Patent Publication Number: US-9843728-B2

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

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a focus control apparatus, a control method therefor, a storage medium therefor, and an image capturing apparatus. 
     Description of the Related Art 
     It is not easy for a photographer to perform precise focus control with respect to a subject by manipulating manual focus (MF manipulation) on a focus control apparatus, such as a high-definition video camera supporting full high-definition, 4K, etc. Especially when performing focus control while checking a viewfinder, a panel, or the like, a slip in focus control may occur that cannot be checked on the viewfinder, the panel, or the like. To correct such a slip in focus control, MF assistance methods are suggested whereby an autofocus (AF) operation is performed after manipulating MF. 
     Japanese Patent Laid-Open No. 2003-241077 suggests a technique to, after detecting completion of MF manipulation and pressing of a release button or the like, perform only one session of detailed focus control through an AF operation within a minute range. 
     Japanese Patent Laid-Open No. 2010-97167 suggests a technique whereby a focus detection frame is automatically set to an area in an in-focus state among a plurality of focus detection areas set on a screen (areas targeted for AF), and if a subject moves on the screen afterwards, the subject is automatically tracked by the focus detection frame. 
     Japanese Patent Laid-Open No. 2007-248615 suggests a technique to display a bar showing an in-focus degree that is calculated while manipulating MF, so as to enable a user to easily check the state of a slip in focus control while manipulating MF. 
     However, with the technique of Japanese Patent Laid-Open No. 2003-241077, once focus control has been performed through AF, subtle shaking of a subject may bring a captured image slightly out of focus. With the technique of Japanese Patent Laid-Open No. 2010-97167, if a small focus detection frame is set, a moving subject easily deviates from the focus detection frame, thereby giving rise to the possibility that AF cannot be performed appropriately. The technique of Japanese Patent Laid-Open No. 2007-248615 does not take into consideration a case where a focus detection area for calculating an in-focus degree is dynamically changed by, for example, a user&#39;s selection; this may disable precise focus control with respect to a desired subject. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the aforementioned problems, and realizes a technique to enable appropriate focus control in accordance with a user&#39;s intention when there are a manual focus control mode and an automatic focus control mode. 
     In order to solve the aforementioned problems, the present invention provides a focus control apparatus having a first mode in which a position of a focus lens is automatically changed, and a second mode in which the position of the focus lens is manually changed, the focus control apparatus comprising: a setting unit configured to set a first area in the first mode and a second area in the second mode, as areas for obtaining signals used in focus detection; a focus detection unit configured to detect a focus state based on signals output from areas of an image capturing unit that correspond to the first area and the second area; and a focus control unit configured to control the position of the focus lens in the first mode based on the focus state of the first area detected by the focus detection unit, wherein upon switching from the first mode to the second mode, the setting unit sets a plurality of the second areas that are each smaller than the first area set in the first mode, and upon switching from the second mode to the first mode again, the setting unit sets the first area based on a second area that is included among the plurality of second areas and that has been determined to be in an in-focus state. 
     In order to solve the aforementioned problems, the present invention provides an image capturing apparatus, comprising: an image capturing unit; and a focus control apparatus having a first mode in which a position of a focus lens is automatically changed, and a second mode in which the position of the focus lens is manually changed, wherein the focus control apparatus includes: a setting unit configured to set a first area in the first mode and a second area in the second mode, as areas for obtaining signals used in focus detection; a focus detection unit configured to detect a focus state based on signals output from areas of the image capturing unit that correspond to the first area and the second area; and a focus control unit configured to control the position of the focus lens in the first mode based on the focus state of the first area detected by the focus detection unit, upon switching from the first mode to the second mode, the setting unit sets a plurality of the second areas that are each smaller than the first area set in the first mode, upon switching from the second mode to the first mode again, the setting unit sets the first area based on a second area that is included among the plurality of second areas and that has been determined to be in an in-focus state, the image capturing unit includes a plurality of pixels, each pixel having a plurality of photoelectric conversion areas corresponding to one microlens, and the focus detection unit detects the focus state based on signal pairs that are each output from a different one of the photoelectric conversion areas in the plurality of pixels. 
     In order to solve the aforementioned problems, the present invention provides a control method of a focus control apparatus having a first mode in which a position of a focus lens is automatically changed, and a second mode in which the position of the focus lens is manually changed, the control method comprising: setting a first area in the first mode and a second area in the second mode, as areas for obtaining signals used in focus detection; detecting a focus state based on signals output from areas of an image capturing unit that correspond to the first area and the second area; and controlling the position of the focus lens in the first mode based on the detected focus state of the first area, wherein upon switching from the first mode to the second mode, a plurality of the second areas are set, the plurality of second areas each being smaller than the first area set in the first mode, and upon switching from the second mode to the first mode again, the first area is set based on a second area that is included among the plurality of second areas and that has been determined to be in an in-focus state. 
     In order to solve the aforementioned problems, the present invention provides a non-transitory computer-readable storage medium storing a program for causing a computer to execute a control method of a focus control apparatus having a first mode in which a position of a focus lens is automatically changed, and a second mode in which the position of the focus lens is manually changed, the control method comprising: setting a first area in the first mode and a second area in the second mode, as areas for obtaining signals used in focus detection; detecting a focus state based on signals output from areas of an image capturing unit that correspond to the first area and the second area; and controlling the position of the focus lens in the first mode based on the detected focus state of the first area, wherein upon switching from the first mode to the second mode, a plurality of the second areas are set, the plurality of second areas each being smaller than the first area set in the first mode, and upon switching from the second mode to the first mode again, the first area is set based on a second area that is included among the plurality of second areas and that has been determined to be in an in-focus 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 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment(s) of the invention, and together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a block diagram showing an exemplary functional configuration of one example of a focus control apparatus according to an embodiment of the present invention, specifically, a camera body and a lens unit. 
         FIGS. 2A and 2B  illustrate an exemplary pixel structure of an imaging surface phase-difference detection method according to the present embodiment. 
         FIG. 3  is a flowchart showing a sequence of operations for image capturing processing according to the present embodiment. 
         FIG. 4  is a flowchart showing a sequence of operations of AF control processing according to the present embodiment. 
         FIG. 5  is a flowchart showing a sequence of operations of focus detection processing according to the present embodiment. 
         FIG. 6  is a flowchart showing a sequence of operations of AF reactivation determination processing according to the present embodiment. 
         FIG. 7  is a flowchart showing a sequence of operations of AF processing according to the present embodiment. 
         FIG. 8  is a flowchart showing a sequence of operations of processing for setting driving of a focus lens according to the present embodiment. 
         FIG. 9  is a flowchart showing a sequence of operations of focus lens driving processing according to the present embodiment. 
         FIG. 10  is a flowchart showing a sequence of operations of search driving processing according to the present embodiment. 
         FIG. 11  is a flowchart showing a sequence of operations of initialization processing according to the present embodiment. 
         FIGS. 12A to 12H  show focus detection areas according to the present embodiment. 
         FIGS. 13A to 13C  show image signals obtained from focus detection areas according to the present embodiment. 
         FIGS. 14A to 14D  illustrate a correlation computation method according to the present embodiment. 
         FIG. 15  is a flowchart showing a sequence of operations of AF frame selection processing according to the present embodiment. 
         FIG. 16A  is a flowchart showing a sequence of operations of normal frame setting processing according to the present embodiment, and  FIG. 16B  is a flowchart showing a sequence of operations of in-focus frame setting processing according to the present embodiment. 
         FIG. 17  is a flowchart showing a sequence of operations of MF control processing according to the present embodiment. 
         FIG. 18  is a flowchart showing a sequence of operations of processing for calculating MF in-focus degrees according to the present embodiment. 
         FIG. 19  is a flowchart showing a sequence of operations of processing for combining MF frames according to the present embodiment. 
         FIG. 20  is a flowchart showing a sequence of operations of image capturing processing according to the present embodiment. 
         FIGS. 21A to 21C  specifically illustrate the AF frame selection processing according to the present embodiment. 
         FIGS. 22A to 22F  specifically illustrate display of focus detection frames according to the present embodiment. 
         FIGS. 23A to 23D  illustrate an overview of the present embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     The following describes an exemplary embodiment of the present invention in detail with reference to the drawings. The following description provides an example in which the present invention is applied to an arbitrary digital camera capable of focus control as one example of a focus control apparatus. However, the present invention is not limited to being applied to a digital camera and is applicable to any electronic device capable of focus control, and examples of such a device may include an information terminal (e.g., a mobile telephone, a personal computer, a tablet, a clock-type device, or an eyeglass-type device), and a vehicle-mounted device. Furthermore, although a body of a digital camera described in the present embodiment includes a display unit and a manipulation unit that enables manipulation of MF, the display unit and the manipulation unit may be externally provided, and manual manipulation may be performed through remote control. 
     Overview of Present Embodiment 
     Before giving a detailed description of the present embodiment, an overview of the present embodiment will now be described with reference to  FIGS. 23A to 23D .  FIG. 23A  shows a predetermined frame of a video capturing a subject. A screen  2301  shows a captured subject  2302 . In this case, an AF frame is set for the subject  2302  based on a face frame  2303 , and AF control is performed using AF evaluation values generated from image signals obtained within the AF frame. However, as the AF frame is set to cover the entire face, performing AF based on the AF evaluation values may result in focus on a high-contrast portion, or on a portion  2304  (specifically, the nose) that is closest from a camera in the case of AF with closest subject priority, within the face frame  2303 . In general, when capturing a person, an eye is often focused on during focus control, and therefore using the AF evaluation values within the AF frame that has been set based on the face frame  2303  may result in focus control that is not in line with a user&#39;s intention. 
     In this case, the user can perform focus control by manipulating MF; however, if AF operates again after such focus control, the closest portion, such as the noise, may be focused on again when the AF frame is set at the face frame. 
