Patent Publication Number: US-7711261-B2

Title: Imaging device, camera and image processing method

Description:
INCORPORATION BY REFERENCE 
   The disclosure of the following priority application is herein incorporated by reference:
     Japanese Patent Application No. 2006-108954 filed Apr. 11, 2006   Japanese Patent Application No. 2006-108957 filed Apr. 11, 2006   

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an imaging device equipped with an image sensor that includes imaging pixels and focus detection pixels, a camera equipped with the imaging device and an image processing method adopted in an image sensor that includes imaging pixels and focus detection pixels. 
   2. Description of the Related Art 
   There is an imaging device known in the related art equipped with an image sensor having imaging pixels and focus detection pixels disposed together on a single substrate, which captures an image formed on the image sensor and also detects the focus adjustment state of the image (see Japanese Laid Open Patent Publication No. 2000-305010). 
   In the imaging device in the related art described above, a virtual imaging pixel output corresponding to the focus detection pixel position is obtained simply by averaging the outputs from the imaging pixels present around the focus detection pixel or simply averaging the focus detection pixel output and the outputs from the imaging pixel around the focus detection pixel. This means that the virtual imaging pixel output obtained through the simple averaging deviates from the output of the imaging pixel that would otherwise occupy the focus detection pixel position if an edge pattern image or a fine line pattern image is superimposed over the focus detection pixel. This, in turn, may lead to the occurrence of color artifacts, a false pattern or a loss of pattern, resulting in poor image quality. 
   SUMMARY OF THE INVENTION 
   According to the 1st aspect of the invention, an imaging device comprises an image sensor that includes a plurality of pixel units disposed in a two-dimensional array, each equipped with a plurality of imaging pixels with varying spectral sensitivity characteristics set regularly and focus detection pixels with sensitivity over a range encompassing the spectral sensitivity of the pixel units, which are set in the array of the pixel units, and an estimating circuit that determines an image output structure corresponding to the position of each focus detection pixel based upon outputs from the imaging pixels present around the focus detection pixel and estimates an image output at the focus detection pixel based upon the image output structure and an output from the focus detection pixel. 
   In the imaging device, the imaging pixels and the focus detection pixels may each include a micro-lens and a photoelectric conversion unit. 
   The focus detection pixels can detect a pair of images formed with a pair of light fluxes passing through a pair of areas at an exit pupil of an optical system that forms an image at the image sensor. 
   According to the 2nd aspect of the invention, an imaging device comprises an image sensor that includes a plurality of imaging pixels with varying spectral sensitivity characteristics and focus detection pixels, an arithmetic operation circuit that determines through arithmetic operation an image output structure corresponding to a position of each focus detection pixel based upon outputs from the imaging pixels, and an output circuit that outputs an image corresponding to the position of the focus detection pixel based upon the image output structure and an output from the focus detection pixel. 
   According to the 3rd aspect of the invention, an imaging device comprises an image sensor that includes a plurality of pixel units each equipped with three different types of pixels set in a Bayer array with sensitivity to red, green and blue, and focus detection pixels disposed in the array of the plurality of pixel units, with sensitivity to all the colors to which the plurality of pixel units are sensitive, an arithmetic operation circuit that determines through arithmetic operation an image color structure corresponding to a position of each focus detection pixel based upon outputs from imaging pixels present around the focus detection pixel, and an output circuit that outputs an image corresponding to the position of the focus detection pixel based upon the image color structure and an output from the focus detection pixel. 
   The focus detection pixels may be disposed at positions on the image sensor that correspond to a horizontal row or a vertical row along which the imaging pixels sensitive to blue and green would otherwise be disposed in a straight line. 
   According to the 4th aspect of the invention, an imaging device comprises an image sensor that includes a plurality of pixel units disposed in a two-dimensional array, each equipped with a plurality of imaging pixels with varying spectral sensitivity characteristics set regularly, and focus detection pixels with sensitivity over a range encompassing the spectral sensitivity of the pixel units, which are set in the array of the pixel units, and an estimating circuit that determines an image output structure corresponding to the position of each focus detection pixel based upon outputs from the imaging pixels present around the focus detection pixel and estimates an image output at the focus detection pixel based upon the image output structure and an output from the focus detection pixel. The estimating circuit estimates the image output at the focus detection pixel by dividing the output from the focus detection pixel in correspondence to the image output structure. 
   According to the 5th aspect of the invention, an imaging device comprises an image sensor that includes a plurality of imaging pixels with varying spectral sensitivity characteristics and focus detection pixels, an arithmetic operation circuit that determines through arithmetic operation an image output structure corresponding to a position of each focus detection pixel based upon outputs from the imaging pixels, and an output circuit that outputs an image corresponding to the position of the focus detection pixel based upon the image output structure and an output from the focus detection pixel. The output circuit outputs the image corresponding to the position of the focus detection pixel, which is obtained by multiplying the output from the focus detection pixel by a ratio corresponding to the image output structure. 
   A camera can be equipped with the imaging device. 
   According to the 6th aspect of the invention, an image processing method comprises output structure ratio calculation processing for determining through arithmetic operation an image output structure corresponding to a position of each focus detection pixel in an image sensor that includes a plurality of pixel units disposed in a two-dimensional array, each equipped with a plurality of imaging pixels with varying spectral sensitivity characteristics set regularly, and focus detection pixels with sensitivity over a range encompassing the spectral sensitivity of the pixel units, which are disposed in the array of the pixel units, based upon outputs from the imaging pixels present around the focus detection pixel, and image output estimation processing for estimating an image output at the focus detection pixel based upon the image output structure and an output from the focus detection pixel. 
   According to the 7th aspect of the invention, an image processing method comprises determining an image output structure corresponding to the position of each focus detection pixel based upon outputs from imaging pixels in an image sensor that includes a plurality of imaging pixels with varying spectral sensitivity characteristics and focus detection pixels and determining an image output at the focus detection pixel based upon the image output structure and an output from the focus detection pixel. 
   According to the 8th aspect of the invention, an imaging device comprises an image sensor that includes a plurality of pixel units disposed in a two-dimensional array, each equipped with a plurality of imaging pixels with varying spectral sensitivity characteristics set regularly, and focus detection pixels with sensitivity over a range encompassing the spectral sensitivity of the pixel units, which are set in the array of the pixel units, and an estimating circuit that corrects outputs from the imaging pixels disposed at low arraying density based upon outputs from the imaging pixels disposed at high arraying density among the imaging pixels present around each focus detection pixel and estimates an image output at the focus detection pixel based upon the corrected outputs from the imaging pixels disposed at low arraying density. 
   The arraying density of the imaging pixels may be arraying density of the imaging pixels corresponding to each of spectral sensitivity characteristics. 
   The imaging pixels and the focus detection pixels may each include a micro-lens and a photoelectric conversion unit. 
   The focus detection pixels may detect a pair of images formed with a pair of light fluxes passing through a pair of areas at an exit pupil of an optical system that forms an image at the image sensor. 
   According to the 9th aspect of the invention, an imaging device comprises an image sensor that includes a plurality of imaging pixels with varying spectral sensitivity characteristics and focus detection pixels, a correction circuit that corrects outputs from the imaging pixels with spectral sensitivity characteristics corresponding to low arraying density based upon outputs from the imaging pixels with spectral sensitivity characteristics corresponding to high arraying density among the imaging pixels present around each focus detection pixels, and an output circuit that outputs an image at the focus detection pixel based upon the outputs from the imaging pixels with the spectral sensitivity characteristics corresponding to the low arraying density, which have been corrected by the correction circuit. 
   According to the 10th aspect of the invention, an imaging device comprises an image sensor that includes a plurality of pixel units each equipped with three different types of pixels set in a Bayer array with sensitivity to red, green and blue, and focus detection pixels disposed in the array of the plurality of pixel units, sensitive to all the colors to which the plurality of pixel units are sensitive, a correction circuit that corrects outputs from the imaging pixels set with low arraying density based upon outputs from the imaging pixels set with high arraying density among the imaging pixels present around each focus detection pixel, and an output circuit that outputs an image at the focus detection pixel based upon the output from the imaging pixels set with the low arraying density, which have been corrected by the correction circuit. 
   The focus detection pixels may be disposed at positions on the image sensor that correspond to a horizontal row or a vertical row along which the imaging pixels sensitive to blue and green would otherwise be disposed in a straight line. 
   According to the 11th aspect of the invention, an imaging device comprises an image sensor that includes a plurality of pixel units disposed in a two-dimensional array, each equipped with a plurality of imaging pixels with varying spectral sensitivity characteristics set regularly, and focus detection pixels with sensitivity over a range encompassing the spectral sensitivity of the pixel units, which are set in the array of the pixel units, and an estimating circuit that converts outputs from the imaging pixels set with low arraying density to outputs from pixels near each of the focus detection pixel based upon outputs from the imaging pixels set with high arraying density, which are present near the imaging pixels set with the low arraying density, among the imaging pixels present around the focus detection pixel and outputs from the imaging pixels set with the high arraying density, which assume positions closer to the focus detection pixel than the imaging pixels set with the low arraying density, and estimates an image output at the focus detection pixel based upon the converted pixel output. 
