Patent Publication Number: US-10313602-B2

Title: Image capture apparatus and method for controlling the same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to image capture apparatuses and methods for controlling the same. 
     Description of the Related Art 
     Known autofocus (AF) techniques for automatically adjusting the in-focus distance in an imaging optical system based on signals obtained from an image sensor include contrast detection AF (contrast AF) and an imaging-plane phase-difference detection method (phase-difference AF). 
     The accuracy of these AF techniques (AF accuracy) is affected by the quality of signals obtained from the image sensor. For example, AF accuracy decreases if the exposure level in a focusing area suddenly changes during an AF operation. If the aperture is adjusted while performing the AF operation such that the amount of exposure during the AF operation does not significantly change, the change in the aperture may affect the contrast evaluation value and/or the defocus amount obtained from an image, resulting in a decrease in the AF accuracy. 
     Japanese Patent Laid-Open No. 2013-242589 proposes that, if an operation to change the aperture is performed during an AF operation, the AF operation is redone to suppress a decrease in AF accuracy due to the change in the aperture. 
     However, the method described in Japanese Patent Laid-Open No. 2013-242589 has a problem in that the time required by AF lengthens because the AF operation is redone when an operation to change the aperture is performed during the AF operation. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image capture apparatus and a method for controlling the image capture apparatus capable of suppressing a decrease in AF accuracy without redoing the AF operation even if an exposure condition is changed during the AF operation. 
     According to an aspect of the present invention, there is provided an image capture apparatus comprising: a focus detection unit configured to detect a defocus amount in an imaging optical system based on a signal obtained from an image sensor; an exposure control unit configured to control exposure based on the signal obtained from the image sensor; and a control unit configured to control driving of a focusing lens based on the defocus amount, wherein: the defocus amount detection and the exposure control are concurrently performed, and the control unit differentiates control of driving of the focusing lens depending on whether (i) the defocus amount detected by the focus detection unit meets a first condition or a second condition or (ii) the defocus amount detected in the detecting does not meet any of the first condition and the second condition; the first condition is that it is determined that the defocus amount is based on a signal obtained before the exposure control is completed, and the second condition is that it is determined that the defocus amount is based on a signal affected by driving of an aperture in the exposure control. 
     According to another aspect of the present invention, there is provided an image capture apparatus comprising: a focus detection unit configured to detect a defocus amount in an imaging optical system based on a signal obtained from an image sensor; an exposure control unit configured to control exposure based on the signal obtained from the image sensor; and a control unit configured to control driving of a focusing lens based on the defocus amount, wherein the defocus amount detection and the exposure control are concurrently performed, and if the defocus amount detected by the focus detection unit meets a first condition or a second condition, the first condition being that a difference between a target exposure condition and a current exposure state is greater than or equal to a given value, and the second condition being that it is determined that the defocus amount is based on a signal affected by driving of an aperture in the exposure control, the control unit does not determine that the image capture apparatus is in an in-focus state, during the control of driving of the focusing lens that is based on the defocus amount. 
     According to a further aspect of the present invention, there is provided a method for controlling an image capture apparatus, comprising: detecting a defocus amount in an imaging optical system based on a signal obtained from an image sensor; controlling exposure based on the signal obtained from the image sensor; and controlling driving of a focusing lens based on the defocus amount, wherein: the detecting and the controlling exposure are concurrently performed, and in the controlling driving, the driving of the focusing lens is differentiated depending on whether (i) the defocus amount detected in the detecting meets a first condition or a second condition or (ii) the defocus amount detected in the detecting does not meet any of the first condition and the second condition; the first condition is that it is determined that the defocus amount is based on a signal obtained before the exposure control is completed, and the second condition is that it is determined that the defocus amount is based on a signal affected by driving of an aperture in the exposure control. 
     According to another aspect of the present invention, there is provided a method for controlling an image capture apparatus, comprising: detecting a defocus amount in an imaging optical system based on a signal obtained from an image sensor; controlling exposure based on the signal obtained from the image sensor; and controlling driving of a focusing lens based on the defocus amount, wherein the defocus amount detection and the exposure control are concurrently performed, and if, in the controlling of driving of the focusing lens, the defocus amount detected in the defocus amount detection meets a first condition or a second condition, the first condition being that a difference between a target exposure condition and a current exposure state is within a given value, and the second condition being that it is determined that the defocus amount is based on a signal affected by driving of an aperture in the exposure control, it is not determined that the image capture apparatus is in an in-focus state during the control of driving of the focusing lens that is based on the defocus amount. 
     According to a further aspect of the present invention, there is provided a non-transitory computer-readable medium storing a program for causing a computer provided in an image capture apparatus to function as an image capture apparatus comprising: a focus detection unit configured to detect a defocus amount in an imaging optical system based on a signal obtained from an image sensor; an exposure control unit configured to control exposure based on the signal obtained from the image sensor; and a control unit configured to control driving of a focusing lens based on the defocus amount, wherein: the defocus amount detection and the exposure control are concurrently performed, and the control unit differentiates control of driving of the focusing lens depending on whether (i) the defocus amount detected by the focus detection unit meets a first condition or a second condition or (ii) the defocus amount detected in the detecting does not meet any of the first condition and the second condition; the first condition is that it is determined that the defocus amount is based on a signal obtained before the exposure control is completed, and the second condition is that it is determined that the defocus amount is based on a signal affected by driving of an aperture in the exposure control. 
     According to another aspect of the present invention, there is provided a non-transitory computer-readable medium storing a program for causing a computer provided in an image capture apparatus to function as an image capture apparatus comprising: a focus detection unit configured to detect a defocus amount in an imaging optical system based on a signal obtained from an image sensor; an exposure control unit configured to control exposure based on the signal obtained from the image sensor; and a control unit configured to control driving of a focusing lens based on the defocus amount, wherein the defocus amount detection and the exposure control are concurrently performed, and if the defocus amount detected by the focus detection unit meets a first condition or a second condition, the first condition being that a difference between a target exposure condition and a current exposure state is greater than or equal to a given value, and the second condition being that it is determined that the defocus amount is based on a signal affected by driving of an aperture in the exposure control, the control unit does not determine that the image capture apparatus is in an in-focus state, during the control of driving of the focusing lens that is based on the defocus amount. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an exemplary functional configuration of an interchangeable-lens digital camera, which serves as an example of an image capture apparatus according to an embodiment. 