     In view of this, in the present embodiment, as shown in  FIG. 23B , segment frames  2305  having a higher level of segmentation than the AF frame are set as focus detection frames while manipulating MF. Focus detection information is obtained from each of the segment frames  2305 , and as shown in  FIG. 23C , an in-focus display frame  2306  is displayed only at a portion that is in focus. Subsequently, an AF operation is performed with an AF frame set based on the focus detection frame that has been focused on by manipulating MF. As a result, a portion that the user tried to focus on by manipulating MF can be focused on also after the AF operation. That is to say, upon returning to the AF operation, as shown in  FIG. 23D , an AF frame  2307  is set at the same area as the in-focus display frame  2306 ; this makes it possible to maintain the state where, for example, an eye of a person is in focus. 
     However, even with the foregoing technique, a significant movement of the subject may cause the subject to deviate from the focus detection frame that has already been focused on by an MF operation. This may result in a situation where an AF frame that has been set based on such a focus detection frame covers an unintended subject, thereby focusing on the unintended subject. Consequently, if the image features satisfy a predetermined condition(s), it is determined that the AF frame has deviated from the subject, and the intended AF is continuously performed by enlarging the AF frame to the face frame  2303  shown in  FIG. 23A . The present embodiment will now be described in detail. 
     (Configuration of Digital Camera) 
       FIG. 1  is a block diagram showing an exemplary functional configuration of one example of a focus control apparatus according to the present embodiment, specifically, a digital interchangeable lens camera that includes a lens unit and a camera body. Note that at least one of function blocks shown in  FIG. 1  may be realized by hardware, such as an ASIC and a programmable logic array (PLA), or may be realized by execution of software by a programmable processor, such as a CPU and an MPU. Alternatively, at least one of the function blocks may be realized by a combination of software and hardware. Therefore, operations that are explained to be executed by different function blocks in the following description may be executed by the same hardware. 
       FIG. 1  is a block diagram showing a configuration of a digital camera according to the present embodiment. A digital camera  30  is composed of a lens unit  10  and a camera body  20 . A lens control unit  106  that controls the entire operations of the lens unit  10  and a camera control unit  207  that controls the entire operations of the camera body  20  perform data communication via a non-illustrated transmission channel, such as a bus. Although the lens unit  10  is configured as an exchangeable lens system that is attachable to and detachable from the camera body  20  in the description of the present embodiment, the lens unit  10  and the camera body  20  may be integrated. 
     &lt;Configuration of Lens Unit  10 &gt; 
     The lens unit  10  includes an imaging optical system composed of a fixed lens  101 , a zoom lens  108 , a diaphragm  102 , and a focus lens  103 . The diaphragm  102  is driven by a 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 a focus lens driving unit  105 , and is moved on an optical axis, either automatically or manually, to perform focus control. More specifically, an image formation position of an optical image of a subject incident on the image sensor  201  is changed in an optical axis direction by controlling a position of the focus lens  103 . The zoom lens  108  is driven by a zoom lens driving unit  109  to control a zoom position. 
     The diaphragm driving unit  104 , the focus lens driving unit  105 , and the zoom lens driving unit  109  are controlled by the lens control unit  106  to decide on an aperture of the diaphragm  102  and the positions of the focus lens  103  and the zoom lens  108 . When a user manipulates focus, zoom, and the like via a lens manipulation unit  107 , the lens control unit  106  performs control according to the user&#39;s manipulation. The lens control unit  106  controls the diaphragm driving unit  104 , the focus lens driving unit  105 , and the zoom lens driving unit  109  in accordance with a control command and control information received from the later-described camera control unit  207 , and thereafter transmits lens information to the camera control unit  207 . 
     &lt;Configuration of Camera Body  20 &gt; 
     The camera body  20  can obtain image capturing signals from a beam of light that has passed through the imaging optical system of the lens unit  10 . The image sensor  201  is constituted by, for example, a CCD sensor or a CMOS sensor. A beam of light that has passed through the imaging optical system forms an image on a light receiving surface of the image sensor  201 , and the formed image of the subject is converted by photodiodes into charges corresponding to an amount of incident light (photoelectric conversion). In accordance with an instruction from the camera control unit  207  and driving pulses fed from a timing generator  209 , the charges accumulated in the photodiodes are sequentially read out from the image sensor  201  as voltage signals corresponding to the amounts of the charges. 
     An image sensor that does not support focus detection according to an imaging surface phase-difference method has a pixel structure with a Bayer array as shown in, for example,  FIG. 2A . On the other hand, as shown in  FIG. 2B , the image sensor  201  according to the present embodiment has a plurality of (in the present embodiment, two) photodiodes (photoelectric conversion areas) per pixel to perform focus detection according to the imaging surface phase-difference method. A beam of light is divided by a microlens that is provided in the vicinity of two photodiodes, and each of the two photodiodes photoelectrically converts the image of the subject; in this way, a signal for image capture and two signals for focus detection can be obtained. A sum of signals from the two photodiodes (A+B) serves as the image capturing signal, and signals from the individual photodiodes (A, B) serve as the two image signals for focus detection. 
     Note that the configuration in which each of the two image signals is read out, which is described in the present embodiment by way of example, may be replaced by other configurations. For example, in consideration of processing load, it is permissible to adopt a configuration in which the sum of signals (A+B) and one of the image signals (e.g., A) are read out, and the other of the image signals (e.g., B) is obtained from a difference between the signals that have been read out. In the present embodiment, correlation computation is carried out with respect to the two image signals thus obtained for focus detection so as to calculate an image shift amount (also referred to as an out-of-focus amount or a focus state) and various types of reliability information as a result of focus detection. 
     Furthermore, although two photodiodes are provided per pixel in the present embodiment, two or more photodiodes may be provided per pixel. Moreover, an image sensor that supports focus detection according to the imaging surface phase-difference method is not limited to having a plurality of photodiodes per pixel as in the present embodiment, and may include pixels for focus detection as in Japanese Patent Laid-Open No. 2010-97167 mentioned earlier. 
     The image capturing signals and the signals for focus detection that have been read out from the image sensor  201  are input to a CDS/AGC circuit  202  to perform correlated double sampling for removing reset noise, control gain, and digitalize signals. The CDS/AGC circuit  202  outputs the image capturing signals to a camera signal processing unit  203 , and outputs the signals for focus detection according to the imaging surface phase-difference method (hereinafter also referred to simply as focus detection signals) to a focus detection signal processing unit  204 . 
     The camera signal processing unit  203  transmits the image capturing signals output from the CDS/AGC circuit  202  to a display unit  205 . The display unit  205  is a display device, such as an LCD and an organic EL display, and displays the image capturing signals. In a mode for recording the image capturing signals, the image capturing signals are recorded to a recording unit  206 . 
     The focus detection signal processing unit  204  obtains two image signals for focus detection from the CDS/AGC circuit  202 , and carries out correlation computation with respect to the image signals to calculate an image shift amount 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 image shift amount and reliability information to the camera control unit  207 . The details of correlation computation will be described later. 
     The camera control unit  207  is, for example, a CPU or an MPU, and controls various components of the camera body  20  by deploying a program stored in an internal ROM to a working area for a RAM and executing the deployed program. As this time, the camera control unit  207  exchanges information with various components. It also executes a camera function corresponding to the user&#39;s manipulation in accordance with input from a camera manipulation unit  208 ; examples of the user&#39;s manipulation include turning ON/OFF power, changing a setting, starting recording, starting AF control, checking a recorded image, and selecting an AF frame. The camera control unit  207  has a mode switching unit that switches between focus control modes (AF and MF) in accordance with input via the camera manipulation unit  208 . The camera control unit  207  exchanges information with the lens control unit  106  in the lens unit  10 , specifically, transmits a control command and control information for controlling the imaging optical system based on a focus state (focus control), and obtains information within the lens unit. The camera control unit  207  also performs display control to display, on the display unit  205 , an AF frame or a later-described combined frame superimposed over the image capturing signals. Note that the camera control unit  207  may receive, as input, the image signals for focus detection directly from the CDS/AGC circuit  202 , and execute processing executed by the focus detection signal processing unit  204 . 
     (Operations for Image Capturing Processing in Camera Body  20 ) 
     A description is now given of a sequence of operations for image capturing processing in the camera body  20  with reference to  FIG. 3 .  FIG. 3  is a flowchart showing an image capturing processing sequence according to the present embodiment. Note that the present processing is started when, for example, the user instructs the camera manipulation unit  208  to start the image capturing processing or change an image capturing mode. The present processing is realized by the camera control unit  207  deploying a program stored in the ROM to the working area for the RAM and executing the deployed program. 
     In step S 301 , the camera control unit  207  executes initialization processing, and then proceeds to step S 302 . The details of the initialization processing will be described later with reference to  FIG. 11 . In step S 302 , the camera control unit  207  determines a focus mode. If the focus mode is set to a OneShotAF mode (AF control is performed only once), the present processing sequence is ended. On the other hand, if the focus mode is not the OneShotAF mode, that is to say, if the focus mode is ContinuousAF (AF control is performed continuously) or manual (MF manipulation), processing proceeds to step S 303 . In the following description, processing executed during OneShotAF will be omitted. 
     In step S 303 , the camera control unit  207  determines whether a focus control mode is automatic (AF operation) or manual (MF manipulation); if the focus control mode is set to AF, processing proceeds to step S 304 , and if not, processing proceeds to step S 309 . 
     In step S 304 , the camera control unit  207  executes AF frame selection processing for setting an AF frame. The AF frame selection processing is processing for deciding on one of the following frames as a basis for selection of the AF frame: a focus detection frame that was determined to be in focus while manipulating MF in later-described step S 313  (hereinafter also referred to as an in-focus frame), and a normal AF frame (hereinafter also referred to as a normal frame). In step S 305 , whether the frame selected in step S 304  is the normal frame or the in-focus frame is determined; if the selected frame is the normal frame, processing proceeds to step S 306 , and if the selected frame is the in-focus frame, processing proceeds to step S 307 . 
     In step S 306 , the camera control unit  207  sets the AF frame at the position of the selected normal frame. The details will be described later with reference to  FIG. 16A . On the other hand, in step S 307 , the camera control unit  207  sets the AF frame at the position of the in-focus frame. The details will be described later with reference to  FIG. 16B . 