   According to the 12th aspect of the invention, an imaging device comprises an image sensor that includes a plurality of imaging pixels with varying spectral sensitivity characteristics and focus detection pixels, a conversion circuit that converts outputs from the imaging pixels with spectral sensitivity characteristics corresponding to low arraying density to an output at each focus detection pixel based upon outputs from the imaging pixels with spectral sensitivity characteristics corresponding to a high arraying density, which are present near the imaging pixels with spectral sensitivity characteristics corresponding to low arraying density, among the imaging pixels present around the focus detection pixel and outputs from the imaging pixels set with the high arraying density, which assume positions closer to the focus detection pixel than the imaging pixels with the spectral sensitivity characteristics corresponding to the low arraying density, and an output circuit that outputs an image at the focus detection pixel based upon the outputs from the imaging pixels with the spectral sensitivity characteristics corresponding to the low arraying density, having been converted by the conversion circuit. 
   A camera can be equipped with an imaging device. 
   According to the 13th aspect of the invention, an image processing method comprises correction processing for correcting outputs from imaging pixels set at low arraying density based upon outputs from imaging pixels set at high arraying density among imaging pixels present around each focus detection pixel in an image sensor that includes a plurality of pixel units disposed in a two-dimensional array, each equipped with a plurality of imaging pixels with varying spectral sensitivity characteristics set regularly, and focus detection pixels with sensitivity over a range encompassing the spectral sensitivity of the pixel units, which are disposed in the array of the pixel units, and estimation processing for estimating an image output at the focus detection pixel based upon the outputs from the imaging pixels set at the low arraying density, which have been corrected through the correction processing. 
   According to the 14th aspect of the invention, an image processing method comprises correcting outputs from imaging pixels with spectral sensitivity characteristics corresponding to low arraying density based upon outputs from imaging pixels with spectral sensitivity characteristics corresponding to high arraying density among imaging pixels present around each focus detection pixel in an image sensor that includes a plurality of imaging pixels with varying spectral sensitivity characteristics and focus detection pixels, and determining an image output at the focus detection pixel based upon the corrected outputs from the imaging pixels with the spectral sensitivity characteristics corresponding to the low arraying density. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the structure adopted in the digital still camera in an embodiment of the present invention; 
       FIG. 2  shows the focus-detection areas on the imaging plane set on the predetermined imaging plane of the exchangeable lens; 
       FIG. 3  shows a Bayer array in color filters; 
       FIG. 4  shows the spectral characteristics of the individual color filters; 
       FIG. 5  shows the spectral characteristics of the green pixels; 
       FIG. 6  shows the spectral characteristics of the focus detection pixels; 
       FIG. 7  shows in detail the structure of the image sensor; 
       FIG. 8  shows the structure of the imaging pixels; 
       FIG. 9  shows the structure of the focus detection pixels; 
       FIG. 10  shows an arrangement that may be adopted for complementary color filters; 
       FIG. 11  is a sectional view of an imaging pixel; 
       FIG. 12  is a sectional view of a focus detection pixel; 
       FIG. 13  illustrates focus detection executed through a pupil division-type method; 
       FIG. 14  is a front view, showing the relationship between the projection areas at the exit pupil; 
       FIG. 15  shows the imaging pixel/focus detection pixel output ratio coefficient relative to the aperture F value; 
       FIG. 16  presents a flowchart of the operations executed in the digital still camera in the embodiment; 
       FIGS. 17A-17C  present a flowchart of the image offset detection calculation processing (correlational algorithm); 
       FIG. 18  presents a flowchart of the interpolation processing; 
       FIG. 19  is a partial enlargement of the image sensor, provided to facilitate the explanation of the interpolation processing; 
       FIG. 20  shows the image sensor in a variation; 
       FIG. 21  shows the image sensor in yet another variation; 
       FIG. 22  is a diagram provided to facilitate the explanation of the interpolation processing executed in the image sensor in a variation; 
       FIG. 23  shows the image sensor in yet another variation; 
       FIG. 24  shows the image sensor in yet another variation; and 
       FIGS. 25A and 25B  show a variation of the focus detection pixels. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An explanation is now given on an embodiment by adopting the present invention in a digital still camera as an imaging device.  FIG. 1  shows the structure of the digital still camera in the embodiment. The digital still camera  201  in the embodiment comprises an exchangeable lens unit  202  and a camera body  203 . The exchangeable lens unit  202  is mounted on a mount unit  204  at the camera body  203 . 
   The exchangeable lens unit  202  includes lenses  205 - 207 , an aperture  208  and a lens drive control device  209 . It is to be noted that the lens  206  is a zooming lens and that the lens  207  is a focusing lens. The lens drive control device  209 , constituted with a CPU and its peripheral components, controls the drive of the focusing lens  207  and the aperture  208 , detects the positions of the zooming lens  206 , the focusing lens  207  and the aperture  208  and transmits lens information and receives camera information by communicating with a control device in the camera body  203 . 
   An image sensor  211 , a camera drive control device  212 , a memory card  213 , an LCD driver  214 , an LCD  215 , an eyepiece lens  216  and the like are mounted at the camera body  203 . The image sensor  211 , set at the predetermined imaging plane (predetermined focal plane) of the exchangeable lens unit  202 , captures a subject image formed through the exchangeable lens unit  202  and outputs image signals. At the image sensor  211 , pixels used for imaging (hereafter simply referred to as imaging pixels) are disposed two-dimensionally, and rows of pixels used for focus detection (hereafter simply referred to as focus detection pixels), instead of imaging pixels, are disposed in the two-dimensional array over areas corresponding to focus detection positions. 
   The camera drive control device  212 , constituted with a CPU and its peripheral components, controls the drive of the image sensor  211 , processes the captured image, executes focus detection and focus adjustment for the exchangeable lens unit  202 , controls the aperture  208 , controls display operation at the LCD  215 , communicates with the lens drive control device  209  and controls the overall operational sequence in the camera. It is to be noted that the camera drive control device  212  communicates with the lens drive control device  209  via an electrical contact point  217  at the mount unit  204 . 
   The memory card  213  is an image storage device in which captured images are stored. The LCD  215  is used as a display unit of a liquid crystal viewfinder (EVF: electronic viewfinder). The photographer is able to visually check a captured image displayed at the LCD  215  via the eyepiece lens  216 . 
   The subject image formed on the image sensor  211  after passing through the exchangeable lens unit  202  undergoes photoelectric conversion at the image sensor  211  and the post-photoelectric conversion output is provided to the camera drive control device  212 . The camera drive control device  212  determines through arithmetic operation the defocus amount at a focus detection position based upon the outputs from the focus detection pixels and transmits the defocus amount to the lens drive control device  209 . In addition, the camera drive control device  212  provides image signals generated based upon the outputs from the imaging pixels to the LCD driver  214  so as to display the captured image at the LCD  215  and also stores the image signals into the memory card  213 . 
   The lens drive control device  209  detects the positions of the zooming lens  206 , the focusing lens  207  and the aperture  208  and obtains through arithmetic operation the lens information based upon the detected positions. It is to be noted that the lens information corresponding to the detected positions may be selected from a lookup table prepared in advance. The lens information is then provided to the camera drive control device  212 . In addition, the lens drive control device  209  calculates a lens drive amount indicating the extent to which the lens is to be driven based upon the defocus amount received from the camera drive control device  212 , and controls the drive of the focusing lens  207  based upon the lens drive amount. 
     FIG. 2  shows focus detection areas on an imaging plane G set on the predetermined imaging plane of the exchangeable lens unit  202 . Focus detection areas G 1  to G 5  are set on the imaging plane G and the focus detection pixels in the image sensor  211  are arrayed in a straight-line extending along the length of each of the focus detection areas G 1  to G 5 . Namely, focus detection pixel rows on the image sensor  211  sample the image portions in the focus detection areas G 1  to G 5  within the subject image formed on the imaging image plane G. The photographer manually selects a given focus detection area among the focus detection areas G 1  to G 5  in correspondence to a specific photographic composition. 
     FIG. 3  shows the positional arrangement adopted for color filters disposed at the image sensor  211 . Color filters set in a Bayer array, as shown in  FIG. 3 , are each disposed at one of the imaging pixels arrayed two-dimensionally on the substrate of the image sensor  211 . It is to be noted that while  FIG. 3  shows the color filter positional arrangement corresponding to a four-pixel imaging area, this imaging pixel unit adopting the color filter positional arrangement over the four-pixel (2×2) area is two-dimensionally reiterated. In the Bayer array, two pixels, each equipped with a G (green) filter, are disposed diagonally across from each other, and a pair of pixels, one equipped with a B (blue) filter and the other equipped with an R (red) filter, are disposed diagonally across from each other, perpendicular to the G filter pixels. In other words, the green pixels are disposed at higher density compared to the red pixels and the blue pixels in the Bayer array. 
   The overall spectral characteristics of the green pixels, the red pixels and the blue pixels, integrating the spectral sensitivities of the color filters corresponding to the individual colors, the spectral sensitivity of the photodiodes engaged in photoelectric conversion and the spectral characteristics of infrared clipping filters (not shown), are indicated in the graph presented in  FIG. 4 . The green pixels, the red pixels and the blue pixels, at which color separation occurs, have sensitivity over different light wavelength ranges (different spectral sensitivity). The spectral characteristics of the green pixels indicate a sensitivity level close to the peak of the relative luminosity shown in  FIG. 5 . 
   It is to be noted that in order to obtain a sufficient light amount, no color filters are disposed at the focus detection pixels and thus, the focus detection pixels have the spectral characteristics shown in  FIG. 6 . The spectral sensitivity of the photodiodes engaged in photoelectric conversion and the spectral characteristics of the infrared clipping filters (not shown) are integrated in the spectral characteristics shown in  FIG. 6 . Namely, the spectral characteristics in  FIG. 6  are similar to the sum of spectral characteristics of the green pixels, the red pixels and the blue pixels shown in  FIG. 4 , which indicate sensitivity in a light wavelength range containing the wavelength ranges corresponding to all the pixels, i.e., the green pixels, the red pixels in the blue pixels. 
     FIG. 7  is a front view showing in detail the structure of the image sensor  211 . It is to be noted that  FIG. 7  shows part of the image sensor  211 , near a given focus detection area set on the image sensor  211 , in an enlargement. The image sensor  211  comprises imaging pixels  310  and focus detection pixels  311  used for focus detection. 
