Patent Publication Number: US-8988585-B2

Title: Focus adjustment apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a focus adjustment apparatus that performs focusing in an imaging apparatus such as a digital still camera, a video camera or a film-based camera. 
     2. Description of the Related Art 
     As a general system that uses a light flux passed through a photographic lens in a focus detection and adjustment method of a camera, there are a contrast detection system and a phase difference detection system. 
     The contrast detection system is frequently used in the video camera or the digital still camera, and an image sensor is used as a focus detection sensor. This system determines, by focusing on an output signal of the image sensor, particularly information of a high-frequency component (contrast information), a position of the photographic lens where its evaluation value is largest as an in-focus position. 
     However, as it is referred to as a hill-climbing method, the system must acquire an evaluation value by slightly moving a focus position of the photographic lens, and move the focus position until the evaluation value is found to be largest. Hence, the system is not suited to a high-speed focus detection operation. 
     Focus detection of the phase difference detection system is a technology frequently used in a single-lens reflex camera, which has contributed to practical use of a single-lens reflex camera with automatic focus (AF) detection. For example, in a digital single-lens reflex camera, a focus detection unit that includes a secondary imaging optical system performs AF of the phase difference detection system. 
     The focus detection unit includes a pupil dividing unit that divides a light flux passed through an exit pupil of the photographic lens into two areas. The light flux divided into the two areas forms, via an optical path division optical system located in a mirror box, an image on a set of focus detection sensors by the secondary imaging optical system. Then, by detecting a shifting amount of a signal output according to light reception amounts of the sensors, namely, a relative positional shifting amount in a pupil dividing direction, a shifting amount in a focus direction of the photographic lens is directly acquired. 
     Thus, a defocusing amount and a defocusing direction are simultaneously acquired through a storage operation by the focus detection sensor. This enables a high-speed focus adjustment operation. During imaging after the focus detection, the optical path division optical system is retracted outside the imaging light flux, and the image sensor is exposed to acquire a captured image. 
     There is a technology for achieving high-speed AF even during electronic viewfinder observation or moving image capturing where an AF function of the phase difference detection system is provided to the image sensor, and a display unit such as a backside liquid crystal checks an image in real time. For example, there has been developed a technology for providing, in a certain light receiving element (pixels) of the image sensor, a pupil dividing function by setting a sensitivity area of a light reception unit eccentric from an optical axis of an on-chip microlens. 
     AF of the phase difference detection system is performed by using the pixels as focus detection pixels and arranging the pixels at a predetermined interval in a group of imaging pixels. Arranging places of the focus detection pixels correspond to defective portions of the imaging pixels, and hence surrounding imaging pixel information is interpolated to generate image information. In this example, AF of the phase difference detection system can be executed on an imaging plane. Thus, high-speed and highly accurate focus detection can be performed even during electronic viewfinder observation or moving image capturing. 
     During the electronic viewfinder observation or the moving image capturing, an amount of light reaching the image sensor is adjusted or a blur amount of a captured image is adjusted according to a user&#39;s image forming intension. This may necessitate adjustment of a diaphragm aperture diameter of the photographic lens. It is desired that an image that is always placed in in-focus state by focus adjustment is captured even in such a situation. However, since focus detection and adjustment are executed by the light flux passed through the photographic lens, the above-mentioned focus detection and adjustment method is affected in no small part by a change in diaphragm aperture diameter of the photographic lens. 
     To deal with this problem, Japanese Patent Application Laid-Open No. 7-111614 discusses a technology for inhibiting a focus adjustment operation when a diaphragm aperture diameter is adjusted in focus adjustment of the contrast detection system. This technology can prevent an erroneous focus detection operation when the diaphragm aperture diameter is adjusted. 
     Japanese Patent Application Laid-Open No. 03-214133 discusses a technology for correcting, in focus detection of the phase difference detection system, when a diaphragm of the photographic lens blocks (vignettes) a light flux used for the focus detection, an output signal in view of an amount of light that has not reached the focus detection unit due to the blocked light flux. This technology enables highly accurate focus detection by correcting the output signal according to a vignetting state of the light flux used for the focus detection even when the diaphragm aperture diameter is adjusted 
     However, in the technology discussed in Japanese Patent Application Laid-Open No. 7-111614, the inhibition of the focus adjustment operation creates a possibility that when an object moves, the system will not be able to follow the movement, resulting in capturing of an out-of focus image. The out-of focus image may be captured even while a focus adjustment operation is executed again after a predetermined period of time. In both cases, there is a possibility that an image felt unnatural during observation may be recorded as a moving image. 
     In the technology discussed in Japanese Patent Application Laid-Open No. 03-214133, the correction of the light amount of the focus detection signal in view of vignetting of the photographic lens is performed based on design information of the photographic lens side. However, a degree of vignetting is determined by several frame members including a diaphragm aperture of the photographic lens, and each component has a manufacturing error in external shape or arrangement. Hence, even when the amount of light is corrected based on only the design information of the photographic lens, there is a possibility that an error may occur in focus detection result. 
     In other words, during the moving image capturing, when a change in diaphragm aperture diameter is accompanied by a change in degree of vignetting, a focus detection result may vary between before and after the change in diaphragm aperture diameter due to the error. Nevertheless, when the photographic lens is driven according to the acquired focus detection result including the error, discontinuous points may be generated in a focus adjustment state of an image being captured, creating a possibility that an image felt unnatural during observation may be recorded as a moving image. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a focus adjustment apparatus and an imaging apparatus including the same, which can display and record an image without giving to a user of the imaging apparatus any unnatural feeling even when a change in diaphragm aperture diameter of a photographic lens affects focus detection and an adjustment result. 
     According to an aspect of the present invention, a focus adjustment apparatus includes a diaphragm aperture adjustment unit configured to adjust a diaphragm aperture area of a photographic lens, a focus detection unit configured to detect a defocusing amount by using a pair of light fluxes passed through different areas of the photographic lens, a detection result correction unit configured to calculate, during focus detection after the diaphragm aperture area has changed by a value equal to or larger than a predetermined value, a reduced defocusing amount correction value with respect to the defocusing amount, and a focus adjustment unit configured to execute control to adjust a focus based on the defocusing amount correction value. 
     Further features and aspects of the present invention will become apparent from the following detailed 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 exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a block diagram illustrating a configuration of camera system according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a plan view illustrating light receiving pixels of an image sensor on which an object image is formed when seen from a photographic lens side. 
         FIGS. 3A and 3B  illustrate a structure of imaging pixels of the image sensor. 
         FIGS. 4A and 4B  illustrate a structure of focus detection pixels of the image sensor. 
         FIG. 5  schematically illustrates a focus detection configuration in the image sensor and an image processing unit. 
         FIGS. 6A and 6B  illustrate a pair of focus detection signals transmitted to an AF unit and a focus detection area within an imaging range. 
         FIGS. 7A and 7B  are optical sectional views illustrating a lens and the image sensor illustrated in  FIG. 1  when seen from an optical viewfinder side. 
         FIG. 8  illustrates an example of a focus detection result before or after diaphragm aperture adjustment. 
         FIG. 9  is a flowchart illustrating a focus adjustment operation according to the first exemplary embodiment. 
         FIG. 10  is a flowchart illustrating a focus adjustment operation according to a second exemplary embodiment. 
         FIG. 11  is a flowchart illustrating a focus adjustment operation according to a third exemplary embodiment. 
         FIG. 12  illustrates a relationship between a lens frame and a diaphragm. 
         FIGS. 13A and 13B  illustrate the lens frame and the diaphragm seen from the image sensor side. 
         FIG. 14  is a flowchart illustrating a focus adjustment operation according to a fourth exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
     The present invention provides a configuration where during focus detection after a diaphragm aperture area changes by a predetermined value or more, a reduced defocusing amount correction value is calculated for a defocusing amount by a certain method, and focus adjustment is performed based on the defocusing amount correction value. 
     A possibility is not small that the defocusing amount detected after the change in diaphragm aperture area may suddenly increase for various reasons or include errors at certain rates. 
     Thus, according to the present invention, the reduced defocusing adjustment correction amount is solely calculated without executing focus adjustment based on the detected amount and irrespective of reliability of the detected amount and, based on the reduced defocusing amount correction value, the focus adjustment is performed by reducing the discontinuity of the focus adjustment state and the influence of errors. Based on this idea, a focus adjustment apparatus according to the present invention includes various components described above in the summary of the invention. 