     In step S 308 , the focus detection signal processing unit  204  and the camera control unit  207  execute AF control processing using the AF frame set in step S 306  or S 307 . This processing will be described later with reference to  FIG. 4 . When the camera control unit  207  has ended the AF control processing, processing proceeds to step S 314 . 
     In step S 314 , the camera control unit  207  displays, on the display unit  205 , the AF frame or the later-described combined frame superimposed over image capturing signals. 
     In step S 315 , the camera control unit  207  executes the image capturing processing. The details will be described later with reference to  FIG. 20 . When the camera control unit  207  has ended the image capturing processing, processing returns to step S 302 , and the foregoing processing is repeated. 
     On the other hand, if the camera control unit  207  determines in step S 303  that the focus control mode is MF manipulation, the camera control unit  207  executes processing for setting MF segment frames in step S 309 . The details of this processing will be described later with reference to  FIGS. 12A to 12H . In step S 310 , the camera control unit  207  executes MF control processing. The details of this processing will be described later with reference to  FIG. 17 . In step S 311 , the camera control unit  207  calculates in-focus degrees of MF segment frames. This processing will be described later with reference to  FIG. 18 . In step S 312 , the camera control unit  207  executes processing for combining MF segment frames. This processing will be described later with reference to  FIG. 19 . In step S 313 , the camera control unit  207  stores position information of the frame that has been focused on by manipulating MF, as well as position information of the focus lens at that point, to a non-illustrated storage apparatus. At this time, information of the in-focus frame is stored in a manner identifiable by coordinates and a frame number. This information is used in executing step S 304  again. 
     &lt;Initialization Processing (Step S 301 )&gt; 
     A description is now given of a sequence of operations of the initialization processing in step S 301  with reference to  FIG. 11 . The initialization processing initializes, for example, predetermined variables, flags related to a driving state and a driving method of the focus lens, and the like when the image capturing processing has started or the image capturing mode has been changed. 
     In step S 1101 , the camera control unit  207  sets various types of default values of the camera. Upon receiving an instruction for starting the image capturing processing or changing the image capturing mode via the camera manipulation unit  208 , various types of default values are set based on such information as user settings and the image capturing mode at that point. 
     In step S 1102 , the camera control unit  207  sets a focusing stop flag to OFF. The focusing stop flag is set to be ON when the focus lens is currently in a driven state, and OFF when the focus lens is currently in a stopped state. 
     In step S 1103 , the camera control unit  207  sets a search driving flag to OFF, and then ends the initialization processing. In search driving, a subject is searched for by, for example, driving the lens in a certain direction without using a defocus amount. The search driving flag is set to OFF and ON when a defocus amount detected with the imaging surface phase-difference detection method is reliable and unreliable, respectively, during lens driving. The defocus amount is reliable when the precision of the defocus amount is credible, or when a defocus direction is credible (that is to say, reliability is higher than a certain degree). For example, the defocus amount is reliable when the vicinity of a focus point with respect to a main subject is focused on, or when the main subject is already focused on. In this case, the defocus amount is relied upon, and driving is performed accordingly. The defocus amount is not reliable when the defocus amount and the defocus direction are not credible (that is to say, reliability is lower than the certain degree). For example, the defocus amount is not reliable when the subject is significantly out of focus and the defocus amount cannot be calculated accurately. In this case, the defocus amount is not relied upon, and search driving is performed accordingly. 
     &lt;AF Frame Selection Processing (Step S 304 )&gt; 
     A description is now given of a sequence of operations of the AF frame selection processing in step S 304  with reference to  FIG. 15 .  FIG. 15  is a flowchart showing the AF frame selection processing. In the AF frame selection processing, upon switching from MF to AF, whether to select a frame that was determined to be in focus during MF (in-focus frame) as an AF area is determined. Specifically, if predetermined conditions are satisfied, e.g., if the subject has moved significantly, the in-focus frame is cancelled and a normal frame covering a larger area is set as the AF area. Here, the camera control unit  207  functions as a setting unit. 
     In step S 1501 , the camera control unit  207  determines whether in-focus frame information and focus lens position information were stored in step S 313 ; processing proceeds to step S 1502  if they were stored, and processing proceeds to step S 1509  if they were not stored. 
     In step S 1502 , the camera control unit  207  determines whether the present processing is being executed for the first time or the second time onward after switching from MF to AF. Processing proceeds to step S 1503  if it is determined that the present processing is being executed for the first time, and processing proceeds to step S 1506  if it is determined that the present processing is being executed for the second time onward. 
     In step S 1503 , the camera control unit  207  selects a normal frame including the in-focus frame based on the in-focus frame information stored in step S 313 , and then processing proceeds to step S 1504 . Hereinafter, in the present embodiment, the “normal frame including the in-focus frame” is referred to as a “selection normal frame.” 
     In step S 1504 , the camera control unit  207  performs focus detection in the in-focus frame and the selection normal frame, and calculates defocus amounts. 
     In step S 1505 , the camera control unit  207  stores the defocus amount of the selection normal frame calculated in step S 1504  to a non-illustrated storage apparatus. In step S 1506 , the defocus amounts of the in-focus frame and the selection normal frame are calculated, similarly to processing in step S 1504 . 
     Note that the defocus amount of the selection normal frame cannot be stored in step S 1505  if the defocus amount of the selection normal frame is not output or the output defocus amount has low reliability. In this case, it is permissible to store the defocus amount calculated in step S 1506  in the present processing performed for the second time onward after switching to AF. 
     In step S 1507 , based on the defocus amounts of the in-focus frame and the selection normal frame calculated in step S 1506  and on an image capturing condition, the camera control unit  207  determines whether cancellation conditions for cancelling the in-focus frame are satisfied. 
     The following three conditions are specific examples of the cancellation conditions. Regarding the first cancellation condition, the camera control unit  207  calculates the difference between the focus lens position stored in step S 313  and the current focus lens position (that is to say, the focus lens positions before and after switching from MF to AF), and determines whether the difference is larger than or equal to a predetermined value. Here, the “current focus lens position” may be replaced by a “lens position based on a combination of the current focus lens position and the amount of lens movement corresponding to the calculated defocus amount.” That is to say, a determination is made about whether the difference between the focus lens position stored before switching to AF and the focus lens position after switching to AF is larger than or equal to the predetermined value. The in-focus frame is cancelled if the difference is larger than or equal to the predetermined value, and the in-focus frame is not cancelled if the difference is smaller than the predetermined value. This is because, while a subject area intended by the user is brought into focus by focusing on a segment frame obtained while manipulating MF, a significant movement of the intended subject may cause the intended subject to deviate from the segment frame, which gives rise to the possibility that AF cannot be performed appropriately. In view of this, if the difference between the current lens position based on the result of AF with respect to the in-focus frame and the lens position while manipulating MF is larger than or equal to a threshold, the camera control unit  207  determines that the subject has moved significantly, and cancels the in-focus frame. The threshold is set to be approximately ±2-3 Fδ so as to track slight shaking of the subject to be captured, and cancel the in-focus frame in the event of a significant movement. The threshold can be changed by a system as appropriate. The threshold may be changed in accordance with the number of combined segment frames, or may be changed in accordance with a camera setting. 
     Regarding the second cancellation condition, the difference between the defocus amount of the selection normal frame stored in step S 1505  and the current defocus amount of the selection normal frame is calculated, and whether the difference is larger than or equal to a predetermined value is determined. The in-focus frame is cancelled if the difference is larger than or equal to the predetermined value, and the in-focus frame is not cancelled if the difference is smaller than or equal to the predetermined value. This is because, when the defocus amount of the selection normal frame has significantly changed since immediately after switching from MF to AF, there is a possibility that the subject in the in-focus frame within the selection normal frame has also significantly moved. When the defocus amount of the in-focus frame does not satisfy the first cancellation condition despite a significant change in the defocus amount of the selection normal frame, there is a possibility of error in detection of the defocus amount of the in-focus frame. However, in a scene where the subject is small and the background changes significantly, there is a possibility that “the first cancellation condition is not satisfied but the second cancellation condition is satisfied,” and hence the second cancellation condition may be used to change the threshold for the first cancellation condition. A threshold for the second cancellation condition is set to have a larger value than the threshold for the first cancellation condition. 
     Regarding the third cancellation condition, a determination is made about whether the image capturing condition has changed to the extent that panning, zoom manipulation, and brightness change. For example, it is determined that the image capturing condition has changed when panning has been detected based on the amount of camera movement and the like, when the zoom lens has moved (the focal length has changed), when brightness has changed by a predetermined value or more, etc. The in-focus frame is cancelled if the image capturing condition has changed, and the in-focus frame is not cancelled if the image capturing condition has not changed. 
     By making a determination about the aforementioned cancellation conditions, the camera control unit  207  can maintain a subject intended by the user or a part of the subject in focus, and enables appropriate AF operations when the subject and the image capturing condition have changed. 
     In step S 1508 , the camera control unit  207  cancels the in-focus frame. Specifically, it deletes information of the in-focus frame (the coordinates and the frame number). 
     In step S 1509 , upon determining that the information of the in-focus frame has been deleted, the camera control unit  207  selects the normal frame as the AF frame, and then end the processing sequence for selecting the AF frame. 
     In step S 1510 , the information of the in-focus frame is checked and the in-focus frame is selected as the AF frame, and then the processing sequence for selecting the AF frame is ended. 
     A specific example of the aforementioned AF frame selection processing will now be described with reference to  FIGS. 21A to 21C . First, a description is given of an exemplary case where there is one AF frame at the center with reference to  FIGS. 21A and 21C .  FIG. 21A  shows an angle of view  2101  and an AF frame  2102  (one AF frame at the center). The AF frame  2102  may be displayed or may not be displayed on a display apparatus, such as the display unit  205 . 
     Upon switching to an MF mode or when MF is manipulated in the state of  FIG. 21A , focus detection frames shown in  FIG. 21C , which are obtained through segmentalization in the later-described processing for setting MF segment frames (step S 309 ), are set. Thereafter, if MF is manipulated in the state of  FIG. 21C , a frame in an in-focused state (i.e., an in-focus frame) is detected from among the focus detection frames obtained through segmentalization in processing from step S 309  to step S 313  shown in  FIG. 3 , and information of the detected frame is stored to the storage apparatus. 