   As shown in  FIG. 8 , the imaging pixels each comprise a micro-lens  10 , a photoelectric conversion unit  11  and a color filter (not shown). As shown in  FIG. 9 , the focus detection pixels  311  each comprise a micro-lens  10  and a pair of photoelectric conversion units  12  and  13 . The photoelectric conversion unit  11  at each imaging pixel  310  is designed in a shape that allows the photoelectric conversion unit to receive all the light flux passing through the exit pupil of the exchangeable lens at a light setting (e.g., an exit pupil equivalent to F 1.0) via the micro-lens  10 . In addition, the pair of photoelectric conversion units  12  and  13  at each focus detection pixel  311  are designed in a shape that allows the photoelectric conversion units to receive all the light flux passing through a specific exit pupil of the exchangeable lens (e.g., an exit pupil equivalent to F 2.8) via the micro-lens  10 . 
   As shown in  FIG. 7 , color filters corresponding to R, G and B colors are disposed at two-dimensionally arrayed imaging pixels  310  so as to achieve an RGB Bayer array. The focus detection pixels  311  are densely disposed along a straight-line without allowing any interval at a horizontal (or vertical) row that would otherwise be occupied by imaging pixels  310  with B filters and G filters in each of the focus detection areas G 1  to G 5  shown in  FIG. 2 . The following advantage is achieved by disposing the focus detection pixels  311  along the horizontal (or vertical) row that would otherwise be occupied by imaging pixels  310  equipped with B filters and G filters. Namely, a slight error, occurring when generating pixel signals at positions occupied by the focus detection pixels  311  through pixel interpolation to be detailed later, can remain more or less unnoticeable to the human eye, since the human eye is more perceptive to red than to blue and the green pixels, disposed with higher density compared to the blue pixels and the red pixels, have a lower image defect contribution factor per pixel. 
   It is to be noted that the color filters may be arranged in an array other than the Bayer array shown in  FIG. 3 . For instance, an imaging pixel unit that includes complementary color filters, i.e., a G (green) filter, a Ye (yellow) filter, an Mg (magenta) filter and a Cy (cyan) filter, as shown in  FIG. 10 , may be reiterated two-dimensionally. At an image sensor achieved by two-dimensionally reiterating the imaging pixel unit with these complementary color filters, the focus detection pixels  311  should be disposed at pixel positions that would otherwise be occupied by imaging pixels equipped with cyan filters and magenta filters, i.e., imaging pixels equipped with color filters that contain a blue component with which the output error is relatively unnoticeable. 
     FIG. 11  is a sectional view of an imaging pixel  310 . The micro-lens  10  is set to the front of the imaging photoelectric conversion unit  11  at the imaging pixel  310  and, as a result, an image of the photoelectric conversion unit  11  is projected frontward via the micro-lens  10 . The photoelectric conversion unit  11  is formed on a semiconductor circuit substrate  29 , with a color filter (not shown) disposed between the micro-lens  10  and the photoelectric conversion unit  11 . 
     FIG. 12  is a sectional view of a focus detection pixel  311 . In the focus detection pixel  311 , the micro-lens  10  is disposed to the front of the photoelectric conversion units  12  and  13  used for focus detection and thus, images of the photoelectric conversion units  12  and  13  are projected frontward via the micro-lens  10 . The photoelectric conversion units  12  and  13  are formed on the semiconductor circuit substrate  29 . 
   Next, in reference to  FIG. 13 , focus detection executed by adopting a pupil division-type method is explained.  FIG. 13  shows a micro-lens  50 , a pair of photoelectric conversion units  52  and  53 , a micro-lens  60  and a pair of photoelectric conversion units  62  and  63 . The micro-lens of a specific focus detection pixel  311  is set on an optical axis  91  of the exchangeable lens unit  202 . The pair of photoelectric conversion units  52  and  53  are disposed to the rear of the micro-lens  50 . The micro-lens  60  of another focus detection pixel  311  is set off the optical axis  91  of the exchangeable lens unit  202 . The pair of photoelectric conversion units  62  and  63  are disposed to the rear of the micro-lens  60 . 
   An exit pupil  90  of the exchangeable lens unit  202  is set at a position assumed over a distance d 4  to the front of the micro-lenses  50  and  60  disposed on the predetermined imaging plane of the exchangeable lens unit  202 . The distance d 4  takes a value determined in correspondence to the curvature and the refractive index of the micro-lenses  50  and  60 , the distance between the micro-lenses  50  and  60  and the photoelectric conversion units  52 / 53  and  62 / 63  and the like. In the description, the distance d 4  is referred to as a range-finding pupil distance. 
   The micro-lenses  50  and  60  are set at the predetermined imaging plane of the exchangeable lens unit  202 . The shapes of the pair of photoelectric conversion units  52  and  53  are projected via the micro-lens  50  set on the optical axis  91  onto the exit pupil  90  set apart from the micro-lenses  50  by the projection distance d 4  and the projected shapes define range-finding pupils  92  and  93 . The shapes of the pair of photoelectric conversion units  62  and  63  are projected via the micro-lens  60  set off the optical axis  91  onto the exit pupil  90  set apart by the projection distance d 4  and the projected shapes define range-finding pupils  92  and  93 . Namely, the projecting direction for each pixel is determined so that the projected shapes (range-finding pupils  92  and  93 ) of the photoelectric conversion units in the individual pixels are aligned on the exit pupil  90  set over the projection distance d 4 . 
   The photoelectric conversion unit  52  outputs a signal corresponding to the intensity of an image formed on the micro-lens  50  with a focus detection light flux  72  having passed through the range-finding pupil  92  and having advanced toward the micro-lens  50 . The photoelectric conversion unit  53  outputs a signal corresponding to the intensity of an image formed on the micro-lens  50  with a focus detection light flux  73  having passed through the range-finding pupil  93  and having advanced toward the micro-lens  50 . Also, the photoelectric conversion unit  62  outputs a signal corresponding to the intensity of an image formed on the micro-lens  60  with a focus detection light flux  82  having passed through the range-finding pupil  92  and having advanced toward the micro-lens  60 . The photoelectric conversion unit  63  outputs a signal corresponding to the intensity of an image formed on the micro-lens  60  with a focus detection light flux  83  having passed through the range-finding pupil  93  and having advanced toward the micro-lens  60 . It is to be noted that the focus detection pixels  311  are arrayed in a direction matching the direction along which the pair of range-finding pupils are separated from each other. 
   Numerous focus detection pixels each structured as described above are arranged in a straight row and the outputs from the pairs of photoelectric conversion units at the individual pixels are integrated into output groups each corresponding to the range-finding pupils  92  and  93 . Thus, information related to the intensity distribution of the pair of images formed on the focus detection pixel row with the individual focus detection light fluxes passing through the range-finding pupil  92  and the range-finding pupil  93  is obtained. Next, image shift detection calculation processing (correlational processing, phase difference detection processing) to be detailed later is executed by using the information thus obtained so as to detect the image shift amount between the pair of images through the pupil division-type detection method. The image shift amount is then multiplied by a predetermined conversion coefficient and, as a result, the extent of deviation (defocus amount) of the current image forming plane (the image forming plane on which the image is formed at the focus detection position corresponding to a specific micro-lens array position on the predetermined imaging plane) relative to the predetermined imaging plane can be calculated. 
   It is to be noted that  FIG. 13  schematically shows the focus detection pixel (the micro-lens  50  and the pair of photoelectric conversion units  52  and  53 ) disposed on the optical axis  91  and the adjacent focus detection pixel (the micro-lens  60  and the pair of photoelectric conversion units  62  and  63 ). At each of the other focus detection pixels, too, the light fluxes arriving at the micro-lens from the pair of range-finding pupils are received at the pair of photoelectric conversion units. 
     FIG. 14  is a front view, showing the relationship between the projection areas at the exit pupil plane. The circle circumscribing the range-finding pupils  92  and  93  formed by projecting the pair of photoelectric conversion units  12  and  13  in the focus detection pixel  311  onto the exit pupil plane  90  via the micro-lens  10 , viewed from the image forming plane, assumes a specific opening F value (referred to as a range-finding pupil F value in this description, F 2.8 in this example). The photoelectric conversion unit  11  in each imaging pixel  310  is projected onto the exit pupil plane  90  over an area  94  via the micro-lens  10 . The area  94  is large enough to contain both the range-finding pupils  92  and  93 . 
   An area  95  corresponds to an aperture (F 1) at the lightest setting, whereas an area  96  corresponds to an aperture (F 5.6) at a dark setting. The area  94  is greater than the area  95  or  96  and thus, the light flux received at the imaging pixel  310  is restricted at the aperture. In other words, the output from the imaging pixel  310  changes in correspondence to the aperture value. The range-finding pupils  92  and  93  are smaller than the area  95  but are larger than the areas  96 . This means that the outputs from the focus detection pixels  311  remain unchanged even when the aperture value changes, as long as the aperture value corresponds to a setting brighter than F 2.8. However, if the aperture value corresponds to a setting darker than F 2.8, the outputs of the focus detection pixels  311  change in correspondence to the aperture value. 
     FIG. 15  presents a graph of the output ratio coefficient (in a standardized form by assuming the value of 1 in correspondence to the aperture value F 2.8), indicating the ratio; imaging pixel output/focus detection pixel output. As shown in  FIG. 15 , the output ratio coefficient assumes a constant value 1 regardless of the aperture value as long as it indicates a setting darker than F 2.8, whereas the output ratio coefficient increases in proportion to the aperture value once the aperture value assumes a setting brighter than F 2.8. 
     FIG. 16  presents a flowchart of the operations executed in the digital still camera (imaging device) in the embodiment. After the power is turned on at the camera body  203  in step S 100 , the camera drive control device  212  repeatedly executes the processing in step S 110  and subsequent steps. In step S 110 , a culled read of the data from the imaging pixels  310  is executed and the data thus read out through the culled read are displayed at the electronic viewfinder (LCD  215 ). The number of sets of data from the focus detection pixels  311  read out through the culled read of the data from the imaging pixels  310  should be minimized, so as to assure better display quality. However, data from the focus detection pixels  311  may also be read out through the culled read and the focus detection pixel outputs in an uncorrected form may be displayed on the electronic viewfinder (LCD  215 ) so as to enable the user to identify the focus detection position with ease. 
   In step S 120  following step S 110 , data are read out from the focus detection pixel row. It is to be noted that the focus detection area has been selected by the photographer via a selection means (not shown). In step S 130  following step S 120 , the defocus amount is calculated by executing image shift detection•calculation processing based upon the pair of image data corresponding to the focus detection pixel row. 
   The image shift detection calculation processing (correlational algorithm) is now explained in reference to  FIG. 17 . A correlation quantity C(L) is first calculated by using the differential correlational algorithm expressed in (1) below, with ei and fi (i=1 to m) representing the pair of sets of data corresponding to each focus detection pixel row.
 