       FIG. 1  illustrates a configuration of an imaging apparatus, namely, a camera system that includes a camera where a plurality of photographic lens units can be replaced, the photographic lenses, and a focus adjustment apparatus. In  FIG. 1 , there are shown a camera  100  and a photographic lens  300  interchangeably mounted on the camera in the camera system that includes the focus adjustment apparatus according to an exemplary embodiment. First, the camera  100  side is described. 
     The camera  100  can be used for the camera system where a plurality of types of photographic lenses  300  are present, and lenses of the same type but different in manufacturing number can be loaded. Photographic lenses  300  different in focal distance or open F number, or a photographic lens  300  having a zoom function can be mounted. Imaging lenses are interchangeable irrespective of similar or different types. 
     In the camera  100 , a light flux that has passed through the photographic lens  300  is transmitted through a camera mount  106 , and reflected upward by a main mirror  130  to enter an optical viewfinder  104 . Through the optical viewfinder  104 , a user of the camera  100  can capture an image while observing an object as an optical image. In the optical viewfinder  104 , a certain function of a display unit  54 , such as an in-focus display, a camera shake warning display, a diaphragm value display, or an exposure correction display, is arranged. 
     The main mirror  130  includes a half-transmissive half mirror. A part of the light flux entering the main mirror  130  is passed through the half mirror, and reflected downward by a sub-mirror  131  to enter a focus detection device  105 . 
     The focus detection device  105  employs an AF mechanism of a phase difference detection system that includes a secondary imaging optical system, and converts the acquired optical image into an electric signal to transmit it to an AF unit  42 . The AF unit  42  performs focus detection calculation based on the electric signal. 
     Based on a result of the calculation, a system control circuit  50  controls focus adjustment for a focus control unit  342  (described below) of the photographic lens  300  side. According to the present exemplary embodiment, the AF unit  42  also corrects a focus detection result. The AF unit  42  corresponds to a detection result correction unit in claims. 
     When still image capturing, observation at the electronic viewfinder, and moving image capturing are performed after the end of the focus adjustment of the photographic lens  300 , the main mirror  130  and the sub-mirror  131  are retracted outside the imaging light flux by a quick return mechanism (not illustrated). The light flux thus transmitted through the photographic lens  300  enters, via a shutter  12  for controlling an exposure amount, an image sensor  14  that converts the optical image into an electric signal. 
     After the end of such imaging operation, the main mirror  130  and the sub-mirror  131  return to illustrated positions. 
     The electric signal converted by the image sensor  14  is transmitted to an analog/digital (A/D) converter  16 , and an analog output is converted into a digital signal (image data). A timing generation circuit  18  supplies a clock signal or a control signal to the image sensor  14  and A/D converter  16 , and a D/A converter  26 . A memory control circuit  22  and a system control circuit  50  control the timing generation circuit  18 . 
     An image processing circuit  20  executes predetermined pixel interpolation or color conversion for image data from the A/D converter  16  or image data from the memory control circuit  22 . The image processing circuit  20  executes predetermined calculation by using the image data. 
     The image sensor  14  includes a part of the focus detection unit, and can execute phase difference detection system AF even in the retracted state of the maim mirror  130  and the sub-mirror  131  outside the imaging light flux by the quick return mechanism. Image data corresponding to focus detection among the acquired image data is converted into focus detection image data by the image processing unit  20 . 
     The image data is then transmitted to the AF unit  42  via the system control circuit  50 , and the focus adjustment unit focuses the photographic lens  300 . Based on a result of calculating the image data of the image sensor  19  by the image processing circuit  20 , contrast system AF can be performed where the system control circuit  50  controls the focus control unit  342  of the photographic lens  300  to be in focus. 
     Thus, during the electronic viewfinder observation or the moving image capturing, while the main mirror  130  and the sub-mirror  131  are retracted outside the imaging light flux, the phase difference detection system AF and the contrast system AF can both be performed by the image sensor  14 . Particularly, high-speed focusing is enabled because the phase difference detection system AF can be performed. 
     Thus, in the camera  100  according to the present exemplary embodiment, for normal still image capturing where the main mirror  130  and the sub-mirror  131  are in the imaging light flux, the phase difference detection system AF by the focus detection device  105  is used. 
     During the electronic viewfinder observation or the moving image capturing where the main mirror  130  and the sub-mirror  131  are retracted outside the imaging light flux, the phase difference detection system AF and the contrast system AF by the image sensor  14  are used. This enables focus adjustment in any of the still image capturing, electronic viewfinder observation, and the moving image capturing. 
     The memory control circuit  22  controls the A/D converter  16 , the timing generation circuit  18 , the image processing circuit  20 , an image display memory  24 , the D/A converter  26 , a memory  30 , and a compression/decompression circuit  32 . Data of the A/D converter  16  is written in the image display memory  24  or the memory  30  via the image processing circuit  20  and the memory control circuit  22  or directly via the memory control circuit  22 . 
     An image display unit  28  includes a liquid crystal monitor, and displays display image data written in the image display memory  24  via the D/A converter  26 . 
     Sequentially displaying the captured image data by using the image display unit  28  can achieve the electronic viewfinder function. The image display unit  28  can arbitrarily switch displaying ON/OFF according to an instruction from a system control circuit  50 . When the displaying is switched OFF, power consumption of the camera  100  can be greatly reduced. 
     As described above, during the electronic viewfinder observation or the moving image capturing, the main mirror  130  and the sub-mirror  131  are retracted outside the imaging light flux by the quick return mechanism. In this case, therefore, use of focus detection by the focus detection device  105  is inhibited. 
     Thus, the camera  100  according to the present exemplary embodiment is configured to perform AF of the phase difference detection system by the focus detection unit included in the image sensor  14 . This enables focus adjustment of the photographic lens  300  in both of the optical viewfinder and the electronic viewfinder. Needless to say, during the electronic viewfinder observation or the moving image capturing, focus detection of the contrast system can be performed. 
     The memory  30  stores a captured still or moving image, and has a capacity enough to store a predetermined number of still images or moving images of a predetermined period of time. Thus, even in the case of continuous imaging or panoramic imaging, a great amount of images can be written in the memory  30  at a high speed. 
     The memory  30  can be used as a work area of the system control circuit  50 . The compression/decompression circuit  32  has a function of compressing/decompressing image data by adaptive discrete cosine transform (ADCT). The compression/decompression circuit  32  reads the image stored in the memory  30  to compress or decompress it, and writes processed image data in the memory  30 . 
     A shutter control unit  36  controls, based on photometric information from a photometric unit  46 , a shutter  12  in association with a diaphragm control unit  344  that controls a diaphragm  312  of the photographic lens  300  side. An interface unit  38  and a connector  122  electrically interconnects the camera  100  and the photographic lens  300 . 
     There are functions of transmitting a control signal, a state signal, or a data signal, and supplying currents of various voltages between the camera  100  and the photographic lens  300 . Not only electric communication but also optical communication and audio communication can be performed. 
     The photometric unit  46  performs automatic exposure (AE) processing. Entering the light flux passed through the photographic lens  300  to the photometric unit  46  via the camera mount  106 , the mirror  130 , and a photometric lens (not illustrated) enables measurement of an image exposure state. 
     The photometric unit  46  has an FE processing function in association with a flash  48 . Based on a result of calculating the image data of the image sensor  14  by the image processing circuit  20 , the system control circuit  50  can perform AE control for the shutter control unit  36  and the diaphragm control unit  344  of the photographic lens  30 C. The flash  48  has a projection function of AF auxiliary light and a flash light control function. 
     The system control circuit  50  controls the entire camera  100 , and a memory  52  stores a constant, a variable, or a program for operating the system control circuit  50 . A display unit  54  is a liquid crystal display device that displays an operation state or a message by using a character, an image, or a voice according to program execution at the system control circuit  50 . 
     A single or a plurality of display units  54  are installed in easily viewed positions near an operation unit of the camera  100 , and each includes a combination of, for example, a liquid crystal display (LCD) and a light emitting diode (LED). Those among display contents of the display unit  54  to be displayed on the LCD include information regarding the number of photographs such as the number of captured images or a remaining number to be photographed, and information regarding imaging conditions such as a shutter speed, a diaphragm value, exposure correction, and a flash. In addition, a remaining battery level, and a date and time are displayed. 
     The display unit  54  has, as described above, some functions provided in the optical viewfinder  104 . A nonvolatile memory  56  is an electrically erasable and recordable memory. For example, an electrically erasable programmable read-only memory (EEPROM) is used. Operation units  60 ,  62 ,  64 ,  66 ,  68 , and  70  input various operation instructions of the system control circuit  50 , each of which includes a single or a plurality of combinations of a switch or a dial, a touch panel, a pointing device based on line-of-sight detection, and a voice recognition device. 
     The mode dial switch  60  can switch and set function modes including a power-off mode, an automatic imaging mode, a manual imaging mode, a reproduction mode, and a personal computer (PC) connection mode. 
