determining whether a set gain is equal to or larger than a predetermined value. If a value of the set gain is equal to or larger than the predetermined value, it is determined that the image capture is performed with low light intensity; if not, it is determined that the image capture is not performed with low light intensity. 
     Here, it is sufficient to determine that the image capture is performed with low light intensity if a high possibility of the image capture with low light intensity can be determined, i.e., if at least one of the aforementioned conditions is satisfied, or if some or all of the aforementioned conditions are satisfied. 
     Furthermore, although whether the image capture is performed with low light intensity is determined in step S 604 , whether a subject has low contrast may be further determined (low contrast determination). If it is determined that the subject does not have low contrast (NO of step S 604 ), it is determined whether the defocus amounts have been obtained in the second divided areas and their reliability levels are high (step S 605 ). If the defocus amounts have been obtained in the second divided areas and their reliability levels are high (YES of step S 605 ), an effective defocus amount is calculated from the results obtained in the second divided areas (step S 606 ), and the processing is ended. If the determination in step S 605  results in NO, the processing moves to step S 609 . 
     If the subject is determined to have low contrast in step S 604  (YES of step S 604 ), it is determined whether the defocus amounts have been obtained in the third divided areas and their reliability levels are high (step S 607 ). If the defocus amounts have been obtained in the third divided areas and their reliability levels are high (YES of step S 607 ), an effective defocus amount is calculated from the results obtained in the third divided areas (step S 608 ), and the processing is ended. On the other hand, if the determination in step S 607  results in NO, the processing moves to step S 609 . 
     This is because, in the case of image capture of a low-contrast subject, correlation amounts are not likely to exhibit steepness, and the precision of defocus amounts is easily lowered, similarly to the case of low light intensity. Furthermore, a low-contrast subject is susceptible to luminance variations (shading) caused by lens aberration. The influence of shading increases especially as an image height increases. Such shading cannot always be corrected thoroughly, even if processing for correcting shading is executed. Should a part of shading be left uncorrected, a subject image is deformed due to a change in the levels of image signals in some pixels, and the precision of focus detection is lowered. In particular, if the levels of image signals of the subject image are low, the subject image is susceptible to uncorrected shading as the influence of uncorrected shading is noticeable in the levels of image signals, thereby giving rise to a higher possibility of error. In view of this, correlation amounts in the third divided areas are merged; in this way, correlation amounts exhibiting steepness are easily obtained, and the precision of defocus amounts and their reliability levels can be improved. 
     Here, whether the subject has low contrast may be determined on the basis of first determination of whether a difference between the largest value and the smallest value of an A image signal and a B image signal is equal to or smaller than a predetermined value. If the difference between the largest value and the smallest value of the A image signal and the B image signal is equal to or smaller than the predetermined value, it is determined that the subject of image capture has low contrast; if not, it is determined that the subject of image capture does not have low contrast. Alternatively, whether the subject has low contrast may be determined on the basis of second determination of whether a difference between the largest value and the smallest value of luminance levels of the video signal is equal to or smaller than a predetermined value. If the difference between the largest value and the smallest value of the luminance levels of the video signal is equal to or smaller than the predetermined value, it is determined that the subject of image capture has low contrast; if not, it is determined that the subject of image capture does not have low contrast. 
     Here, it is sufficient to determine that the subject of image capture has low contrast if a high possibility of image capture of a low-contrast subject can be determined, i.e., if at least one of the aforementioned conditions is satisfied, or if all of the aforementioned conditions are satisfied. 