 C ( L )=Σ| e ( i+L )− f ( i )|  (1)
 
L in expression (1) is an integer representing the relative shift amount indicated in units corresponding to the pitch at which the pair of sets of data are detected. In addition, L assumes a value within a range Lmin to Lmax (−5 to +5 in the figure). Σ indicates total sum calculation over a range expressed as i=p to q, with p and q satisfying a conditional expression 1≦p&lt;q≦m. The specific values assumed for p and q define the size of the focus detection area.
 
   As shown in  FIG. 17A , the results of the calculation executed as expressed in (1) indicate the smallest correlation quantity C(L) in correspondence to the shift amount L=kj (kj=2 in  FIG. 17A ) indicating a high level of correlation between the pair of sets of data. Next, a shift amount x, which will provide the minimum value C(L)min=C(x) in a continuous curve representing the correlation quantities, is determined through the three-point interpolation method as expressed in (2) to (5) below.
 
 x=kj+D /SLOP  (2)
 
 C ( x )= C ( kj )−| D|   (3)
 
 D={C ( kj− 1)− C ( kj+ 1)}/2  (4)
 
SLOP=MAX{ C ( kj+ 1)− C ( kj ),  C ( kj− 1)− C ( kj )}  (5)
 
   In addition, a defocus amount. DEF representing the extent of defocusing of the subject image plane relative to the predetermined imaging plane can be determined as expressed in (6) below based upon the shift amount x having been calculated as expressed in (2).
 