         FIGS. 2A and 2B  are diagrams showing an exemplary arrangement of color filters and photodiodes on an image sensor according to an embodiment. 
         FIG. 3  is a flowchart relating to an overall operation of the image capture apparatus according to an embodiment. 
         FIGS. 4A and 4B  are flowcharts relating to the details of the AF operation in  FIG. 3 . 
         FIG. 5  is a flowchart relating to the details of focus detection processing in  FIGS. 4A and 4B . 
         FIG. 6  is a diagram schematically showing an example of a focus detection area used in focus detection processing according to an embodiment. 
         FIGS. 7A to 7C  are diagrams showing an example of image signals obtained from the focus detection area shown in  FIG. 6 . 
         FIGS. 8A and 8B  are diagrams showing an exemplary relationship between a shift amount and a correlation amount of the image signals shown in  FIGS. 7A to 7C . 
         FIGS. 9A and 9B  are diagrams showing an exemplary relationship between a shift amount and an amount of correlation change of the image signals shown in  FIGS. 7A to 7C . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. Note that a description will be given below of embodiments of this invention applied to an interchangeable-lens digital camera, which serves as an example of an image capture apparatus. However, this invention is applicable to electronic devices having an automatic focus adjustment function that is based on image signals. Such electronic devices include digital cameras, mobile phones, personal computers (desktop, laptop, tablet etc.), projectors, game consoles, robots, domestic appliances, drive recorders, and the like, but are not limited thereto. 
       FIG. 1  is a block diagram showing an exemplary functional configuration of the digital camera according to the embodiment of this invention. The digital camera is constituted by a main body  200  and a lens unit (interchangeable lens)  100 . The lens unit  100 , which is removable from a mount portion of the main body  200 , is supplied with power from the main body  200  and communicates with the main body  200  through an electrical contact unit  106  that is provided in the mount portion. 
     The lens unit  100  includes an imaging lens  101 , an aperture (shutter)  102 , a focusing lens  103 , a motor  104 , and a lens controller  105 . The imaging lens  101 , the aperture  102 , and the focusing lens  103  form an imaging optical system. The imaging lens  101  may include a magnification lens. The aperture  102  also functions as a shutter. The focusing lens  103  can be moved by the motor  104 , and adjusts the focus of the imaging optical system. The operation of the motor  104  is controlled by the lens controller  105 . Note that the imaging optical system may include a stabilization lens, a filter, or the like. 
     In the main body  200 , an image sensor  201  is a CCD or CMOS image sensor, and has a plurality of pixels that are arranged in a matrix. In this embodiment, the image sensor  201  has a pupil division function, and an AF signal and an imaging signal can be read out therefrom. In the image sensor  201 , an object image, which is formed on an imaging plane by the imaging optical system, is converted into an electrical signal by a photoelectric converter (photodiode) in each pixel, and is then output. 
     An A/D converter  202  includes a correlated double sampling (CDS) circuit, a non-linear amplifier circuit, and an A/D converter circuit. The CDS circuit reduces noise in the electrical signal output by the image sensor  201 . The electrical signal that has been amplified by the non-linear amplifier circuit is converted into a digital signal by the A/D converter circuit. The A/D converter  202  outputs AF signals among signals read out from the image sensor  201  to an AF signal processor  204 , and outputs imaging signals among those read signals to an image processor  203 . 
     The image processor  203  applies various kinds of image processing to the digital signal output by the A/D converter  202 . Image processing that can be performed by the image processor  203  may include white balance adjustment, demosaicing processing, hue correction processing, object recognition processing, object tracking processing, scaling processing, filtering processing, and the like, but is not limited thereto. The image processor  203  outputs digital signals (image data) that have been processed for recording to a codec  205 . Also, the image processor  203  outputs digital signals (image data) that have been processed for display or automatic exposure control (AE) to a system controller  209 . For example, the image data for display may have a lower resolution than the image data for recording. The image data for AE may be the same as the image data for display, or may be image data in a partial area such as a focusing area or an object area. However, these are merely an example, and the invention is not limited thereto. 
     The AF signal processor  204  generates a pair of image signals to be used in phase-difference AF based on the AF signals supplied from the A/D converter  202 , performs correlation calculation while changing relative positions of the image signals, and calculates a phase difference (image shift amount) between the image signals. The AF signal processor  204  also calculates information regarding reliability of an image shift amount (two-image coincidence degree, two-image steepness degree, contrast information, saturation information, damage information etc.). The AF signal processor  204  outputs, to the system controller  209 , the calculated image shift amount as-is, or after converting the image shift amount into the amount and direction of defocus, together with the reliability information. 
     The system controller  209  can change settings for correlation calculation and reliability information calculation in the AF signal processor  204  based on the image shift amount or the defocus amount obtained from the AF signal processor  204 , as well as the reliability information. For example, if it is determined that the defocus amount is greater than a threshold value, the system controller  209  can increase the maximum shift amount in the correlation calculation, and can change the type of band pass filter to be used by the AF signal processor  204  in accordance with the contrast information. 
     The codec  205  encodes image data and audio data and decodes encoded image data or audio data in accordance with a predetermined coding method (e.g. JPEG or MPEG method). 
     An internal memory  206  is a random access memory, for example, and is also mentioned as a DRAM. The internal memory  206  is used as a buffer to be used to temporarily store an image, or a work memory during coding processing and decoding processing performed by the codec  205 . The internal memory  206  is also used as a work memory for the system controller  209 , which will be described later. 
     An image recorder  207  is constituted by a recording medium such as a memory card, and an interface for reading and writing data from/to the recording medium. 
     A timing generator  208  supplies a timing signal to control the operation of the image sensor  201  in accordance with control performed by the system controller  209 . Note that the timing generator  208  also supplies a timing signal to functional blocks other than the image sensor  201  as required. 
     The system controller  209  includes, for example, a CPU (not shown), and a nonvolatile memory for storing a program to be performed by the CPU, and controls overall operation of the digital camera, including a later-described AF operation, for example. 
     A lens communication unit  210  is an interface for communication between the main body  200  (system controller  209 ) and the lens unit  100  (lens controller  105 ). 
     An AE processor  211  performs AE processing based on the image data obtained from the image processor  203 . AE processing is roughly divided into the following processes:
     process 1—generation of an evaluation value;   process 2—determination of an exposure condition;   process 3—change of the exposure time for the next frame; and   process 4—change of the f-number (aperture diameter).   