     The operation unit  62  that is a shutter switch SW  1  is turned ON by half-pressing a shutter button (not illustrated) to instruct a start of AF, AE, automatic white balance (AWB) processing, or EF processing. The operation unit  64  that is a shutter switch SW  2  is turned ON by fully pressing the shutter button to instruct a start of series of imaging processes. 
     The imaging processes include exposure, development, and recording processing. In the exposure, a signal read from the image sensor  14  is written as image data in the memory  30  via the A/D converter  16  and the memory control circuit  22 . 
     The development is executed by using calculation at the image processing circuit  20  or the memory control circuit  22 . In the recording, the image data is read from the memory  30 , compressed by the compression/decompression circuit  32 , and written in a recording medium  200  or  210 . 
     The image display ON/OFF switch  66  can set the image display unit  28  ON/OFF. This function enables power saving by blocking current supplied to the image display unit including the liquid crystal monitor during imaging executed by using the optical viewfinder  104 . 
     The quick review ON/OFF switch  68  sets a quick review function for automatically reproducing image data captured immediately after imaging. The operation unit  70  includes various buttons and a touch panel. Various buttons include a menu button, a flash setting button, a single imaging/continuous imaging/self-timer switching button, and an exposure correction button. 
     A power control unit  80  includes a battery detection circuit, a direct current (DC)/DC converter, and a switch circuit for switching a block to be energized. The power control unit  80  detects presence of a loaded battery, a type of the battery, and a battery remaining level, controls the DC/DC converter based on a detection result and an instruction from the system control circuit  50 , and supplies a necessary voltage to the units including the recording medium for a necessary period of time. 
     Connectors  82  and  84  connect a primary battery such as an alkaline battery or a lithium battery, a secondary battery such as NiCd battery or a NiMH battery, and a power source unit  86  including an alternate current (AC) adaptor to the camera  100 . Interfaces  90  and  94  have connection functions with a recording medium such as a memory card or a hard disk, and connectors  92  and  96  physically connect with the recording medium such as a memory card or a hard disk. 
     A recording medium loading/unloading detection unit  98  detects loading of the recording medium on the connector  92  or  96 . According to the present exemplary embodiment, there are two systems of interfaces and connectors to load the recording medium. However, a single or a plurality of systems of interfaces and connectors can be installed. Interfaces and connectors of different standards can be installed. 
     Connecting various communication cards such as a local-area-network (LAN) card to the interface and the connector enables transfer of the image data and management information attached to the image data with other peripheral devices such as a computer and a printer. A communication unit  110  has various communication functions such as wire communication and wireless communication. 
     A connector  112  connects the camera  100  to the other devices by the communication unit  110 , and serves as an antenna in the case of wireless communication. The recording media  200  and  210  are memory cards or hard disks. Each of the recording media  200  and  210  includes a recording unit  202  including a semiconductor memory or a magnetic disk, an interface  204  with the camera  100 , and a connector  206  for connection with the camera  100 . 
     Next, the photographic lens  300  side is described. The photographic lens  300  is detachable from the camera  100 . A lens mount  306 , which mechanically couples the photographic lens  300  with the camera  100 , is interchangeably fixed to the camera  100  via the camera mount  106 . 
     The camera mount  106  and the lens mount  306  include functions of a connector  122  and a connector  322  for electrically connecting the photographic lens  300  to the camera  100 . A lens  311  includes a focus lens for focusing an object, and a diaphragm  312  controls an amount of an imaging light flux. 
     The connector  322  and an interface  338  electrically connect the photographic lens  300  to the connector  122  of the camera  100 . The connector  322  has functions of transferring a control signal, a state signal, or a data signal between the photographic lens  300  and the camera  100 , and receiving or supplying currents of various voltages. 
     The connector  322  can be configured to execute not only electric communication but also optical communication and audio communication. A zoom control unit  340  controls zooming of the lens  311 , and a focus control unit  342  controls an operation of the focus lens of the lens  311 . When the photographic lens  300  is a single-focus lens type having no zoom function, the zoom control unit  340  can be removed. 
     The diaphragm control unit  344  controls, based on the photometric information from the photometric unit  46 , the diaphragm  312  in association with the shutter control unit  36  that controls the shutter  12 . The diaphragm  312  and the diaphragm control unit  344  correspond to a diaphragm aperture adjustment unit in claims. 
     A lens system control unit  346  controls the entire photographic lens  300 . The lens system control unit  346  has a memory function of storing a constant, a variable, or a program for a photographic lens operation. 
     A nonvolatile memory  348  stores identification information such as a number unique to the photographic lens, management information, function information such as a full-aperture F value, a minimum diaphragm value or a focal distance, and present and past setting values. According to the present exemplary embodiment, the nonvolatile memory  348  also stores lens frame information according to a state of the photographic lens  300 . This lens frame information is information regarding a distance of a frame opening from the image sensor  14  and a radius of the frame opening that determine a light flux passed through the photographic lens. 
     The diaphragm  312  is included in a frame that determines the light flux passed through the photographic lens, and others such as an opening of a lens frame component for holding the lens correspond to frames. The frame that determines the light flux passed through the photographic lens varies depending on a focus position or a zoom position of the lens  311 , and hence a plurality of frames are prepared corresponding to the focus position and the zoom position of the lens  311 . 
     When the camera  100  performs focus detection by using the focus detection unit, optimal lens frame information corresponding to the focus position and the zoom position of the lens  311  is selected to be transmitted through the connector  322  to the camera  100 . 
     The configuration of the camera system that includes the camera  100  and the photographic lens  300  has been described. Next, the focus detection unit that includes the image sensor  14  is described in detail. The focus detection unit employs AF of the phase difference detection system as in the case of the focus detection device  105 . A configuration of the focus detection unit is described. 
       FIG. 2  is a plan view illustrating light receiving pixels where an object image is formed in the image sensor  14  illustrated in the camera system block diagram of  FIG. 1  when seen from the photographic lens  300  side. Specifically,  FIG. 2  illustrates an imaging range  400  of all pixels including m pixels in a horizontal direction and n pixels in a vertical direction on the image sensor  14 , and one pixel portion  401 . 
     Each pixel portion includes primary color filters formed in an on-chip Bayer arrangement, and pixels are arranged at a cycle of four pixels of 2×2. In  FIG. 2 , to eliminate complexity, only an upper left pixel portion including 10 pixels×10 pixels is displayed while other pixel portions are omitted. 
       FIGS. 3A and 3B  and  FIGS. 4A and 4B  illustrate structures of imaging pixels and focus detection pixels included in the pixel portion.  FIGS. 3B and 4B  are optical sectional views illustrating the lens  311  and the image sensor  14  illustrated in  FIG. 1  when seen from the optical viewfinder  104  side. Members unnecessary for description are omitted. 
     The present exemplary embodiment employs the Bayer arrangement where diagonal two of the four pixels of 2×2 are pixels having green (G) spectral sensitivities and the other two are pixels respectively having red (R) and blue (B) spectral sensitivities. In the Bayer arrangement, a focus detection pixels having a structure described below is disposed. 
       FIGS. 3A and 3B  illustrate an arrangement and a structure of imaging pixels.  FIG. 3A  is a plan view illustrating imaging pixels of 2×2. In the Bayer arrangement, G pixels are located in a diagonal direction, and the other two are pixels of R and B. This structure of 2×2 pixels is repeatedly arranged. 
       FIG. 3B  that is a sectional view cut along the line A-A illustrated in  FIG. 3A  illustrates an on-chip microlens ML located at the foreground of each pixel, a R color filter CF R , and a G color filter CF G . 
     A photodiode (PD) is a schematically illustrated photoelectric conversion element of a complimentary metal-oxide semiconductor (CMOS) image sensor. A contact layer (CL) is a wiring layer to form a signal line for transmitting each of various signals in the CMOS image sensor.  FIGS. 3A and 3B  illustrate pixels near a center in the image sensor  14 , namely, a pixel structure near an axis of the photographic lens  300 . 
     The on-chip microlens ML and the photoelectric conversion element PD for the imaging pixels are configured to capture the light flux passed through the photographic lens  300  as effectively as possible. In other words, an exit pupil  411  of the photographic lens  300  and the photoelectric conversion element PD are in conjugate relationship with each other because of the microlens ML, and an effective area of the photoelectric conversion element is designed to be large. 
     Alight flux  410  illustrated in  FIG. 3B  indicates this status, and an entire area of the exit pupil  411  is captured by the photoelectric conversion element PD. The incident light flux of the R pixel has been described referring to  FIG. 3B . The G pixel and the B pixel have similar structures. Members around the microlens ML are illustrated in an enlarged manner for easier understanding. In reality, these members are micrometers in size. 
       FIGS. 4A and 4B  illustrate an arrangement and a structure of focus detection pixels for horizontal (lateral) pupil division of the photographic lens  300 . The horizontal direction corresponds to a longitudinal direction of the image sensor  14  illustrated in  FIG. 2 .  FIG. 4A  is a plan view illustrating pixels of 2×2 including focus detection pixels. 
     To obtain an image signal for recording or viewing, a main component of luminance information in the G pixel is acquired. Human image recognition characteristics are sensitive to the luminance information, and hence image deterioration is easily recognized when the G pixel is damaged. 
