Patent Publication Number: US-8526807-B2

Title: Focus detecting apparatus

Description:
This is a Continuation of International Patent Application No. PCT/JP2009/061019 filed Jun. 17, 2009, which claims priority to Japanese Patent Application No. 2008-211645 filed Aug. 20, 2008. The disclosures of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Incorporation by Reference 
     The disclosure of the Japanese Patent Application No. 2008-211645 filed on Aug. 20, 2008 is herein incorporated by reference. 
     1. Field of the Invention 
     This invention relates to a focus detecting apparatus. 
     2. Related Background of the Invention 
     An apparatus is known in which micro lenses arranged two-dimensionally and a plurality of detecting elements (photoelectric conversion elements) for each micro lens are provided to generate a pair of signal sequences corresponding to respective images caused from light fluxes having passed through different pupil areas of an imaging optical system on the basis of received light outputs obtained from the plurality of detecting elements, and to detect a phase difference in this pair of signal sequences thereby detecting a focus adjustment status of the imaging optical system (Japanese Patent Application, Publication No. 2007-11314). 
     SUMMARY OF THE INVENTION 
     However, the prior art focus detecting apparatus has caused the following problem when being used for an imaging apparatus with an interchangeable shooting lens, such as a single-lens reflex digital camera. That is, the prior art focus detecting apparatus has been required to be designed corresponding to the minimum aperture value among conceivable aperture values in order to be compatible with a plurality of interchangeable lenses having different aperture values. As a consequence, the base line length, which is a space between a pair of groups of detecting elements to be used for focus detecting, has come to be small thereby to cause a problem that the accuracy in focus detecting is deteriorated. 
     Problems to be solved by the present invention include appropriately selecting a pair of groups of detecting elements to be used for focus detecting thereby providing a focus detecting apparatus and an imaging apparatus which are capable of improving the accuracy in focus detecting. 
     According to the first aspect of the present invention, there is configured a focus detecting apparatus comprising: a micro lens array arranged with a plurality of micro lenses; a photo detector that has a plurality of detecting elements provided in correspondence with the micro lenses and receives light flux from an optical system via the micro lenses; and a focus detector that selects a pair of groups of detecting elements from the plurality of detecting elements based on an F-value of the optical system and a brightness of the light flux from the optical system and detects a focus adjustment status of the optical system based on a pair of light receiving signals obtained in the groups of detecting elements. 
     According to the second aspect of the present invention, the focus detector may be configured to determine a space between the pair of groups of detecting elements depending on the F-value. 
     According to the third aspect of the present invention, the focus detecto may be configured to decrease, when the brightness of the light flux is higher, a number of the detecting elements included in the group of detecting elements compared with when the brightness of the light flux is lower. 
     According to the fourth aspect of the present invention, the focus detector may be configured to set a selecting number of the detecting elements in response to outputs of the detecting elements. 
     According to the fifth aspect of the present invention, the focus detector may be configured to select, as the groups of detecting elements, a plurality of detecting elements symmetrical about a position corresponding to a pupil center of the optical system, from the plurality of detecting elements. 
     According to the sixth aspect of the present invention, the focus detector may be configured to convert a relative shift amount in the pair of light receiving signals having been selected to a defocus amount of the optical system, and determines a conversion factor at a time of the converting depending on a space between the pair of groups of detecting elements. 
     According to the present invention, a pair of detecting elements or a pair of groups of detecting elements is appropriately selected to be used for focus detecting thereby allowing to improve the accuracy in focus detecting. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a single-lens reflex digital camera in an embodiment according to the present invention; 
         FIG. 2  is a block diagram illustrating the configuration of a focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 3A  is a view illustrating an optical arrangement in the focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 3B  is a cross-sectional view of a focus detecting optical system and a focus detecting sensor of the camera illustrated in  FIG. 1 ; 
         FIG. 4A  is a plan view illustrating an arrangement status of the focus detecting optical system and the focus detecting sensor of the camera illustrated in  FIG. 1 ; 
         FIG. 4B  is an enlarged plan view illustrating one element of the focus detecting optical system and the focus detecting sensor of the camera illustrated in  FIG. 1 ; 
         FIG. 5  illustrates exemplary matrices for differentiation filters to be used for the focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 6  is a view illustrating a shooting screen to be observed through a view finder of the camera illustrated in  FIG. 1 ; 
         FIG. 7A  is a view for explaining a determining method of the group of elements for focus detecting in the focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 7B  is a view for explaining a determining method of the group of elements for focus detecting in the focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 7C  is a view for explaining a determining method of the group of elements for focus detecting in the focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 7D  is a view for explaining a determining method of the group of elements for focus detecting in the focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 7E  is a view for explaining a determining method of the group of elements for focus detecting in the focus detecting apparatus of the camera illustrated in  FIG. 1 ; 
         FIG. 7F  is a view for explaining a determining method of the group of elements for focus detecting in the focus detecting apparatus of the camera illustrated in  FIG. 1 ; and 
         FIG. 8  is a graph for describing a method for calculating a shift amount D in the focus detecting apparatus of the camera illustrated in  FIG. 1 . 
     