     The AE evaluation value (photometric value) generated in the process 1 may be an evaluation value related to the luminance of an image, for example, and typically, the evaluation value is generated through processing including integration of pixel values, conversion of an RGB component values into a luminance value, and the like. 
     In the process 2, for example, a program chart is referenced based on the evaluation value generated in the process 1, and a combination of the exposure time, the f-number, and the ISO speed is determined. For example, because a change in the f-number may affect the depth of field, the exposure time can be preferentially adjusted. The ISO speed can be adjusted when the exposure time and the f-number exceed their adjustable ranges. However, these are merely an example, and in this embodiment, there is no particular limitation on how to change the exposure condition in accordance with a change in the evaluation value. 
     The processes 3 and 4 are performed only when necessary, e.g. when an exposure condition that is different from the current setting is determined in the process 2. Note that, in this embodiment, the setting of the exposure time and control of driving of the aperture  102  are performed by the system controller  209 . For this reason, when the process 3 and/or 4 is necessary, the AE processor  211  outputs the exposure time and the f-number to the system controller  209 , and the system controller  209  performs the actual control for the change. 
     Note that the processes 1 to 4 in AE processing can be performed solely by the system controller  209 , or can be performed by a combination of the image processor  203  and the system controller  209 . In this case, the AE processor  211  does not need to be provided independently. 
     An image display memory (VRAM)  212  is a memory for storing display image data that is to be displayed on an image display  213 . The image display  213  displays a captured image and a reproduced image. The image display  213  also superimposes, on the display, an image for assisting in operation, an image indicating camera status, an image indicating a focusing area, or the like on the captured image and the reproduced image, and displays a GUI image of a menu screen or the like. The image display  213  also functions as a monitor for displaying a live view. 
     An operation unit  214  is an input device (key, button, touch panel, dial etc.) with which a user makes an instruction to the digital camera. The operation unit  214  includes a menu switch for displaying a menu screen for configuring various settings, a zoom lever for making an instruction to perform a zoom operation of the imaging lens, a switch for switching the operation mode (shooting mode and reproduction mode), an up-down/left-right direction key, and the like, but is not limited thereto. If the image display  213  is a touch panel display, the image display  213  also functions as part of the operation unit  214 . 
     A shooting mode switch  215  is a switch for selecting one of various shooting modes provided in the digital camera. The shooting modes may include, for example, a macro mode, a sports mode, a firework mode, a portrait mode, or the like, but are not limited thereto. A main switch  216  is a power switch. 
     A SW 1   217  is a switch that turns on when a shutter button is half-pressed, for example. The turning on of the SW 1   217  is an instruction to start a shooting standby operation. Upon the SW 1   217  turning on, the system controller  209  starts the shooting standby operation. The shooting standby operation includes AF processing and AE processing, for example. A SW 2   218  is a switch that turns on when the shutter button is full-pressed, for example. The turning on of the SW 2   218  is an instruction to start a shooting operation for recording. Upon the SW 2   218  turning on, the system controller  209  starts a shooting operation and a recording operation that are based on the result of shooting standby operation. 
     The image sensor  201  according to this embodiment is provided with regularly arranged color filters of multiple colors, and a color filter of one of the multiple colors is arranged on each pixel. Here, it is assumed that color filters of three colors, namely red (R), green (G), and blue (B) are provided in the Bayer arrangement, as shown in  FIG. 2A . Note that green filters are provided both between red filters and between blue filters, and the former and latter green filters will be denoted respectively as Gr and Gb. 
     A microlens array is also provided in the image sensor  201  according to this embodiment, and one microlens corresponds to one pixel. Each pixel is provided with a plurality of photodiodes (photoelectric converters). In this embodiment, each pixel is provided with two photodiodes A and B having the same size.  FIG. 2B  shows an arrangement of the photodiodes A and B that corresponds to the color filter arrangement shown in  FIG. 2A . Although each pixel here has two photodiodes that are separated in the horizontal direction, the number of separated photodiodes and the separating direction may be different. For example, three or more photodiodes that are separated in the horizontal direction, or photodiodes that are separated in the vertical direction, or photodiodes separated in a plurality of directions may be employed. Pixels with different number of separated photodiodes or a different separating direction may be included. 
