Patent Publication Number: US-2021181461-A1

Title: Image sensor and image-capturing device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. application Ser. No. 16/067,165 filed Jun. 29, 2018, which is a National Stage of PCT/JP2017/000145, filed Jan. 5, 2017, and which is based on and claims priority under 35 U.S.C. 119 from Japanese Patent Application No. 2016-002729 filed on Jan. 8, 2016. The entire contents of the above applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image sensor and an image-capturing device. 
     BACKGROUND ART 
     Image sensors having disposed therein pixels each of which includes an organic photoelectric conversion film are known in the related art. 
     The image sensors in the related art, however, are limited in that the size of the light receiving area at each pixel cannot be adjusted. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Laid Open Patent Publication No. 2014-67948 
     SUMMARY OF INVENTION 
     An image sensor according to a first aspect of the present invention comprises: a photoelectric conversion film that performs photoelectric conversion on light having entered therein; at least two electrodes, including a first electrode and a second electrode, disposed at a surface of the photoelectric conversion film; and at least two electrodes, including a third electrode and a fourth electrode, disposed at another surface of the photoelectric conversion film. 
     According to a second aspect of the present invention, in the image sensor according to the first aspect, it is preferable that a position of a boundary between the first electrode and the second electrode is different from a position of a boundary between the third electrode and the fourth electrode, viewed from a side to which the light enters. 
     According to a third aspect of the present invention, in the image sensor according to the second aspect, it is preferred to further comprise: an electric charge readout unit that reads out an electric charge generated in the photoelectric film from the first electrode, the second electrode, the third electrode and the fourth electrode. 
     According to a fourth aspect of the present invention, in the image sensor according to the third aspect, it is preferable that the first electrode overlaps the entire third electrode via the photoelectric conversion film and also overlaps part of the fourth electrode via the photoelectric conversion film; and the second electrode does not overlap the third electrode via the photoelectric conversion film but overlaps part of the fourth electrode via the photoelectric conversion film. 
     According to a fifth aspect of the present invention, in the image sensor according to the fourth aspect, it is preferable that the electric charge readout unit is able to read out from the first electrode an electric charge generated in an area of the photoelectric conversion film located between the first electrode and the third electrode and between the first electrode and the fourth electrode, is able to read out from the first electrode or the fourth electrode an electric charge generated in an area of the photoelectric conversion film located between the first electrode and the fourth electrode, and is able to read out from the fourth electrode an electric charge generated in an area of the photoelectric conversion film located between the fourth electrode and the first electrode and between the fourth electrode and the second electrode. 
     According to a sixth aspect of the present invention, in the image sensor according to the fourth or fifth aspect, it is preferable that the electric charge readout unit is able to read out from the second electrode or the fourth electrode an electric charge generated in an area of the photoelectric conversion film located between the second electrode and the fourth electrode, and is able to read out from the first electrode or the third electrode an electric charge generated in an area of the photoelectric conversion film located between the first electrode and the third electrode. 
     According to a seventh aspect of the present invention, in the image sensor according to any one of the fourth to sixth aspects, it is preferable that the electric charge readout unit is able to read out from the second electrode an electric charge generated in an area of the photoelectric conversion film located between the second electrode and the fourth electrode, while concurrently reading out from the first electrode an electric charge generated in an area of the photoelectric conversion film located between the first electrode and the third electrode and between the first electrode and the fourth electrode. 
     According to an eighth aspect of the present invention, in the image sensor according to the seventh aspect, it is preferable that the electric charge readout unit is able to read out from the fourth electrode an electric charge generated in an area of the photoelectric conversion film located between the fourth electrode and the first electrode and between the fourth electrode and the second electrode, while concurrently reading out from the third electrode an electric charge generated in an area of the photoelectric conversion film located between the first electrode and the third electrode. 
     An image-capturing device according to a ninth aspect of the present invention comprises: a first image sensor having arrayed therein a plurality of first pixels, each of the first pixels includes a photoelectric conversion film, a first electrode and a second electrode disposed at a surface of the photoelectric conversion film and a third electrode and a fourth electrode disposed at another surface of the photoelectric conversion film, and receives first and second light fluxes having passed through first and second areas of a pupil of a photographic optical system, and outputs first and second photoelectric conversion signals; a second image sensor having arrayed therein a plurality of second pixels, each of the second pixels receives third and fourth light fluxes having passed through third and fourth areas of the pupil of the photographic optical system and having been transmitted through the first image sensor, and outputs third and fourth photoelectric conversion signals; and a focus detection unit that executes focus detection by using the third and fourth photoelectric conversion signals provided from the second image sensor as focus detection signals corresponding to a central area of a photographic image plane formed via the photographic optical system, and executes focus detection by using the first and second photoelectric conversion signals provided from the first image sensor as focus detection signals corresponding to a peripheral area of the photographic image plane, wherein: at the first image sensor, a first photoelectric conversion area and a second photoelectric conversion area are formed with the first electrode, the second electrode, the third electrode and the fourth electrode in each of the first pixels corresponding to a peripheral area located on one side relative to the central area of the photographic image plane, and a third photoelectric conversion area and a fourth photoelectric conversion area are formed with the first electrode, the second electrode, the third electrode and the fourth electrode in each of the first pixels corresponding to a peripheral area located on another side relative to the central area of the photographic image plane; and at the first image sensor, the first pixels corresponding to the peripheral area located on the one side each output photoelectric conversion signals from the first photoelectric conversion area and the second photoelectric conversion area as the first and second photoelectric conversion signals, and the first pixels corresponding to the peripheral area located on the other side each output photoelectric conversion signals from the third photoelectric conversion area and the fourth photoelectric conversion area as the first and second photoelectric conversion signals. 
     According to a tenth aspect of the present invention, in the image-capturing device according to the ninth aspect, it is preferable that each of the first pixels in the first image sensor include a microlens, and the first and second light fluxes having passed through the microlens are received at the photoelectric conversion film; a boundary between the first photoelectric conversion area and the second photoelectric conversion area is offset toward one side relative to an optical axis of the microlens and a boundary between the third photoelectric conversion area and the fourth photoelectric conversion area is offset toward another side relative to the optical axis of the microlens at the first image sensor; and each of the second pixels in the second image sensor include a pair of photoelectric conversion units that receive the third and fourth light fluxes, respectively, and a boundary of the pair of photoelectric conversion units is substantially in alignment with the optical axis of the microlens. 
     According to an eleventh aspect of the present invention, in the image-capturing device according to the tenth aspect, it is preferable that at the first image sensor, the first photoelectric conversion area and the second photoelectric conversion area correspond to the first electrode and the second electrode, and the third photoelectric conversion area and the fourth photoelectric conversion area correspond to the third electrode and the fourth electrode. 
     According to a twelfth aspect of the present invention, in the image-capturing device according to the tenth or eleventh aspect, it is preferable that if an exit pupil position at the photographic optical system is set at a first predetermined position relative to a focus detection pupil position, the focus detection unit executes focus detection by using the third and fourth photoelectric conversion signals provided from the second image sensor in the peripheral area of the photographic image plane as well as in the central area of the photographic image plane. 
     According to a thirteenth aspect of the present invention, in the image-capturing device according to any one of the tenth to twelfth aspects, it is preferable that if an exit pupil position at the photographic optical system is set at a second predetermined position relative to a focus detection pupil position, at the first image sensor, the first photoelectric conversion area and the second photoelectric conversion area are formed with the first electrode, the second electrode, the third electrode and the fourth electrode in each of the first pixels corresponding to the peripheral area located on the one side, and the third photoelectric conversion area and the fourth photoelectric conversion area are formed with the first electrode, the second electrode, the third electrode and the fourth electrode in each of the first pixels corresponding to the peripheral area located on the other side; and if the exit pupil position at the photographic optical system is set at a third predetermined position relative to the focus detection pupil position, at the first image sensor, the third photoelectric conversion area and the fourth photoelectric conversion area are formed with the first electrode, the second electrode, the third electrode and the fourth electrode in each of the first pixels corresponding to the peripheral area located on the one side, and the first photoelectric conversion area and the second photoelectric conversion area are formed with the first electrode, the second electrode, the third electrode and the fourth electrode in each of the first pixels corresponding to the peripheral area located on the other side. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram presenting an example of a structure that is adopted in a digital camera achieved in an embodiment. 
         FIG. 2  shows an overview of first and second image sensors. 
         FIG. 3A  is a diagram indicating the positional arrangement with which pixels are disposed over a range of 10 rows×6 columns at part of the first image sensor, and  FIG. 3B  is a diagram indicating the positional arrangement with which pixels are disposed over a range of 10 rows×6 columns at part of the second image sensor. 
         FIG. 4A  shows a plan view of a pixel in the first image sensor, viewed from the subject side,  FIG. 4B  is a side elevation, and  FIG. 4C  is a sectional view through c 1 -c 1  in  FIG. 4B . 
         FIGS. 5A and 5B  show schematic illustrations of the structure of pixels in the first image sensor,  FIG. 5C  shows a schematic illustration of the structure of pixels in the second image sensor. 
         FIG. 6  is a sectional view showing the structure of a pixel in the first image sensor and the structure of a pixel in the second image sensor. 
         FIG. 7  is a diagram presenting a structural example that may be adopted for the signal readout circuit at a pixel in the first image sensor. 
         FIGS. 8A to 8E  show illustrations of photoelectric conversion areas where electric charges are read out at the organic photoelectric conversion film  230  in a pixel  210 , viewed from the subject side. 