    
    
     DISCRIPTION OF THE PREFERRED EMBODIMENTS 
     While illustrative embodiments will be hereinafter described with reference to the accompanying drawings wherein the present invention is applied to a single-lens reflex digital camera having an interchangeable lens, the present invention is applicable to any imaging apparatus and non-retractable lens type camera which perform focus adjustment of a shooting lens. 
       FIG. 1  is a block diagram illustrating a configuration where an embodiment of the present invention is applied to a single-lens reflex digital camera  1  (hereinafter referred to as simply “camera  1 ”). 
     The camera  1  according to the present embodiment is provided with a camera body  100  and a lens barrel  200 . The camera body  100  and lens barrel  200  are detachably connected to each other by means of a mount  300 . In the camera  1  according to the present embodiment, the lens barrel  200  is interchangeable depending on the purpose of shooting etc. 
     The lens barrel  200  is provided therein with a shooting optical system which comprises shooting lenses  210  including a focus lens  211  and a zoom lens  212 , an aperture device  220 , and other components thereof. 
     The focus lens  211  is provided movably along an optical axis L 1  thereof, and the position of focus lens  211  is adjusted by a lens driving motor  230  while the position or the travel distance of focus lens  211  is detected by an encoder  260 . The focus lens  211  is movable in the direction of the optical axis L 1  by the rotation of a rotating barrel from one end position facing the camera body (near end) to the other end position facing a subject (far end). Note that information regarding the position or the travel distance of focus lens  211  detected by the encoder  260  is transmitted via a lens controller  250  to a lens driving controller  165 . Also note that the lens driving motor  230  is driven by a driving signal, which is received from the lens driving controller  165  via the lens controller  250 , in accordance with a driving distance and a driving speed calculated based on a focus detecting result as will be described later. 
     The aperture device  220  is configured such that the diameter of an aperture centering the optical axis L 1  is adjustable in order to limit an amount of light flux that reaches an image sensor  110  through the shooting lenses  210 . Adjustment of the aperture diameter by the aperture device  220  may be performed through obtaining a signal relevant to an aperture value calculated in an automatic exposure mode, for example, and transmitting the obtained signal from a camera controller  170  to an aperture driver  240  via the lens controller  250 . Alternatively, adjustment of the aperture diameter may also be accomplished through setting an aperture value by manual operation at an operation board  150  provided on the camera body  100 , and transmitting a signal relevant to the set aperture value from the camera controller  170  to the aperture driver  240  via the lens controller  250 . The aperture diameter of aperture device  220  is detected by an aperture diameter detector not shown, and the current aperture diameter is recognized by the lens controller  250 . 
     The lens controller  250  is provided in the lens barrel  200 . The lens controller  250 , which is configured with a microprocessor and peripheral components such as memories, is electrically connected with the camera controller  170  to receive information regarding a defocus amount, an aperture controlling signal and the like from the camera controller  170  and transmit lens information such as aperture value (F value) to the camera controller  170 . 
     In turn, the camera body  100  is provided with a mirror system  120  for guiding the light flux from a subject to the image sensor  110 , a view finder  135 , a photometric sensor  137  and a focus detecting optical system  161 . This mirror system  120  comprises a quick return mirror  121  adapted to pivotally move around a pivot axis  123  within a predetermined angular range between an observing position to a subject and a shooting position to the subject, and a sub mirror  122  pivotally provided at the quick return mirror  121  and adapted to move pivotally in synchronization with the quick return mirror  121 . 
       FIG. 1  illustrates two statuses of the mirror system  120 , one for the observing position to a subject indicated by solid line and the other for the shooting position to the subject indicated by dashed two dotted line. The mirror system  120  moves pivotally between the two statuses, that is, in the observing position to the subject, the mirror system  120  is positioned on the optical path of optical axis L 1 , while in the shooting position to the subject, the mirror system  120  escapes from the optical path of optical axis L 1 . 
     The quick return mirror  121  is configured as a half mirror. In the status of the observation position to a subject, the quick return mirror  121  reflects certain light fluxes (optical axes L 2  and L 3 ) extracted from the light flux from the subject (optical axis L 1 ) to guide respectively toward the view finder  135  and the photometric sensor  137 , and transmits the residual light flux (optical axis L 4 ) to guide toward the sub mirror  122 . In contrast, the sub mirror  122  is configured as a total reflection mirror, and guides the residual light flux (optical axis L 4 ) transmitted through the quick return mirror  121  toward the focus detecting optical system  161 . 
     Accordingly, when the mirror system  120  is positioned at the observing position, the light flux from a subject (optical axis L 1 ) is guided to the view finder  135 , the photometric sensor  137  and the focus detecting optical system  161 , thereby allowing a photographer to observe the subject, and a calculation for exposure and a detection for a focus adjustment status of the focus lens  211  may be performed. Thereafter, if the photographer fully presses a release button, then the mirror system  120  moves pivotally to the shooting position thereby to guide the light flux (optical axis L 1 ) from the subject toward the image sensor  110 , and the image data having been shot is stored into a memory not shown. 
     The image sensor  110  is provided in the camera body  100  so as to be located on the optical axis L 1  of the light flux from a subject and at a position to be a possible focusing plane, and a shutter  111  is provided to face the front surface of the image sensor  110 . The image sensor  110  comprises a plurality of photoelectric conversion elements arranged two-dimensionally, and may be configured as a two-dimensional CCD image sensor, a MOS sensor, a CID, or the like. 