     As a result of a plurality of photodiodes sharing one microlens, individual photodiodes receive different part of a light beam that exits from an exit pupil in the imaging optical system. Accordingly, in the case of the configuration shown in  FIG. 2B , phase-difference AF can be performed using an image signal (image A signal) that is formed by a signal group obtained from photodiodes A in the plurality of pixels, and an image signal (image B signal) that is formed by a signal group obtained from photodiodes B. In the following description, a signal obtained from some of the plurality of photodiodes that share one microlens will be called an AF signal or a focus detection signal. On the other hand, a signal obtained from all of the plurality of photodiodes that share one microlens is the same signal as one obtained when the photodiodes are not separated, and accordingly will be called an imaging signal or an added signal. 
     In the example in  FIG. 2B , a signal (signal A (B)) obtained from one photodiode A (B) is an AF signal, and an added signal (also called an A+B signal) obtained by adding a signal A and a signal B acquired from the same pixel is an imaging signal. An image signal formed by a plurality of signals A is an image A signal, and an image signal formed by a plurality of signals B is an image B signal. As will be described later, the AF signal processor  204  detects the defocus amount and the defocus direction based on a phase difference (image shift amount) between the image A signal and the image B signal. 
     Next, a description will be given, using the flowchart shown in  FIG. 3 , of the overall operation in the shooting mode of the digital camera according to this embodiment. The operation shown in  FIG. 3  is performed when a shooting mode is set and the digital camera is in a shooting standby state. In the shooting standby state, the system controller  209  continuously shoots a moving image and displays the shot moving image on the image display  213 , thereby causing the image display  213  to function as an EVF. The image processor  203  supplies part of or the entire image data of a captured moving image frame to the AE processor  211  through the system controller  209 . 
     In step S 301 , the AE processor  211  performs AE processing based on the image data obtained from the image processor  203 , and advances the processing to step S 302 . The AE processor  211  determines an exposure condition (aforementioned processes 1 and 2) and performs exposure control (aforementioned processes 3 and 4). Thus, the exposure of the live view image that is being displayed can be maintained at a correct level. 
     In step S 302 , the system controller  209  determines whether the SW 1   217  is ON, advances the processing to step S 303  if it is determined that the SW 1   217  is ON, and returns the processing to step S 301  if not. 
     In step S 303 , the system controller  209  initializes a flag (AE for AF completion Flg) that indicates whether AE processing for AF has been completed such that the flag is FALSE (uncompleted). 
     In step S 304 , the AF signal processor  204  performs later-described AF processing, outputs an image shift amount (or an amount and direction of defocus) and reliability information to the system controller  209 , and advances the processing to step S 305 . 
     Note that, in step S 304 , AE processing for AF and AF processing are concurrently performed. Note that, because the aperture is mechanically driven, this driving takes time. Further, the aperture is driven concurrently with and independently of AF processing, and therefore, the aperture driving period possibly spans over charge accumulation periods for a plurality of frames. 
     In step S 305 , the system controller  209  determines whether the SW 1   217  is ON, advances the processing to step S 306  if it is determined that the SW 1   217  is ON, and returns the processing to step S 301  if not. 
     In step S 306 , the system controller  209  determines whether the SW 2   218  is ON, advances the processing to step S 307  if it is determined that the SW 2   218  is ON, and returns the processing to step S 305  if not. 
     In step S 307 , the system controller  209  performs a shooting operation, and returns the processing to step S 301 . 
       FIGS. 4A and 4B  are flowcharts illustrating the AF operation in step S 304  in  FIG. 3 . 
     In step S 401 , the system controller  209  starts AE processing for AF, and advances the processing to step S 402 . After AE processing for AF is started here, AE processing is performed concurrently with AF control. 
     In step S 402 , the system controller  209  checks whether the system controller  209  has been notified of a target exposure condition determined through AE control, advances the processing to step S 403  if so, and advances the processing to step S 405  if not. 
     In step S 403 , the system controller  209  determines whether the difference between the target exposure condition and the current exposure state is smaller than a given exposure value, advances the processing to step S 404  if it is determined that the difference is smaller than the given exposure value, and advances the processing to step S 405  if not. If, for example, the given exposure value is set to 1.5, the system controller  209  determines in step S 403  whether the following condition is met:
 