         FIG. 9  shows an illustration of vignetting, showing how a pair of light fluxes arriving at pixels disposed on the first and second image sensors are restricted by the exit pupil of the photographic optical system in correspondence to the positional relationship between the exit pupil plane and the focus detection pupil plane. 
         FIG. 10  is a front view of the image-capturing surface of the first or second image sensor. 
         FIG. 11  is a front view indicating the range over which light enters the first and second photoelectric conversion units in a pixel disposed at a peripheral position. 
         FIG. 12  is an illustration of a condition assumed when the exit pupil distance d at the photographic optical system matches the focus detection pupil distance d. 
         FIGS. 13A to 13C  show illustrations of a condition assumed when the exit pupil distance do at the photographic optical system is smaller than the focus detection pupil distance d. 
         FIGS. 14A and 14B  show illustrations of a condition assumed when the exit pupil distance df at the photographic optical system is greater than the focus detection pupil distance d. 
         FIG. 15  is an illustration of a state of pronounced defocus. 
         FIGS. 16A to 16D  show illustrations of a variation. 
         FIGS. 17A to 17C  show illustrations of a variation. 
         FIGS. 18A to 18E  show illustrations of a variation. 
         FIGS. 19A to 19C  show illustrations of a variation. 
         FIGS. 20A and 20B  show illustrations of a variation. 
         FIG. 21  is an illustration of a variation. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a diagram presenting a structural example for a digital camera  1  achieved in an embodiment of the present invention. The digital camera  1  includes a photographic optical system  10 , an image-capturing unit  11 , a control unit  12 , an operation unit  13 , an image processing unit  14 , a liquid crystal monitor  15  and a buffer memory  16 . In addition, a memory card  17  is loaded in the digital camera  1 . The memory card  17 , constituted with a non-volatile flash memory or the like, can be detachably loaded into the digital camera  1 . 
     The photographic optical system  10 , configured with a plurality of lenses, forms a subject image onto the image-capturing surface of the image-capturing unit  11 . The plurality of lenses constituting the photographic optical system  10  includes a focusing lens that is driven along the optical axis for purposes of focus adjustment. The focusing lens is driven along the optical axis by a lens drive unit (not shown). 
     The image-capturing unit  11  includes a first image sensor  21  and a second image sensor  22  laminated one on top of the other, an amplifier circuit  23  and an A/D conversion circuit  24 . The first and second image sensors  21  and  22 , each constituted with a plurality of pixels disposed in a two-dimensional pattern, receive light from a photographic subject via the photographic optical system  10  and output photoelectric conversion signals resulting from photoelectric conversion of the light received therein. As will be described in detail later, the pixels in the first and second image sensors  21  and  22  each output an analog photoelectric conversion signals. These photoelectric conversion signals are then used as signals for focus detection executed through the phase detection method and also as signals for photographic image generation, as will be explained later. The amplifier circuit  23  amplifies the photoelectric conversion signals at a predetermined amplification factor (gain) and outputs the resulting signals to the A/D conversion circuit  24 . The photoelectric conversion signals undergo A/D conversion at the A/D conversion circuit  24 . 
     The control unit or controller  12 , constituted with a microprocessor and its peripheral circuits, executes various types of control for the digital camera  1  by executing a control program installed in a ROM (not shown). In addition, the control unit  12  includes a focus detection unit  12   a  and an image generation unit  12   b  in the form of functional units. These functional units are provided in software by means of the control program mentioned above. It is to be noted that the functional units may instead be constituted as electronic circuits. 
     The control unit  12  stores the photoelectric conversion signals resulting from the A/D conversion at the A/D conversion circuit  24  into the buffer memory  16 . The focus detection unit  12   a  detects the focusing condition at the photographic optical system  10 , individually based upon the photoelectric conversion signals stored in the buffer memory  16 , which have originated at the first image sensor  21 , and based upon the photoelectric conversion signals stored in the buffer memory  16 , which have originated at the second image sensor  22 . The image generation unit  12   b  generates image signals by using the photoelectric conversion signals having originated at the second image sensor  22  and stored in the buffer memory  16 . 
     The image processing unit  14  may be constituted with, for instance, an ASIC. The image processing unit  14  generates image data by executing various types of image processing, such as interpolation processing, compression processing and white balance processing, on the image signals provided from the image generation unit  12   b.  The image data thus generated are brought up on display at the liquid crystal monitor  15  and stored into the memory card  17 . 
     The operation unit  13 , constituted with various types of operation members including a shutter release operation member, a mode selection operation member, a focus detection area setting operation member and a power on/off operation member, is operated by the photographer. The operation unit  13  outputs an operation signal, which corresponds to an operation performed by the photographer at an operation member among the operation members listed above, to the control unit  12 . 
     Description of the First and Second Image Sensors  21  and  22   
       FIG. 2  provides an overview of the first and second image sensors  21  and  22  achieved in the embodiment. The first image sensor  21  includes photoelectric conversion units each constituted with an organic photoelectric conversion film, whereas the second image sensor  22  includes photoelectric conversion units each constituted with a photodiode formed at a semiconductor substrate. The first image sensor  21  is laminated on the second image sensor  22 , and the first and second image sensors  21  and  22  are disposed in the optical path of the photographic optical system  10  so that the optical axis of the photographic optical system  10  shown in  FIG. 1  passes through the centers of the image-capturing surfaces of the first and second image sensors  21  and  22 . It is to be noted that while  FIG. 2  shows pixels  210  and  220  disposed over a range of 4 rows×3 columns at the first and second image sensors  21  and  22  so as to simplify the illustration, pixels are disposed over m rows×n columns at each of the first image sensor  21  and the second image sensor  22  and the size of the pixels in the first image sensor  21  matches the size of the pixels in the second image sensor  22  in the embodiment. 
     The pixels  210  in the first image sensor  21  each include an organic photoelectric conversion film that absorbs (or performs photoelectric conversion on) a predetermined color component of light. Color components of the light that have not been absorbed (that have not undergone photoelectric conversion) at the first image sensor  21 , are transmitted through the first image sensor  21 , and enter the second image sensor  22  to undergo photoelectric conversion at the second image sensor  22 . It is to be noted that the color component of the light that undergoes photoelectric conversion at the first image sensor  21  and the color component of the light that undergoes photoelectric conversion at the second image sensor  22  are complementary to each other. To describe this in further detail, a given pixel  210  at the first image sensor  21  corresponds to a pixel  220  at the second image sensor  22 , disposed at a position directly behind the pixel  210 , i.e., the pixels  210  at the first image sensor  21  each correspond to a pixel  220  at the second image sensor  22 , which receives the light having passed through the particular pixel  210 , and at the pixels  210  and  220  in the first and second image sensors  21  and  22  that correspond to each other as described above, light having complementary color components are absorbed and undergo photoelectric conversion. 
     In  FIGS. 3A and 3B , the positional arrangement with which pixels  210  are disposed over a range of 10 rows×6 columns in part of the first image sensor  21  and the positional arrangement with which pixels  220  are disposed over a range of 10 rows×6 columns at part of the second image sensor  22  are individually illustrated. In the first image sensor  21  in  FIG. 3A , each pixel  210  marked “Mg” is a pixel at which light with a magenta color component is absorbed and undergoes photoelectric conversion, i.e., a pixel having magenta spectral sensitivity. Likewise, each pixel  210  marked “Ye” is a pixel at which light with a yellow color component is absorbed and undergoes photoelectric conversion, i.e., a pixel having yellow spectral sensitivity, and each pixel  210  marked “Cy” is a pixel at which light with a cyan color component is absorbed and undergoes photoelectric conversion, i.e., a pixel having cyan spectral sensitivity. In the first image sensor  21 , the pixel positions in each odd-numbered row are alternately taken up by an “Mg” pixel  210  and a “Ye” pixel  210  and the pixel positions in each even-numbered row are alternately taken up by a “Cy” pixel  210  and an “Mg” pixel  210 . 
     In the second image sensor  22  in  FIG. 3B , each pixel  220  marked “G” is a pixel at which light with a green color component is absorbed and undergoes photoelectric conversion, i.e., a pixel having green spectral sensitivity. Likewise, each pixel  220  marked “B” is a pixel at which light with a blue color component is absorbed and undergoes photoelectric conversion, i.e., a pixel having blue spectral sensitivity, and each pixel  220  marked “R” is a pixel at which light with a red color component is absorbed and undergoes photoelectric conversion, i.e., a pixel having red spectral sensitivity. In the second image sensor  22 , the pixel positions in each odd-numbered row are alternately taken up by a “G” pixel  220  and a “B” pixel  220  and the pixel positions in each even-numbered row are alternately taken up by an “R” pixel  220  and a “G” pixel  220 . Namely, the pixels are disposed in a Bayer array at the second image sensor  22 . 
     In  FIGS. 3A and 3B , the “Mg” pixels  210  in the first image sensor  21  each correspond to one of the “G” pixels  220  in the second image sensor  22 , the “Ye” pixels  210  in the first image sensor  21  each correspond to one of the “B” pixels  220  in the second image sensor  22 , and the “Cy” pixels  210  in the first image sensor  21  each correspond to one of the “R” pixels  220  in the second image sensor  22 . 