 DEF=KX·PY·x   (6)
 
PY in expression (6) represents the detection pitch, whereas KX in expression (6) represents the conversion coefficient that is determined in correspondence to the opening angle formed with the gravitational centers of the pair of range-finding pupils.
 
   The judgment as to whether or not the calculated defocus amount DEF is reliable is made as follows. As shown in  FIG. 17B , the interpolated minimum value C(X) of the correlation quantity increases when the level of correlation between the pair of sets of data is low. Accordingly, if C(X) is equal to or greater than a predetermined value, the defocus amount is judged to be less reliable. Alternatively, C(X) may be standardized with regard to the data contrast, and in such a case, if the value obtained by dividing C(X) by SLOP indicating a value in proportion to the contrast is equal to or greater than a predetermined value, the defocus amount should be judged to be not reliable. As a further alternative, if SLOP indicating the value in proportion to the contrast is equal to or less than a predetermined value, the subject should be judged to be a low contrast subject and, accordingly, the reliability of the calculated defocus amount DEF should be judged to be low. 
   If the level of correlation between the pair of sets of data is low and the correlation quantity C (L) does not dip at all over the shift range Lmin to Lmax, as shown in  FIG. 17C , the minimum value C(X) cannot be determined. Under such circumstances, it is judged that the focus detection is disabled. 
   To continue the explanation of the camera operations in reference to  FIG. 16 , a decision is made in step S 140  as to whether or not the focusing lens  207  is set near the focus match position i.e., whether or not the absolute value of the defocus amount having been calculated is equal to or less than a predetermined value. If it is decided that the focusing lens  207  is not near the focus match position, the operation proceeds to step S 150 . In step S 150 , the defocus amount is transmitted to the lens drive control device  209  in the exchangeable lens unit  202 . The lens drive control device  209  drives the focusing lens  207  in the exchangeable lens unit  202  to the focus match position. Once the processing in step S 150  is executed, the operation returns to step S 110  to repeatedly execute the operations described above. 
   It is to be noted that if it is decided in step S 140  that the focus detection is not possible, too, the operation proceeds to step S 150 . In this situation, a scan drive instruction is transmitted to the lens drive control device  209  in the exchangeable lens unit  202 . In response, the lens drive control device  209  drives the focusing lens  207  in the exchangeable lens unit  202  to scan through the infinity to close-up range. Once the processing in step S 150  is executed, the operation returns to step S 110  to repeatedly execute the operations described above. 
   If, on the other hand, it is decided in step S  140  that the focusing lens  207  is set near the focus match position, the operation proceeds to step S 160 . In step S 160 , a decision is made as to whether or not a shutter release has been executed through an operation at a shutter release button (not shown). If it is decided that a shutter release has not been executed, the operation returns to step S 110  to repeatedly execute the operations described above. If, on the other hand, it is decided that a shutter release has been executed, the operation proceeds to step S 170 . 
   In step S 170 , an aperture adjustment instruction is transmitted to the lens drive control device  209  in the exchangeable lens unit  202  to adjust the aperture value at the exchangeable lens unit  202  to a control F value (an F value set by the user or an automatically selected F value). Once the aperture control ends, the image sensor  211  is engaged in imaging operation and image data from the imaging pixels  310  and all the focus detection pixels  311  at the image sensor  211  are read out. 
   In step S 180  following step S 170 , image data at pixel positions occupied by the focus detection pixels in the focus detection pixel row are interpolated based upon the data output from the focus detection pixels  311  and the data output from the surrounding imaging pixels  310 . This interpolation processing is to be described in detail later. In step S 190  following step S 180 , image data constituted with the data output from the imaging pixels  310  and the interpolation data at the focus detection pixels  311  are saved into the memory card  213 . Once the image data are saved, the operation returns to step S 110  to repeatedly execute the operations described above. 
     FIG. 18  presents a flowchart of the interpolation processing. To facilitate the explanation, the output from a green pixel (a pixel with the green filter) in the imaging pixel  310  is referred to as Gn, the output from a red pixel (a pixel with the red filter) in the imaging pixel  310  is referred to as Rn, the output from a blue pixel (a pixel with the blue filter) in the imaging pixel  310  is referred to as Bn and the output from a focus detection pixel  311  is referred to as AFn (n=1, 2, . . . ), as shown in  FIG. 19 . It is to be noted that while the flow of the interpolation processing is explained in reference to a specific target pixel, similar processing is executed for all the focus detection pixels  311 . The following explanation focuses on the interpolation executed to obtain image data (a virtual imaging pixel output at a focus detection pixel X) at the focus detection pixel X (output AF  3 ) that would otherwise be occupied by a blue pixel and on the interpolation executed to generate image data (a virtual imaging pixel output at a focus detection pixel Y) at the focus detection pixel Y (output AF  2 ) that would otherwise be occupied by a green pixel. 
   After starting the interpolation processing in step S 200 , information related to the characteristics of the image ranging around the focus detection pixel row is obtained through calculation in step S  210 . Namely, the parameters (Gau, Gad, Rau, Rad, Bau and Bad) indicating the average values of the individual colors on the two sides above and below the focus detection pixel row, the parameters (Gnu, Gnd, Rnu, Rnd, Bnu and Bnd) indicating the extents of variance with regard to the individual colors on the two sides above and below the focus detection pixel row, and the color composition ratios (Kg, Kr and Kb) around the focus detection pixels are calculated.
 
Gau=( G 1 +G 2 +G 3 +G 4 +G 5)/5,
 
Gad=( G 6 +G 7 +G 8 +G 9 +G 10)/5,
 
Rau=( R 1 +R 2)/2,
 
Rad=( R 3 +R 4)/2,
 
Bau=( B 1 +B 2 +B 3)/3,
 
Bad=( B 4 +B 5 +B 6)/3,
 
Gnu=(| G 3 −G 1 |+|G 1 −G 4 |+|G 4 −G 2 |+|G 2 −G 5|)/(4×Gau),
 
Gnd=(| G 6 −G 9 |+|G 9 −G 7 |+|G 7 −G 10 |+|G 10 −G 8|)(4×Gad),
 
Rnu =|R 1 −R 2/Rau,
 
Rnd =|R 3 −R 4|/Rad,
 
Bnu=(| B 1 −B 2 |+|B 2 −B 3|)/(2×Bau),
 
Bnd=(| B 4 −B 5 |+|B 5 −B 6|)/(2×Bad),
 
 Kg =(Gau+Gad)/(Gau+Gad+Rau+Rad+Bau+Bad),
 
 Kr =(Rau+Rad)/(Gau+Gad+Rau+Rad+Bau+Bad),
 
 Kb =(Bau+Bad)/(Gau+Gad+Rau+Rad+Bau+Bad)  (7)
 
   It is to be noted that the color composition ratios Kg, Kr and Kb each indicate the output composition ratio of the imaging pixel output that would be provided by the specific color imaging pixel that would otherwise be positioned at the target focus detection pixel, relative to the outputs from the imaging pixels corresponding to all the colors, calculated based upon the outputs from the imaging pixels surrounding the focus detection pixel. 
   In step S 220  following step S 210 , the data from the imaging pixels  310  surrounding the focus detection pixel  311  are statistically processed to calculate image data S 1  at the particular focus detection pixel  311 . The image data S 1 , which may be either image data S 1  (X) corresponding to the focus detection pixel X or image data S 1  (Y) corresponding to the focus detection pixel Y, can be obtained as expressed in (8) below.
 