     The R pixel and the B pixel are for acquiring color information (color difference information). Human visual characteristics are insensitive to the color information, and hence image deterioration is difficult to be recognized even when some damages occur in the pixel for acquiring the color information. 
     Thus, according to the present exemplary embodiment, among the pixels of 2×2, the R pixel and the B pixel are replaced by focus detection pixels while the G pixel is left as the imaging pixel.  FIG. 4A  illustrates focus detection pixels S HA  and S HB . 
       FIG. 9B  is a sectional view cut along the line A-A illustrated in  FIG. 4A . A microlens ML and a photoelectric conversion element PD are similar in structure to those of the imaging pixel. Pixels near the center of the image sensor  14 , namely, a pixel structure near the axis of the photographic lens  300 , is illustrated. 
     According to the present exemplary embodiment, no signal of the focus detection pixel is used for image generation, and hence a transparent film CF (white) is disposed in place of a color separation color filter. Since the exit pupil  911  is divided by the image sensor, an opening portion of the wiring layer CL is set eccentric in one direction with respect to a center line of the microlens ML. 
     Specifically, an opening portion OP HA  of the pixel S HA  is eccentric to the right by  421   HA  with respect to the center line of the microlens ML. Hence, a light flux  420   HA  passed through a left exit pupil  422   HA  sandwiching an optical axis L of the lens  311  is received. Similarly, since an opening portion OP HB  of the pixel S HB  is eccentric to the left by  421   HB  with respect to the center line of the microlens ML, a light flux  420   HB  passed through a right exit pupil  422   HB  sandwiching the optical axis L of the lens  311  is received. 
     As clear from  FIG. 4B , the eccentric amount  421   HA  is equal to the eccentric amount  421   HB . Thus, the eccentricity between the opening portion OP and the microlens ML enables extraction of light fluxes  420  passed through different pupil areas of the photographic lens  300 . 
     In this configuration, a plurality of pixels S HA  are horizontally arranged, and an object image acquired in the group of pixels is set as an A image. Pixels S HB  are also horizontally arranged, and an object image acquired in the group of pixels is set as a B image. Detecting relative positions of the A image and the B image enables detection of out-of-focus amounts (defocusing amounts) of the object images. 
     The image sensor  19  accordingly has a function as a second focus detection unit, and simultaneously a second pupil division unit. 
       FIGS. 4A and 4B  illustrate the focus detection pixels near the center of the image sensor  14 . In other than the center, setting the opening portions OP HA  and OP HB  of the microlens ML and the wiring layer CL eccentric in a state different from that illustrated in  FIG. 4B  enables division of the pupil  911 . 
     Specifically, taking the opening portion OP HA  as an example, the opening portion OP HA  is set eccentric in a manner of aligning a spherical core of the roughly spherical microlens ML with a line connecting a center of the opening portion OP HA  with a center of the exit pupil area. Hence, even around the image sensor  14 , pupil division almost similar to that for the focus detection pixel near the center illustrated in  FIGS. 4A and 4B  can be executed. 
     In the pixels S HA  and S HB , a focus can be detected for an object having a luminance distribution in a horizontal direction on an imaging plane, for example, a vertical line. However, a focus cannot be detected for a horizontal line having a luminance distribution in a vertical direction. To enable this focus detection, pixels for vertically dividing the pupil of the photographic lens can be disposed. According to the present exemplary embodiment, focus detection pixel structures are arranged in both vertical and horizontal directions. 
     The focus detection pixels have no original color information. Hence, to form a captured image, interpolation calculation is performed from surrounding pixel signals to generate a signal. Focus detection pixels are accordingly arranged not continuously but discretely in the image sensor  14 . As a result, image quality of the captured image is not deteriorated. 
     As described above referring to  FIG. 2 ,  FIGS. 3A and 3B , and  FIGS. 4A and 4B , the image sensor  14  has not only the function of imaging but also the function as the focus detection unit. For a focus detection method, the inclusion of the focus detection pixel for receiving the light flux used for dividing the exit pupil  411  enables AF of the phase difference detection system. 
       FIG. 5  schematically illustrates a focus detection configuration in the image sensor  14  and the image processing unit  20 . In the camera system illustrated in the block diagram of  FIG. 1 , the image data acquired in the image sensor  14  is transmitted to the image processing unit  20  via the A/D converter  16 . For easier description, in  FIG. 5 , the A/D converter  16  is omitted. 
     The image sensor  14  includes a plurality of focus detection units  901 , each of which includes pupil-divided focus detection pixels  901   a  and  901   b . The focus detection unit  901  corresponds to the portion illustrated in  FIG. 4A , and the focus detection pixel  901   a  corresponds to the pixel S HA , and the focus detection pixel  901   b  corresponds to the pixel S HB . The image sensor  14  includes a plurality of imaging pixels for photoelectrically converting object images formed by the photographic lens. 
     The image processing unit  20  includes a synthesizing unit  902  and a coupling unit  903 . The image processing unit  20  allocates, to include a plurality of focus detection units  901 , a plurality of sections (areas) CST to the imaging plane of the image sensor  14 . The image processing unit  20  can appropriately change sizes, arrangement or the number of sections CST. 
     The synthesizing unit  902  synthesizes, in each of the plurality of sections CST allocated to the image sensor  14 , output signals from the focus detection pixel  901   a  to acquire a first synthesized signal of one pixel. The synthesizing unit  902  synthesizes, in each section CST, output signals from the focus detection pixel  901   b  to acquire a second synthesized signal of one pixel. 
     The coupling unit  903  couples, in the plurality of sections CST, the first synthesized signals of the pixels to acquire a first coupled signal, and the second synthesized signals to acquire a second coupled signal. Thus, a coupled signal where the pixels corresponding to the number of sections are coupled together is acquired for each of the focus detection pixels  901   a  and  901   b.    
     A calculation unit  904  calculates a defocusing amount of the photographic lens  300  based on the first and second coupled signals. Thus, to synthesize the output signals from the focus detection pixels in the same pupil-division direction, which have been arranged in the section, even when luminance of each focus detection unit is small, a luminance distribution of the object can be sufficiently detected. 
       FIG. 6A  illustrates a pair of focus detection signals formed by the focus detection unit  901 , the synthesizing unit  902 , and the coupling unit  903  illustrated in  FIG. 5  and transmitted to the AF unit  42 . In  FIG. 6A , a horizontal axis indicates a pixel array and a direction of the coupled signal, and a vertical axis indicates intensity of the signal. 
     A focus detection signal  430   a  and a focus detection signal  430   b  are respectively generated by the focus detection pixel  901   a  and the focus detection pixel  901   b . The photographic lens  300  is in a defocused state with respect to the image sensor  14 , and hence the focus detection signal  430   a  is shifted left while the focus detection signal  430   b  is shifted right. 
     Calculating shifting amounts of the focus detection signals  430   a  and  430   b  at the AF unit  42  based on well-known correlation calculation enables determination of a defocusing level of the photographic lens  300 . Thus, the focus adjustment unit can perform focusing. 
       FIG. 6B  illustrates a focus detection area within an imaging range. The image sensor  14  executes AF of the phase difference detection system in this focus detection area. The focus detection area illustrated in  FIG. 6B  includes, in addition to a focus detection unit including the pixel for horizontal pupil division of the photographic lens illustrated in  FIG. 5 , a focus detection unit including a pixel for vertical pupil division of the photographic lens. 
     In  FIG. 6B , a dotted-line rectangle  217  indicates the imaging range where the pixels of the image sensor  14  are formed. Within the imaging range, three horizontal focus detection areas  218   ah ,  218   bh , and  218   ch , and three vertical focus detection areas  218   av ,  218   bv , and  218   cv  are respectively formed. The vertical and horizontal focus detection areas are arranged to intersect each other, constituting a cross type focus detection area. According to the exemplary embodiment, cross type focus detection areas are located at totally three places, namely, a center and left and right sides of the imaging range  217 . 
     In this configuration, the image sensor  14  achieves AF of the phase difference detection system. In the AF of the phase difference detection system, focus detection is executed by using, among light fluxes passed through the exit pupil  411  of the photographic lens  300 , light fluxes passed through two different places. 
     This may cause, depending on an aperture size of the diaphragm  312 , vignetting where the light fluxes used for the AF are blocked. Hereinafter, an influence of aperture adjustment by the diaphragm  312  on the phase difference AF of the image sensor  14  is described. 
       FIGS. 7A and 7B  are optical sectional views of the lens  311  and the image sensor  14  seen from the optical viewfinder  104  side in the camera system block diagram of  FIG. 1 , illustrating a imaging light flux to form an image at the center of the image sensor  14  and a focus detection light flux of the AF of the phase difference detection system executed by the image sensor  14 . Members other than the lens  311  and the image sensor  14  unnecessary for description are omitted. 
     In  FIG. 7A , a solid-line light flux  401  is an imaging light flux passed through the lens  311  and the diaphragm  312  of the photographic lens  300  to form an image near a center of a light receiving surface of the image sensor  14 . A pair of diagonal-line light fluxes  440   a  and  440   b  are, among the focus light fluxes received by the focus detection pixels  901   a  and  901   b  illustrated in  FIG. 5 , focus detection light fluxes to form images near the center of the light receiving surface of the image sensor  14 . In  FIG. 7A , the focus detection light fluxes are not vignetted by the diaphragm  312 . 