(target  Av +target  Tv −target Gain)−(current  Av +current  Tv −current Gain)&lt;1.5
 
Av: f-number v: shutter speed Gain: delta gain.
 
     In step S 404 , the system controller  209  sets, to TRUE, the flag (AE for AF completion Flg) indicating whether AE processing for AF has been completed, and advances the processing to step S 406 . 
     In step S 405 , the system controller  209  sets the AE for AF completion Flg to FALSE, and advances the processing to step S 406 . Thus, different AF processing can be performed between the case where AF processing is based on a signal obtained before exposure control is completed and the case where AF processing is based on a signal obtained after exposure control is completed. 
     In step S 406 , in the case where the aperture is driven during AE processing for AF, the system controller  209  determines whether the aperture driving period overlaps the charge accumulation period for the image to be used in AF processing. The system controller  209  can comprehend the charge accumulation period for the individual frames based on the timing of a vertical synchronizing signal and the length of the charge accumulation period that is set for the timing generator  208 , for example. Also, the system controller  209  can comprehend the aperture driving period based on the timing at which the system controller  209  transmitted a request (command) to drive the aperture  102  with a given f-number to the lens controller  105 , and the timing at which the system controller  209  received a drive-end notification from the lens controller  105 . Based on that information, the system controller  209  can determine whether the aperture driving period overlaps the charge accumulation period for the frame to be used in focus detection processing in step S 409  (i.e. whether the aperture was driven during the charge accumulation period). Note that the above described determination method is merely an example, and the determination may be made based on any other methods. 
     The system controller  209  advances the processing to step S 407  if it is determined that the aperture driving period overlaps the charge accumulation period, and advances the processing to step S 408  if the aperture was not driven during AE processing for AF, or it is not determined that the aperture driving period overlaps the charge accumulation period. 
     In step S 407 , the system controller  209  sets, to TRUE (overlap), a flag (aperture controlling Flg) that indicates whether the aperture driving period overlaps the charge accumulation period, and advances the processing to step S 409 . 
     In step S 408 , the system controller  209  sets the aperture controlling Flg to FALSE (not overlap), and advances the processing to step S 409 . 
     In step S 409 , the AF signal processor  204  performs focus detection processing, detects a defocus direction and a defocus amount, obtains reliability of the defocus amount (AF reliability), outputs the results to the system controller  209 , and advances the processing to step S 410 . The details of focus detection processing in step S 409  will be described later. 
     In step S 410 , the system controller  209  determines whether the AF reliability generated by the AF signal processor  204  in step S 409  is greater than or equal to a preset second reliability threshold value. If it is determined that the AF reliability is greater than or equal to the second reliability threshold value, the system controller  209  advances the processing to step S 411 , and if not, the system controller  209  advances the processing to step S 423 . Here, regarding the second reliability threshold value, a reliability being smaller than the second reliability threshold value indicates that the accuracy of the defocus amount cannot be guaranteed, but the defocus direction (direction in which the focusing lens is to be moved) can be guaranteed. The second reliability threshold value can be set in advance using an experimental method, for example. 
     In step S 411 , the system controller  209  determines whether the defocus amount obtained through focus detection processing is smaller than or equal to a preset second Def amount threshold value, advances the processing to step S 412  if it is determined that the obtained defocus amount is smaller than or equal to the second Def amount threshold value, and advances the processing to step S 422  if not. Here, regarding the second Def amount threshold value, a defocus amount being smaller than or equal to the second Def amount threshold value is a defocus amount (e.g. five depths) with which the number of times that the lens thereafter needs to be driven to drive the focusing lens into the focal depth is within a given number of times (e.g. three times or less). The second Def amount threshold value can be set in advance based on the largest defocus amount obtained through focus detection processing. 
     In step S 412 , the system controller  209  determines whether the focusing lens  103  is in a stopped state, advances the processing to step S 413  if it is determined that the focusing lens is in a stopped state, and advances the processing to step S 419  if not. For example, the system controller  209  can comprehend the state of the focusing lens  103  by making an inquiry thereabout to the lens controller  105 . 
     In step S 413 , the system controller  209  determines whether the AF reliability is greater than or equal to a preset first reliability threshold value. If it is determined that the AF reliability is greater than or equal to the first reliability threshold value, the system controller  209  advances the processing to step S 414 , and if not, the system controller  209  advances the processing to step S 419 . Here, regarding the first reliability threshold value, if a reliability is greater than or equal to the first reliability threshold value, settings have been configured such that a fluctuation in the accuracy of the defocus amount falls within a given range (e.g. less than or equal to one depth). That is to say, being greater than or equal to the first reliability threshold value indicates a greater reliability than being greater than or equal to the second reliability threshold value does. The first reliability threshold value can be set in advance using an experimental method, for example. 