 S 1( X )=( B 2 +B 5)/2,
 
 S 1( Y )=( G 3 +G 4 +G 6 +G 7)/4  (8)
 
The blue pixels, disposed at lower arraying density compared to the green pixels, are located further away from the focus detection pixel X (output AF  3 ). For this reason, a pattern with fine lines or the like may be present between the focus detection pixel X (output AF  3 ) and the surrounding blue pixels, which would greatly alter the image pattern to result in an error in S 1  (X) calculated as expressed in (8).
 
   This problem may be prevented by first correcting the blue pixel outputs to blue pixel outputs that would be generated at the green pixels around the focus detection pixel X (output AF  3 ), in correspondence to the outputs from the green pixels near the blue pixels and the outputs from the green pixels near the focus detection pixel X (output AF  3 ) and then taking the average of the corrected blue pixel outputs.
 
 S 1( X )= B 2 ×G 4/( G 1 +G 2)+ B 5 ×G 7/( G 9 +G 10)  (9)
 
The extent of the error in the data generated as expressed in (9) is reduced, since the average is taken after converting the blue pixel outputs from the blue pixels disposed at low density to blue pixel outputs that would be provided at positions in the vicinity of the focus detection pixel X (output AF  3 ) in correspondence to the changes occurring in the outputs from the green pixels disposed at higher density.
 
   The method adopted when calculating through statistical processing the image data S 1  at the focus detection pixel X or Y is not limited to the simple averaging expressed in (9). For instance, image data corresponding to the focus detection pixel X or Y may be obtained through linear interpolation by using the outputs from nearby pixels assuming a plurality of positions along the vertical direction, through interpolation executed by using expressions of multiple degrees, i.e., expressions of at least second degrees, or through median processing. 
   In step S  230 , the output from the focus detection pixel  311  is corrected in correspondence to the color composition ratios calculated by using the data from the imaging pixels  310  surrounding the focus detection pixel  311 , and thus, image data S 2  at the focus detection pixel  311  are calculated. The image data S 2  may be either image data S 2  (X) (a virtual imaging pixel output calculated in correspondence to the position occupied by the focus detection pixel X) corresponding to the focus detection pixel X or image data S 2  (Y) (a virtual image pixel output calculated in correspondence to the position occupied by the focus detection pixel Y) corresponding to the focus detection pixel Y, and are calculated as expressed in (10) below.
 
 S 2( X )= AF 3 ×Kb×Ks×kc,  
 
 S 2( Y )= AF 2 ×Kg×Ks×Kc   (10)
 
The coefficient Ks in expression (10) is the output ratio coefficient having been explained in reference to  FIG. 15 , which assumes a value selected in correspondence to the control F value (aperture F value) set for the imaging operation. More specifically, a lookup table containing paired-up F value data and output ratio coefficient data, as indicated in the graph in  FIG. 15 , which is stored in advance in the camera drive control device  212 , is used. If the output ratio coefficient curve changes in correspondence to the position of the focus detection area (the distance from the optical axis) or the exit pupil distance (the distance between the image sensor and the exit pupil) of the exchangeable lens unit  202 , a plurality of lookup tables each corresponding to a specific set of the focus detection area position and the exit pupil distance should be prepared. Then, based upon the focus detection position and the exit pupil distance of the exchangeable lens unit  202  (indicated in the lens information), the matching lookup table should be selected and used.
 
   The coefficient Kc in expression (10) is an adjustment coefficient used to adjust the discrepancy in the quantity of received light, which is attributed to the difference between the spectral characteristics of the focus detection pixels  311  and the spectral characteristics of the imaging pixels  310 . A value obtained through advance measurement is stored as the coefficient Kc in the camera drive control device  212 . The adjustment coefficient Kc may be calculated as expressed in (11) below with Sg, Sr, Sb and Sa respectively representing the outputs from the green pixels, the red pixels, the blue pixels and the focus detection pixels generated as an image of a planar light source achieving flat light emission characteristics over the visible light range is captured at the image sensor  211  via an imaging optical system with an aperture at a setting darker than F 2.8.
 
 Kc =( Sg+Sr+Sb )/ Sa   (11)
 
   In step S 240 , a decision is made as expressed in the following expression in (12) to determine whether or not the image area around the focus detection pixel  311  is uniform. When the focus detection pixel X is the target pixel, the surrounding image area is judged to be uniform if the following conditions (12) are satisfied.
 
Bnu&lt;T1 and Bnd&lt;T1  (12)
 
T 1  in expression (12) represents a predetermined threshold value. If, on the other hand, the focus detection pixel Y is the target pixel, the surrounding image is judged to be uniform if the following conditions (13) are satisfied.
 
Gnu&lt;T2 and Gnd&lt;T2  (13)
 
T 2  in expression (13) represents a predetermined threshold value. If the surrounding image is judged to be nonuniform, the operation proceeds to step S 290  to designate the data S 1  having been obtained through the statistical processing executed in step S 220  as the image data S at the focus detection pixel  311 . Namely, if the image is not uniform, the data S 1  obtained through the statistical processing which is simple averaging processing are used, since the processing for calculating the color composition ratio data S 2  would be complicated under such circumstances. Even if there is an error in the data S 1  generated through the statistical processing, the error will not be noticeable since the surrounding image area is not uniform and a significant change occurs over the surrounding image area.
 
   If, on the other hand, it is decided that the surrounding image is uniform, the operation proceeds to step S 250 . In step S 250 , a decision is made by comparing the sets of information sampled around the focus detection pixel row, i.e., from the sides above and below the focus detection pixel row, as to whether or not there is an edge pattern that indicates a change in the pixel outputs along the direction perpendicular to the focus detection pixel row. For the target focus detection pixel X, it is determined that an edge pattern indicating a change in the pixel outputs along the direction perpendicular to the focus detection pixel row is present if the conditions expressed in (14) are satisfied.
 
|Bau−Bad |&gt;T 3  (14)
 
T 3  in expression (14) represents a predetermined threshold value. For the target focus detection pixel Y, it is determined that an edge pattern indicating a change in the pixel outputs along the direction perpendicular to the focus detection pixel row is present if the conditions expressed in (15) are satisfied.
 
|Gau−Gad |&gt;T 4  (15)
 
T 4  in expression (15) represents a predetermined threshold value.
 
   If it is decided that there is an edge pattern indicating a change in the imaging pixel outputs along the direction perpendicular to the focus detection pixel row, the operation proceeds to step S 260 . In step S 260 , the data S 1  resulting from the statistical processing and the color composition ratio data S 2  are weighted in correspondence to an edge level Kbe or Kge and image data S at the focus detection pixel  311  are obtained through weighted addition. Once the processing in step S 260  is executed, the operation returns from step S 300  to the program shown in  FIG. 16 , thereby ending the interpolation processing. 
   The edge levels Kbe and Kge, each indicating the steepness of the edge slope and the increments with which the edge is staged, are calculated as follows. If the focus detection pixel X is the target pixel, the edge level Kbe is calculated as;
 
Kbe=|Bnu−Bnd|/( T 5 −T 3),
 
IF Kbe&gt;1 THEN Kbe=1,
 
 S =(1−Kbe)× S 1( X )+Kbe× S 2( X )  (16)
 
T 5  in expression (16) represents a predetermined threshold value (&gt;T 3 ). If the edge level Kbe is high (=1), S=S 2 (X). If the focus detection pixel Y is the target pixel, the edge level Kg is calculated as;
 
Kge=|Gnu−Gnd|/( T 6 −T 4),
 
IF Kge&gt;1 THEN Kge=1,
 
 S =(1−Kge)× S 1( Y )+Kge× S 2( Y )  (17)
 
T 6  in expression (17) represents a predetermined threshold value (&gt;T 4 ). If the edge level Kge is high (=1), S=S 2 (Y).
 