     When a shutter button included in the operation board  150  is fully pressed (i.e. at the time of shutter release), the shutter  111  provided to face the front surface of the image sensor  110  is released based on an exposure calculation result or only during the time period corresponding to the shutter speed set by a photographer thereby to expose the image sensor  110 . The image sensor  110  photo-electrically converts the incident optical image into an electrical image signal, and the electrical image signal is stored into a memory not shown after being image processed in the camera controller  170 . It is to be noted that the memory for storing the electrical image signal may be configured as a built-in memory, a card-type memory, or the like. 
     On the other hand, the light flux from the subject reflected by the quick return mirror  121  forms an image on a focal plane plate  131  arranged at a plane optically equivalent to the image sensor  110 , and is then guided into an ocular globe of the photographer via a pentagonal prism  133  and an ocular lens  134 . At this time, a transmissive-type liquid crystal display device  132  superimposes an indication such as a focus detecting area mark onto the image of a subject on the focal plane plate  131 , and displays relevant information for shooting, such as a shutter speed, an aperture value, and the number of shootings, on an outer area not overlapping the image of the subject. This allows the photographer to observe both the subject and the back-ground thereof, and the relevant information for the shooting and the like, through the view finder  135  in a shooting standby status. 
     The photometric sensor  137 , which is configured as a two-dimensional color CCD image sensor or the like, divides the shooting image into a plurality of areas to output a photometric signal in response to the luminance of each divided area. Image information obtained in the photometric sensor  137  is output to the camera controller  170  thereby to be utilized for an automatic exposure control. 
     The operation board  150 , which includes the shutter release button and input switches for the photographer to set various operating modes, allows a photographer to select from AUTO FOCUS MODE/MANUAL MODE, or select from ONE SHOT MODE/CONTINUOUS MODE selectable especially in the AUTO FOCUS MODE. The shutter release button causes the shutter to be switched ON when being fully pressed. Other than this, when the shutter release button is half-pressed in the AUTO FOCUS MODE, the focusing operation of the focus lens is switched ON, whereas pulling away from the shutter release button turns OFF the focusing operation. Information relevant to various modes set by means of the operation board  150  is sent to the camera controller  170 , and the camera controller  170  controls generally the operation of the camera  1 . 
     The camera body  100  is provided therein with the camera controller  170 . The camera controller  170 , which is configured with a microprocessor and peripheral components such as memories, is electrically connected with the lens controller  250  via an electric signal connector provided on the mount  300 , receives lens information from the lens controller  250 , and sends information including the defocus amount, the aperture controlling signal and the like to the lens controller  250 . In addition, the camera controller  170  reads out image information from the image sensor  110  as described above, and after a certain information processing if required, the image information is output to a memory not shown. Furthermore, the camera controller  170  corrects the shooting image information and detects a status of focus adjustment of the lens barrel  200 , a status of aperture adjustment and the like, thereby ruling general control of the camera  1 . 
     The focus detecting optical system  161 , a focus detecting sensor  162 , a focus detecting calculation unit  163  and a lens driving amount calculation unit  164  shown in  FIG. 1  constitute the focus detecting apparatus of phase difference detecting type, which detects a defocus amount representing the focus adjustment status of the shooting lenses  210 . 
     The focus detecting apparatus according to the present embodiment will be described with reference to  FIGS. 2 to 4B . 
       FIG. 2  is a block diagram illustrating the configuration of the focus detecting apparatus, wherein the configuration of the focus detecting calculation unit  163  illustrated in  FIG. 1  is depicted in detail according to a processing procedure thereof.  FIG. 3A  illustrates in turn an optical arrangement of the focus detecting apparatus,  FIG. 3B  is a cross-sectional view illustrating the focus detecting optical system  161  and the focus detecting sensor  162 ,  FIG. 4A  is a plan view illustrating an arrangement status of the focus detecting optical system  161  and the focus detecting sensor  162 , and  FIG. 4B  is an enlarged plan view illustrating one element of the focus detecting optical system  161  and the focus detecting sensor  162 . 
     The focus detecting optical system  161  is, as illustrated in  FIG. 4A , provided as a micro lens array adapted to have a plurality of micro lenses  161   a  arranged densely in a two-dimensional plane (in a honeycomb structure), and is allocated adjacent to a position P 1  to be a possible focusing plane of the shooting lenses  210 , as illustrated in  FIG. 3A . Hereinafter, the focus detecting optical system  161  is also referred to as the micro lens array  161 . While the micro lens array  161  may be positioned just on the position P 1  to be the possible focusing plane, the micro lens array  161  may be positioned alternatively on a point shifted away from the point P 1  to be the possible focusing plane. Positioning just on the position P 1  may causes a dead zone where the contrast in an image of a subject exists between micro lenses  161   a , whereas shifted positioning from the position P 1  may avoid the appearance of such dead zones. 
     The focus detecting sensor  162  is, as illustrated in  FIG. 4A , provided as a photo-detector array adapted to have a plurality of photoelectric conversion elements  162   a  arranged densely in a two-dimensional plane, and is allocated approximately on focal points of the micro lenses  161   a  constituting the micro lens array  161 , as illustrated in  FIG. 3B . Hereinafter, the focus detecting sensor  162  is also referred to as the photo-detector array  162 . It is to be noted that  FIG. 3B  illustrates beam focusing of each light flux to be received by the photoelectric conversion element  162   a  corresponding to the center of each micro lens  161   a  or the area adjacent to the center. 