     In step S 414 , the system controller  209  determines whether the aperture controlling Flg is TRUE, returns the processing to step S 402  if it is determined that the aperture controlling Flg is TRUE, and advances the processing to step S 415  if not. If the aperture controlling Flg is TRUE, it indicates that the defocus amount detected in step S 409  is based on an image affected by the aperture being driven. If the aperture controlling Flg is TRUE, it is possible to avoid driving the focusing lens and performing focus determination based on a defocus amount affected by the driving of the aperture, by returning the processing to step S 402 . 
     In step S 415 , the system controller  209  determines whether the defocus amount detected by the AF signal processor  204  is smaller than or equal to a preset first Def amount threshold value. If it is determined that the defocus amount is smaller than or equal to the first Def amount threshold value, the system controller  209  advances the processing to step S 416 , and if not, the system controller  209  advances the processing to step S 418 . Here, the first Def amount threshold value is set to a value indicating that, if the defocus amount is smaller than or equal to the first Def amount threshold value, the focusing lens  103  has been driven into a range in which an object is brought into focus in the imaging optical system (i.e. an image distance in the imaging optical system falls within the focal depth). That is to say, being smaller than or equal to the first Def amount threshold value indicates a higher degree of focus than being smaller than or equal to the second Def amount threshold value does. 
     In step S 416 , the system controller  209  determines whether the AE for AF completion Flg is TRUE, advances the processing to step S 417  if it is determined that the AE for AF completion Flg is TRUE, and returns the processing to step S 402  if not. Thus, it is possible to avoid performing focus determination based on the defocus amount that was detected with AE processing for AF uncompleted. 
     In step S 417 , the system controller  209  determines that the digital camera is in an in-focus state, and ends focus detection processing. 
     In step S 418 , the system controller  209  drives the focusing lens  103  by a moving amount in a moving direction corresponding to the defocus amount and the defocus direction that were detected by the AF signal processor  204  in step S 409 , and returns the processing to step S 402 . For example, the system controller  209  transmits, to the lens controller  105 , a request to drive the focusing lens  103  including the direction and amount of driving. The lens controller  105  then drives the motor  104  in accordance with the driving request, and moves the focusing lens  103 . 
     As a result of the processing in steps S 412  to S 418 , in the case where the AF reliability is greater than or equal to the first reliability threshold value, the defocus amount can be detected again with the lens stopped. 
     In step S 419 , the system controller  209  determines whether the aperture controlling Flg is TRUE, advances the processing to step S 421  if it is determined that the aperture controlling Flg is TRUE, and advances the processing to step S 420  if not. When the defocus amount is smaller than or equal to the second Def amount threshold value, it is possible to avoid driving the focusing lens based on the defocus amount affected by the driving of the aperture, by skipping step S 420  when it is determined that the aperture controlling Flg is TRUE. 
     In step S 420 , the system controller  209  drives the focusing lens  103  by an amount smaller than the moving amount corresponding to the defocus amount in a direction corresponding to the defocus direction, and advances the processing to step S 421 . For example, the system controller  209  drives the focusing lens by an amount corresponding to a given percentage (e.g. 80%) of the defocus amount. 
     In step S 421 , the system controller  209  stops the focusing lens  103  through the lens controller  105 , and returns the processing to step S 402 . 
     In step S 422 , the system controller  209  drives the focusing lens  103  by an amount smaller than the moving amount corresponding to the defocus amount in a direction corresponding to the defocus direction, and returns the processing to step S 402 . For example, the system controller  209  can set a smaller driving amount of the focusing lens  103  than the amount corresponding to the defocus amount by setting a lower driving speed than the driving speed at which the focusing lens  103  moves by an amount corresponding to the defocus amount within the time corresponding to a one-frame period during moving image shooting. 
     By driving the focusing lens  103  at that speed, it is possible to prevent the focusing lens  103  from being moved beyond the focus position of the object when the defocus amount is incorrect. Furthermore, the driving can be continuously performed based on the defocus amount that is based on the next frame in a state of driving the focusing lens without stop (overlap control). When the defocus amount is greater than the second Def amount threshold value, the focusing lens can be driven based on the defocus amount that was affected by the driving of the aperture. 
     In step S 423 , the system controller  209  determines whether a not-in-focus condition is met, advances the processing to step S 424  if it is determined that the not-in-focus condition is met, and advances the processing to step S 425  if not. Note that meeting the not-in-focus condition unit meeting a condition under which it is determined that no object to be brought into focus is present. For example, the not-in-focus condition may be the case where the focusing lens  103  has been driven over the entire movable range, i.e. the case where the position of the focusing lens  103  has reached both lens ends on the telephoto side and the wide angle side and then returned to the initial position. The system controller  209  can acquire the information regarding the position of the focusing lens  103  through the lens controller  105 . 
     In step S 424 , the system controller  209  determines that the digital camera is in an out-of-focus state, and ends focus detection processing. 
     In step S 425 , the system controller  209  determines whether the focusing lens  103  has reached an end (limit) of its movable range, advances the processing to step S 426  if it is determined that the focusing lens  103  has reached an end, and advances the processing to step S 427  if not. The system controller  209  can perform the determination based on the information regarding the position of the focusing lens  103 . When the focusing lens  103  has reached an end (limit) of the movable range, the lens controller  105  may notify the system controller  209  of that effect. 