     In the present embodiment, in merging the correlation amounts in areas along the direction perpendicular to the direction of phase-difference detection, the correlation amounts in all of the areas included in the focus detection range are merged; however, the present invention is not limited in this way. The following configuration may be adopted: in an arbitrary area, the correlation amounts in the included areas arranged along the direction perpendicular to the direction of phase-difference detection are compared with one another, and only the correlation amounts in areas in which shift amounts corresponding to local minima of the correlation amounts are equal to or smaller than a predetermined value are merged. For example, in the case of  514  shown in  FIG. 5D  in which the correlation amounts in the areas  502 ,  505 , and  508  are merged, if a shift amount corresponding to a local minimum of the correlation amount in  508  is different by at least a predetermined shift amount from shift amounts corresponding to local minima of the correlation amounts in  502  and  505 , then only the correlation amounts in  502  and  505  are merged. 
     The same goes for a method in which conversion into defocus amounts is performed on the basis of local minima of correlation amounts in the areas, and then merger is performed only in areas in which the defocus amounts fall within a predetermined range. 
     It is sufficient to set the aforementioned predetermined shift amount or predetermined range as a ratio to the depth of focus, e.g., convert the predetermined shift amount or the predetermined range into a shift amount or a defocus amount that is twice as large as the depth of focus. 
     In this case, the predetermined shift amount or the predetermined range is set to be larger when image capture is performed with low light intensity than when image capture is not performed with low light intensity. For example, when it is determined that image capture is performed with low light intensity, the predetermined shift amount or the predetermined range is converted into a shift amount or a defocus amount that is four times as large as the depth of focus. 
     The foregoing description applies not only to a case in which it is determined that image capture is performed with low light intensity, but also to a case in which a subject is determined to have low contrast. In this way, a conflict between a far point and a near point can be eliminated. It will be assumed that the correlation amounts in at least two areas are merged. 
     &lt;Correlation Computation&gt; 
     Below is a description of computation of correlation as correlation information, correlation amount waveforms, and the like, followed by a description of a method of calculating the aforementioned effective defocus amount. 
       FIG. 9D  is a conceptual diagram showing examples of areas from which image signals are obtained, the areas being included in a pixel array in the image sensor  201 . In a pixel array  701  in which pixel units are arrayed, an area  702  serves as a target of computation described below. A shift area  704  necessary for correlation computation is obtained by combining the area  702  with shift areas  703  that are necessary for correlation computation for calculation of a defocus amount in the area  702 . 
     In  FIGS. 9A to 9D, 10A, 10B, 11A, and 11B , p, q, s, and t represent coordinates along an x-axis direction, a section from p to q represents the shift area  704 , and a section from s to t represents the area  702 . 
       FIGS. 9A, 9B, and 9C  show image signals obtained from the shift area  704  that is set as shown in  FIG. 9D . Referring to the image signals, a section from s to t corresponds to the area  702 , and a section from p to q corresponds to the shift area  704 , which is a range necessary for calculation of a defocus amount based on a shift amount.  FIG. 9A  is a conceptual diagram showing waveforms of a pre-shift A image signal and B image signal for correlation computation. A solid line  801  represents the A image signal, and a dash line  802  represents the B image signal.  FIG. 9B  is a conceptual diagram showing image waveforms obtained by shifting the pre-shift image waveforms shown in  FIG. 9A  in a positive direction, and  FIG. 9C  is a conceptual diagram showing image waveforms obtained by shifting the pre-shift image waveforms shown in  FIG. 9A  in a negative direction. A correlation amount indicating a degree of correlation between two images is calculated by, for example, shifting the A image signal  801  and the B image signal  802  bit by bit in the directions of arrows. 
     A description is now given of a method of calculating a correlation amount COR. First, for example, the A image signal and the B image signal are shifted bit by bit as shown in  FIGS. 9B and 9C , and a sum of absolute values of differences between the shifted A image signal and B image signal is obtained as the correlation amount COR. A shift amount is expressed as i, the smallest shift amount is p−s shown in  FIG. 10A , and the largest shift amount is q−t shown in  FIG. 10A . In addition, x represents the coordinates at which a focus detection area starts, and y represents the coordinates at which the focus detection area ends. Using the foregoing elements, the correlation amount COR is given by the following Expression 1. 
     