       FIG. 7B  illustrates a state where the diaphragm  312  illustrated in  FIG. 7A  is narrowed to reduce an aperture area. As in the case illustrated in  FIG. 7A , a solid-line light flux  401  is an imaging light flux passed through the lens  311  and the diaphragm  312  of the photographic lens  300  to form an image near the center of the light receiving surface of the image sensor  14 . 
     A pair of diagonal-lire light fluxes  440   a - 2  and  440   b - 2  illustrated in  FIG. 7B  are blocked by the diaphragm  312  with respect to the focus detection light fluxes illustrated in  FIG. 7A . In  FIG. 7B , broken-line light fluxes indicate the focus detection light fluxes  440   a  and  440   b  illustrated in  FIG. 7A , namely, focus detection light fluxes that have not been vignetted. 
     During the electronic viewfinder observation or the moving image capturing, the diaphragm  312  is always subjected to aperture expansion or reduction control to adjust a light receiving amount of the image sensor  14  according to brightness of an environment including an object during the capturing or to express a blurring level of an object image intended by the user. 
     The vignetting state of the pixels near the center of the image sensor  14  by the diaphragm  312  has been described referring to  FIGS. 7A and 7B . However, depending on an aperture state of the diaphragm  312 , vignetting is generated not only by the diaphragm  312  but also by a mechanical frame component for holding the lens  311 , and the level of vignetting varies from one pixel position to another of the image sensor  14 . 
     Thus, in the focus adjustment apparatus according to the present exemplary embodiment, lens frame information is transmitted from the photographic lens  300  to the camera  100 . The camera  100  (e.g., vignetting amount calculation unit included in the AF unit  42 ) calculates, based on the information, a vignetting correction value corresponding to the pixel position of the image sensor  14 . 
     An output signal from each pixel is corrected by using the vignetting correction value. This correction is well-known as peripheral light amount correction in the imaging apparatus, and thus description thereof is omitted. 
     As described above, each time focus detection and diaphragm aperture adjustment are simultaneously executed during electronic viewfinder displaying or moving image displaying, a vignetting correction value must be calculated to correct a pixel output. However, in the correction, data is simplified, a calculation amount is reduced, and no manufacturing error is added. As a result, correction errors may occur. 
     Each time the diaphragm aperture is adjusted, a focus detection result may lose its continuity and be discontinuous. Conventionally, during still image capturing, such a correction error has occurred. However, as it is sufficiently small, the correction error has caused no problem to satisfy quality of the still image. 
     However, during the electronic viewfinder observation or the moving image capturing, the image is displayed and captured in real time, and hence unnatural motion becomes conspicuous due to the discontinuity of the focus detection result.  FIG. 8  illustrates an example of focus detect results before and after diaphragm aperture adjustment, and the discontinuity of the focus detection result is described. 
     In  FIG. 8 , a vertical axis indicates a focus detection result. In the case of 0 (on an X axis), the result is not an out-of-focus state but an in-focus state. A horizontal axis indicates time, and results  601  to  610  are results of sequentially executed focus detection operations. 
     In  FIG. 8 , the focus detection results  601  to  604  are near 0, maintaining in-focus states. The diaphragm is changed between the focus detection results  604  and  605 . The focus detection result  605  indicates a state where the AF unit  42  has determined defocusing. The focus lens included in the lens  31  must accordingly be driven to adjust a focus. 
     Some reasons are conceivable for the defocusing determination of the AF unit  42  in the focus detection result  605 , such as a correction error caused by the diaphragm change and movement of the object. However, reasons cannot be identified. As a result, when the lens is driven based on the focus detection result  605 , points discontinuous in an in-focus state may be generated in electronic viewfinder displaying or a recorded moving image. 
     An error included in the focus detection result is sufficiently small for still image recording. Thus, during the electronic viewfinder displaying or the moving image recording, there is no problem for driving the lens based on the focus detection result. However, during the electronic viewfinder displaying or the moving image recording, images continuous in time are displayed or recorded. As a result, when points discontinuous in an in-focus state are generated, unnaturalness becomes conspicuous. 
     Thus, according to the present exemplary embodiment, the focus detection result after the adjustment of the diaphragm aperture is corrected in a direction for reducing a lens driving amount calculated therefrom. This can make points discontinuous in an in-focus state generated in the electronic viewfinder displaying or the recorded moving image difficult to be conspicuous, and reduce unnaturalness. 
     A configuration may be employed where correction is always executed in a direction for reducing the lens driving amount irrespective of a change in diaphragm aperture. With this configuration, however, since the lens is not sufficiently driven even when the object moves, followability to the movement of the object is lost. According to the present exemplary embodiment, by changing processing before and after the aperture change of the diaphragm, reduction of discontinuity in the in-focus state and followability to the movement of the object can both be achieved. 
       FIG. 8  illustrates, with respect to the focus detection result  605 , the focus detection result  606  after driving of the lens by about 40% of the lens driving amount calculated therefrom. The focus detection result  606  indicates a change in an in-focus direction with respect to the focus detection result  605 . However, the focus lens must still be driven to adjust a focus. 
     Similarly, the focus detection results  607  and  608  are results of driving the lens by about 40% of a lens driving amount calculated from a last detected focus detection result, indicating gradual changes in the in-focus direction. In the focus detection result  608 , a defocusing amount is smaller. Hence, the focus detection result  609  is a result after driving of the lens by a lens driving amount calculated therefrom. 
     The focus detection results  609  and  610  are near 0, maintaining an in-focus state. Thus, correcting the focus detection result after the diaphragm change in the direction for reducing the lens driving amount calculated thereform enables a gradual change in in-focus state of the electronic viewfinder displaying or the recorded moving image. Thus, unnaturalness during viewing can be reduced. 
     Next, an operation in the camera  100  is described.  FIG. 9  is a flowchart illustrating a focus adjustment operation stored in the system control unit  50 . The flowchart illustrates a focus adjustment operation during the electronic viewfinder displaying or the moving image capturing where the main mirror  130  and the sub-mirror  131  are retracted outside an imaging light flux, and the image sensor  104  performs AF of the phase difference detection system. In other words, the focus adjustment operation is performed in parallel with electronic viewfinder displaying or moving image displaying. 
     First, in step S 501 , it is determined whether a focus detection start button has been pressed by operating the shutter switch SW  1  or the operation unit  70 . When pressed (YES in step S 501 ), the processing proceeds to step S 502 . In this case, determination is executed based on the focus detection start button. However, focus detection can be started in response to a change to electronic viewfinder displaying or moving image recording. 
     In step S 502 , various pieces of lens information such as the lens frame information or the focus lens position of the photographic lens  300  are acquired via the interface units  38  and  338 , and the connectors  122  and  322 . 
     In step S 503 , the synthesizing unit  902  and the coupling unit  903  of the image processing unit  20  generate a pair of focus detection signals from sequentially read image data. The focus detection signals are transmitted to the AF unit  42 , and then the processing proceeds to step S 504 . 
     The AF unit  42  executes light amount correction or vignetting correction to reduce the influence of vignetting. According to the present exemplary embodiment, the image sensor  14  performs focus detection during the electronic viewfinder displaying or the moving image capturing, and hence the focus detection pixels  901   a  and  901   b  are discretely arranged corresponding to thinned reading. 
     In step S 504 , the AF unit  42  calculates a shifting amount between the pair of focus detection signals by using a well-known correlation calculation unit, and converts it into a defocusing amount. 
     In step S 505 , whether a diaphragm aperture area by the diaphragm aperture adjustment unit has changed by at least a predetermined value from that of last focus detection is determined. The determination is made based on the change in diaphragm aperture area for the purpose of determining whether a change of a vignetting state of the focus detection light flux is large. 
     When the change of the vignetting state is large, a vignetting error is estimated to be relatively large. When the change of the vignetting state is small, a vignetting error is estimated to be relatively small. Thus, by setting a certain threshold value for determination of a change in diaphragm aperture area, a focus detection result can be corrected only when a vignetting correction error is likely to be large. 
     Presence of a change in diaphragm aperture area can be determined based on only information at last focus detection. However, the change can be determined based on information at a plurality of previous focus detection operations. 
     When presence of a change in diaphragm aperture area is determined based on only information at last focus detection, after the change of the diaphragm aperture area, at second focus detection and after, No is selected in step S 505 , and the focus lens is driven relatively fast. To increase followability to the movement of the object, this configuration is preferred. 
     When presence of a change in diaphragm aperture area is determined based on information at several focus detection operations, after the change of the diaphragm aperture area, at several focus detection operations, YES is selected in step S 505 , and the focus lens is driven relatively slow. 
     When continuity in the in-focus state during the electronic viewfinder displaying or the moving image recording is prioritized, this configuration is preferred. When the diaphragm aperture area has changed by a predetermined value or more, the processing proceeds to step S 506 . When the diaphragm aperture area has not changed by more than the predetermined value, the processing proceeds to step S 507 . 
     In step S 506 , a focus detection result is corrected. A focus detection result P′ after correction is calculated by the following expression (1):
 