     As described above, the first image sensor  21 , which includes organic photoelectric conversion films, fulfils a function of color filters for the second image sensor  22 , and a color image (a Bayer array image in the example presented in  FIGS. 3A and 3B ), which is complementary to the color image provided via the first image sensor  21 , is obtained from the second image sensor  22 . This means that a CMY image, composed with the three colors, Cy, Mg and Ye, can be obtained from the first image sensor  21 , whereas an RGB image composed with the three colors R, G and B can be obtained from the second image sensor  22 . 
       FIGS. 4A to 4C  and  FIGS. 5A and 5B  schematically illustrate the structure of the pixels  210  in the first image sensor  21 .  FIG. 4A  shows a pixel  210  in the image sensor  21  in a plan view, taken from the subject side,  FIG. 4B  is a side elevation of the pixel  210  viewed at a side surface thereof and  FIG. 4C  is a sectional view taken through c 1 -c 1  in  FIG. 4B . The pixels  210  in the first image sensor  21  each include an organic photoelectric conversion film  230  that absorbs light with the magenta color component, the yellow color component or the cyan color component, transparent first and second partial electrodes  231   a  and  231   b  formed on the upper surface of the organic photoelectric conversion film  230 , i.e., on the surface of the organic photoelectric conversion film  230  located toward the subject, and transparent third and fourth partial electrodes  232   a  and  232   b  formed on the lower surface of the organic photoelectric conversion film  230 . 
     The first and the second partial electrodes  231   a  and  231   b  are disposed along the direction in which the pixels  210  are disposed in a row, as shown in  FIG. 5A , i.e., along the left/right direction in  FIGS. 4A and 4B . Likewise, the third and fourth partial electrodes  232   a  and  232   b  are disposed along the direction in which the pixels  210  are disposed in a row, i.e., along the left/right direction in  FIG. 4C . The length of the first partial electrode  231   a,  measured along the row direction, and the length of the second partial electrode  231   b  measured along the row direction are different from each other. The length W 1  of the first partial electrode  231   a  located on the left side in the figures measured along the row direction, is greater than the length W 2  of the second partial electrode  231   b  located on the right side in the figures, measured along the row direction. Likewise, the length of the third partial electrode  232   a,  measured along the row direction, and the length of the fourth partial electrode  232   b  measured along the row direction are different from each other. The length W 2  of the third partial electrode  232   a  located on the left side in the figures measured along the row direction is smaller than the length W 1  of the fourth partial electrode  232   b  located on the right side in the figures, measured along the row direction. In other words, viewed from the side on which light enters, the position of a separation area G 1  separating the first partial electrode  231   a  from the second partial electrode  231   b,  i.e., the position of the boundary, is different from the position of a separation area G 2 , i.e., the position of the boundary separating the third partial electrode  232   a  from the fourth partial electrode  232   b.  It is to be noted that the length W 1  of the first partial electrode  231   a  measured along the row direction matches the length W 1  of the fourth partial electrode  232   b  measured along the row direction, and the length W 2  of the second partial electrode  231   b  measured along the row direction matches the length W 2  of the third partial electrode  232   a  measured along the row direction. 
     Thus, the first partial electrode  231   a  overlaps, along the optical axis, the entire third partial electrode  232   a  and part of the fourth partial electrode  232   b  located further to the left in the figures. The entire second partial electrode  231   b  overlaps, along the optical axis, the fourth partial electrode  232   b.  The entire third partial electrode  232   a  overlaps, along the optical axis, the first partial electrode  231   a.  The fourth partial electrode  232   b  overlaps, along the optical axis, part of the first partial electrode  231   a  located further to the right in the figures and the entire second partial electrode  231   b.  In the following description, the first and second partial electrodes  231   a  and  231   b  formed at the upper surface of the organic photoelectric conversion film  230  may also be referred to as upper partial electrodes  231   a  and  231   b  and the third and fourth partial electrodes  232   a  and  232   b  formed at the lower surface of the organic photoelectric conversion film  230  may also be referred to as lower partial electrodes  232   a  and  232   b.    
     In each pixel  210  structured as described above, an area from which an electric charge is to be read out from the organic photoelectric conversion film  230  can be set by selecting a specific combination of partial electrodes among the upper partial electrodes  231   a  and  231   b  and the lower partial electrode  232   a  and  232   b.    
     Next, the positional relationship of the partial electrodes in each pixel  210  at the first image sensor  21  to first and second photoelectric conversion units in the corresponding pixel  220  at the second image sensor  22  will be explained.  FIG. 5A  schematically illustrates the positions of the first and second partial electrodes  231   a  and  231   b  at the individual pixels  210  in the first image sensor  21  viewed from the subject side, whereas  FIG. 5B  schematically illustrates the positions of the third and fourth partial electrodes  232   a  and  232   b  at the individual pixels  210  in the first image sensor  21  viewed from the subject side.  FIG. 5C  schematically illustrates the position of first and second photoelectric conversion units  220   a  and  220   b  at the individual pixels  220  in the second image sensor  22  viewed from the subject side. It is to be noted that  FIGS. 5A to 5C  only show pixels  210  and  220  disposed in the first and second image sensors  21  and  22  respectively over a range of 5 rows×6 columns, in order to simplify the illustrations. 
     As explained earlier, the pixels  210  in the first image sensor  21  in  FIGS. 5A and 5B  each include the first and second partial electrodes  231   a  and  231   b  disposed side-by-side along the row direction and the third and fourth partial electrodes  232   a  and  232   b  also disposed side-by-side along the row direction, i.e., along the left/right direction in  FIGS. 5A and 5B . The pixels  220  in the second image sensor  22  shown in  FIG. 5C  each include the first and second photoelectric conversion units  220   a  and  220   b.  The first and second photoelectric conversion units  220   a  and  220   b  are disposed side-by-side along the row direction, i.e. along the left/right direction in  FIG. 5C . The first and second photoelectric conversion units  220   a  and  220   b  have identical shapes and matching sizes. Namely, the length of the first photoelectric conversion unit  220   a  measured along the row direction is equal to the corresponding length of the second photoelectric conversion unit  220   b.    
       FIG. 6  shows a pixel  210  and a pixel  220  in the first and second image sensors  21  and  22  in a sectional view. As  FIG. 6  indicates, the second image sensor  22  is formed on a semiconductor substrate  50 , and the pixels  220  each include a first photoelectric conversion unit  220   a  and a second photoelectric conversion unit  220   b  set side-by-side along the left/right direction on the drawing sheet. At the surface, i.e., the upper surface, of the second image sensor  22 , the first image sensor  21  is laminated via a planarization layer  55 . A wiring layer (not shown) is formed inside the planarization layer  55 . 
     In addition, a microlens  233  is disposed above each of the pixels  210  in the first image sensor  21 , and the microlens  233 , the corresponding pixel  210  in the first image sensor  21  and the corresponding pixel  220  in the second image sensor  22  are disposed in an orderly alignment along the optical axis of the microlens  233 . 
     The first and second photoelectric conversion units  220   a  and  220   b  in the second image sensor  22  are set at positions achieving symmetry relative to an optical axis  233   a  of the microlens  233 . However, the boundary G 1  of the upper partial electrodes  231   a  and  231   b  and the boundary G 2  of the lower partial electrodes  232   a  and  232   b  in the first image sensor  21  are offset in opposite directions relative to the optical axis  233   a  of the microlens  233 . 
       FIG. 7  presents an example of a circuit structure that may be adopted in the signal readout circuit for a pixel  210  in the first image sensor  21 . The pixel  210  includes the organic photoelectric conversion film  230 , the first and second partial electrodes  231   a  and  231   b  and the third and fourth partial electrodes  232   a  and  232   b.  The signal readout circuit for each pixel  210  includes electrode selector transistors  301  through  308 , reset transistors  311  and  312 , output transistors  313  and  314  and row selector transistors  315  and  316 . The first partial electrode  231   a  is connected to the ground via the electrode selector transistor  301 , the second partial electrode  231   b  is connected to the ground via the electrode selector transistor  302 , the third partial electrode  232   a  is connected to the ground via the electrode selector transistor  303 , and the fourth partial electrode  232   b  is connected to the ground via the electrode selector transistor  304 . 
     The first partial electrode  231   a  and the gate of the output transistor  314  are connected via the electrode selector transistor  305 , the second partial electrode  231   b  and the gate of the output transistor  313  are connected via the electrode selector transistor  306 , the third partial electrode  232   a  and the gate of the output transistor  314  are connected via the electrode selector transistor  307 , and the fourth partial electrode  232   b  and the gate of the output transistor  313  are connected via the electrode selector transistor  308 . 
     The output transistor  313  amplifies a voltage signal generated based upon an electric charge from the second partial electrode  231   b,  read out via the electrode selector transistor  306 . In addition, the output transistor  313  amplifies a voltage signal generated based upon an electric charge from the fourth partial electrode  232   b  read out via the electrode selector transistor  308 . A signal having been amplified at the output transistor  313  is read out from a terminal R_Vout via the row selector transistor  315 . 
     The output transistor  314  amplifies a voltage signal generated based upon an electric charge from the first partial electrode  231   a,  read out via the electrode selector transistor  305 . In addition, the output transistor  314  amplifies a voltage signal generated based upon an electric charge from the third partial electrode  232   a  read out via the electrode selector transistor  307 . A signal having been amplified at the output transistor  314  is read out from a terminal L_Vout via the row selector transistor  316 . The reset transistors  311  and  312  each allow excess electric charge to be discharged (i.e., so as to reset to a predetermined potential) in response to a reset signal ϕRST. 