       FIG. 4A  is a plan view illustrating of the micro lens array  161  and the photo-detector array  162  seen from the sub mirror  122  to the micro lens array  161 . The photoelectric conversion elements  162   a  are illustrated in the same figure behind only some of the micro lenses  161   a , it is to be understood, however, that the photoelectric conversion elements  162   a  are arranged in the similar manner behind other micro lenses  161   a.    
     Each micro lens  161   a  according to the present embodiment is formed as being cut out from a circular formed micro lens with a lens surface indicated by dashed dotted line into a regular hexagon, and has similar functional capability with the circular formed micro lens. Thus, the micro lens array  161  is formed as being arranged with such regular hexagonal micro lenses  161   a  in a honeycomb structure. Arranging regular hexagonal micro lenses  161   a  in a honeycomb structure in such a manner enables to avoid dead zones of focus detecting which may occur in the case of arranging circular formed micro lenses. Directions of up-and-down and left-and-right indicated in the same figure are similar to those in an image picked up by the image sensor  110 . 
     In contrast, the photo-detector array  162  provided behind the micro lens array  161  is adapted such that square shaped photoelectric conversion elements  162   a  are arranged squarely. Each of the photoelectric conversion elements  162   a  is formed smaller than each micro lens  161   a , and therefore, as enlarged and illustrated in  FIG. 4B , a plurality of photoelectric conversion elements  162   a  are included in an area to which one micro lens  161   a  is projected perpendicularly. Thus, these photoelectric conversion elements  162   a  are photoelectric conversion elements  162   a  provided in correspondence with each micro lens  161   a . Note that the number and the arrangement of photoelectric conversion elements  162   a  are not limited to those illustrated in  FIG. 4B , and may be arbitrarily modified. 
     Incidentally, because the micro lens array  161  is positioned just on or in the vicinity of the position P 1  (a plane being optically equivalent to an imaging plane of the image sensor  110 ) to be the possible focusing plane of the shooting lenses  210 , an optical image is projected onto the micro lens array  161  similarly onto the image sensor  110 . There is obtained a pupil image of the shooting lenses  210  focused onto the photo-detector array  162  by each micro lens  161   a . Each photoelectric conversion element  162   a  of the photo-detector array  162  corresponds to each part of the pupil. Therefore, by selecting appropriate photoelectric conversion elements  162   a  of photo-detector array  162  for each micro lens  161   a  to receive light from that micro lens  161   a  and combining signals therefrom, it is enabled to obtain an image picked up at a certain aperture determined by the selected photoelectric conversion elements  162   a.    
     In the present embodiment, focus detecting is performed according to the procedure described hereinafter. 
     The focus detecting calculation unit  163  illustrated in  FIG. 2  includes an A/D converter  163 A which converts an analog image signal output from the focus detecting sensor (photo-detector array)  162  to a digital image signal to be output into a memory  163 B. The memory  163 B outputs the digital image signal in response to a demand from a two-dimensional image generating unit  163 C and an image signal extracting unit  163 F. 
     At this time, if a focus detecting area AFP depicted in  FIG. 6  (depicted by dashed line in  FIG. 6 ) is selected, then the outputs are read out only from the photoelectric conversion elements  162   a  which are covered by the micro lenses  161   a  within a specific area corresponding to the selected focus detecting area. 
       FIG. 6  illustrates a shooting screen  135 A to be observed through the view finder  135 , and it is assumed that the focus detecting area may be set at an arbitrary location within the shooting screen  135 A in the present embodiment. The focus detecting area is, for example, set to correspond to a location which is selected as the location where a highest contrast is obtained in the image based on outputs from the photoelectric conversion elements  162   a  of the photo-detector array  162 . In this case, if the focus detecting area AFP depicted by dashed line in  FIG. 6  is selected, then the outputs are read out as signals for focus detecting from the photoelectric conversion elements  162   a  corresponding to the micro lenses  161   a  within the specific area centered on the focus detecting point AFP. 
     Note that the focus detecting area may comprise a plurality of areas preliminarily allocated within the shooting screen  135 A. In this case, the liquid crystal display device  132  superimposes marks representing the locations of the plurality of focus detecting areas onto a subject image projected on the focal plane plate  131  thereby to provide an indication. In an operation, a photographer may select a desired focus detecting area AFP using the operation board  150 , or an appropriate focus detecting area AFP is automatically selected based on the contrast of image in the similar manner as the above. 
     Referring again to  FIG. 2 , the two-dimensional image generating unit  163 C determines the center of optical axis for each micro lens  161   a , and generates a pan-focus image from data reflecting the output of focus detecting sensor  162  stored in the memory  163 B and the determined center of optical axis for each micro lens. Thereafter, the two-dimensional image generating unit  163 C receives from the camera controller  170  information relevant to the focus detecting area (AFP in  FIG. 6 , for example) selected as a focus detecting location, and extracts image data within the specific area centered on the focus detecting area from the generated pan-focus image to cut out a selective region. 
     Preceding the process for cutting out the selective region, it is required to calculate a pupil center position of the shooting lenses  210  in order to ensure the conjugate relationship with pupil as described above. The reason why of this is that the micro lens array  161  and the photo-detector array  162  are assembled in usual after being manufactured independently, and as such it is uncertain that which photoelectric conversion element  162   a  corresponds to which micro lens  161   a  and to which position on the corresponding micro lens  161   a . Moreover, because it is expected that the lens barrel  200  may be interchanged for the single-lens reflex camera  1 , the position of pupil of the shooting lenses  210  observed from each micro lens  161   a  possibly changes. Therefore, the position of the photoelectric conversion element  162   a  having a conjugate relationship with the center position of the pupil of shooting lenses  210  is determined as the center of micro lens  161   a.    