 P′=K×P   (1)
 
P: defocusing amount that is a focus correction result before correction
 
K: coefficient that is a positive number less than 1, corresponding to a correction coefficient in claims
 
Thus, without changing the sign, the focus detection result is corrected so that a value can approach 0.
 
     Lens driving sensitive to a focus detection error caused by the diaphragm aperture change can be reduced, and electronic viewfinder displaying or moving image recording where an in-focus state is discontinuous can be alleviated. For example, to perform driving as described above referring to  FIG. 8 , the correction is performed by setting K as 0.4. 
     In step S 507 , based on the focus detection result calculated in step S 504  or the corrected focus detection result calculated in step S 506 , a lens driving amount of the photographic lens  300  is calculated. In step S 508 , the lens driving amount is transmitted to the focus control unit  342  of the photographic lens  300  via the interface units  38  and  338  and the connectors  122  and  322 , and the focus lens is driven to adjust a focus of the photographic lens  300 . 
     According to the present exemplary embodiment, one threshold value is determined to identify a size of a change in diaphragm aperture area, and one type of correction coefficient is used. However, the number of correction coefficients is not limited to one. By setting a plurality of threshold values for determining a size of a change in diaphragm aperture area and using corresponding correction coefficients, correction corresponding to finer statuses can be performed. The focus adjustment operation of the camera  100  according to the present exemplary embodiment has been described. 
     As described above, according to the present exemplary embodiment, the processing is performed to correct the focus detection result according to the change in diaphragm aperture area and reduce the lens driving amount calculated based on the focus detection result before the correction. As a result, even during the electronic viewfinder observation or the moving image capturing, natural displaying or recording can be performed with little discontinuity of the in-focus state. 
     The present exemplary embodiment has been described by taking the example of the AF of the phase difference system executed by the image sensor  19 . However, the present exemplary embodiment can be applied to AF of the phase difference system executed by the focus detection device  105 . 
     The present exemplary embodiment has been applied to both of the electronic viewfinder observation and the moving image capturing. However, the present exemplary embodiment can be applied only to the moving image capturing. Specifically, correction by the detection result correction unit is inhibited during the electronic viewfinder observation, while correction by the detection result correction unit is executed during the moving image capturing. Thus, priority can be placed on followability to the movement of the object during the electronic viewfinder observation. Priority can be placed on continuity of the in-focus state during the moving image capturing. 
     The example of focusing by driving the focus lens of the photographic lens  300  has been described. However, the image sensor  14  can be configured to move back and forth in the optical axis direction of the photographic lens  300 , and the focus can be adjusted by driving the image sensor  14 . 
     Particularly, when a photographic lens not good at small-amount driving or low-speed driving is attached to the camera  100 , smooth focusing can be performed by driving the image sensor  14 . In other words, based on a defocusing amount correction value, the focus can be adjusted by driving at least one of the photographic lens and the image sensor. 
     A method for calculating a reduced defocusing amount correction value for a defocusing amount during focus detection after a change of a diaphragm aperture area by at least a predetermined value is not limited to the method of multiplication by the fixed correction coefficient. 
     For example, a correction coefficient proportional to a size of a defocusing amount can be acquired, and the defocusing amount can be multiplied by the correction coefficient. Then, when the defocusing amount is large, a largely reduced defocusing amount correction value can be set. According to the present invention, any method for calculating a defocusing amount correction value can be employed as long as it can reduce a defocusing amount of a focus detection result. 
     A second exemplary embodiment of the present invention is a modified example of the first exemplary embodiment of the present invention, and directed to a case where a change in an in-focus state varies depending on whether or not a change in diaphragm aperture area is in an enlarging direction. A difference from the first exemplary embodiment is that a correction coefficient of a focus detection result is changed depending on whether the change in diaphragm aperture area is in the enlarging direction. 
     A configuration of the second exemplary embodiment enables changing of a lens driving amount according to a change in depth of field, and achievement of both reduction in discontinuity of the in-focus state and followability to movement of an object. 
       FIG. 1 , which is a block diagram illustrating the configuration of the imaging apparatus,  FIG. 2  to  FIGS. 6A and 6B , which illustrate the focus detection execution method,  FIG. 7 , which illustrates the change in focus detection light flux at the time of the diaphragm aperture area change, and  FIG. 8 , which illustrates the focus detection method before and after the diaphragm aperture area change according to the first exemplary embodiment can be applied to the second exemplary embodiment. 
     Referring to  FIG. 10 , an operation in a camera  100  according to the second exemplary embodiment is described.  FIG. 10  is a flowchart illustrating a focus adjustment operation stored in a system control unit  50 . 
     In the flowchart, the focus adjustment operation is performed in parallel with electronic viewfinder displaying or moving image recording. In steps having suffixes the same as those of the first exemplary embodiment illustrated in  FIG. 9 , similar processing is executed, and thus description thereof is omitted. 
     Step S 1000  is executed when it is determined that a diaphragm aperture area has changed by at least a predetermined value (YES in step S 505 ). Whether the diaphragm aperture area has increased is determined therein. 
     Generally, when the diaphragm aperture area increases, a depth of field is reduced to cause a change in in-focus state to be conspicuous. When the diaphragm aperture area decreases, in contrast thereto, the change in in-focus state is not conspicuous. Thus, according to the present exemplary embodiment, a correction level of a focus detection result is changed according to enlargement or reduction of the diaphragm aperture area. 
     Instep S 1001 , a focus detection result is corrected. A focus detection result P′ after correction is calculated by the following expression (2):
 
 P′=Ka×P   (2)
 
Ka: coefficient that is a positive number less than 1
 
Thus, without changing the sign, the focus detection result is corrected so that a value can approach 0. Ka corresponds to a first correction coefficient in claims.
 