     It is to be noted that since the signal readout circuit for each pixel  220  in the second image sensor  22  is achieved by adopting a structure of the known art, an explanation is not provided. 
     —Area Through Which an Electric Charge is Read Out from a Pixel  210 — 
     In reference to  FIGS. 8A to 8E , an example of pairs of photoelectric conversion areas formed in the organic photoelectric conversion film  230  at the pixel  210 , in which pairs of light fluxes resulting from pupil splitting are received, in correspondence to the on/off states of the electrode selector transistors  301  through  308 , will be explained.  FIGS. 8A to 8E  each show photoelectric conversion areas, viewed from the subject side, through which electric charges are read out at the organic photoelectric conversion film  230  in the pixel  210 . As explained below, the photoelectric conversion areas through which electric charges generated at the organic photoelectric conversion film  230  can be read out in the first image sensor  21  are part of the area located between the upper partial electrodes  231   a  and  231   b  and the lower partial electrodes  232   a  and  232   b,  over which the upper partial electrodes and the lower partial electrodes, being used for purposes of readout, overlap. 
     (1) Electric Charge Readout Area Pattern in  FIG. 8A   
       FIG. 8A  shows an example in which a first photoelectric conversion area  251  and a second photoelectric conversion area  252  to receive a pair of light fluxes resulting from pupil splitting are formed at the organic photoelectric conversion film  230 . The first photoelectric conversion area  251  corresponds to the area of the organic photoelectric conversion film  230  which is covered by the upper partial electrode  231   a,  whereas the second photoelectric conversion area  252  corresponds to the area of the organic photoelectric conversion film  230  which is covered by the upper partial electrode  231   b.  In order to read out photoelectric conversion signals from the first and second photoelectric conversion areas  251  and  252 , the electrode selector transistors  303 ,  304 ,  305  and  306  are turned on with control signals ϕP 1 , ϕP 2 , ϕP 3  and ϕP 4 . 
     As the electrode selector transistors  305  and  306  are turned on in response to the control signals ϕP 3  and ϕP 4  respectively, the first partial electrode  231   a  is connected to the gate of the output transistor  314  and the second partial electrode  231   b  is connected to the gate of the output transistor  313 . As the electrode selector transistors  303  and  304  are turned on in response to the control signals ϕP 1  and ϕP 2  respectively, the third partial electrode  232   a  and the fourth partial electrode  232   b  are connected to the ground. 
     As a result, the electric charge generated in the overlapping area where the first partial electrode  231   a  overlaps the third and fourth partial electrodes  232   a  and  232   b  in the organic photoelectric conversion film  230  is output to the gate of output transistor  314 . Namely, the electric charge generated in the first photoelectric conversion area  251  in the organic photoelectric conversion film  230 , which corresponds to the first partial electrode  231   a,  is output to the gate of the output transistor  314 , as shown in  FIG. 8A . A photoelectric conversion signal generated based upon the electric charge generated in the first photoelectric conversion area  251  is thus read out from the terminal L_Vout. 
     Likewise, the electric charge generated in the overlapping area where the second partial electrode  231   b  overlaps the fourth partial electrodes  232   b  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  313 . In other words, the electric charge generated in the second photoelectric conversion area  252  in the organic photoelectric conversion film  230 , which corresponds to the second partial electrode  231   b,  is output to the gate of the output transistor  313 , as shown in  FIG. 8A . As a result, a photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252  is read out from the terminal R_Vout. 
     This means that the photoelectric conversion signal generated based upon the electric charge generated in the first photoelectric conversion area  251  and the photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252  are read out through the terminal L_Vout and the terminal R_Vout as a pair of photoelectric conversion signals in the electric charge read-out area pattern shown in  FIG. 8A . 
     In each pixel  210 , the organic photoelectric conversion film  230 , the first partial electrode  231   a,  the third partial electrode  232   a  and the fourth partial electrode  232   b  together form a first photoelectric conversion unit  261  from which the electric charge generated in the first photoelectric conversion area  251  is read out, whereas the organic photoelectric conversion film  230 , the second partial electrode  231   b  and the fourth partial electrode  232   b  together form a second photoelectric conversion unit  262  from which the electric charge generated in the second photoelectric conversion area  252  is read out 
     The photoelectric conversion signal generated based upon the electric charge generated in the first photoelectric conversion area  251 , which is output through the terminal L_Vout, and the photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252 , which is output through the terminal R_Vout, are a pair of photoelectric conversion signals generated based upon a pair of light fluxes having passed through different pupil areas of the photographic optical system  10 . These photoelectric conversion signals are used for focus detection executed through the phase method. 
     (2) Electric Charge Readout Area Pattern in  FIG. 8B   
       FIG. 8B  shows an example in which a third photoelectric conversion area  253  and a fourth photoelectric conversion area  254  to receive a pair of light fluxes resulting from pupil splitting are formed at the organic photoelectric conversion film  230 . The third photoelectric conversion area  253  corresponds to the area of the organic photoelectric conversion film  230  which is covered by the lower partial electrode  232   a,  whereas the fourth photoelectric conversion area  254  corresponds to the area of the organic photoelectric conversion film  230  which is covered by the lower partial electrode  232   b.  In order to read out photoelectric conversion signals from the third and fourth photoelectric conversion areas  253  and  254 , the electrode selector transistors  302 ,  301 ,  308  and  307  are turned on with control signals ϕN 1 , ϕN 2 , ϕN 3  and ϕN 4 , respectively. 
     As the electrode selector transistors  307  and  308  are turned on in response to the control signals ϕN 4  and ϕN 3  respectively, the third partial electrode  232   a  is connected to the gate of the output transistor  314  and the fourth partial electrode  232   b  is connected to the gate of the output transistor  313 . As the electrode selector transistors  301  and  302  are turned on in response to the control signals ϕN 2 , ϕN 1  respectively, the first partial electrode  231   a  and the second partial electrode  231   b  are connected to the ground. 
     As a result, the electric charge generated in the overlapping area where the third partial electrode  232   a  overlaps the first partial electrode  231   a  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  314 . Namely, the electric charge generated in the third photoelectric conversion area  253  in the organic photoelectric conversion film  230 , which corresponds to the third partial electrode  232   a,  is output to the gate of the output transistor  314 , as shown in  FIG. 8B . A photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253  is thus read out from the terminal L_Vout. 
     Likewise, the electric charge generated in the overlapping area where the fourth partial electrode  232   b  overlaps the first and second partial electrodes  231   a  and  231   b  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  313 . In other words, the electric charge generated in the fourth photoelectric conversion area  254  in the organic photoelectric conversion film  230 , which corresponds to the fourth partial electrode  232   b,  is output to the gate of the output transistor  313 , as shown in  FIG. 8B . As a result, a photoelectric conversion signal generated based upon the electric charge generated in the fourth photoelectric conversion area  254  is read out from the terminal R_Vout. 
     This means that the photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253  and the photoelectric conversion signal generated based upon the electric charge generated in the fourth photoelectric conversion area  254  are read out through the terminal L_Vout and the terminal R_Vout as a pair of photoelectric conversion signals in the electric charge read-out area pattern shown in  FIG. 8B . 
     In each pixel  210 , the organic photoelectric conversion film  230 , the first partial electrode  231   a  and the third partial electrode  232   a  together form a third photoelectric conversion unit  263  from which the electric charge generated in the third photoelectric conversion area  253  is read out, whereas the organic photoelectric conversion film  230 , the first partial electrode  231   a,  the second partial electrode  231   b  and the fourth partial electrode  232   b  together form a fourth photoelectric conversion unit  264  from which the electric charge generated in the fourth photoelectric conversion area  254  is read out. 
     The photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253 , which is output through the terminal L_Vout, and the photoelectric conversion signal generated based upon the electric charge generated in the fourth photoelectric conversion area  254 , which is output through the terminal R_Vout, are a pair of photoelectric conversion signals generated based upon a pair of light fluxes having passed through different pupil areas of the photographic optical system  10 . These photoelectric conversion signals are used for focus detection executed through the phase method. 
     (3) Electric Charge Readout Area Pattern in  FIG. 8C   
       FIG. 8C  shows an example in which the second photoelectric conversion area  252  and the third photoelectric conversion area  253  to receive a pair of light fluxes resulting from pupil splitting are formed at the organic photoelectric conversion film  230 . In order to read out photoelectric conversion signals from the second and third photoelectric conversion areas  252  and  253 , the electrode selector transistors  304 ,  301 ,  306  and  307  are turned on with control signals ϕP 2 , ϕN 2 , ϕP 4  and ϕN 4 . 
     As the electrode selector transistors  306  and  304  are turned on in response to the control signals ϕP 4  and ϕP 2  respectively, the second partial electrode  231   b  is connected to the gate of the output transistor  313  and the fourth partial electrode  232   b  is connected to the ground. As the electrode selector transistors  307  and  301  are turned on in response to the control signals ϕN 4 , ϕN 2  respectively, the third partial electrode  232   b  is connected to the gate of the output transistor  314  and the first partial electrode  231   a  is connected to the ground. 
     As a result, the electric charge generated in the overlapping area where the second partial electrode  231   b  overlaps the fourth partial electrode  232   b  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  313 . Namely, the electric charge generated in the second photoelectric conversion area  252  in the organic photoelectric conversion film  230 , which corresponds to the second partial electrode  231   b,  is output to the gate of the output transistor  313 , as shown in  FIG. 8C . A photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252 , i.e., the photoelectric conversion signal provided from the second photoelectric conversion unit  262 , is thus read out from the terminal R_Vout. 