     Upon switching from MF to AF in this state, a selection normal frame is selected in step S 1503 . In  FIG. 21A , the selection normal frame corresponds to  2102 . From then on, processing from step S 1506  to step S 1510  is executed with  2103  set as the in-focus frame, and the position of the frame  2102  set as the selection normal frame. 
     Next, a description is given of an exemplary case where there are three-by-three, i.e., nine areas serving as AF frames during AF with reference to  FIGS. 21B and 21C .  FIG. 21B  shows nine AF frames  21020  to  21028  during AF. Upon switching from MF in the state of  FIG. 21C  to AF, a selection normal frame is selected in step S 1503 . In the example of  FIG. 21B , the selection normal frame corresponds to  21024 . From then on, processing from step S 1506  to step S 1510  is executed with  2103  set as the in-focus frame, and  21024  set as the selection normal frame. 
     When the in-focus frame overlaps two or more frames included among the three-by-three frames, AF frames including the in-focus frame are set as the selection normal frame. When the selection normal frame is set using a method other than the above-described method, a predetermined area centered at the in-focus frame may be set as the selection normal frame instead of selecting one of the AF frames at fixed positions during AF. 
     &lt;Normal Frame Setting Processing (Step S 306 )&gt; 
     The normal frame setting processing in step S 306  will now be described with reference to  FIGS. 12A to 12H, 16A, and 16B . 
       FIGS. 12A to 12H  show examples of an area from which image signals handled in focus detection processing are obtained (focus detection range). During AF, a focus detection range is equivalent to the range of an AF frame. Focus detection is performed based on image signals output from an area corresponding to the focus detection range within an area of the image sensor  201 .  FIG. 12A  shows a focus detection range  1202  in an image signal  1201 . An area  1204  necessary for carrying out correlation computation is obtained by adding the focus detection range  1202  and shift areas  1203  necessary for correlation computation. In  FIG. 12A , p, q, s, and t denote coordinates in the x-axis direction. Here, the area  1204  extends from p to q, and the focus detection range  1202  extends from s to t. 
       FIG. 12B  shows focus detection areas  1205  to  1209  obtained by dividing the focus detection range  1202  into five areas. In the present embodiment, out-of-focus amounts are calculated in one-to-one correspondence with the focus detection areas to perform focus detection by way of example. The present embodiment selects a computation result in the most reliable area among the plurality of focus detection areas obtained through the division, and uses the out-of-focus amount calculated in the most reliable area for AF and an in-focus determination. Note that the focus detection range is not limited to being divided into the aforementioned number of areas. 
       FIG. 12C  shows a tentative focus detection area  1210  obtained by merging the focus detection areas  1205  to  1209  shown in  FIG. 12B . In an example of the embodiment, an out-of-focus amount calculated from such an area obtained by merging the focus detection areas may be used for AF. 
     When a restriction is placed on focus detection areas, or when a plurality of focus detection areas cannot be arranged on a screen due to a restriction on a time period of focus detection processing and the like, it is permissible to use a method whereby one focus detection area is composed of a plurality of areas having different lengths as shown in  FIG. 12D , for example.  FIG. 12D  shows an exemplary arrangement of focus detection areas, specifically, seven focus detection areas  1211  to  1217 . In this figure, the following areas are arranged at the center of an image capturing screen: two areas ( 1211 ,  1217 ), a ratio of which to the image capturing screen in the horizontal direction is 25%, and five areas ( 1212  to  1216 ); a ratio of which to the image capturing screen in the horizontal direction is 12.5%. In this way, a plurality of focus detection areas having different sizes are arranged in such a manner that the number of areas having a ratio of 12.5% to the image capturing screen is larger than the number of areas having a ratio of 25% to the image capturing screen. Then, one effective defocus amount and one effective defocus direction are calculated by combining the computation results obtained from the seven focus detection areas  1211  to  1217 . An in-focus state is achieved by driving the focus lens  103  using this effective defocus amount or effective defocus direction. 
     As such, in the example of  FIG. 12D , a subject at the center of the image capturing screen can be brought into better focus by arranging many focus detection areas having a small ratio to the image capturing screen. Furthermore, the influence exerted by subjects of different distances on AF is alleviated by reducing the ratio of the focus detection areas to the image capturing screen. Moreover, by arranging not only focus detection areas having a small ratio to the image capturing screen but also focus detection areas having a large ratio to the image capturing screen, unstable focusing caused by deviation of a subject from a focus detection area is alleviated. That is to say, even if the subject has temporarily deviated from a focus detection area, the subject can be maintained in focus as long as the subject is covered by the focus detection areas having a large ratio to the image capturing screen. 
     When a face detection function is effective, an AF frame  1219  can be set at the position of a detected face  1220  as shown in  FIG. 12E . In this case, one or more of the frames shown in  FIGS. 12A to 12D  are set with respect to the detected face frame, and one effective defocus amount and one effective defocus direction are calculated by combining the computation results obtained from the focus detection areas. Then, an in-focus state is achieved by driving the focus lens  103  using this effective defocus amount or effective defocus direction. 
     In the case of a camera with a touch AF function or the like, the position of an AF frame may be freely designated by the user. The AF frame can be set at a designated position  1221  as shown in  FIG. 12F . Note that the arrangement, the size, and the like of the focus detection areas are not limited to those described in the present embodiment as long as they do not depart from the principles of the invention. 
     With reference to  FIG. 16A , a description is now given of processing for setting an AF frame (here, a normal frame) exemplarily shown in  FIGS. 12A to 12F .  FIG. 16A  is a flowchart showing processing for setting a normal frame. 
     In step S 1601 , the camera control unit  207  determines whether the position of an AF frame has been designated; if the position has been designated, processing proceeds to step S 1605 , and if not, processing proceeds to step S 1602 . Note that the AF frame can be designated by, for example, touching a touchscreen of the camera manipulation unit  208 , and manipulating arrow keys of the camera manipulation unit  208 . 
     In step S 1602 , the camera control unit  207  determines whether face detection AF is being performed. If face detection is being performed, processing proceeds to step S 1604 , and if not, processing proceeds to step S 1603 . It will be assumed that face detection is performed using a known detection method in the present embodiment, and the details of a face detection method will be omitted. 
     In step S 1603 , the camera control unit  207  sets the AF frame at the center of a screen, and then ends the processing sequence for setting the normal frame. The camera control unit  207  sets the AF frame with respect to a face area as shown in  FIG. 12E  in step S 1604 , or sets the AF frame at the designated position as shown in  FIG. 12F  in step S 1605 , and then ends the processing sequence for setting the normal frame. 
     &lt;In-Focus Frame Setting Processing (Step S 307 )&gt; 
     With reference to  FIG. 16B , a description is now given of in-focus frame setting processing in step S 307 , that is to say, processing for setting the AF frame at the in-focus frame.  FIG. 16B  is a flowchart showing the in-focus frame setting processing. 
     In step S 1610 , the camera control unit  207  obtains the position of the in-focus frame. This is processing for calling up the information of the in-focus frame stored in step S 313 . The in-focus frame may be designated using any method, e.g., by designating coordinates, by designating a frame number, etc. In step S 1611 , the camera control unit  207  sets the AF frame based on the information of the in-focus frame obtained in step S 1610 . The foregoing processing enables the AF frame to be set at the position of a frame that has been brought into focus by manipulating MF. 
     &lt;Processing for Setting MF Segment Frames (Step S 309 )&gt; 
     In order to provide an ordered description of processing during MF (from step S 309  to step S 313 ), the processing for setting MF segment frames in step S 309  will be described first with reference to  FIGS. 12G and 12H . 
       FIGS. 12G and 12H  show examples of a focus detection range while manipulating MF. In the present embodiment, upon switching from AF to MF, the camera control unit  207  sets segment frames having a higher level of segmentation than a normal AF frame set during AF (that is to say, small focus detection areas). For example, these segment frames may be arranged as segment frames  1222  across the entire screen as shown in  FIG. 12G , or may be arranged as segment frames  1223  obtained by segmentalizing an AF frame  1219  set during AF as shown in  FIG. 12H . 
     &lt;MF Control Processing (Step S 310 )&gt; 
     With reference to  FIG. 17 , a description is now given of the MF control processing in step S 310 , that is to say, a sequence of operations of control processing for the focus lens  103  and the like based on MF manipulated by the user.  FIG. 17  is a flowchart showing the MF control processing. 
     In step S 1701 , the camera control unit  207  determines whether MF has been manipulated. If MF has been manipulated, processing proceeds to step S 1702 , and if not, the MF control processing is ended. 
     In step S 1702 , the camera control unit  207  obtains MF manipulation information via the camera manipulation unit  208  to specify a driving direction and a driving amount of the focus lens. In step S 1703 , the camera control unit  207  converts the obtained MF manipulation information into the driving amount of the focus lens. In step S 1704 , the camera control unit  207  transmits a driving command including the driving amount of the focus lens. The driving command for the focus lens is transmitted to the focus lens driving unit  105 , and the focus lens  103  is driven accordingly. Then, the camera control unit  207  ends the MF control processing sequence. 
     &lt;Processing for Calculating MF In-Focus Degrees (Step S 311 )&gt; 
     With reference to  FIG. 18 , a description is now given of processing for calculating MF in-focus degrees in step S 311 , that is to say, processing for calculating in-focus degrees in one-to-one correspondence with focus detection frames obtained through segmentalization for MF.  FIG. 18  is a flowchart showing the processing for calculating the MF in-focus degrees. 
     In step S 1801 , the camera control unit  207  performs focus detection processing. The details of the focus detection processing will be described later with reference to  FIG. 5 . In step S 1802 , the camera control unit  207  determines an in-focus degree by determining whether a defocus amount calculated in step S 1801  is smaller than or equal to a predetermined value. If the defocus amount is smaller than or equal to a predetermined threshold, processing proceeds to step S 1803 , and if not, processing proceeds to step S 1804 . In step S 1803 , the camera control unit  207  stores information of an in-focus area to, for example, a non-illustrated storage apparatus. As mentioned earlier, the information of the in-focus area may be stored by storing coordinates of a frame whose defocus amount has been determined to be smaller than or equal to the threshold, or by storing a frame number. 
     In step S 1804 , the camera control unit  207  determines whether focus detection and calculation of an in-focus degree have been completed with respect to all areas (that is to say, all segment frames). If the focus detection and calculation have been completed, processing proceeds to step S 1805 , and if not, processing returns to step S 1801  and continues until the focus detection and calculation are completed with respect to all areas. 
     In step S 1805 , the camera control unit  207  stores a focus lens position that has been decided on by a user&#39;s manipulation to a non-illustrated storage apparatus, and then ends the processing sequence for calculating the in-focus degrees. Although the MF focus detection processing (step S 1801 ) is executed by the camera control unit  207  in the description of the present embodiment, it may be executed by the focus detection signal processing unit  204  instead. 
     &lt;Processing for Combining MF Frames&gt; 
     With reference to  FIG. 19 , a description is now given of processing for combining MF segment frames in step S 312  shown in  FIG. 3 .  FIG. 19  is a flowchart showing the processing for combining MF frames. 