       
         
           
             
               
                 
                   
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       FIG. 10A  is a conceptual diagram showing a waveform of the correlation amount COR as a graph. In the graph, a horizontal axis represents a shift amount, and a vertical axis represents the correlation amount COR. The graph pertains to an example in which a correlation amount waveform  901  has extrema  902 ,  903 . A smaller one of the extrema of the correlation amount indicates a higher degree of match between an A image and a B image. 
     A description is now given of a method of calculating a correlation change amount ΔCOR. First, using the conceptual diagram of  FIG. 10A  showing the correlation amount waveform, a correlation change amount ΔCOR is calculated on the basis of, for example, changes in the correlation amount COR at an interval corresponding to one shift. A shift amount is expressed as i, the smallest shift amount is p−s shown in  FIG. 10A , and the largest shift amount is q−t shown in  FIG. 10A . Using the foregoing elements, ΔCOR is given by the following Expression 2.
 
ΔCOR[ i ]=COR[ i− 1]−COR[ i+ 1]{( p−s+ 1)&lt; i &lt;( q−t− 1)}  Expression 2
 
       FIG. 10B  is a conceptual diagram showing a waveform of the correlation change amount ΔCOR as a graph. In the graph, a horizontal axis represents a shift amount, and a vertical axis represents the correlation change amount. A correlation change amount waveform  1001  has points  1002 ,  1003  at which the sign of the correlation change amount changes from positive to negative. From the point  1002 , at which the correlation change amount ΔCOR hits zero, obtainment of a shift amount of the A image signal and the B image signal that relatively increases the degree of match between the A image and the B image is possible. This shift amount corresponds to a defocus amount. 
       FIG. 11A  is an enlarged view of the point  1002  shown in  FIG. 10B , and shows a part of the waveform  1001  of the correlation change amount ΔCOR as a waveform  1101 . With reference to  FIG. 11A , the following describes a method of calculating an out-of-focus amount PRD corresponding to a defocus amount. Assume that the out-of-focus amount is divided into an integer part β and a decimal part α. The decimal part α can be calculated on the basis of a similarity relationship between a triangle ABC and a triangle ADE in the figure, using the following Expression 3. 
     
       
         
           
             
               
                 
                   
                       
                   
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     On the other hand, the integer part β can be calculated on the basis of  FIG. 11A  using the following Expression 4.
 
β= k− 1  Expression 4
 
     A sum of α and β described above is the out-of-focus amount PRD. 
     When there are multiple zero-crossings as shown in  FIG. 10B , a zero-crossing exhibiting significant steepness, maxder, in the changing correlation amount is regarded as a first zero-crossing. The steepness is an index showing how easily AF is performed; the larger the value of the steepness is, the more easily AF is performed. The steepness is given by the following expression 5.
 
max der =|ΔCOR[ k− 1]|+|ΔCOR[ k]|   Expression 5
 
     As described above, when there are multiple zero-crossings, a first zero-crossing is determined on the basis of the steepness. 
     A description is now given of a method of calculating a reliability level of the out-of-focus amount. This corresponds to reliability of a defocus amount; the following description is one example, and the reliability level may be calculated using other known methods. Reliability can be defined by the aforementioned steepness and a degree of match, fnclvl, between two image signals, i.e., an A image signal and a B image signal (hereinafter referred to as a degree of match between two images). The degree of match between two images is an index showing the precision of the out-of-focus amount; the smaller the value of the degree of match between two images, the higher the precision.  FIG. 11B  is an enlarged view of the vicinity of the extremum  902  shown in  FIG. 10A , and shows a part of the correlation amount waveform  901  as a waveform  1201 . The degree of match between two images is calculated on the basis of this figure. The degree of match between two images is given by the following Expression 6.
 