     Likewise, the electric charge generated in the overlapping area where the third partial electrode  232   a  overlaps the first partial electrode  231   a  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  314 . In other words, the electric charge generated in the third photoelectric conversion area  253  in the organic photoelectric conversion film  230 , which corresponds to the third partial electrode  232   a,  is output to the gate of the output transistor  314 , as shown in  FIG. 8C . As a result, a photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253 , i.e., the photoelectric conversion signal provided from the third photoelectric conversion unit  263 , is read out from the terminal L_Vout. 
     The photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253 , which is output through the terminal L_Vout, and the photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252 , which is output through the terminal R_Vout, are a pair of photoelectric conversion signals generated based upon a pair of light fluxes having passed through different pupil areas of the photographic optical system  10 . These photoelectric conversion signals are used for focus detection executed through the phase method. 
     (4) Electric Charge Readout Area Pattern in  FIG. 8D   
       FIG. 8D  shows an example in which the second photoelectric conversion area  252  and a fifth photoelectric conversion area  255  to receive a pair of light fluxes resulting from pupil splitting are formed at the organic photoelectric conversion film  230 . The fifth photoelectric conversion area  255  corresponds to the area of the organic photoelectric conversion film  230  where the upper partial electrode  231   a  and the lower partial electrode  232   b  overlap each other. In order to read out photoelectric conversion signals from the second and fifth photoelectric conversion areas  252  and  255 , the electrode selector transistors  304 ,  305  and  306  are turned on with control signals ϕP 2 , ϕP 3  and ϕP 4 . 
     As the electrode selector transistors  305 ,  306  and  304  are turned on in response to the control signals ϕP 3 , ϕP 4  and ϕP 2  respectively, the first partial electrode  231   a  is connected to the gate of the output transistor  314 , the second partial electrode  231   b  is connected to the gate of the output transistor  313 , and the fourth partial electrode  232   b  is connected to the ground. 
     As a result, the electric charge generated in the overlapping area where the first partial electrode  231   a  overlaps the fourth partial electrode  232   b  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  314 . Namely, the electric charge generated in the fifth photoelectric conversion area  255 , which corresponds to the overlapping area where the first partial electrode  231   a  and the fourth partial electrode  232   b  overlap each other at the organic photoelectric conversion film  230 , is output to the gate of the output transistor  314 , as shown in  FIG. 8D . A photoelectric conversion signal generated based upon the electric charge generated in the fifth photoelectric conversion area  255  is thus read out from the terminal L_Vout. 
     Likewise, the electric charge generated in the overlapping area where the second partial electrode  231   b  overlaps the fourth partial electrode  232   b  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  313 . In other words, the electric charge generated in the second photoelectric conversion area  252  in the organic photoelectric conversion film  230 , which corresponds to the second partial electrode  231   b,  is output to the gate of the output transistor  313 , as shown in  FIG. 8D . As a result, a photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252  is read out from the terminal R_Vout. 
     In each pixel  210 , the organic photoelectric conversion film  230 , the first partial electrode  231   a  and the fourth partial electrode  232   b  together form a fifth photoelectric conversion unit  265  from which the electric charge generated in the fifth photoelectric conversion area  255  is read out. 
     The photoelectric conversion signal generated based upon the electric charge generated in the fifth photoelectric conversion area  255 , which is output through the terminal L_Vout, and the photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252 , which is output through the terminal R_Vout, are a pair of photoelectric conversion signals generated based upon a pair of light fluxes having passed through different pupil areas of the photographic optical system  10 . These photoelectric conversion signals are used for focus detection executed through the phase method. 
     (5) Electric Charge Readout Area Pattern in  FIG. 8E   
       FIG. 8E  shows an example in which the third photoelectric conversion area  253  and the fifth photoelectric conversion area  255  to receive a pair of light fluxes resulting from pupil splitting are formed at the organic photoelectric conversion film  230 . In order to read out the photoelectric conversion signals from the third and fifth photoelectric conversion areas  253  and  255 , the electrode selector transistors  301 ,  308  and  307  are turned on with control signals ϕN 2 , ϕN 3  and ϕN 4 . 
     As the electrode selector transistors  307 ,  308  and  301  are turned on in response to the control signals ϕN 4 , ϕN 3  and ϕN 2  respectively, the third partial electrode  232   a  is connected to the gate of the output transistor  314 , the fourth partial electrode  232   b  is connected to the gate of the output transistor  313 , and the first partial electrode  231   a  is connected to the ground. 
     As a result, the electric charge generated in the overlapping area where the third partial electrode  232   a  overlaps the first partial electrode  231   a  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  314 . Namely, the electric charge generated in the third photoelectric conversion area  253  in the organic photoelectric conversion film  230 , which corresponds to the third partial electrode  232   a  is output to the gate of the output transistor  314 , as shown in  FIG. 8E . A photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253  is thus read out from the terminal L_Vout. 
     Likewise, the electric charge generated in the overlapping area where the first partial electrode  231   a  overlaps the fourth partial electrode  232   b  in the organic photoelectric conversion film  230  is output to the gate of the output transistor  313 . In other words, the electric charge generated in the fifth photoelectric conversion area  255 , which corresponds to the overlapping area where the first partial electrode  231   a  and the fourth partial electrode  232   b  overlap each other at the organic photoelectric conversion film  230 , is output to the gate of the output transistor  313 , as shown in  FIG. 8E . As a result, a photoelectric conversion signal generated based upon the electric charge generated in the fifth photoelectric conversion area  255  is read out from the terminal R_Vout. 
     The photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253 , which is output through the terminal L_Vout, and the photoelectric conversion signal generated based upon the electric charge generated in the fifth photoelectric conversion area  255 , which is output through the terminal R_Vout, are a pair of photoelectric conversion signals generated based upon a pair of light fluxes having passed through different pupil areas of the photographic optical system  10 . These photoelectric conversion signals are used for focus detection executed through the phase method. 
     The photoelectric conversion signals read out from the individual pixels  210  in the first image sensor  21  as described above are used to detect the focusing condition at the photographic optical system  10  in correspondence to the positional relationship between the focus detection pupil plane and the plane of the exit pupil at the photographic optical system  10 , as will be explained later. 
     It is to be noted that formation patterns of electric charge readout areas, through which the electric charges are read out from the individual pixels  210 , are not limited to those described above. For instance, photoelectric conversion signals may be obtained from the second photoelectric conversion areas  252  alone, photoelectric conversion signals may be obtained through the third photoelectric conversion areas  253  alone or photoelectric conversion signals may be obtained through the fifth photoelectric conversion areas  255  alone. 
     In addition, an electric charge readout area pattern at the individual pixels  210  in the first image sensor  21  and a range of pixels  210  for photoelectric conversion signal readout may be selected in a desired combination. For instance, a uniform electric charge readout area pattern may be set for all the pixels  210  in the first image sensor, or electric charge readout area patterns may differ depending on the position taken at the image-capturing surface, as described below. 
     As described above, photoelectric conversion signals can be obtained from the first through fifth photoelectric conversion areas  251  through  255  formed with the first and second partial electrodes  231   a  and  231   b  disposed at one surface of the organic photoelectric conversion film  230  and the third and fourth partial electrodes  232   a  and  232   b  disposed at the other surface of the organic photoelectric conversion film  230  in the embodiment. As a result, the electric charge readout areas can be adjusted in an optimal manner by using the upper partial electrodes and the lower partial electrodes in a specific combination for purposes of charge readout, which makes it possible to assure better convenience in the use of the first image sensor  21 . 
     It is to be noted that a photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252  may be read out through the terminal R_Vout, as described below. As the electrode selector transistors  308  and  302  are turned on in response to the control signals ϕN 3  and ϕN 1 , the fourth partial electrode  232   b  is connected to the gate of the output transistor  313  and the second partial electrode  231   b  is connected to the ground. 
     As a result, the electric charge generated in the overlapping area where the second partial electrode  231   b  and the fourth partial electrode  232   b  overlap at the organic photoelectric film  230  is output to the gate of the output transistor  313 . In other words, the electric charge generated in the second photoelectric conversion area  252  corresponding to the second partial electrode  231   b  at the organic photoelectric film  230  is output to the gate of the output transistor  313 . A photoelectric conversion signal generated based upon the electric charge generated in the second photoelectric conversion area  252  is thus read out through the terminal R_Vout. 
     In addition, a photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253  may be read out through the terminal L_Vout, as described below. As the electrode selector transistors  305  and  303  are turned on in response to the control signals ϕP 3  and ϕP 1 , the first partial electrode  231   a  is connected to the gate of the output transistor  314  and the third partial electrode  232   a  is connected to the ground. 
     As a result, the electric charge generated in the overlapping area where the third partial electrode  232   a  and the first partial electrode  231   a  overlap at the organic photoelectric film  230  is output to the gate of the output transistor  314 . In other words, the electric charge generated in the third photoelectric conversion area  253  corresponding to the third partial electrode  232   a  at the organic photoelectric film  230  is output to the gate of the output transistor  314 . A photoelectric conversion signal generated based upon the electric charge generated in the third photoelectric conversion area  253  is thus read out through the terminal L_Vout. 