     In step S 1901 , the camera control unit  207  determines whether an AF frame was selected by the user during AF. If the AF frame was selected, processing proceeds to step S 1905 , and if not, processing proceeds to step S 1902 . A detailed description of a method of selecting the AF frame will be omitted as a known method can be used thereas; for example, the user selects the AF frame by manipulating the touchscreen and the arrow keys of the camera manipulation unit  208 . 
     In step S 1902 , the camera control unit  207  determines whether face detection AF is being performed. If face detection is being performed, processing proceeds to step S 1904 , and if not, processing proceeds to step S 1903 . In the present embodiment, face detection is performed using a known method, as in step S 1602  described earlier, and hence the details of a face detection method will be omitted. 
     In steps S 1903  to S 1905 , the camera control unit  207  sets in-focus frames as a selection range. In step S 1903 , the entire area of an AF frame is set as the selection range, whereas in step S 1904 , an area in which a face has been detected is set as the selection range. In step S 1905 , the selected AF frame is set as the selection range. 
     In step S 1906 , in-focus frames within the set selection range are extracted. Here, based on the information of the in-focus area stored in step S 1803 , segment frames whose defocus amounts have been determined to be smaller than or equal to the threshold are extracted as the in-focus frames. In step S 1907 , among the extracted in-focus frames, in-focus frames having adjacent coordinates are combined into one frame (also referred to a combined frame), and then the processing sequence for combining MF frames is ended. 
     With reference to  FIGS. 22A to 22F , a description is now given of a specific example of the aforementioned processing sequences during MF (that is to say, from the processing for setting the MF segment frames to the processing for combining MF frames).  FIG. 22A  shows subjects and AF frames during AF. An AF area  2202  is provided with respect to an angle of view  2201 , and this AF frame  2202  is divided into, for example nine AF frames  22020  to  22028 . 
     In the state of  FIG. 22A , if it is determined that there has been a switchover from AF to MF or that MF has been manipulated, the processing for setting the MF segment frames (step S 309 ) is executed, and focus detection frames  2203  are set through segmentalization, as shown in  FIG. 22B . 
     Then, if MF control (step S 310 ) is performed in the state of  FIG. 22B  where the focus detection frames have been set through segmentalization, the processing for calculating the MF in-focus degrees (step S 311 ) is executed, and hence an in-focus degree (defocus amount) of each focus detection frame is calculated. In accordance with the calculation result, in-focus areas are set as in-focus frames. Furthermore, in the processing for combining MF frames (step S 312 ), in-focus frames having adjacent coordinates are reconstructed as one combined frame, e.g., a combined frame  2204  and a combined frame  2205 . The reconstructed combined frames  2204  and  2205  are displayed on the display unit  205  in the display processing (step S 314 ). 
     Meanwhile,  FIG. 22C  shows a state where an AF frame  22024  is selected by the user during AF. If there is a switchover to the MF mode or MF is manipulated in this state of  FIG. 22C , the processing for setting the MF segment frames (step S 309 ) is executed, and hence focus detection frames  2203  are set through segmentalization, as shown in  FIG. 22D . If MF is further manipulated in the state of  FIG. 22D , in-focus areas calculated in the processing for calculating the MF in-focus degrees (step S 311 ) are set as in-focus frames. In this case, in-focus frames calculated within the range of the selected AF frame  22024  are extracted. Furthermore, in-focus frames having adjacent coordinates are combined and reconstructed as a combined frame  2204  in step S 1907 . Thereafter, the combined frame  2204  is displayed on the display unit  205  in the display processing (step S 314 ). 
     Although an AF frame is selected by a user&#39;s manipulation in the above-described example, a face detection function may be used as will be described below with reference to  FIGS. 22E and 22F . 
     In the example of  FIG. 22E , face areas of subjects  2210  and  2211  are detected from an image. In  FIG. 22E , among the faces of the subjects, the face of the subject  2210  is differentiated as a main face, and a face frame  2212  is set in accordance with the area of the detected main face. During AF, the range of this face frame serves as an AF frame. One example of methods of selecting a main face is to allocate the highest priority level to a face that has been selected by an instruction issued by a photographer, and allocate subsequent priority levels in such a manner that a higher priority level is allocated to a face that is located closer to the center of the screen has a larger size. Note that the priority levels may be allocated to faces using other methods. 
     If there is a switchover to the MF mode or MF is manipulated in the state of  FIG. 22E  where the AF frame has been set at the face frame  2212 , the processing for setting the MF frames (step S 309 ) is executed, and hence focus detection frames  2213  are set within the range of the set face frame  2212  through segmentalization, as shown in  FIG. 22F . Furthermore, if MF is manipulated in the state of  FIG. 22F , in-focus areas calculated in the processing for calculating the MF in-focus degrees (step S 311 ) are set as in-focus frames. In-focus frames having adjacent coordinates are combined and reconstructed as a combined frame  2214  in step S 1907 . Thereafter, the combined frame  2214  is displayed on the display unit  205  in the display processing (step S 314 ). 
     When face detection has low reliability, e.g., when a target face cannot be detected, the entire focus detection areas may be set as a selection range for an in-focus frame. 
     &lt;Image Capturing Processing (Step S 315 )&gt; 
     A description is now given of the image capturing processing in step S 315  with reference to  FIG. 20 .  FIG. 20  is a flowchart showing the image capturing processing (here, moving image capturing processing). In step S 2001 , the camera control unit  207  determines whether a moving image recording switch is ON; if the moving image recording switch is ON, processing proceeds to step S 2002 , and if the moving image recording switch is not ON, processing proceeds to step S 2305 . 
     In step S 2002 , the camera control unit  207  determines whether moving images are currently recorded. If moving images are not currently recorded, processing proceeds to step S 2003 , recording of moving images is started, and the sequence of operations of the image capturing processing is ended. On the other hand, if moving images are currently recorded, processing proceeds to step S 2004 , recording of moving images is stopped, and the image capturing processing sequence is ended. Although recording of moving images is started and stopped by pressing down the moving image recording switch in the present embodiment, recording may be started and stopped using other methods, e.g., using a changeover switch. Furthermore, although  FIG. 20  only illustrates the case of recording of moving images, still images may be captured and recorded upon issuance of an instruction for capturing still images during live-view image capture. 
     &lt;AF Control Processing (Step S 308 )&gt; 
     The AF control processing in step S 308  will now be described with reference to  FIG. 4 .  FIG. 4  is a flowchart showing the AF control processing. 
     In step S 401 , the camera control unit  207  causes the focus detection signal processing unit  204  to execute focus detection processing, and obtains defocus information and reliability information for performing focus detection according to the imaging surface phase-difference method. This processing is similar to processing in step S 1801  shown in  FIG. 18 . The details of the focus detection processing will be described later with reference to  FIG. 5 . 
     In step S 402 , the camera control unit  207  determines whether focusing is currently stopped (that is to say, a main subject is in focus and the focus lens is in a stopped state); if focusing is not currently stopped, processing proceeds to step S 403 , if focusing is currently stopped, processing proceeds to step S 404 . More specifically, the camera control unit  207  determines whether focusing is currently stopped based on the ON/OFF state of the flag (focusing stop flag) for stopping driving of the focus lens while the main subject is in focus. Note that ON/OFF of the focusing stop flag is set in, for example, step S 606  described later. 
     AF processing is executed in step S 403 , and then the AF control processing is ended. The AF processing is executed based on the defocus information and the reliability information obtained in step S 401 . The details will be described later with reference to  FIG. 7 . 
     An AF reactivation determination is made in step S 404 , and then the AF control processing is ended. 
     In step S 404 , a determination is made about whether to re-start AF control due to a change in a subject since focusing was stopped. The details will be described later with reference to  FIG. 6 . 
     &lt;Focus Detection Processing (Step S 401 )&gt; 
     The focus detection processing in step S 401  will now be described with reference to  FIG. 5 .  FIG. 5  is a flowchart showing the focus detection processing. First, in step S 501 , the focus detection signal processing unit  204  obtains a pair of image signals from a set focus detection range (AF frame, focus detection frame). Next, in step S 502 , the focus detection signal processing unit  204  calculates a correlation amount from the pair of image signal obtained in step S 501 . 
     In step S 503 , the focus detection signal processing unit  204  calculates a correlation change amount based on the correlation amount calculated in step S 502 . In step S 504 , an out-of-focus amount is calculated based on the correlation change amount calculated in step S 503 . 
     In step S 505 , the focus detection signal processing unit  204  calculates reliability indicating how reliable the out-of-focus amount calculated in step S 504  is. Note that the focus detection signal processing unit  204  executes this processing for calculating reliability for every individual focus detection area within the focus detection range. 
     In step S 506 , the focus detection signal processing unit  204  converts the out-of-focus amount into a defocus amount for each individual focus detection area. Finally, in step S 507 , the camera control unit  207  decides on a focus detection area used in the AF processing, and then ends the focus detection processing sequence. Note that processing in step S 507  is executed only during AF, and skipped while manipulating MF. 
     With reference to  FIGS. 13A to 13C and 14A  to  14 D, the following provides a more detailed description of the focus detection processing illustrated in  FIG. 5 . 
       FIGS. 13A to 13C  show examples of image signals obtained from the focus detection range  1202  shown in  FIG. 12B . A horizontal axis indicates, for example, positions of the image signals in the horizontal direction. That it so say, the focus detection range  1202  shown in  FIG. 12B  extends from s to t, and an area obtained by adding the same and the shift areas  1203  necessary for correlation computation extends from p to q. The shift areas are the ranges necessary for focus detection computation that takes shift amounts into consideration. One of the focus detection areas  1205  to  1209  shown in  FIG. 12B  extends from x to y, and an area obtained by adding all of the focus detection areas  1205  to  1209  serves as the focus detection range  1202 . 
       FIG. 13A  shows waveforms of pre-shift image signals. A solid line  1301  denotes an image signal A, and a dash line  1302  denotes an image signal B.  FIG. 13B  shows examples of image waveforms obtained by shifting each image waveform shown in  FIG. 13A  in a positive direction, whereas  FIG. 13C  shows examples of image waveforms obtained by shifting each image waveform shown in  FIG. 13A  in a negative direction. That is to say, to calculate correlation amounts, the AF signal processing unit  204  shifts each of the image signal A  1301  and the image signal B  1302 , bit by bit, in the direction of a corresponding arrow. 
     A description is now given of processing for calculating correlation amounts COR. The focus detection signal processing unit  204  shifts the obtained image signal A and image signal B bit by bit as illustrated in  FIGS. 13B and 13C , and calculates a sum of absolute values of differences between the shifted image signal A and image signal B. A shift amount is expressed as i, the smallest shift amount is p−s shown in  FIGS. 13A to 13C , and the largest shift amount is q−t shown in  FIGS. 13A to 13C . Also, x represents the coordinates at which a focus detection area starts, and y represents the coordinates at which the focus detection area ends. With the foregoing elements, the correlation amounts COR can be calculated using the following Expression 1. 
     