     Thereafter, from the image data stored in the memory  16313 , the image data of photoelectric conversion element(s)  162   a  corresponding to the obtained optical axis center of each micro lens  161   a  or corresponding to adjacent area around the center is extracted. 
     The two-dimensional image generated in the two-dimensional image generating unit  163 C in such a manner is identical with an image shot with the aperture corresponding to the photoelectric conversion element  162   a . Assuming for example that the size of photoelectric conversion element  162   a  is 3 μm, the focal distance of micro lens  161   a  is 200 μm, and the distance from the micro lens array  161  to the pupil is 70 mm, then the equivalent size of photoelectric conversion element  162   a  at the pupil is calculated as 1 mm, and a two-dimensional image is to be generated as being substantially equal to an image obtained through an aperture of 1 mmφ. For example, the focal distance of 50 min for the shooting lenses  210  gives the F-value of 50, thereby generating a pan-focus image with deep focal depth. 
     Now, the micro lens array  161  according to the present embodiment has, as illustrated in  FIG. 4A , regular hexagonal micro lenses  161   a  arranged in a honeycomb structure, and therefore the sequence or the arrangement of image data comes to have a honeycomb structure. Consequently, at the time of generating a two-dimensional image, the image data may not be converted as it stands or directly to a pixel arrangement of square arrangement with equal intervals. That is, the positions of centers of respective micro lenses  161   a  in micro lens array  161  are arranged alternatively between even-numbered row and odd-numbered row, and if the pitch in vertical direction is one (arbitral unit), the pitch in horizontal direction is different as being 2/√3 (arbitral unit). Given the foregoing, the two-dimensional image generating unit  163  according to the present embodiment rearranges the image data of such honeycomb arrangement into a square arrangement with equal intervals by performing an interpolation operation or an extrapolation operation. 
     A feature detecting unit  163 D illustrated in  FIG. 2  detects contrasts in a plurality of directions by convolving the pan-focus two-dimensional image generated in the two-dimensional image generating unit  163 C, and selects a direction providing largest accumulated value in terms of the contrast (“convolving” means herein performing convolution as a binary operation, wherein one function f and other function g are added to each other while the function f being translated). 
     Directions allowing dense images to be extracted in the micro lens array  161  with honeycomb arrangement in the present embodiment are the three directions i.e. the horizontal direction X 1  and directions X 2  and X 3  inclined respectively by ±60° (±π/3 rad) to the X 1  direction, as shown in  FIG. 4A . Therefore, contrasts are detected by convolving two-dimensional images in terms of these three directions X 1  to X 3 . Note that any direction may be adopted for detecting contrasts other than these three directions X 1  to X 3 . 
     Contrast detection for these three directions X 1  to X 3  may be performed by incorporating a differentiation filter into a convolution filter for two-dimensional image thereby to image edges of an image having contrast in each direction.  FIG. 5  shows exemplary matrices for differentiation filters applicable to the present embodiment. (A 1 ) to (A 3 ) in  FIG. 5  are matrices presenting Sobel filters (gradient filters) as first-order differentiation filters for two-dimensional images, which detect edge areas in contrasts in terms of the horizontal direction X 1 , π/3 direction X 2 , and 2π/3 direction X 3 , respectively. In contrast, (B 1 ) to (B 3 ) in  FIG. 5  are matrices presenting Laplacian filters as second-order differentiation filters for two-dimensional images, which detect edge areas in contrasts in terms of the horizontal direction X 1 , π/3 direction X 2 , and 2π/3 direction X 3 , respectively. 
     Any of such differentiation filters may be used for contrast detection for the three directions X 1  to X 3  in the present embodiment as illustrated in  FIG. 4A . In addition, the first-order differentiation filters presented by (A 1 ) to (A 3 ) in  FIG. 5  are given as matrices each forwarding in a direction in each of the three directions X 1  to X 3 , and therefore matrices each forwarding in the reverse direction in each of the three directions X 1  to X 3  may be used as presented by (C 1 ) to (C 3 ) in  FIG. 5 , in which elements are reversed. 
     The feature detecting unit  163 D accumulates each contrast obtained for the three directions X 1  to X 3 . This accumulated value is a value which presents a contrast amount for each direction within the specific area centered on the selected focus detecting area AFP. After that, the accumulated values for the three directions X 1  to X 3  are compared with one another to determine either one direction X 1 , X 2  or X 3  which provides the largest contrast. For example, if the contrast for X 1  direction is largest, a predetermined number of data values are extracted along the X 1  direction within the specific area centered on the selected focus detecting area AFP. Hereinafter, description will be continued assuming that the focus detecting direction is the X 1  direction. 
     After determining the X 1  direction as being the focus detecting direction, the feature detecting unit  163 D calculates luminance differences, i.e. contrasts, among the photoelectric conversion elements  162   a  (elements constituting the two-dimensional image) of each micro lens  161   a  within the determined focus detecting direction X 1 . If the luminance value of two-dimensional image is given by V[i, j] (“i” presents the row number of photoelectric conversion element  162   a  for the X 2  direction, and “j” presents the column number of photoelectric conversion element  162   a  for the X 2  direction), then the contrast C[i, j] between adjacent photoelectric conversion elements  162   a  can be obtained from the following equation (1).
 