       FIG. 9  provides an illustration of vignetting, indicating how a pair of light fluxes arriving at pixels  210  and  220  disposed on the first and second image sensors  21  and  22  are restricted by the exit pupil of the photographic optical system  10  in correspondence to the positional relationship between the exit pupil plane and the focus detection pupil plane. 
     The term “exit pupil” is used to refer to an image of the aperture opening at the photographic optical system  10 , viewed from the image sensor side, and the exit pupil distance is defined in the description of the embodiment as the distance measured from a microlens  233  to an exit pupil  97  ( 97 A through  97 C) of the photographic optical system  10 . A focus detection pupil plane  90  is a plane conjugate to the organic photoelectric conversion film  230  in a pixel  210  at the image sensor  21  relative to the corresponding microlens  233  or a plane conjugate to the first and second photoelectric conversion units  220   a  and  220   b  in a pixel  220  at the second image sensor  22  relative to the corresponding microlens  233 . It is to be noted that the plane conjugate to the organic photoelectric conversion film  230  in the first image sensor  21  relative to the microlens  233  and the plane conjugate to the first and second photoelectric conversion units  220   a  and  220   b  in the pixel  220  at the second image sensor  22  relative to the microlens  233  are not in exact alignment and they are slightly offset relative to each other along the optical axis of the photographic optical system. However, the focus detection pupil plane pertaining to the first image sensor  21  and the focus detection pupil plane pertaining to the second image sensor  22  are both set at a single position  90  in the illustration presented in  FIG. 9 . A focus detection pupil distance d is the distance between the microlens  233  and the focus detection pupil plane  90 . 
     A pair of focus detection pupils  95  and  96  on the focus detection pupil plane  90  are split pupils used for focus detection executed by adopting a method known as the split pupil method. A pair of light fluxes having passed through the focus detection pupils  95  and  96  enter a pair of photoelectric conversion areas (e.g., the first and second photoelectric conversion areas  251  and  252  in  FIGS. 8A to 8E ) in each pixel  210  at the first image sensor  21  and then enter a pair of photoelectric conversion units in each pixel  220  at the second image sensor  22 . 
     A center  191  and positions  194  and  195  set apart from the center in the first and second image sensors  21  and  22  in  FIG. 9  respectively correspond to a center  191  and positions  194  and  195  in  FIG. 10 .  FIG. 10  is a front view of an image-capturing surface  190  in each of the first and second image sensors  21  and  22 . The center  191  of the image-capturing surface  190  in  FIG. 10  is in alignment with an optical axis  91  of the photographic optical system  10 . Positions set apart from the center  191  by a predetermined distance along the row direction are designated as peripheral positions  194  and  195 . The peripheral positions  194  and  195  are symmetrical to each other relative to the center  191 . As  FIG. 9  shows, pairs of light fluxes ( 285 ,  286 ), ( 385 ,  386 ) and ( 485 ,  486 ), each having passed through the pair of detection pupils  95  and  96 , respectively reach the pixels  210  and  220  disposed at the center  191 , the pixels  210  and  220  disposed at the peripheral position  194 , and the pixels  210  and  220  disposed at the peripheral position  195 . 
     When the exit pupil  97 A of the photographic optical system  10  is in alignment with the focus detection pupil plane  90 , i.e., when the exit pupil distance matches the focus detection pupil distance d, the pair of focus detection pupils  95  and  96  are contained within the exit pupil  97 A with a circular shape centered on the optical axis  91 . As a result, the pairs of light fluxes ( 285 ,  286 ), ( 385 ,  386 ) and ( 485 ,  486 ) arriving at the individual pixels  220  on the second image sensor  22  are each restricted symmetrically relative to the optical axis. In this case, the amounts of light received at the first and second photoelectric conversion units  220   a  and  220   b  in each pixel  220  are equal to each other. 
     When the exit pupil  97 B of the photographic optical system  10  takes a position between the focus detection pupil plane  90  and the microlenses  233 , i.e., when the exit pupil distance do is smaller than the focus detection pupil distance d, the pair of light fluxes ( 285 ,  286 ) arriving at the pixel  220  disposed at the center  191  is restricted symmetrically relative to the optical axis, and thus, the first and second photoelectric conversion units  220   a  and  220   b  in the pixel  220  receive light in amounts equal to each other. However, asymmetrical vignetting occurs in the pair of light fluxes ( 385 ,  386 ) or ( 485 ,  486 ) arriving at the pixel  220  disposed at the peripheral positions  194  or  195 , and as a result, the amounts of light received at the first and second photoelectric conversion units  220   a  and  220   b  in the pixel  220  at the peripheral position  194  or  195  are different from each other. 
     When the exit pupil  97 C of the photographic optical system  10  takes a position further toward the subject relative to the detection pixel plane  90 , i.e., when the exit pupil distance df is greater than the focus detection pupil distance d, the pair of light fluxes ( 285 ,  286 ) arriving at the pixel  220  disposed at the center  191  is restricted symmetrically relative to the optical axis, and thus, the first and second photoelectric conversion units  220   a  and  220   b  in the pixel  220  receive light in amounts equal to each other. However, asymmetrical vignetting occurs in the pair of light fluxes ( 385 ,  386 ) or ( 485 ,  486 ) arriving at the pixel  220  disposed at the peripheral position  194  or  195 , and as a result, the amounts of light received at the first and second photoelectric conversion units  220   a  and  220   b  in the pixel  220  at the peripheral position  194  or  195  are different from each other. 
     It is to be noted that vignetting occurs in one of a pair of light fluxes entering the pixels disposed at the peripheral positions  194  and  195  when the photographic optical system  10  has the exit pupil  97 B, whereas vignetting occurs the other of the pair of light fluxes entering the pixel disposed at the peripheral position  194  or  195  when the photographic optical system  10  has the exit pupil  97 C. 
     As explained above, when the exit pupil plane of the photographic optical system  10  is not in alignment with the focus detection pupil plane and the pixel receiving a pair of light fluxes is not disposed at the center  191  of the second image sensor  22 , i.e., a pair of light fluxes enters a pixel disposed at a peripheral position  194  or  195 , the pair of light fluxes is restricted asymmetrically by the exit pupil  97 B or  97 C of the photographic optical system  10 . The extent to which the light fluxes are restricted asymmetrically changes in correspondence to the difference between the exit pupil distance at the photographic optical system  10  and the focus detection pupil distance d and the distance between the peripheral position  194  or  195  and the center  191 . 
       FIG. 11  is a front view of the pixel  220  disposed at the peripheral position  194  in  FIGS. 9 and 10 , indicating the range over which the light fluxes enter the first and second photoelectric conversion units  220   a  and  220   b  at the pixel  220  when the exit pupil distance do at the photographic optical system  10  is smaller than the focus detection pupil distance d. On the first and second photoelectric conversion units  220   a  and  220   b  at the pixel  220 , a circular image  271  of the exit pupil of the photographic optical system  10  is formed via the microlens  233 . The asymmetrical restriction of the pair of light fluxes described above corresponds to an offset of a central position C of the circular image  271  of the exit pupil relative to a central position G of an element separation area  220   c  equivalent to the position of the geographical center of the first and second photoelectric conversion units  220   a  and  220   b.  When the exit pupil distance dn in the photographic optical system  10  is smaller than the focus detection pupil distance d, the central position C of the image of the exit pupil moves further toward the greater image height side, i.e., toward the peripheral edge of the second image sensor  22 , as the image height increases. 
     When the exit pupil distance df is greater than the focus detection pupil distance d, the positional relationship between the first and second photoelectric conversion units  220   a  and  220   b  and the circular shape  271  in the pixel disposed at the peripheral position  194  is reversed from that shown in  FIG. 11 . Specifically, the central position C of the circular shape  271  is offset further to the left in the figure relative to the central position G of the element separation area  220   c.  Namely, when the exit pupil distance df in the photographic optical system  10  is greater than the focus detection pupil distance d, the central position C of the image of the exit pupil moves further toward the center  191  of the second image sensor  22 , as the image height increases. 
     When the exit pupil distance dn is smaller than the focus detection pupil distance d, the positional relationship of the first and second photoelectric conversion units  220   a  and  220   b  to the circular shape  271  in the pixel disposed at the position  195  symmetry to the position  194  relative to the center  191  along the horizontal direction as shown in  FIG. 11  is reversed from the positional relationship shown in  FIG. 11 . Namely, the central position C of the circular shape  271  is offset further to the left side in the figure relative to the central position G of the element separation area  220   c.    
     When the exit pupil distance df is greater than the focus detection pupil distance d, the positional relationship of the first and second photoelectric conversion units  220   a  and  220   b  to the circular shape  271  in the pixel disposed at the position  195  symmetry to the position  194  along the horizontal direction relative to the center  191 , is identical to the positional relationship shown in  FIG. 11 . 
     As described above, the central position C of the exit pupil image formed at a pixel  220  is offset relative to the central position G of the element separation area  220   c  in correspondence to the positional relationship between the exit pupil plane and the focus detection pupil plane, by a greater extent with increase in the image height. Accordingly, the range of pixels  210  and  220 , from which photoelectric conversion signals are to be read out for purposes of detecting the focusing condition at the photographic optical system  10 , is adjusted as described below in correspondence to the positional relationship between the exit pupil plane and the focus detection pupil plane in the embodiment. 