     In step S 1002 , a focus detection result is corrected. A focus detection result P′ after correction is calculated by the following expression (3):
 
 P′=Kb×P   (3)
 
Kb: coefficient that is a positive number less than 1
 
Thus, without changing the sign, the focus detection result is corrected so that a value can approach 0. Kb corresponds to a second correction coefficient in claims
 
     A relationship between Ka and Kb is described. In step S 1001 , the diaphragm aperture area increases to cause the change in in-focus state to be conspicuous. Hence, sudden lens driving easily generates discontinuous points in the in-focus state during the electronic viewfinder displaying or the moving image recording. In step S 1002 , the reverse occurs because of the reduced diaphragm aperture area. 
     According to the second exemplary embodiment, therefore, Ka is set to a value smaller (value near 0) than that of Kb. Thus, when the change in in-focus state after the diaphragm aperture area change is conspicuous, a lens can be driven more slowly. When the change in in-focus state after the diaphragm aperture area change is not conspicuous, priority is placed on followability to movement of the object, and the lens can be driven faster. The focus adjustment operation of the camera  100  according to the second exemplary embodiment of the present invention has been described. 
     As described above, according to the present exemplary embodiment, based on the enlarged or reduced diaphragm aperture area, the coefficient for correcting the focus detection result is changed, and the focus detection result is corrected in a manner of driving the lens more slowly when the change in the in-focus state is conspicuous. As a result, even during the electronic viewfinder observation or the moving image capturing, natural displaying or recording can be performed with little discontinuity of the in-focus state. 
     According to the second exemplary embodiment, the correction coefficient for the focus detection result is selected based on only the enlargement or the reduction of the diaphragm aperture area. However, the correction coefficient can be selected by using other information. 
     For example, the depth of field changes depending on the lens focal distance. Thus, the correction coefficient for the focus detection result can be selected in view of the focal distance among the pieces of lens information acquired in step S 502 . As a result, a correction value corresponding to the change in depth of field can be selected more accurately. 
     A third exemplary embodiment of the present invention is a modified example of the first exemplary embodiment of the present invention, and directed to a case where a change in an in-focus state varies depending on a detected defocusing amount. A difference from the first exemplary embodiment is that a correction coefficient of a focus detection result is changed depending on the detected defocusing amount. 
     A configuration of the third exemplary embodiment enables changing of a lens driving amount according to the detected defocusing amount, and achievement of both reduction in discontinuity of the in-focus state and followability to movement of an object.  FIG. 1 , which is a block diagram illustrating the configuration of the imaging apparatus according to the first exemplary embodiment can be applied to the third exemplary embodiment. 
     Referring to  FIG. 11 , an operation in a camera  100  according to the third exemplary embodiment is described.  FIG. 11  is a flowchart illustrating a focus adjustment operation stored in a system control unit  50 . 
     In the flowchart, the focus adjustment operation is performed in parallel with electronic viewfinder displaying or moving image recording. In steps having suffixes to the same as those of the first exemplary embodiment illustrated in FIG.  9 , similar processing is executed, and thus description thereof is omitted. 
     Step S 1100  is executed when it is determined that a diaphragm aperture area has changed by at least a predetermined value (YES in step S 505 ). Whether the defocusing amount calculated in step S 504  is larger than a predetermined value is determined. 
     Generally, when the calculated defocusing amount is large, a necessary lens driving amount is also large. When the lens is driven in this state, a change in in-focus state becomes conspicuous. Thus, according to the present exemplary embodiment, a correction level of a focus detection result is changed according to the detected defocusing amount. 
     In step S 1101 , a focus detection result is corrected. A focus detection result P′ after correction is calculated by the following expression (4):
 
 P′=Kc×P   (4)
 
Kc: coefficient that is a positive number less than 1
 
Thus, without changing the sign, the focus detection result is corrected so that a value can approach 0. Kc corresponds to a third correction coefficient in claims.
 
     In step S 1102 , a focus detection result is corrected. A focus detection result P′ after correction is calculated by the following expression (5):
 
 P′=Kd×P   (5)
 
Kd: coefficient that is a positive number less than 1
 
Thus, without changing the sign, the focus detection result is corrected so that a value can approach 0. Kd corresponds to a fourth correction coefficient in claims
 