( i ) when|ΔCOR[ k− 1]|×2&lt;max der,fnclvl =COR[ k− 1]+ΔCOR[ k− 1]/4
 
( ii ) when|ΔCOR[ k− 1]|×2&gt;max der,fnclvl =COR[ k ]−ΔCOR[ k]/ 4  Expression 6
 
     This concludes the description of calculation of the degree of match between two images. 
     &lt;Calculation of Defocus Amount&gt; 
       FIG. 12  is a flowchart of processing ending with calculation of a defocus amount. Note that in the following exemplary description, an out-of-focus amount and a defocus amount will be distinguished from each other. A defocus amount according to a technical idea of the present embodiment may be an absolute distance from a point of focus, the number of pulses, or a (relative) value of a different level/unit. The defocus amount is a value that indicates whether determination of the extent of deviation from an in-focus state is possible, and whether determination of the extent of focus control that should be performed to make a transition to the in-focus state is possible. Obtainment of defocus information within the foregoing concept is referred to as obtainment of focus information. 
     In step S 1001 , an A image signal and a B image signal are obtained from pixels in each of areas that are set as shown in  FIGS. 5A to 5E . Next, a correlation amount is calculated from the obtained image signals (step S 1002 ). Subsequently, a correlation change amount is calculated from the calculated correlation amount (step S 1003 ). Then, an out-of-focus amount is calculated from the calculated correlation change amount (step S 1004 ). Furthermore, a reliability level, which indicates how reliable the calculated out-of-focus amount is, is calculated (step S 1005 ). The foregoing processes are executed in each one of the areas within the focus detection range. 
     Then, in each one of the areas within the focus detection range, the out-of-focus amount is converted into a defocus amount (step S 1006 ). Furthermore, an effective defocus direction and an effective defocus amount are calculated (steps S 1007 , S 1008 ). 
     &lt;Calculation of Effective Defocus Amount&gt; 
       FIG. 13  is a flowchart of processing for calculating one defocus amount corresponding to the focus detection range as the effective defocus amount shown in the aforementioned step S 1008 . In the present embodiment, processing is executed using a defocus amount; alternatively, processing may be executed using a shift amount corresponding to a local minimum of a correlation amount. That is to say, as stated earlier, either of them may be used as long as it, in concept, indicates whether it is possible to determine the extent of deviation from an in-focus state, and whether it is possible to determine the extent of focus control that should be performed to make a transition to the in-focus state. 
     First, the camera control unit  207  searches the plurality of areas within the focus detection range for areas in which defocus amounts have been obtained and their reliability levels are high. It calculates an average value of the defocus amounts in the areas that have been found to satisfy the conditions as a result of the search (step S 1101 ). 
     Next, the camera control unit  207  calculates differences between the defocus amounts in the areas and the average value calculated in step S 1101  (step S 1102 ). Then, it is determined whether the largest one of the calculated differences in the areas is equal to or larger than a predetermined difference. That is to say, it is determined whether the defocus amount corresponding to the largest difference exhibits a large deviation among the defocus amounts in the plurality of areas within the focus detection range. If the largest one of the calculated differences in the areas is smaller than the predetermined difference (NO of step S 1103 ), the camera control unit  207  sets the average value calculated in step S 1101  as the effective defocus amount (step S 1104 ). On the other hand, if the largest one of the calculated differences in the areas is equal to or larger than the predetermined difference (YES of step S 1103 ), the defocus amount in the area corresponding to the largest difference is excluded from the operands used in calculation of the average value (step S 1105 ). That is to say, among the defocus amounts in the plurality of areas within the focus detection range, a defocus amount exhibiting a large deviation is excluded from the operands. 
     In step S 1106 , the camera control unit  207  determines whether there are defocus amounts in the remaining areas; if there are defocus amounts in the remaining areas (NO of step S 1106 ), it proceeds to step S 1101  again and repeats the processing. If the number of the defocus amounts in the remaining areas is two (YES of step S 1106 ), it proceeds to step S 1107  and determines whether a difference between the defocus amounts in the remaining two areas is equal to or larger than the predetermined difference (step S 1107 ). 
     If the difference between the defocus amounts is equal to or larger than the predetermined difference (YES of step S 1107 ), the camera control unit  207  selects a smaller one of defocus amounts that are in the same direction as the effective defocus direction, and uses the selected defocus amount as the effective defocus amount in step S 1109 . If not, the camera control unit  207  calculates an average of the two defocus amounts and uses the average as the effective defocus amount (step S 1108 ). 
     In a case where only one area has been set, one defocus amount that has been obtained is used as the effective defocus amount. In a case where no area yields a defocus amount with a high reliability level, no effective defocus amount is set. The foregoing description applies not only to the effective defocus amount, but also to the effective defocus direction. Furthermore, for example, areas in which the defocus amounts have been obtained and their reliability levels are high or medium are searched for from among the plurality of areas, and a direction that appears most frequently among the directions in the discovered areas is used as the effective defocus direction. 