 C[i, j]=|V[i, j]−V[i+ 1 , j]|   (1)
 
     Thereafter, the position of photoelectric conversion element  162   a  is extracted as a feature point, which corresponds to the center position of micro lens  161   a  where the calculated contrast C[i, j] is relatively large. Note that the feature extraction is not limited to only those by the above equation (1), and any method may be adopted as long as capable of detecting physical quantities relevant to the contrast. 
     Referring still again to  FIG. 2 , a region setting unit  163 E selects, from feature points extracted by the feature detecting unit  163 D, a feature point adjacent to the center of the focus detecting area AFP, and sets a focus detecting region centered on the selected feature point. If, as shown in  FIG. 4A , the extracted feature point for the X 1  direction corresponds to two micro lenses  161 X, then a focus detecting region AFA is assigned centered around those, as shown by dashed dotted line. 
     It is to be noted that, even if the feature point exists at a position apart from the focus detecting area AFP, the focus detecting region AFA may be assigned centered on that feature point. Such assignment allows a part with high contrast to be set as the focus detecting region AFA regardless of the contrast within the selected focus detecting area AFP. 
     The image signal extracting unit  163 F shown in  FIG. 2  reads out from the memory  163 B the output signals from the plurality of photoelectric conversion elements  162   a  corresponding to micro lenses  161   a  within the focus detecting region AFA set by the region setting unit  163 E, and generates a pair of signal sequences for focus detecting, which presents an image shift amount caused from a pair of light fluxes having passed through different pupil areas of the shooting lenses  210 . 
     At the time of generating the pair of signal sequences for focus detecting, the image signal extracting unit  163 F initially determines a group of elements for focus detecting to obtain the pair of signal sequences for focus detecting, from the photoelectric conversion elements  162   a  constituting the photo-detector array  162 , which corresponds to the micro lenses  161   a  within the focus detecting region AFA set by the region setting unit  163 E. The group of elements for focus detecting is determined based on the aperture value of the lens barrel  200 , luminance or brightness of a subject, and the focus detecting direction determined by the feature detecting unit  163 D. Note that the aperture value of the lens barrel  200  is obtained from the lens controller  250 . Also note that the luminance of a subject may be determined based on the outputs from the photo-detector array  162 , which are read out from the memory  163 B. Further the focus detecting direction is a direction having been determined by the feature detecting unit  163 D and is supposed to be the X 1  direction in the present embodiment. 
       FIG. 7A  is a view for explaining a determining method of the group of elements for focus detecting, and more specifically an enlarged plan view of one element of the focus detecting optical system  161  and the focus detecting sensor  162 , wherein a group of elements for focus detecting  162 X has exemplarily determined by the image signal extracting unit  163 F. Hereinafter, a specific determining method for a group of elements for focus detecting will be described referring to the example shown in  FIG. 7A . 
     The image signal extracting unit  163 F initially determines, depending on the aperture value of the lens barrel  200 , photoelectric conversion elements  162   a  which are detectable for light fluxes from a subject, among the photo-detector array  162 . For example, as the example shown in  FIG. 7A , when the aperture value of the lens barrel  200  is F4.0, the size of each photoelectric conversion element  162   a  is 5 μm, the diameter of each micro lens  161   a  is 60 μm, and the focal distance of each micro lens  161   a  is 150 μm, the photoelectric conversion elements  162   a  detectable for light fluxes from a subject are restricted to be within a circle having diameter of 37.5 μm centered on the photoelectric conversion element  162   a  which is located at the center, among the photo-detector array  162 . Note that the circle R 1  shown by dashed line in  FIG. 7A  is an area where the photoelectric conversion elements  162   a  detectable for light fluxes from a subject may exist. 
     Then, the image signal extracting unit  163 F determines, based on the luminance of a subject, the number of photoelectric conversion elements  162   a  which constitute a pair of the groups of elements for focus detecting  162 X.  FIG. 7A  illustrates an example in which the number of photoelectric conversion elements  162   a  constituting each group of elements for focus detecting  162 X is set as being three based on the luminance of a substance. In addition, as shown in  FIG. 7A , the pair of the groups of elements for focus detecting  162 X are formed as being a symmetric pattern. 
     Thereafter, the image signal extracting unit  163 F determines the pair of the groups of elements for focus detecting  162 X, based on the area where the photoelectric conversion elements  162   a  detectable for light fluxes from a subject exist (i.e. the area within the circle R 1  shown by dashed line in  FIG. 7A ), the number of photoelectric conversion elements  162   a  constituting the pair of the groups of elements for focus detecting  162 X, and the focus detecting direction determined by the feature detecting unit  163 D, from the photoelectric conversion elements  162   a  which are detectable for light fluxes from a subject (the photoelectric conversion elements  162   a  which exist within the circle R 1  shown by dashed line in  FIG. 7A ). The pair of the groups of elements for focus detecting  162 X is required to be selected to have a certain base line length and such that this base line length is substantially identical with the X 1  direction as the focus detecting direction. It is preferred that the base line, which is the distance between the pair of the groups of elements for focus detecting  162 X, is large as much as possible because of allowing high accuracy in focus detecting. For this reason, in the case of  FIG. 7A , the pair of the group of elements for focus detecting  162 X is to be selected as shown in the same figure. 