       
         
           
             
               
                 
                   
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     The obtained correlation amounts COR have a waveform shown in  FIG. 14A , for example. In the graph, a horizontal axis indicates the shift amounts, whereas a vertical axis indicates the correlation amounts. The correlation amounts COR have a correlation amount waveform  1401 , in which  1402  and  1403  indicate the vicinities of extrema of correlation values. In the exemplary case of the correlation amount waveform  1401 , the smaller a correlation amount, the higher a degree of match between an A image and a B image. 
     Next, based on the obtained correlation amounts COR, correlation change amounts ΔCOR are calculated. More specifically, the focus detection signal processing unit  204  calculates a correlation change amount from a difference between correlation amounts that have an interval of one shift among the obtained correlation amounts (i.e., the correlation amounts shown in  FIG. 14A ). Provided that a shift amount is expressed as i, the smallest shift amount is p−s shown in  FIGS. 13A to 13C , and the largest shift amount is q−t shown in  FIGS. 13A to 13C , the correlation change amounts ΔCOR can be calculated using the following Expression 2.
 
   COR[i]=COR[i− 1]− COR[i+ 1]{( p−s+ 1)&lt; i &lt;( q−t− 1)}  Expression 2
 
     The correlation change amounts ΔCOR thus obtained have a waveform shown in  FIG. 14B , for example. Here, a horizontal axis indicates the shift amounts, whereas a vertical axis indicates the correlation change amounts. The correlation change amounts ΔCOR have a correlation change amount waveform  1404 , and the sign of the correlation change amounts changes from positive to negative in the vicinities of shift amounts  1405  and  1406 . A correlation change amount of zero at  1405  (this state is referred to as a zero-crossing) gives the A image and the B image the highest degree of match, and thus a shift amount corresponding thereto can be used as an out-of-focus amount. 
     Processing for calculating this out-of-focus amount (PRD) will now be described in more detail.  FIG. 14C  is an enlarged view of the vicinity of  1405  shown in  FIG. 14B . A portion of the correlation change amount waveform  1404  is enlarged and presented as  1407 , and a shift amount at a zero-crossing is indicated by a point C. First of all, the out-of-focus amount (i.e., the position of the point C) is divided into an integer part β and a decimal part α. The decimal part α can be calculated based on a similarity relationship between a triangle ABC and a triangle ADE shown in  FIG. 14C , using the following Expression 3. 
     