     In the foregoing description, the defocus amount in the area that yields the largest difference is considered a defocus amount exhibiting a large deviation, and is excluded from the operands used in calculation of the average value (step S 1105 ). However, certain effects can be achieved by reducing a weight for the defocus amount exhibiting a large deviation, instead of excluding such a defocus amount from the operands. In this case, there is a possibility that the main subject image is defocused by an amount corresponding to the weight. 
     As described above, among the defocus amounts in the plurality of areas within the focus detection range, a defocus amount exhibiting a large deviation is not used. This is because, as one defocus amount corresponding to the focus detection range is calculated from the defocus amounts in the plurality of areas, there is a relatively high possibility of the occurrence of a so-called conflict between a far point and a near point depending on the size of the focus detection range. 
     In the foregoing description, averaging processing is used as an example of a method of obtaining one piece of defocus information corresponding to the focus detection range using a plurality of pieces of focus information output in the focus detection range. Alternatively, the defocus amount may be calculated by applying predetermined weighting instead of the averaging processing. As described above, one piece of defocus information corresponding to the focus detection range is obtained using the plurality of output pieces of defocus information, and such obtainment is carried out for the following reason. If a defocus amount in one area is selected from among the defocus amounts in the areas within the focus detection range, the subject image is captured as a “line” or “point”. As a result, focus control is performed also with respect to a difference between defocus amounts in areas in which the subject image is captured as a “line” or “point”, thereby increasing the possibility that the resultant live-view images and moving images are not in an appropriate form. On the other hand, the technical idea of averaging the defocus amounts in the areas enables the subject image to be captured as a “plane”, thereby alleviating the negative effects of focus control based on a difference between defocus amounts in areas in which the subject image is captured as a “line” or “point”. This technical idea also secures the precision at which a subject image targeted by the user is brought into focus, as stated earlier. The same goes for a case in which one piece of defocus information corresponding to the areas is obtained using a plurality of pieces of focus information that have been output under the influence of weighting, instead of using the averaging processing. 
     As described above, by averaging a plurality of defocus amounts in the areas arranged for a subject image that should be captured as one entity, stable focus control can be achieved while suppressing a fluctuation in a defocus amount in each area within the focus detection range. 
     Furthermore, the predetermined difference in steps S 1103  and S 1107  may be changed depending on whether the image capturing state of a subject indicates low light intensity. In the case of image capture with low light intensity, the ISO film speed may be increased in the image capture, and a subject image may have low contrast. As a result, the signal-to-noise ratio of an A image signal and a B image signal may be lowered, correlation amounts may not be likely to exhibit steepness, a fluctuation in defocus amounts may increase, and the precision may be lowered. Thus, if a threshold equivalent to a threshold for normal image capture is used, the possibility of containment within the range decreases. For this reason, whether the image capture is performed with low light intensity is determined as stated earlier, and when the image capture is performed with low light intensity, the value of the predetermined difference is made larger than when the image capture is performed not with low light intensity. 
     Similarly, the predetermined difference may vary depending on whether the correlation amounts in the plurality of areas have been merged, that is to say, between the case of the first divided areas and other cases. To be more specific, in the case of divided areas other than the first divided areas, the value of the predetermined difference is made larger than in the case of the first divided areas. The correlation amounts in the plurality of areas are merged when the precision is low because the following conditions are satisfied: the defocus amounts are low in reliability; and image capture is performed with low light intensity, or a subject has low contrast. In view of this, if a threshold that is equivalent to a threshold for a case in which the correlation amounts in the plurality of areas are not merged is used, the possibility of containment within the range decreases, and hence capturing of a subject and stabilization of focus control are expected to be difficult. The aforementioned method makes it possible to capture a subject, maintain the precision of defocus amounts, and stabilize focus control regardless of the image capturing state of the subject. 
     As described above, the embodiment of the present invention uses an imaging surface phase-difference AF method in which focus detection areas are arranged by dividing a focus detection range of an image capturing screen in a direction of phase-difference detection and in a direction at a right angle thereto. In accordance with the result of focus detection and the image capturing state of a subject, a method of merging correlation amounts in the plurality of areas is changed. This makes it possible to realize focus control that is more stable and more appropriate for a live-view mode and moving image capture than focus control of conventional image capturing apparatuses. 
     Although the present invention has been elaborated thus far on the basis of a preferred embodiment thereof, the present invention is not limited to such a specific embodiment and encompasses various embodiments within the spirit of the present invention. A part of the foregoing embodiment may be implemented in combinations as appropriate. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-008957, filed Jan. 20, 2015, which is hereby incorporated by reference herein in its entirety.