     Note that, at the time of determining the groups of elements for focus detecting  162 X, the reason why the number of photoelectric conversion elements  162   a  constituting the groups of elements for focus detecting  162 X is determined depending on the luminance of a subject is as follows. That is, if the number of the photoelectric conversion elements  162   a  constituting the groups of elements for focus detecting  162 X is determined regardless of the luminance of a subject, then the accuracy in focus detecting varies depending on the luminance of a subject thereby to deteriorate the accuracy in focus detecting in case of low luminance of a subject, for example. To this end, in the present embodiment, if the luminance of a subject is low, then the number of the photoelectric conversion elements  162   a  constituting the groups of elements for focus detecting  162 X is increased in order to gain a high sensitivity with expanded light receiving surface area, while if the luminance of a subject is high, then the number of the photoelectric conversion elements  162   a  constituting the groups of elements for focus detecting  162 X is decreased. 
     For example, when the luminance of a subject is low, the number of the photoelectric conversion elements  162   a  constituting each group of elements for focus detecting  162 X may be set to nine, for example shown in  FIG. 7B , to ensure a sensitivity three times higher than that of the example shown in  FIG. 7A . In contrast, when the luminance of a subject is high, the number of the photoelectric conversion element  162   a  constituting each group of elements for focus detecting  162 X may be set to one, for example shown in  FIG. 7C , and in this case the sensitivity becomes to be one third of the example shown in  FIG. 7A . 
     On the other hand, in the case where the same conditions as those for  FIG. 7A  are adopted except that the aperture value of the lens barrel  200  is F3.0, as shown in  FIG. 7D , the area where the photoelectric conversion elements  162   a  detectable for light fluxes from a subject exist (the area within the circle R 1  shown by dashed line in  FIG. 7D ) comes to be large compared with the case of  FIG. 7A . As described above, it is preferred that the base line, which is the distance between the pair of the groups of elements for focus detecting  162 X, is large as much as possible because of allowing high accuracy in focus detecting. Therefore, in the case of  FIG. 7D , the pair of the group of elements for focus detecting  162 X is to be selected as shown in the same figure. 
     Similarly,  FIG. 7E  illustrates an example where the same conditions as those for  FIG. 7A  are adopted except that the aperture value of the lens barrel  200  is F2.8, and the area where the photoelectric conversion elements  162   a  detectable for light fluxes from a subject exist (the area within the circle R 1  shown by dashed line in  FIG. 7E ) comes to be more large compared with the cases of  FIG. 7A  and  FIG. 7D , as shown in  FIG. 7E . Therefore, in the case of  FIG. 7E , the pair of the groups of elements for focus detecting  162 X is to be selected as shown in the same figure. 
     Moreover, in the case where the aperture value of the lens barrel  200  is F2.8 similarly with the case of  FIG. 7E  and the luminance of a subject is low, the number of the photoelectric conversion elements  162   a  constituting each group of elements for focus detecting  162 X may be set to 27, for example shown in  FIG. 7F , to ensure a sensitivity nine times higher than that of the case shown in  FIG. 7E . 
     It is preferred that, at the time of determining the pair of the groups of elements for focus detecting  162 X, a method is employed in which an arrangement pattern table for groups of elements for focus detecting is preliminarily prepared to have a relationship between each of the aperture values of the lens barrel  200  and the luminance values of a subject and each of the arrangement patterns of the photoelectric conversion elements  162   a  constituting the pair of the groups of elements for focus detecting  162 X on the assumption that the focus detecting direction is the X 1  direction, and the table is used. Using such arrangement pattern table, based on each information for the aperture value of the lens barrel  200  and the luminance of a subject and information for the focus detecting direction determined by the feature detecting unit  163 D, the pair of the groups of elements for focus detecting  162 X matching these information may be determined from the arrangement pattern table. Note that such arrangement pattern table may be stored in the camera controller  170 . 
     Thereafter, the image signal extracting unit  163 F generates a pair of signal sequences for focus detecting, i.e. a first signal sequence {aj} and a second signal sequence {bj} a is natural number), from the output signals of the photoelectric conversion elements  162   a  constituting the pair of the groups of elements for focus detecting  162 X, and outputs these signal sequences to an image shift amount calculation unit  163 G. Here, the suffix j in the first signal sequence {aj} and the second signal sequence {bj} is a natural number depending on the number of the photoelectric conversion elements  162   a  constituting the pair of the groups of elements for focus detecting  162 X. 
     Different from the examples shown in  FIG. 7A  to  FIG. 7F  on the other hand, there may be possibly a case where the feature detecting unit  163 D sets the focus detecting direction as being a direction other than the X 1  direction, i.e. the X 2  direction or the X 3  direction. In this case, for example when the focus detecting direction is set to be the X 2  direction, the above-described arrangement pattern table may not be adopted without modification. For this reason, the arrangement pattern table is used after the coordinate rotation of 60° centering the center position of each micro lens  161   a  according to the following method. 
     More specifically, similarly with the above case of setting the focus detecting direction of the X 1  direction, an arrangement pattern table is prepared based on information including the aperture values of the lens barrel  200 , the focus detecting direction determined by the feature detecting unit  163 D, and luminance values of a subject. Then, the prepared arrangement pattern table is subjected to coordinate rotation of 60° centered on the center position of each micro lens  161   a.    
     Note that, after the coordinate rotation of 60° for the arrangement pattern table, if the optical axis center position of each micro lens  161  is expressed as pm(xm, ym), the relative position of each photoelectric conversion element  162   a  to the center of the pair of the groups of elements for focus detecting  162 X is expressed as P(p, q), then the relative position P(Xr, Yr) of each photoelectric conversion elements  162   a  after the coordinate rotation of 60° may be expressed by the following equation (2), where xm and ym are numerical values having values after the decimal points, and n and u are integers. 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           Xr 
                         