     In step S 426 , the system controller  209  transmits, to the lens controller  105 , a command to reverse the driving direction of the focusing lens  103 , and returns the processing to step S 402 . 
     In step S 427 , the system controller  209  determines whether the AE for AF completion Flg is TRUE (completed), advances the processing to step S 428  if it is determined that the AE for AF completion Flg is TRUE, and returns the processing to step S 402  if not. 
     In step S 428 , the system controller  209  drives the focusing lens  103  in the direction according to the current setting, irrespective of the defocus amount, and returns the processing to step S 402 . Here, for example, the system controller  209  sets the focusing lens driving speed to the fastest speed within a range in which the focusing lens does not pass through the in-focus position, after the defocus amount becomes detectable. Through the processing in steps S 427  and S 428 , it is possible to avoid driving the focusing lens  103  in a given direction with AE for AF uncompleted. For this reason, the focusing lens being unnecessarily driven can be suppressed in the case where AE for AF has not been completed, and where the AF reliability is low because focus detection has been performed based on an image obtained under an exposure condition that significantly deviates from a correct exposure. 
     Next, a description will be given, using the flowchart in  FIG. 5 , of the details of focus detection processing performed in step S 409  in  FIG. 4A . 
     Initially, in step S 501 , the AF signal processor  204  generates a pair of AF image signals (image A signal and image B signal) using signals of the pixels included in a set focus detection area among signals obtained from the A/D converter  202 , and advances the processing to step S 502 . 
     In step S 502 , the AF signal processor  204  calculates a correlation amount between the image signals, and advances the processing to step S 503 . 
     In step S 503 , the AF signal processor  204  calculates an amount of correlation change based on the correlation amount calculated in step S 502 , and advances the processing to step S 504 . 
     In step S 504 , the AF signal processor  204  calculates an image shift amount based on the amount of correlation change, and advances the processing to step S 505 . 
     In step S 505 , the AF signal processor  204  calculates a reliability of the image shift amount, and advances the processing to step S 506 . This reliability is used as the reliability of the defocus amount (AF reliability) obtained by converting the corresponding image shift amount. 
     The AF signal processor  204  performs the processing in steps S 501  to S 505  for each focusing area that is present within the focus detection area. Then, in step S 506 , the AF signal processor  204  converts the image shift amount calculated for each focusing area into a defocus amount, and advances the processing to step S 507 . 
     In step S 507 , the AF signal processor  204  determines a focusing area to be used in AF, sets the defocus amount in the determined focusing area and the corresponding AF reliability as a focus detection processing result, and ends focus detection processing. 
     Focus detection processing described in  FIG. 5  will now be described in more detail using  FIGS. 6 to 8B . 
       FIG. 6  is a diagram schematically showing an example of a focus detection area and a focusing area that are dealt with in focus detection processing. A focus detection area  602  is set to a portion of a pixel array  601  in the image sensor  201 . The size and position of the focus detection area  602  shown in  FIG. 6  are merely an example. Shift areas  603  that are present to the left and right of the focus detection area  602  are areas necessary for correlation calculation. Accordingly, a pixel area  604 , which is a combination of the focus detection area  602  and the shift areas  603 , is a pixel area necessary for correlation calculation. p, q, s, and t in the diagram denote coordinates in the x-axis direction, where p and q denote x coordinates of a starting point and an end point of the pixel area  604 , and s and t denote x coordinates of a starting point and an end point of the focus detection area  602 , respectively. 
       FIGS. 7A to 7C  show exemplary AF image signals generated using the pixels included in the focus detection area  602  that was set in  FIG. 6 . Solid lines indicate an image A signal  701 , and broken lines indicate an image B signal  702 . 
       FIG. 7A  shows an example of pre-shift image signals.  FIGS. 7B and 7C  show states where the pre-shift image signals in  FIG. 7A  are shifted in a positive direction and a negative direction. When the correlation amount is calculated, both the image A signal  701  and the image B signal  702  are shifted by one bit in the respective arrow directions. 
     A method for calculating a correlation amount COR will now be described. As shown in  FIGS. 7B and 7C , the image A signal  701  and the image B signal  702  are shifted by one bit, and the sum of absolute values of differences between the image A signal and the image B signal at the respective times is calculated. Assuming that the shift amount is i, the smallest shift amount is p-s, the largest shift amount is p-t, the starting coordinate of the focus detection area  602  is x, and the end coordinate of the focus detection area  602  is y, the correlation amount COR can be calculated using Equation (1) below: 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 8A  is a diagram showing an exemplary relationship between the shift amount i and the correlation amount COR. The horizontal and vertical axes indicate the shift amount i and the correlation amount COR, respectively. Regarding zones  802  and  803  of minimum values on a correlation amount waveform  801 , the degree of coincidence between the image A signal and the image B signal is higher in the zone  802 , where the correlation amount is smaller than that in the zone  803 . 
     Subsequently, a method for calculating the amount of correlation change ΔCOR will be described. Initially, regarding the correlation amount indicated by the correlation amount waveform  801  in  FIG. 8A , the amount of correlation change is calculated based on a difference between correlation amounts spanning two shift amounts. Assuming that the shift amount is i, the smallest shift amount is p-s, and the largest shift amount is p-t, the amount of correlation change ΔCOR can be calculated using Equation (2) below.
 