     The photographic optical system  10  may be an interchangeable lens that is detachably mounted at a camera body and, in such a case, the position of the exit pupil changes in correspondence to the interchangeable lens that is currently mounted. The position of the exit pupil may also change in correspondence to the zoom position when the photographic optical system  10  includes a zoom lens. In the embodiment, either the pair of photoelectric conversion areas  251  and  252  or the pair of photoelectric conversion areas  253  and  254 , for instance, is selected at the first image sensor  21  in correspondence to the exit pupil distance at the photographic optical system  10 , so as to use the optimal pair of photoelectric conversion areas best suited for the particular exit pupil position taken at the photographic optical system. The concept will be described in detail below. 
     (a) When the exit pupil distance d at the photographic optical system  10  matches the focus detection pupil distance d 
       FIG. 12  shows pixels at an image sensor used for purposes of focus detection when the exit pupil distance d matches the focus detection pupil distance d. It is to be noted that in  FIG. 12  and  FIGS. 13A through 15 , in reference to which a description will be provided later, pixels  210  and  220  disposed over a range of 5 rows×6 columns are shown to represent the respective image-capturing surfaces in their entirety for purposes of illustration simplification. When the exit pupil distance d in the photographic optical system  10  matches the focus detection pupil distance d, the pairs of light fluxes ( 285 ,  286 ), ( 385 ,  386 ) and ( 485 ,  486 ) arriving at the various pixels are each restricted symmetrically relative to the optical axis as explained with reference to  FIG. 9 . As a result, the central position C of the exit pupil image is aligned with the central position G of the element separation area  220   c  equivalent to the position of the geographical center of the first and second photoelectric conversion units  220   a  and  220   b.  For this reason, the amount of light received at the first photoelectric conversion unit  220   a  and the amount of light received at the second photoelectric conversion unit  220   b  in each pixel  220  at the second image sensor are substantially equal to each other, even when the image height increases. 
     Accordingly, the photoelectric conversion signals provided from the first and second photoelectric conversion units  220   a  and  220   b  in each pixel  220  at the second image sensor  22  are used as focus detection signals, as shown in  FIG. 12 . This means that when the exit pupil distance d in the photographic optical system is equal to the focus detection pupil distance d, the focus detection unit  12   b  shown in  FIG. 1  executes focus detection by using the photoelectric conversion signals provided from the pixels  220  disposed over the entire image-capturing surface  190  in  FIG. 10  at the second image sensor  22 , without using photoelectric conversion signals from the pixels disposed at the first image sensor  21 . It is to be noted that even when there is a difference between the exit pupil distance d in the photographic optical system  10  and the focus detection pupil distance d, the exit pupil distance d in the photographic optical system  10  and the focus detection pupil distance d are regarded to be the same as long as the difference is within a predetermined range. 
     (b) When the exit pupil distance dn in the photographic optical system  10  is smaller than the focus detection pupil distance d 
       FIGS. 13A to 13C  illustrate a condition where the exit pupil distance dn in the photographic optical system  10  is smaller than the focus detection pupil distance d. As shown in  FIG. 13A , the image-capturing surface  190  is divided into three sections, i.e., a central portion  190 A, a left peripheral portion  190 B and a right peripheral portion  190 C. In the left and right peripheral portions  190 B and  190 C, photoelectric conversion signals from the first through fourth photoelectric conversion areas in pixels in the first image sensor  21  are used as focus detection signals and photoelectric conversion signals from the first and second photoelectric conversion units in pixels in the second image sensor  22  are used as focus detection signals in the central portion  190 A of the image-capturing area  190 , as shown in  FIG. 13B . In other words, focus detection is executed by using the photoelectric conversion signals provided from the first through fourth photoelectric conversion areas of pixels in the first image sensor  21 , as focus detection signals corresponding to the peripheral areas in the photographic image plane achieved via the photographic optical system  10 , whereas focus detection is executed by using the photoelectric conversion signals from the first and second photoelectric conversion units in pixels in the second image sensor  22  as focus detection signals corresponding to the central area of the photographic image plane. 
       FIG. 13B  shows the various photoelectric conversion areas  251 ,  252 ,  253  and  254  formed in the individual pixels  210  in the first image sensor. The electrode selector transistors  303 ,  304 ,  305  and  306  in the readout circuit shown in  FIG. 7  are turned on so as to form the first and second photoelectric conversion areas  251  and  252 , as shown in FIG.  8 A, in each pixel  210  present in the right peripheral portion  190 C of the image-capturing surface  190  at the first image sensor  21 . In addition, the electrode selector transistors  302 ,  301 ,  308  and  307  in the readout circuit shown in  FIG. 7  are turned on so as to form the third and fourth photoelectric conversion areas  253  and  254 , as shown in  FIG. 8B  in each pixel  210  present in the left peripheral portion  190 B of the image-capturing surface  190  at the first image sensor  21 . 
     The focus detection unit  12   a  executes focus detection for the left peripheral portion  190 B of the image-capturing surface  190  by using the photoelectric conversion signals provided from the third and fourth photoelectric conversion areas  253  and  254  in the pixels  210  present in the left peripheral portion of the first image sensor  21  as focus detection signals, executes focus detection for the right peripheral portion  190 C of the image-capturing surface  190  by using the photoelectric conversion signals provided from the first and second photoelectric conversion areas  251  and  252  in the pixels  210  present in the right peripheral portion of the first image sensor  21  as focus detection signals, and executes focus detection for the central portion  190 A of the image-capturing surface  190  by using the photoelectric conversion signals provided from the first and second photoelectric conversion units  220   a  and  220   b  in the pixels  220  present in the central portion at the second image sensor  22  as focus detection signals, as shown in  FIG. 13C . As a result, an improvement is achieved in the detection accuracy with which the focusing condition of the photographic optical system  10  in the peripheral areas of the image-capturing surface  190  in the row direction is detected. This means that the range over which focus detection can be executed in a desirable manner through the phase detection method can be expanded toward the peripheral edges of the image-capturing surface  190 . 
     (c) When the exit pupil distance df in the photographic optical system  10  is greater than the focus detection pupil distance d 
       FIGS. 14A and 14B  illustrate a condition where the exit pupil distance df in the photographic optical system  10  is greater than the focus detection pupil distance d. When the exit pupil distance df in the photographic optical system  10  is greater than the focus detection pupil distance d, the photoelectric conversion signals from the first through fourth photoelectric conversion areas of pixels in the first image sensor  21  are used as focus detection signals in the left and right peripheral portions  190 B and  190 C of the image-capturing surface  190  and the photoelectric conversion signals from the first and second photoelectric conversion units in pixels in the second image sensor  22  are used as focus detection signals in the central portion  190 A of the image-capturing surface  190 , as shown in  FIG. 14A . The concept will be described in detail below. 
       FIG. 14A  shows the various photoelectric conversion areas  251 ,  252 ,  253  and  254  formed in the individual pixels  210  in the first image sensor. The electrode selector transistors  302 ,  301 ,  308  and  307  in the readout circuit shown in  FIG. 7  are turned on so as to form the third and fourth photoelectric conversion areas  253  and  254 , as shown in  FIG. 8B , in each pixel  210  present in the right peripheral portion  190 C of the image-capturing surface  190  at the first image sensor  21 . In addition, the electrode selector transistors  303 ,  304 ,  305  and  306  in the readout circuit shown in  FIG. 7  are turned on so as to form the first and second photoelectric conversion areas  251  and  252 , as shown in  FIG. 8A , in each pixel  210  present in the left peripheral portion  190 B of the image-capturing surface  190  at the first image sensor  21 . 
     The focus detection unit  12   a  executes focus detection for the left peripheral portion  190 B of the image-capturing surface  190  by using the photoelectric conversion signals provided from the first and second photoelectric conversion areas  251  and  252  in the pixels  210  present in the left peripheral portion of the first image sensor  21  as focus detection signals, executes focus detection for the right peripheral portion  190 C of the image-capturing surface  190  by using the photoelectric conversion signals provided from the third and fourth photoelectric conversion areas  253  and  254  in the pixels  210  present in the right peripheral portion of the first image sensor  21  as focus detection signals, and executes focus detection for the central portion  190 A of the image-capturing surface  190  by using the photoelectric conversion signals provided from the first and second photoelectric conversion units  220   a  and  220   b  in the pixels  220  present in the central portion at the second image sensor  22  as focus detection signals. As a result, an improvement is achieved in the detection accuracy with which the focusing condition of the photographic optical system  10  in the peripheral areas of the image-capturing surface  190  in the row direction, is detected. This means that the range over which focus detection can be executed in a desirable manner through the phase detection method can be expanded toward the peripheral edges of the image-capturing surface  190 . 
     It is to be noted that the positions at which the image-capturing surface  190  is split into the central portion  190 A, the left peripheral portion  190 B and the left peripheral portion  190 C as described above may be adjusted in correspondence to the difference between the exit pupil distance do or df in the photographic optical system  10  and the focus detection pupil distance d. Namely, as the difference between the exit pupil distance dn or df in the photographic optical system  10  and the focus detection pupil distance d increases, the left peripheral portion  190 B and the right peripheral portion  190 C may be set to have a greater width measured along the left/right direction in the figures, so as to expand the range over which the photoelectric conversion signals from the first through fourth photoelectric conversion areas in the pixels at the first image sensor  21  are used as focus detection signals. 
     —In a State of Pronounced Defocus— 
       FIG. 15  illustrates a state of pronounced defocus. In a state of pronounced defocus, the focusing condition at the photographic optical system  10  is detected based upon the photoelectric conversion signals provided from the second and third photoelectric conversion areas  252  and  253  in the pixels  210  at the first image sensor  21 , as explained below. 