   Over the range in which the edge level expressed in (16) or (17) shifts from low to high, the image data S at the focus detection pixel  311  are obtained as the sum of the data S 1  resulting from the statistical processing and the data S 2  calculated based upon the color composition ratio, which are individually weighted by using the edge level Kbe or Kge. As a result, stable image data can be obtained since the image data S does not change abruptly regardless of whether the edge judging results in an affirmative-judgment or a negative judgment. Values that will provide the optimal image quality are selected for the predetermined threshold values T 3  to T 6  in correspondence to the characteristics and the like of the optical low pass filter (not shown) mounted at the image sensor  211 . For instance, if an optical low pass filter achieving a significant filtering effect is mounted, the edge pattern will be blurred and accordingly, less rigorous (smaller) values should be selected for the predetermined threshold values T 3  to T 6 . 
   If it is decided that there is no edge pattern indicating a change in the imaging pixel outputs along the direction perpendicular to the focus detection pixel row, the operation proceeds to step S 270 . In step S 270 , the focus detection pixel output and the information sampled from the surrounding pixels present above and below the focus detection pixel are compared to make a decision as to whether or not a fine line pattern is present over the focus detection pixel row or over an area near the focus detection pixel row. The term “fine line pattern” in this context refers to a peak pattern showing a pixel output spiking upward, away from the average value of the pixel output, or a bottom pattern showing a pixel output spiking downward away from the average value of the pixel output. If the focus detection pixel X is the target pixel, a fine line pattern is judged to be present when the conditions in (18) are satisfied.
 
| S 1( X )− S 2( X )|&gt; T 7  (18)
 
T 7  in expression (18) represents a predetermined threshold value. Alternatively, the decision may be made as expressed in (19) below instead of expression (18).
 
|(Bau+Bad)/2 −S 2( X )|&gt; T 7  (19)
 
T 7  in expression (19) represents a predetermined threshold value.
 
   If the focus detection pixel Y is the target pixel, a fine line pattern is judged to be present when the conditions in (20) are satisfied.
 
| S 1( Y )− S 2( Y )|&gt; T 8  (20)
 
T 8  in expression (20) represents a predetermined threshold value. Alternatively, the decision may be made as expressed in (21) below instead of expression (20).
 
|(Gau+Gad)/2 −S 2( Y )|&gt; T 8  (21)
 
T 8  in expression (21) represents a predetermined threshold value.
 
   If it is decided that there is no fine line pattern, the operation proceeds to step S 290  to designate the data S 1  obtained through the statistical processing executed in step S 220  as the image data at the focus detection pixel  311 , since the data obtained through the statistical processing do not manifest a significant error as long as the image surrounding the focus detection pixel  311  is uniform and no image pattern is present over the focus detection pixel row. 
   If, on the other hand, it is decided that a fine line pattern is present, the operation proceeds to step S 280 . In step S 280 , the data S 1  obtained through the statistical processing and the color composition ratio data S 2  are weighted in correspondence to a peak/bottom level Kbp or Kgp and the image data S at the focus detection pixel  311  are obtained as the sum of the weighted data S and S 2 . Subsequently, the operation returns from step S 300  to the program shown in  FIG. 16 , thereby ending the interpolation processing. 
   The peak/bottom levels Kbp and Kgp, each indicating the level of the peak or the bottom and also the steepness of the peak or the bottom, are calculated as follows. If the focus detection pixel X is the target pixel, Kbp is calculated as;
 
Kbp=| S 1( X )− S 2( X )|/( T 9 −T 7),
 
or
 
Kbp=|(Bau+Bad)/2 −S 2( X )|/( T 9 −T 7),
 
IF Kbp&gt;1 THEN Kbp=1,
 
 S =(1−Kbp)× S 1( X )+Kbp× S 2( X )  (22)
 
T 9  in (22) represents a predetermined threshold value (&gt;T 7 ) As indicated above, if the peak/bottom level Kbp is high (=1), S=S 2  (X).
 
   If the focus detection pixel Y is the target pixel, Kgp is calculated as;
 
Kgp=| S 1( Y )− S 2( Y )|/( T 10 −T 8),
 
or
 
Kgp=|(Gau+Gad)/2 −S 2( Y )|/( T 10 −T 8),
 
IF Kgp&gt;1 THEN Kgp=1,
 
 S =(1−Kgp)× S 1( Y )+Kgp× S 2( Y )  (23)
 
T 10  in (23) represents a predetermined threshold value (&gt;T 8 ). As indicated above, if the peak or bottom level Kgp is high (=1), S=S 2  (Y).
 
   As indicated in expressions (22) and (23), over the range in which the peak/bottom level shifts from low to high, the image data S at the focus detection pixel  311  are obtained as the sum of the data S 1  resulting from the statistical processing and the data S 2  calculated based upon the color composition ratio, which are individually weighted by using the peak/bottom level Kbp or Kgp. As a result, stable image data can be obtained since the image data S does not change abruptly regardless of whether the fine line judging results in an affirmative judgment or a negative judgment. 
   Values that will provide the optimal image quality are selected for the predetermined threshold values T 7  to T 10  in correspondence to the characteristics and the like of the optical low pass filter (not shown) mounted at the image sensor  211 . For instance, if an optical low pass filter achieving a significant filtering effect is mounted, the fine line pattern will be blurred and accordingly, less rigorous (smaller) values should be selected for the predetermined threshold values T 7  to T 10 . 
   In step S 290 , the data S 1  obtained by statistically processing the data from only the imaging pixels  310  around the focus detection pixel  311  are designated as the image data S at the focus detection pixel, and the operation proceeds to step S 300 . In step S 300 , the interpolation processing ends and the operation returns to the program shown in  FIG. 16 . 
   (Variations of the Embodiment) 
   While the image sensor  211  in  FIG. 7  includes the focus detection pixels  311  disposed without allowing any interval, the focus detection pixels  311  may each be set so as to occupy every other pixel position as in an image sensor  211 A in  FIG. 20  or an image sensor  211 B in  FIG. 21 , instead.  FIG. 20  is a partial enlargement of the image sensor  211 A which includes the focus detection pixels  311  arrayed in a single row so as to occupy positions that would otherwise be occupied by blue pixels.  FIG. 21  is a partial enlargement of the image sensor  211 B which includes the focus detection pixels  311  arrayed in a single row so as to occupy positions that would otherwise be occupied by green pixels. While the focus detection accuracy is somewhat lowered when the arraying pitch (arraying interval) with which the focus detection pixels  311  are disposed is increased, the quality of the image having undergone the interpolation processing explained earlier is improved since the focus detection pixels  311  are set with a lower density at the image sensor  211 A or  211 B. 
   At the image sensor  211 A in  FIG. 20 , a greater number of green pixels are present around the focus detection pixels AF  1  to AF  5  disposed at positions that would otherwise be occupied by blue pixels, as shown in  FIG. 22 . Thus, the image data for the position occupied by the target focus detection pixel can be calculated with an even higher level of accuracy by using the outputs G 11  and G 12  from the green pixels to further improve the quality of the image resulting from the interpolation processing. In addition, since imaging pixels  310  are disposed between the individual focus detection pixels  311 , the decisions as to whether or not there is an edge pattern and whether or not there is a fine line pattern superimposed over the focus detection pixel row can be made more accurately by comparing the outputs from the imaging pixels  310  present in the row of the focus detection pixels  311  with the outputs from the imaging pixels  310  present around the focus detection pixel row. 
   For instance, at the image sensor shown in  FIG. 22 , the data S 1  (X) may be obtained through statistical processing for the position occupied by the focus detection pixel  311  with the output AF 3 , as expressed in (24) instead of expression (9).
 