ΔCOR[ i ]=COR[ i− 1]−COR[ i+ 1]{( p−s+ 1)&lt; i &lt;( q−t− 1)}  (2)
 
       FIG. 9A  is a diagram showing an exemplary relationship between the shift amount and the amount of correlation change ΔCOR. The horizontal and vertical axes indicate the shift amount i and the amount of correlation change ΔCOR, respectively. The sign of the amount of correlation change ΔCOR switches from plus to minus in zones  902  and  903  on an amount-of-correlation change waveform  901 . A state where the amount of correlation change ΔCOR is 0 is called a zero-cross, where the degree of coincidence between the image A signal and the image B signal is highest. That is to say, the shift amount at the time of a zero-cross is the image shift amount (phase difference). 
     A method for calculating an image shift amount PRD will now be described using  FIG. 9B , which enlarges the zone  902  in  FIG. 9A . 
     Here, a shift amount (k−1+α) at the time of the zero-cross is divided into an integer portion β (=k−1) and a decimal fraction portion α. Because a triangle ABC and a triangle ADE in the diagram are in a similarity relation, the decimal fraction portion α can be calculated using Equation (3) below. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     The integer portion β can be calculated using Equation (4) below, based on  FIG. 9B .
 
β= k− 1  (4)
 
     Then, the image shift amount PRD can be calculated based on the sum of α and β. 
     Note that, if the amount of correlation change ΔCOR has a plurality of zero-crosses as shown in  FIG. 9A , the zero-cross at which the amount of correlation change ΔCOR is steepest is considered as a first zero-cross. Accurate detection can be more easily achieved with a zero-cross at which the amount of correlation change ΔCOR is steeper. The steepness maxder can be calculated using Equation (5) below.
 
maxder=|ΔCOR[ k− 1]|+|ΔCOR[ k]|   (5)
 
     As described above, if there are a plurality of zero-crosses, the first zero-cross is determined based on the steepness, and the shift amount corresponding to the first zero-cross is considered as the image shift amount. 
     Subsequently, a method for calculating reliability of an image shift amount will be described. The reliability can be defined by, for example, the aforementioned steepness, or the degree of coincidence fnclvl between the image A signal and the image B signal (hereinafter called a two-image coincidence degree). The two-image coincidence degree is an index indicating the accuracy of an image shift amount, and in the calculation method according to this embodiment, a smaller value indicates a higher accuracy. 
     Using  FIG. 8B , which enlarges the zone  802  in  FIG. 8A , the two-image coincidence degree fnclvl can be calculated using Equation (6) below. 
     (i) When IΔCOR[k−1]|×2≤maxder,
 
fnclvl=COR[ k− 1]+ΔCOR[ k− 1]/4
 
     (ii) When IΔCOR[k−1]|×2&gt;maxder,
 
fnclvl=COR[ k ]−ΔCOR[ k]/ 4  (6)
 
     The AF signal processor  204  obtains the two-image coincidence degree fnclvl as the reliability (AF reliability) of an image shift amount (and of the defocus amount obtained by converting this image shift amount). 
     The AF operation according to this embodiment can be summarized as follows, mainly in terms of the threshold values.
         If the AF reliability is greater than or equal to the first reliability threshold value, the defocus amount is smaller than or equal to the first defocus amount threshold value, the focus detection result is not affected by aperture control, and AE processing for AF has been completed, then it is determined that the digital camera is in an in-focus state (S 417 ), and the AF operation ends.   If the AF reliability is greater than or equal to the second reliability threshold value and is greater than or equal to the first reliability threshold value, the focus detection result is not affected by aperture control, and the defocus amount is greater than the first defocus amount threshold value and smaller than or equal to the second defocus amount threshold value, then the focusing lens is driven correspondingly to the defocus amount (and direction) obtained through focus detection processing (S 418 ).   If the AF reliability is greater than or equal to the second reliability threshold value, and the defocus amount is greater than the second defocus amount threshold value, then the focusing lens is not stopped, and continues to be moved in the focusing direction by an amount smaller than the amount corresponding to the defocus amount (S 422 ).   If the AF reliability is smaller than the second reliability threshold value, and AE processing for AF has been completed, then the focusing lens continues to be moved in a set direction (irrespective of the defocus amount) (S 428 ).       

     Meanwhile, the AF operation according to this embodiment can be summarized as follows, mainly in terms of AE processing for AF.
         If AE processing for AF has not been completed,       

     focus determination is not performed even if the AF reliability is greater than or equal to the first reliability threshold value, and the defocus amount is smaller than or equal to the first defocus amount threshold value, and 
     the focusing lens is not moved even if a reliable result has been obtained regarding the defocus direction.
         Further, if the focus detection result is affected by aperture control in AE processing for AF,       

     focus determination is not performed even if the AF reliability is greater than or equal to the first reliability threshold value, and the defocus amount is smaller than or equal to the first defocus amount threshold value, and 
     the focusing lens is not driven correspondingly to the defocus amount (and direction) obtained through focus detection processing even if the AF reliability is greater than or equal to the first reliability threshold value, and the defocus amount is greater than the first defocus amount threshold value and is smaller than or equal to the second defocus amount threshold value. 
     With this AF operation, the influence of the AE operation exerted on AF accuracy can be suppressed while the AE operation and the AF operation are concurrently carried out. In addition, even if the object&#39;s luminance changes during the AF operation, the AF operation can be performed under a more appropriate exposure condition due to the AE operation that is concurrently performed. 
     Other Embodiments 
     Although the AF operation according to the above embodiment ( FIGS. 4A and 4B ) employs a configuration in which AF processing starts after AE processing for AF has started, a configuration may alternatively be employed in which AE processing starts after AF processing has started. 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-121222, filed on Jun. 17, 2016, which is hereby incorporated by reference herein in its entirety.