     A relationship between Kc and Kd is described. In step S 1101 , when the defocusing amount detected in step S 504  is equal to or more than a predetermined value (YES instep S 1100 ), and the lens is driven without any correction, a lens driving amount becomes relatively large. When the lens driving amount is large, because of the conspicuous change in in-focus state, sudden lens driving easily generates discontinuous points in the in-focus state during the electronic viewfinder displaying or moving image recording. 
     In step S 1002 , when the defocusing amount detected in step S 504  is smaller than the predetermined value (NO in step S 1100 ), and the lens is even driven without any correction, a lens driving amount is relatively small. According to the third exemplary embodiment, therefore, Kd is set to a value smaller (value near 0) than that of Kc. Thus, when the change in in-focus state after the diaphragm aperture area change is conspicuous, in other words, when the detected defocusing amount is large, the lens is driven more slowly. 
     When the change in in-focus state is not conspicuous, in other words, when the detected defocusing amount is small, priority is placed on followability to movement of the object, and the lens is driven faster. The focus adjustment operation of the camera  100  according to the third exemplary embodiment of the present invention has been described above. 
     As described above, according to the present exemplary embodiment, based on the size of the detected defocusing amount, the coefficient for correcting the focus detection result is changed, and the focus detection result is corrected in a manner of driving the lens more slowly when the change in the in-focus state is conspicuous. 
     As a result, even during the electronic viewfinder observation or the moving image capturing, natural displaying or recording can be performed with little discontinuity of the in-focus state. According to the third exemplary embodiment, one threshold value is determined to identify the size of the defocusing amount, and two types of correction coefficients are used. However, the number of correction coefficients is not limited to two. 
     By setting a plurality of threshold values for determining a size of a defocusing amount and using corresponding correction coefficients, correction corresponding to finer statuses can be performed. 
     A fourth exemplary embodiment of the present invention is a modified example of the first exemplary embodiment of the present invention, and directed to a case where a size of an error generated during vignetting correction varies because of a difference in vignetting state between a pair of focus detection light fluxes. A difference from the first exemplary embodiment is that a correction coefficient of a focus detection result is changed based on a ratio of vignetting states between the pair of focus detection light fluxes. 
     A configuration of the fourth exemplary embodiment enables changing of a lens driving amount according to a size of an expected vignetting correction error, and achievement of both reduction in discontinuity of the in-focus state and followability to movement of an object.  FIG. 1 , which is a block diagram illustrating the configuration of the imaging apparatus according to the first exemplary embodiment applies to the fourth exemplary embodiment. 
     Referring to  FIGS. 12 and 1 , a vignetting status of a focus detection light flux is described.  FIG. 12  illustrates a lens frame for determining a vignetting status in a certain state of a photographic lens  300 . 
     One lens frame EntW generates vignetting in the focus detection light flux. The lens frame EntW generates vignetting according to pixel positions on an image sensor  14  in association with a diaphragm  312 , which is an exit pupil. The photographic lens  300  has an optical axis L.  FIG. 12  illustrates points of intersection  218   a ,  218   b , and  218   c  between the cross focus detection areas illustrated in  FIG. 6B . The point of intersection  218   a  is between the focus detection areas  218   av  and  218   ah , and similarly the points of intersection  218   b  and  218   c  are between the respective focus detection areas. 
     The lens frame EntW and the diaphragm  312  are different in distance from the image sensor  14  and aperture diameter, and a light flux reaching the image sensor  14  must pass through these two openings. Thus, a light flux reaching a pixel portion other than near the point of intersection  218   a  of the image sensor  14  is affected by not only the diaphragm  312  but also the lens frame EntW. 
       FIGS. 13A and 13B  schematically illustrate a difference in vignetting status of a focus detection light flux between positions on the image sensor  14 .  FIG. 13A  illustrates pixels near the point of intersection  218   a  of the image sensor  14 , and  FIG. 13B  illustrates pixels near the point of intersection  218   b  of the image sensor  14 . Both illustrate focus detection pixels to divide a pupil in a longitudinal direction of the image sensor  14 . 
       FIG. 13A  illustrates two lens frames EntW and  312  that affect vignetting as in the case illustrated in  FIG. 12 , and exit pupil areas  422   HA  and  422   HB  similar to those illustrated in  FIGS. 4A and 4B , through which a pair of focus detection light fluxes pass. The light fluxes passed through the focus detection areas reach pixels S HA  and S HB . 
     However, the exit pupil areas are partially blocked by the diaphragm  312  to generate vignetting, and only light fluxes passed through diagonal-line portions illustrated in  FIG. 13A  reach the pixel. In this case, light amounts of the pair of focus detection light fluxes are nearly equal to each other as indicated by areas of the shaded areas. 
     In  FIG. 13B , how light fluxes are blocked by a diaphragm  312  and a lens frame EntW are different between focus detection areas  422   HA  and  422   HB . Similarly, in  FIG. 13B , light fluxes passed through the shaded areas reach the image sensor  14  without vignetting. 
     In the focus detection area  422   HA , focus detection light flux vignetting occurs due to the lens frame EntW. In the focus detection area  422   HB , focus detection light flux vignetting occurs due to the diaphragm  312 . In this case, as indicated by areas of the shaded areas, light amounts of the pair of focus detection light fluxes are smaller in the area  422   HA  than in the area  422   HB . 
     As described above, when vignetting occurs, based on lens frame information transmuted from the photographic lens  300  to the camera  100 , a vignetting correction value corresponding to a pixel position of the image sensor  14  is calculated, and an output signal from each pixel is corrected. 
     The shaded area in the focus detection area  422   HA  illustrated in  FIG. 13B  is smaller in area than that in the focus detection area  422   HA  illustrated in  FIG. 13A . Generally, therefore, more vignetting occurs to reduce a light amount in a pixel not near the center of the image sensor  14  that in a pixel near the center of the image sensor  14 . 
     Thus, to correct light amounts, in the pixel not near the center, as compared with the pixel near the center of the image sensor  14 , its output must be more greatly amplified in value to be corrected. 
     When pixel outputs contain errors due to shapes of the diaphragm  312  and other lens frames that generate vignetting or manufacturing errors in assembling, when the value is amplified more largely and corrected, the errors contained in the pixel outputs are also amplified. This generates a difference between the pair of acquired focus detection signals. As described above, in AF of a phase difference detection system, focus detection is performed by comparing the pair of focus detection signals acquired from the pair of focus detection light fluxes with each other. 
     Thus, when a degree of matching in shape is low between the pair of focus detection signals, a focus detection error is generated. In other words, the pair of pixels not near the center of the image sensor  14  where nonuniform vignetting occurs as illustrated in  FIG. 13B  more easily contains errors in focus detection result than that near the center of the image sensor  14  where uniform vignetting occurs as illustrated in  FIG. 13A . 
     According to the present exemplary embodiment, therefore, the focus detection result is corrected in view of a difference in vignetting status between the pair of focus detection light fluxes. The lens frame information corresponds to lens information in claims. 
     Referring to  FIG. 14 , an operation in the camera  100  according to the fourth exemplary embodiment is described. FIG.  14  is a flowchart illustrating a focus adjustment operation stored in a system control unit  50 . 
     In the flowchart, the focus adjustment operation is performed in parallel with electronic viewfinder displaying or moving image recording. In steps having suffixes to the same as those of the first exemplary embodiment illustrated in  FIG. 9 , similar processing is executed, and thus description thereof is omitted. 
     Step S 1200  is executed when it is determined that a change in diaphragm aperture area is equal to or more than a predetermined value (YES in step S 505 ). In this case, whether a ratio of a pair of light amount correction values calculated in step S 503  to correct vignetting (i.e., a ratio of vignetting amounts of a pair of light fluxes calculated by a vignetting amount calculation unit (value acquired by dividing a large vignetting amount by a small vignetting amount) is determined. 
     An output of each pixel is multiplied to execute correction by the light amount correction values of the pair of focus detection light fluxes. Thus, the ratio of the pair of light amount correction values indicates a difference in vignetting status between the pair of focus detection light fluxes. 
     As described above, when the difference in vignetting status between the pair of focus detection light fluxes is large, the acquired focus detection result easily contains errors. Thus, according to the present exemplary embodiment, a correction level of a focus detection result is changed according to the difference in vignetting status between the focus detection light fluxes. 
     In step S 1201 , a focus detection result is corrected. A focus detection result P′ after correction is calculated by the following expression (6):
 
 P′=Ke×P   (6)
 
Ke: coefficient that is a positive number less than 1
 
Thus, without changing any code, the focus detection result is corrected so that a value can approach 0. Ke corresponds to a fifth correction coefficient in claims.
 
     In step S 1202 , a focus detection result is corrected. A focus detection result P′ after correction is calculated by the following expression (7):
 
 P′=Kf×P   (7)
 
Kf: coefficient that is a positive number less than 1
 
Thus, without changing any code, the focus detection result is corrected so that a value can approach 0. Kf corresponds to a sixth correction coefficient in claims.
 
     A relationship between Ke and Kf is described. In step S 1201 , the difference in vignetting status between the focus detection light fluxes is large, creating a possibility that the focus detection result may contain errors. Thus, when a lens is driven based on a lens driving amount calculated from the acquired focus detection result, discontinuous points are easily generated in the in-focus state during the electronic viewfinder displaying or the moving image recording. 
     In step S 1202 , the difference in vignetting status between the focus detection light fluxes is small with a low possibility that the focus detection result may contain errors, hence a status is reverse. According to the fourth exemplary embodiment, therefore, Ke is set to a value smaller (value near 0) than that of Kf. 
     When the focus detection result easily contains errors, the lens is driven more slowly. When the focus detection result does not easily contain any errors, priority is placed on followability to movement of the object, and the lens can be driven faster. The focus adjustment operation of the camera  100  according to the fourth exemplary embodiment of the present invention has been described. 
     As described above, according to the present exemplary embodiment, based on the difference in vignetting status between the focus detection light fluxes, the coefficient for correcting the focus detection result is changed, and the focus detection result is corrected in a manner of driving the lens more slowly when the focus detection result easily contains errors. As a result, even during the electronic viewfinder observation or the moving image capturing, natural displaying or recording can be performed with little discontinuity of the in-focus state. 
     According to the fourth exemplary embodiment, one threshold value is determined to determine the ratio of the vignetting correction amounts, and two types of correction coefficients are used. However, the number of correction coefficients is not limited to two. By setting a plurality of threshold values for determining a ratio of vignetting correction amounts and using corresponding correction coefficients, correction corresponding to finer statuses can be performed. 
     According to the fourth exemplary embodiment, the focus detection result is always corrected when the diaphragm aperture area change is equal to or more than the predetermined value. However, this is not necessary. Even when the diaphragm aperture area change is equal to or more than the predetermined value, if a ratio of vignetting correction values is near 1, correction may not be necessary because errors contained in the focus detection result are small. Thus, focusing can be performed at a higher speed following movement of the object. 
     The exemplary embodiments of the present invention have been described above. However, the embodiments are in no way limitative of the present exemplary embodiments. Various changes and modifications can be made within the gist of the invention. 
     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 modifications, equivalent structures, and functions. 
     This application claims priority from Japanese Patent Application No. 2010-205293 filed Sep. 14, 2010, which is hereby incorporated by reference herein in its entirety.