       
         
           
             
               
                 
                   
                       
                   
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     On the other hand, as shown in  FIG. 14C , the integer part β can be calculated using the following Expression 4.
 
β= k− 1  Expression 4
 
     As described above, the focus detection signal processing unit  204  can calculate the out-of-focus amount PRD from a sum of α and β. 
     When there are multiple zero-crossings as shown in  FIG. 14B , a shift amount corresponding to a zero-crossing exhibiting a change in correlation amounts with greater steepness, max der (hereinafter referred to as steepness), can be used as a superior zero-crossing. The steepness can be used as an index showing how easily focus detection is performed, and the steepness having a larger value indicates that focus detection is more easily performed. The steepness can be calculated using the following Expression 5.
 
max der=|   COR[k− 1]|+|/   COR[k]|   Expression 5
 
     As described above, when there are multiple zero-crossings, a first zero-crossing (that is to say, a superior zero-crossing) can be decided on by calculating the steepness. 
     A description is now given of the processing for calculating the reliability of an out-of-focus amount. The reliability of an out-of-focus amount can be defined using, for example, the aforementioned steepness and fnclvl, which is a degree of match between two images, that is to say, the image signal A and the image signal B (hereinafter referred to as an image match degree). Therefore, the reliability of an out-of-focus amount can be rephrased as the reliability of image signals. An image match degree can be used as an index showing the precision of an out-of-focus amount; the smaller the value of an image match degree, the higher the precision. 
     An image match degree can be calculated using a change in correlation amounts (that is to say, a correlation change amount ΔCOR) per unit shift amount, and such a correlation change amount is shown in, for example,  FIG. 14D  which is an enlarged view of  1402  shown in  FIG. 14A . Note that  1408  is a part of the correlation amount waveform  1401 . The focus detection signal processing unit  204  can calculate an image match degree using the following Expression 6.
 
( i ) when |   COR[k− 1]|×2≦max der  fnclvl=COR[k− 1]+   COR[k− 1]/4
 
( ii ) when |   COR[k− 1]×2&gt;max der  fnlcl=COR[k]−     COR[k]/ 4  Expression 6
 
&lt;AF Reactivation Determination Processing (Step S 404 )&gt;
 