 S 1( X )=( B 2×( G 11 +G 12)/( G 1 +G 2)+ B 5×( G 11 +G 12)/(G9 +G 10))/2  (24)
 
   In the image sensor  211 B shown in  FIG. 21 , the extent to which the image quality is affected by the correction-related error can be minimized as the focus detection pixels  311  are disposed at positions that would otherwise be occupied by green pixels arrayed with a higher level of density in the Bayer array. 
   At an image sensor  211 C shown in  FIG. 23 , focus detection pixels  311  are set by allowing two-pixel intervals. While the focus detection accuracy is somewhat compromised as the arraying pitch (arraying interval) for the focus detection pixels  311  increases, the quality of the corrected image is further improved since the focus detection pixels  311  are not disposed as densely. In addition, a green pixel and a blue pixel are set over each interval between the focus detection pixels  311 . Accordingly, by comparing the green pixel outputs/blue pixel outputs from the green and blue pixels present in the row of focus detection pixels  311  with the green pixel output/blue pixel outputs from the green and blue pixels present around the focus detection pixel row, decisions as to whether or not a green or blue edge pattern is present over the focus detection pixel row and whether or not a green or blue fine line pattern is present over the focus detection pixel row can be made with better accuracy. 
   In the image sensor  211  shown in  FIG. 7 , the focus detection pixels  311  each include a pair of photoelectric conversion units  12  and  13 .  FIG. 24  shows an alternative image sensor  211 D with focus detection pixels  313  and  314  each equipped with a single photoelectric conversion unit. Each focus detection pixel  313  and the adjacent focus detection pixel  314  in  FIG. 24  form a pair, and this pair of focus detection pixels  313  and  314  is equivalent to one focus detection pixel  311  shown in  FIG. 7 . 
     FIGS. 25A and 25B  show in detail the structures of the focus detection pixels  313  and  314 . As shown in  FIG. 25A , the focus detection pixel  313  includes a micro-lens  10  and a single photoelectric conversion unit  16 . As shown in  FIG. 25B , the focus detection pixel  314  includes a micro-lens  10  and a single photoelectric conversion unit  17 . The pair of photoelectric conversion unit  16  and  17  in the pair of focus detection pixels  313  and  314  are projected onto the exit pupil  90  of the exchangeable lens unit  202  via the micro-lenses  10  and form a pair of range-finding pupils  92  and  93 , as shown in  FIG. 13 . Thus, a pair of image outputs to be used in focus detection can be obtained from the pair of focus detection pixels  313  and  314 . By installing a single photoelectric conversion unit  16  ( 17 ) in each focus detection pixel  313  ( 314 ) as in the image sensor  211 D shown in  FIG. 24 , the read circuit structure in the image sensor  211 D can be simplified. 
   While the focus detection pixels  311  at the image sensor  211  shown in  FIG. 7  are not equipped with color filters, the present invention may also be adopted in an image sensor with focus detection pixels equipped with color filters corresponding to one of the colors (e.g., green) assumed for the color filters mounted at the imaging pixels  310 . In such a case, image data for the position occupied by each focus detection pixel  311 , which would otherwise be occupied by a green pixel, can be obtained through interpolation processing executed as expressed in (25) instead of expression (10), further improving the accuracy with which the image data at the focus detection pixel  311 , which would otherwise be occupied by a green pixel, are generated through interpolation.
 
 S 2( Y )= AF 2 ×Ks×Kc   (25)
 
The image data at a focus detection pixel  311  that would otherwise be occupied by a blue pixel may be generated through interpolation as expressed in (26) below instead of expression (10).
 
 S 2( X )= AF 3×((Bau+Bad)/(Gau+Gad))× Ks×Kc   (26)
 
   The focus detection pixels  311  in the image sensor  211  shown in  FIG. 7  are not equipped with color filters. However, the present invention may also adopted in an image sensor that includes focus detection pixels equipped with color filters in colors matching the colors of the filters mounted at the imaging pixels  310  that would otherwise occupy the positions occupied by the focus detection pixels in the Bayer array. In such an image sensor, image shift detection will be executed in correspondence to each group of focus detection pixels  311  equipped with filters in the same color. As a result, the image interpolation accuracy will be further improved and an even higher quality image will be provided through the correction. The image data at each focus detection pixel  311 , which would otherwise be occupied by a green pixel, can be calculated as expressed in (25), whereas the image data at each focus detection pixel  311 , which would otherwise be occupied by a blue pixel, can be calculated as expressed in (27).
 
 S 2( X )= AF 3 ×Ks×Kc   (27)
 
   While the corrected image data are saved into the memory card  213  in the operational flow shown in  FIG. 16 , the corrected image data may also be displayed on the electronic viewfinder or a rear monitor screen (not shown) installed at the rear side of the camera body. 
   While the information related to the characteristics of the image around the focus detection pixel row is obtained through calculation as expressed in (7), the range of the pixels to be used for the calculation is not limited to this example and the pixel range size may be adjusted as necessary. For instance, if an optical low pass filter with an intense filtering effect is mounted, the extent of image blur becomes significant and, accordingly, the range of the pixels used in the calculation expressed in (7) should be increased. 
   An example in which the image data at the focus detection pixel  311  that would otherwise be occupied by a green pixel are obtained by averaging the outputs from the four green pixels set on the diagonals around the focus detection pixel  311  is explained in reference to expression (8). However, this method gives rise to a problem in that if an edge pattern indicating a pixel output change along the direction in which the focus detection pixels  311  are arrayed is present over the focus detection pixel  311 , a significant error manifests. Accordingly, if the relationship expressed in (28) below is satisfied, it may be judged that an edge pattern indicating a pixel output change along the focus detection pixel  311  arraying direction is present over the focus detection pixels  311  and the following processing may be executed. Namely, the image data for the position that would otherwise be occupied by a green pixel may be obtained by averaging the outputs from the green pixels present above and below the focus detection pixel  311 , as expressed in (29) below.
 
|( G 3 G 6)−( G 6 +G 7)|&gt; T 11  (28)
 
T 11  in expression (28) represents a predetermined threshold value.
 
 S 1( Y )=( G 1 +G 9)/2  (29)
 
   It is to be noted that the image sensors  211 ,  211 A,  211 B,  211 C and  211 D may each be constituted with a CCD image sensor or a CMOS image sensor. In addition, while an explanation is given above in reference to the embodiment on an example in which the imaging device according to the present invention is a digital still camera  201  with the exchangeable lens unit  202  mounted at its camera body  203 . However, the present invention may also be adopted in a digital still camera with an integrated lens or in a video camera, as well as in the digital still camera  201  in the embodiment. Furthermore, the present invention may be adopted in a compact camera module built into a portable telephone or the like, a surveillance camera or the like as well. 
   The imaging device in an embodiment according to the present invention includes an image sensor having a plurality of pixel units, disposed in a two-dimensional array, each equipped with a plurality of imaging pixels with varying spectral sensitivity characteristics arrayed with a specific rule and focus detection pixels with sensitivity over a range encompassing the spectral sensitivity of the pixel units, which are set in the array of pixel units. The imaging device further includes an estimating circuit that determines through arithmetic operation the image output structure at the focus detection pixels based upon the outputs from the imaging pixels present around the focus detection pixel and estimates the image output at the focus detection pixel based upon the output structure and the output from the focus detection pixel. Since the output structure corresponding to the imaging pixels, which includes the image output at the focus detection pixel position, i.e., the virtual imaging pixel output, is made to match the output structure corresponding to the imaging pixels surrounding the focus detection pixel by adjusting this structure, the occurrence of color artifacts can be prevented. In addition, since the image output at the focus detection pixel position is estimated based upon the actual output from the focus detection pixel, occurrence of color artifacts, false pattern or pattern loss can be prevented even when an edge pattern image or a fine line pattern image manifesting an image change along the direction perpendicular to the focus detection pixel row is superimposed over the focus detection pixel. Thus, a desirable level of image quality can be maintained. 
   In the imaging device in the embodiment, the focus detection pixel output is categorized in correspondence to the output structure when estimating the image output at the focus detection pixel position, i.e., the output from the virtual imaging pixel. As a result, an edge pattern image or a fine line pattern image superimposed over the focus detection pixel can be accurately detected so as to effectively prevent occurrence of color artifacts, false pattern or pattern loss.