                       
                       
                         
                           Yr 
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       
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     Here, since Pr(Xr, Yr) calculated by the above equation (2) is a numerical value having a value after the decimal point, respective fractional portions s and t of Xr and Yr are obtained by s=Xr−[Xr] and t=Yr−[Yr], respectively, where [Xr] and [Yr] are respective integer portions of Xr and Yr. 
     Then, supposing that the signal output of the photoelectric conversion element  162   a  corresponding to ([Xr], [Yr]) is given by O(xr, yr), the output Od of the photoelectric conversion element  162   a  corresponding to ([Xr], [Yr]) may be obtained by the following equations (3) to (5).
 
 Od 1=(1 −s )· O ( xr, yr )+ s·O ( xr+ 1 , yr )  (3)
 
 Od 2=(1 −s )· O ( xr, yr )+ s·O ( xr+yr )  (4)
 
 Od =(1 −t )· Od 1 +t·Od 2  (5)
 
     Thereafter, by obtaining the output Od for each photoelectric conversion element  162   a  and calculating the value given by the following equation (6) using the above results, a pair of signal sequences for focus detecting i.e. the first signal sequence {aj} and the second signal sequence {bj}, which are based on the pair of the groups of elements for focus detecting  162 X after the coordinate rotation of 60°, may be obtained. 
     
       
         
           
             
               
                 
                   
                     ∑ 
                     
                       p 
                       , 
                       q 
                     
                     
                         
                     
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Od 
                 
               
               
                 
                   ( 
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     Note that, also in the case where the focus detecting direction is set for the X 3  direction, the first signal sequence {aj} and the second signal sequence {bj} may be obtained similarly with the above. 
     Returning to  FIG. 2 , the image shift amount calculation unit  163 G executes an image shift calculation using the first signal sequence {aj} and the second signal sequence {bj}, thereby to calculate a defocus amount. In this calculation, a correlation calculation value Dk with respect to a pair of images (signal sequences) is initially calculated from the following equation (7) utilizing the first signal sequence {aj} and the second signal sequence {bj}. 
     
       
         
           
             
               
                 
                   
                     D 
                     k 
                   
                   = 
                   
                     
                       ∑ 
                       i 
                       
                           
                       
                     
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                           a 
                           
                             i 
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                         - 
                         
                           b 
                           i 
                         
                       
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                   ( 
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                   ) 
                 
               
             
           
         
       
     
     As shown in  FIG. 8 , let the minimum Dk be Di, and Dks adjacent thereto be Di+1 and Di−1. Greater one is selected from Di+1 and Di−1. In this example illustrated in the same figure, Di−1 greater than Di+1 is selected. Thereafter, the selected Di−1 and Di are connected by a straight line L 1  having a slope α. Then, a straight line L 2  having a slope −α and passing through Di+1 is assumed, and the intersection point of straight lines L 1  and L 2  is obtained. Thus, value Y of the intersection point corresponds to a relative shift amount of received light signal. 
     In order to convert the relative shift amount Y to an actual shift amount D (distance to the focal point), the calculation may be performed using the following equation (8) and a factor K depending on the base line length of the photoelectric conversion elements  162   a  constituting the pair of the groups of elements for focus detecting  162 X.
 
 D=K·Y   (8)
 
     Note that the factor K depending on the base line length may be obtained by calculating a sum of moments from respective optical axis centers in terms of the photoelectric conversion elements  162   a  constituting the pair of the groups of elements for focus detecting  162 X. 
     Return again to  FIG. 2 , the lens driving amount calculation unit  164  receives the actual shift amount D transmitted from the focus detecting calculation unit  163 , and calculates a lens driving amount Δd for causing the actual shift amount D to be zero, thereafter outputting the calculated result to the lens driving controller  165 . 
     The lens driving controller  165  transmits a driving command to the lens driving motor  230  while receiving the lens driving amount Δd transmitted from the lens driving amount calculation unit  164 , and drives the focus lens  211  in response to the lens driving amount Δd. 
     As described above, in the camera  1  according to the present embodiment, the pair of the groups of elements for focus detecting  162 X is determined based on the aperture value of the lens barrel  200  and the luminance of a subject, and the focus detecting is performed based thereon. Therefore, a remarkable accuracy in focus detecting can be achieved. 
     It is to be noted that the embodiments as explained above are described to facilitate understanding of the present invention and are not described to limit the present invention. Therefore, it is intended that the elements disclosed in the above embodiments include all design changes and equivalents to fall within the technical scope of the present invention. 
     Although the above embodiments involve hexagonal micro lenses  161   a  arranged in a honeycomb structure, circular micro lenses arranged squarely may also be used. 
     Moreover, the focus detecting sensor  162  is provided as a two-dimensional sensor separate from the image sensor  110  in the present embodiments. Alternatively, micro lenses  161   a  and photoelectric conversion elements  162   a  may be provided as a portion of the image sensor  110  in a similar manner, thereby to enable focus detecting through the procedure described above.