     A description is now given of a sequence of operations of the AF reactivation determination processing in step S 404  with reference to a flowchart of  FIG. 6 .  FIG. 6  is a flowchart showing the AF reactivation determination processing. The AF reactivation determination processing is processing for determining whether to drive the focus lens again in an in-focus state with the focus lens being stopped. When a defocus amount is larger than a predetermined value, or when reliability is lower than a predetermined value, there is a possibility that a main subject being captured has changed. In view of this, a counter for controlling reactivation of AF (AF reactivation counter) is provided, and if there is a possibility that the main subject has changed, reactivation of AF is prepared by incrementing a value of the AF reactivation counter in the AF reactivation determination processing. On the other hand, when a detected defocus amount is smaller than the predetermined value and high reliability is maintained, the AF reactivation counter is reset so as to maintain the focus lens in the stopped state. Below is a specific description of the steps in the flowchart of  FIG. 6 . 
     In step S 601 , the camera control unit  207  determines whether the defocus amount calculated by the focus detection signal processing unit  204  is smaller than a threshold (e.g., a value that is a predetermined number of times larger than a depth). The camera control unit  207  proceeds to step S 602  if the defocus amount is smaller than the threshold, and proceeds to step S 604  if the defocus amount is larger than or equal to the threshold. 
     In step S 602 , the camera control unit  207  determines whether the reliability calculated in step S 505  is higher than or equal to a threshold. The camera control unit  207  proceeds to step S 603  if the calculated reliability is higher than or equal to the threshold, and proceeds to step S 604  if the calculated reliability is lower than the threshold. 
     The camera control unit  207  resets the AF reactivation counter in step S 603 , or increments the value of the AF reactivation counter in step S 604 , and then processing proceeds to step S 605 . 
     In step S 605 , the camera control unit  207  determines whether the value of the AF reactivation counter is larger than or equal to a threshold for AF reactivation. If the value of the AF reactivation counter is larger than or equal to the threshold for AF reactivation, processing proceeds to step S 606 , and if the value of the AF reactivation counter is smaller than the threshold for AF reactivation, the sequence of processing for determining AF reactivation is ended. In step S 606 , the camera control unit  207  sets the focusing stop flag to OFF. In this way, AF is reactivated, and driving of the focus lens can be started again. Thereafter, the camera control unit  207  ends the sequence of operations. 
     Note that the threshold for the defocus amount set in step S 601  (the value that is a predetermined number of times larger than the depth) can be adjusted as appropriate so as to enable easy reactivation when the main subject has changed, and make inadvertent reactivation less likely to occur when the main subject has not changed. For example, the threshold can be set to be equivalent to a depth with which the out-of-focus state of the main subject is visible. The threshold for the reliability may be set in step S 602  in such a manner that, for example, the harder it is to rely on the defocus direction, the smaller the set value of the reliability is. In this way, the main subject can be assumed to have changed with the use of the threshold for the reliability. As described above, the thresholds set in steps S 601  and S 602  can be adjusted as appropriate depending on how the change in the main subject is determined. 
     &lt;AF Processing (Step S 403 )&gt; 
     The following describes a sequence of operations of the AF processing in step S 403  with reference to a flowchart of  FIG. 7 . The AF processing is processing for making a determination about driving of the focus lens and cessation of focusing in a state where focusing is not stopped. 
     In step S 701 , the camera control unit  207  determines whether the following conditions are satisfied: the defocus amount is smaller than or equal to the depth, and the reliability calculated in step S 505  shown in  FIG. 5  is higher than or equal to a threshold. If these conditions are satisfied, processing proceeds to step S 702 , and if not, processing proceeds to step S 703 . Although the threshold used in step S 701  is, for example, equivalent to the depth in the description of the present embodiment, the threshold can be increased or reduced as necessary. 
     In step S 702 , the camera control unit  207  sets the focusing stop flag to ON, and then ends the AF processing sequence. As described above, when a subject is determined to be in focus, a transition is made from the state where the focus lens is driven to the state where the focus lens is stopped, and then the reactivation determination is made in step S 409  shown in  FIG. 4 , i.e., whether to drive the focus lens again is determined. 
     On the other hand, in step S 703 , the camera control unit  207  sets driving of the focus lens. Here, a driving speed and a driving method of the focus lens are decided on, as will be described later in detail with reference to  FIG. 8 , and in step S 704 , focus lens driving processing is executed in accordance with the settings that were decided on in step S 703 . Upon completion of the focus lens driving processing, the camera control unit  207  ends the AF processing sequence. The details of the focus lens driving processing in step S 704  will be described later with reference to  FIG. 9 . 
     &lt;Processing for Setting Driving of Focus Lens (Step S 703 )&gt; 
     With reference to a flowchart of  FIG. 8 , a description is now given of a sequence of operations of the processing for setting driving of the focus lens in step S 703 . In the processing for setting driving of the focus lens, a determination is made about a transition to the above-described search driving in accordance with the reliability of an out-of-focus amount, and a driving speed at which the focus lens is driven is set in accordance with the reliability of the out-of-focus amount. 
     In step S 801 , the camera control unit  207  determines whether the reliability is higher than or equal to a predetermined threshold α; if the reliability is higher than or equal to the predetermined threshold α, processing proceeds to step S 802 , and if the reliability is lower than the predetermined threshold, processing proceeds to step S 804 . In step S 802 , the camera control unit  207  resets a search driving counter to, for example, 0, and then proceeds to step S 803 . 
     In step S 803 , the camera control unit  207  sets a predetermined speed A as the driving speed of the focus lens, and then proceeds to step S 808 . 
     Next, in step S 804 , the camera control unit  207  increments a counter value of a search driving transition counter for determining whether a low-reliability state has continued. For example, the counter value is incremented by one, and then processing proceeds to step S 805 . In step S 805 , whether the counter value of the search driving transition counter is larger than or equal to a predetermined value is determined; if the counter value is larger than or equal to the predetermined value, processing proceeds to step S 806 , and if the counter value is not larger than or equal to the predetermined value, processing proceeds to step S 807 . 
     In step S 806 , as it is determined that the low-reliability state has continued, the camera control unit  207  sets the search driving flag to ON to perform search driving, and then proceeds to step S 808 . On the other hand, in step S 807 , as it is determined that the counter value of the search driving transition counter is not larger than or equal to the predetermined value, i.e., the low-reliability state has not continued, the camera control unit  207  sets a speed Z as the driving speed of the focus lens. Thereafter, processing proceeds to step S 808 . 
     In step S 808 , the camera control unit  207  determines whether the search driving flag is set to ON after processing in steps S 803 , S 806 , and S 807 ; if the search driving flag is set to ON, processing proceeds to step S 809 , and if the search driving flag is not set to ON, the processing for setting driving of the focus lens is ended. In step S 809 , a driving speed S for search driving is set, and the processing sequence for setting driving of the focus lens is ended. 
     Note that the threshold α for reliability, which is used in step S 801 , is set to have a value that makes at least the defocus direction reliable. When the defocus direction is reliable, the focus lens is driven at the set driving speed A based on the defocus amount. 
     In the present processing for setting driving of the focus lens, the search driving flag is set so as to perform search driving when an unreliable state of the direction of the defocus amount has continued. In the present embodiment, search driving is performed using a driving method in which the defocus direction is set independently of the defocus amount, and the focus lens is driven at a set speed in the defocus direction. For example, if the direction of the defocus amount is not reliable in step S 801 , the counter value of the search driving transition counter is incremented in step S 804 . Therefore, by determining whether the counter value of the search driving transition counter has reached or exceeded the predetermined value in step S 805 , it is possible to determine whether there is a possibility that the subject is out of focus due to a continued low-reliability state, and search driving is performed only if there is such a possibility. As a search driving method does not use the defocus amount, low-quality focusing may be performed, which temporarily causes a significantly out-of-focus state. In view of this, in the present embodiment, whether or not there is continuity is determined so as to prevent an immediate transition to search driving after a decline in the reliability. This can prevent inadvertent execution of search driving attributed to hypersensitive reaction to the influences of noise and the like. Furthermore, if the reliability reaches or exceeds the predetermined threshold α in the course of incrementing the counter value of the search driving transition counter for determining whether to make a transition to search driving (e.g., in steps S 804  and S 805 ), the search driving transition counter is reset in step S 802 . 
     Note that the driving speed S for search driving is set to be higher than the driving speed A in step S 809 , whereas the driving speed Z is set to be, for example, extremely low or zero in step S 807 . That is to say, the driving speeds of the focus lens set in  FIG. 8  satisfy the following relationship: the driving speed Z&lt;the driving speed A&lt;the driving speed S. During search driving, the subject is expected to be significantly out of focus, and it is necessary to quickly bring the subject into focus; this is why the driving speed S is set to be higher than the driving speed A in step S 809 . Furthermore, when a determination is made about a transition to search driving based on the search driving transition counter in step S 805  as the reliability of the out-of-focus amount is lower than the threshold α, the precision of defocus detection is low, and therefore inadvertent lens driving may trigger low-quality focusing. For this reason, the driving speed Z is set to be extremely low or zero in step S 807  so as to prevent a low-quality focusing operation in a low-reliability state. 
     &lt;Focus Lens Driving Processing (Step S 704 )&gt; 
     With reference to a flowchart of  FIG. 9 , a description is now given of a sequence of operations of the focus lens driving processing in step S 704 . In step S 901 , the camera control unit  207  determines the state of the search driving flag set in the processing for setting driving of the focus lens (ON or OFF). If the search driving flag is set to OFF, search driving need not be performed, and thus processing proceeds to step S 902 ; if the search driving flag is set to ON, processing proceeds to step S 903  to perform search driving. 
     In step S 902 , the camera control unit  207  performs distance driving based on the defocus amount that has been calculated according to the phase-difference detection method using image signals. Upon completion of distance driving, the camera control unit  207  ends the lens driving processing. In this processing for distance driving, the focus lens is driven by an amount equivalent to the calculated defocus amount. For example, the defocus amount calculated by the camera control unit  207  is converted into a driving amount of the focus lens  103 , and a driving command based on the converted driving amount is issued to the focus lens driving unit  105 . On the other hand, in step S 903 , later-described search driving is performed based on the defocus direction. Once the camera control unit  207  has performed search driving, it ends the processing sequence for driving the focus lens. 
     &lt;Search Driving Processing (Step S 903 )&gt; 
     With reference to a flowchart of  FIG. 10 , a description is now given of a sequence of operations of the search driving processing in step S 903 . In the search driving processing, a driving direction is set if search driving is performed for the first time, and the focus lens is driven until a later-described condition for ending search driving is satisfied. 
     In step S 1001 , the camera control unit  207  determines whether search driving is performed for the first time. If search driving is performed for the first time, processing proceeds to step S 1002 , and if search driving is not performed for the first time, processing proceeds to step S 1005 . 
     Below-described steps S 1002  to S 1004  represent processing for setting a driving direction as search driving is performed for the first time. In step S 1002 , the camera control unit  207  determines whether the current lens position is close to a near end. If the current lens position is close to the near end, processing proceeds to step S 1003 , and if the current lens position is close to a far end, processing proceeds to step S 1004 . In step S 1003 , the camera control unit  207  sets a near direction as the driving direction of the focus lens at the start of search driving. On the other hand, in step S 1004 , it sets a far direction as the driving direction of the focus lens at the start of search driving. Setting the driving direction in the foregoing manner can reduce a time period of search driving across the entire driving area of the focus lens, and also reduce the maximum time period required to find an in-focus position through search driving. Once the camera control unit  207  has set the driving direction of the focus lens, processing proceeds to step S 1005 . 
     In step S 1005 , the camera control unit  207  starts control to drive the focus lens based on the set driving direction and driving speed. In step S 1006 , the camera control unit  207  determines whether the focus lens has reached the near end or the far end. If the focus lens has reached one of the ends, processing proceeds to step S 1007 , and if the focus lens has not reached one of the ends, processing proceeds to step S 1008 . In step S 1007 , the camera control unit  207  reverses the driving direction of the focus lens. 
     In step S 1008 , the camera control unit  207  determines whether reliability is higher than or equal to a predetermined threshold γ. If reliability is higher than or equal to the predetermined threshold γ, processing proceeds to step S 1010 , and if not, processing proceeds to step S 1009 . In step S 1009 , the camera control unit  207  determines whether the focus lens has reached both of the near end and the far end; if the focus lens has reached both ends, processing proceeds to step S 1010 , and if the focus lens has not reached both ends, the search driving processing sequence is ended. In step S 1010 , the camera control unit  207  sets the search driving flag to OFF to end search driving. Thereafter, the camera control unit  207  ends the search driving processing sequence. 
     In the present embodiment, the condition for ending search driving is that reliability is higher than or equal to the predetermined threshold γ in step S 1008 , or that both of the near end and the far end of the focus lens have been reached in step S 1009 . The threshold γ for reliability, which is set in step S 1008 , indicates that at least the defocus amount calculated based on a phase difference between image signals is reliable, similarly to the threshold α set in step S 801  shown in  FIG. 9 . If reliability is higher than or equal to the threshold γ, it can be determined that the current position is close to the in-focus position, and thus the camera control unit  207  can stop search driving and switch again to control for distance driving based on the defocus amount (step S 902 ). Furthermore, if both of the near end and the far end have been reached in step S 1009 , it could possibly mean that a subject was not able to be specified after driving across the entire focus driving area. In this case, the camera control unit  207  sets the search driving flag to OFF to restore the state before the start of search driving. When a subject is not able to be specified in a manner different from the present embodiment, control may be performed to continue search driving without setting the search driving flag to OFF. 
     As described above, in the present embodiment, upon switching from AF to MF, an AF frame set for AF is segmentalized for MF (that is to say, a plurality of areas are set within an AF focus detection area). Then, in-focus degrees are determined in one-to-one correspondence with focus detection frames of the segment areas, and in-focus display is performed based on an area that is in focus. Such in-focus display enables the user to bring more detailed areas into focus, on an area-by-area basis, during MF. 
     Furthermore, in-focus display is performed with respect to an enlarged area by combining adjacent focus detection frames among the plurality of focus detection frames obtained through segmentalization. This makes it possible to prevent a plurality of adjacent frames from obstructing a subject, and perform in-focus display with increased visibility. That is to say, a specific subject or a specific location of a subject can be brought into precise focus more smoothly. 
     Furthermore, upon switching from MF to AF, an AF frame is set based on a focus detection frame that was in focus while manipulating MF, and tracking is performed according to AF. In this way, a subject intended by the user can be continuously brought into focus. Moreover, when it is determined that a subject being tracked has changed within an AF frame, or that the subject has moved to the outside of the AF frame, the AF frame is restored to a normal size set for AF. This can increase the precision of focus detection through AF, and enables appropriate AF operations. 
     As described above, the present invention enables appropriate focus control in accordance with the user&#39;s intention when there are a manual focus control mode and an automatic focus control mode. 
     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. 
     Examples of a storage medium for providing program codes include a flexible disk, a hard disk, an optical disc, and a magneto-optical disc. Other examples include a CD-ROM, a CD-R, a CD-RW, a DVD-ROM, a DVD-RAM, a DVD-RW, a DVD-R, a magnetic tape, a nonvolatile memory card, and a ROM. The functions of each of the above-described embodiment(s) are realized by enabling execution of the program codes read out by the computer. In an alternative case, an operating system (OS) and the like running on the computer executes a part or all of actual processing based on instructions of the program codes, and the functions of each of the above-described embodiment(s) are realized by such processing. The following alternative case is also possible. First, the program codes read out from the storage medium are written to a memory provided to a function expansion board inserted into the computer or a function expansion unit connected to the computer. Thereafter, based on the instructions of the program codes, a CPU or the like of the function expansion board or the function expansion unit execute a part or all of actual processing. 
     While the present invention has been described with reference to exemplary embodiment(s), it is to be understood that the invention is not limited to the disclosed exemplary embodiment(s). 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. 2015-086209, filed Apr. 20, 2015, which is hereby incorporated by reference herein its entirety.