     In a state of pronounced defocus, the image is blurred to a greater extent, which makes it difficult to detect the focusing condition at the photographic optical system  10 . In addition, when the F-number set at the photographic optical system  10  is small, a greater extent of image blurring occurs in a defocused state due to a small depth of field, which also makes it difficult to detect the focusing condition at the photographic optical system  10 . 
     If the width of a photoelectric conversion area, measured along the direction running parallel to the direction in which the pupil is split, i.e., the width measured along the row direction, is reduced, the light flux to enter the photoelectric conversion area is restricted, resulting in a smaller extent of image blurring. As a result, the focusing condition at the photographic optical system  10  can be detected with better ease even if the F-number at the photographic optical system  10  is small or in a state of pronounced defocus. 
     Accordingly, the focusing condition at the photographic optical system  10  is detected in the embodiment based upon the photoelectric conversion signals provided from the second and third photoelectric conversion areas  252  and  253  having a smaller width measured along the row direction, i.e. the direction running parallel to the direction in which the pupil is split, in a state of pronounced defocus. For instance, if the focusing condition at the photographic optical system  10  cannot be detected based upon the photoelectric conversion signals based upon the electric charges generated in the first through fourth photoelectric conversion areas  251  through  254  in the pixels  210  at the first image sensor  21  or the photoelectric conversion signals output from the individual pixels  220  at the second image sensor  22 , as in (a) through (c) explained earlier, the focusing condition at the photographic optical system  10  is detected based upon the photoelectric conversion signals provided from the second and third photoelectric conversion areas  252  and  253 . 
     Namely, if the focusing condition at the photographic optical system  10  cannot be detected through any of the methods (a) through (c) described above, the focus detection unit  12   a  shown in  FIG. 1  switches the on/off states of the electrode selector transistors in the readout circuits in  FIG. 7  in all the pixels  210  at the first image sensor  21  so as to output photoelectric conversion signals from the second and third photoelectric conversion areas  252  and  253 . The focus detection unit  12   b  then executes focus detection by using the photoelectric conversion signals provided from the second and third photoelectric conversion areas  252  and  253  in all the pixels  210  at the first image sensor  21  over the entire image-capturing surface  190  shown in  FIG. 10 . Through these measures, the focusing condition at the photographic optical system  10  can be detected with better accuracy even in a state of pronounced defocus or when the F-number at the photographic optical system  10  is small. 
     As focus detection is executed by using the photoelectric conversion signals provided from the second and third photoelectric conversion areas  252  and  253  in each pixel  210  at the first image sensor  21  as focus detection signals and the focusing lens is driven along the optical axis via a lens drive unit (not shown), the defocus quantity is reduced. It is desirable that once the focusing lens has been driven along the optical axis and the extent of defocus has been reduced, the focus detection unit  12   a  switch the on/off states of the electrode selector transistors in the readout circuits in  FIG. 7  so as to re-attempt to detect the focusing condition at the photographic optical system  10  through one of the methods (a) through (c) explained earlier. 
     It is to be noted that when executing focus detection by using a pair of photoelectric conversion signals provided from the second and third photoelectric conversion areas  252  and  253  in each pixel as focus detection signals, the sensitivity is bound to become lower since the second and third photoelectric conversion areas  252  and  253  take up smaller areas. In such a case, focus detection may also be executed by using the photoelectric conversion signals provided from the first and second photoelectric conversion units  220   a  and  220   b  in each pixel  220  at the second image sensor  22  as focus detection signals. 
     —Variations of the Positional Arrangement for the Partial Electrodes— 
     In the embodiment described above, the pixels  210  in the first image sensor  21  each include first and second partial electrodes  231   a  and  231   b  disposed side-by-side along the row direction and third and fourth partial electrodes  232   a  and  232   b  disposed side-by-side along the row direction, as shown in  FIGS. 4A to 4C and 5A to 5C . However, the various partial electrodes may be disposed along a direction other than the direction described above. For instance, the partial electrodes may be disposed along the column direction, as illustrated in  FIGS. 16A through 16D .  FIGS. 16A to 16D  schematically illustrate the structure adopted in a pixel  210  in the first image sensor  21  in a variation, with  FIG. 16A  showing a pixel  210 A in a plan view taken from the subject side,  FIG. 16B  showing the pixel  210 A in a side elevation taken from a side surface along the column direction,  FIG. 16C  showing the pixel  210 A in a sectional view taken through c 2 -c 2  in  FIG. 16B  and  FIG. 16D  showing the pixel  210 A in a side elevation taken from a side surface along the row direction. 
     First and second partial electrodes  231   c  and  231   d  in the pixel  210 A in this variation are set side-by-side along the column direction, i.e., along the up/down direction in  FIG. 16A . Likewise, third and fourth partial electrodes  232   c  and  232   d  are set side-by-side along the column direction, i.e., along the up/down direction in  FIG. 16C . The length of the first partial electrode  231   c,  measured along the column direction and the length of the second partial electrode  231   d  measured along the column direction are different from each other. The length W 3  of the first partial electrode  231   c  located on the upper side in  FIG. 16A  and  FIG. 16D  measured along the column direction, is smaller than the length W 4  of the second partial electrode  231   d  located on the lower side in the figures, measured along the column direction. Likewise, the length of the third partial electrode  232   c,  measured along the column direction and the length of the fourth partial electrode  232   d  measured along the column direction are different from each other. The length W 4  of the third partial electrode  232   c  located on the upper side in the figures, measured along the column direction, is greater than the length W 3  of the fourth partial electrode  232   d  located on the lower side in the figures, measured along the column direction. In other words, viewed from the side on which light enters, the position of a separation area G 3  separating the first partial electrode  231   c  from the second partial electrode  231   d,  i.e., the position of the boundary, is different from the position of a separation area G 4 , i.e., the position of the boundary separating the third partial electrode  232   c  from the fourth partial electrode  232   d.  It is to be noted that the length W 3  of the first partial electrode  231   c  measured along the column direction matches the length W 3  of the fourth partial electrode  232   d  measured along the column direction, and the length W 4  of the second partial electrode  231   d  measured along the column direction matches the length W 4  of the third partial electrode  232   c  measured along the column direction. 
     Thus, the entire first partial electrode  231   c  overlaps, along the optical axis, the third partial electrode  232   c.  The second partial electrode  231   d  overlaps, along the optical axis, part of the third partial electrode  232   c  located toward the lower side in the figures, and the entire fourth partial electrode  232   d.  The third partial electrode  232   c  overlaps, along the optical axis, the entire first partial electrode  231   c  and part of the second partial electrode  231   d  located toward the upper side in the figures. The entire fourth partial electrode  232   d  overlaps, along the optical axis, the third partial electrode  231   d.  In the following description, the first and second partial electrodes  231   c  and  231   d  formed at the upper surface of the organic photoelectric conversion film  230  may be alternatively referred to as upper partial electrodes  231   c  and  231   d  and the third and fourth partial electrodes  232   c  and  232   d  formed at the lower surface of the organic photoelectric conversion film  230  may be alternatively referred to as lower partial electrodes  232   c  and  232   d.    
       FIG. 17A  schematically illustrates the positions of the first and second partial electrodes  231   c  and  231   d  at the individual pixels  210 A in the first image sensor  21  viewed from the subject side, whereas  FIG. 17B  schematically illustrates the positions of the third and fourth partial electrodes  232   c  and  232   d  at the individual pixels  210 A at the first image sensor  21  viewed from the subject side.  FIG. 17C  schematically illustrates the positions of first and second photoelectric conversion units  220   a  and  220   b  at the individual pixels  220  in the second image sensor  22  viewed from the subject side. It is to be noted that  FIGS. 17A to 17C  only show pixels  210 A and  220  disposed in the first and second image sensors  21  and  22  respectively over a range of 5 rows×6 columns, in order to simplify the illustrations. 
     As explained earlier, the pixels  210 A in the first image sensor  21  in  FIGS. 17A and 17B  each include the first and second partial electrodes  231   c  and  231   d  disposed side-by-side along the column direction, i.e., along the up/down direction in  FIGS. 17A and 17B , and the third and fourth partial electrodes  232   c  and  232   d,  also disposed side-by-side along the column direction. The pixels  220  in the second image sensor  22  shown in  FIG. 17C  are identical to the pixels  220  in the embodiment described earlier. 
       FIGS. 18A to 18E  illustrate first through fifth photoelectric conversion areas  251 A through  255 A, which are formed in the organic photoelectric conversion film  230  in each pixel  210 A, viewed from the subject side. As  FIGS. 18A to 18E  show, the first through fifth photoelectric conversion areas  251 A through  255 A are formed in the pixel  210 A.  FIG. 18A  shows the first photoelectric conversion area  251 A. The first photoelectric conversion area  251 A corresponds to the area of the organic photoelectric conversion film  230 , which is covered by the upper partial electrode  231   c.    FIG. 18B  shows the second photoelectric conversion area  252 A. The second photoelectric conversion area  252 A corresponds to the area of the organic photoelectric conversion film  230  which is covered by the upper partial electrode  231   d.    FIG. 18C  shows the third photoelectric conversion area  253 A. The third photoelectric conversion area  253 A corresponds to the area of the organic photoelectric conversion film  230  which is covered by the lower partial electrode  232   c.    FIG. 18D  shows the fourth photoelectric conversion area  254 A. The fourth photoelectric conversion area  254 A corresponds to the area of the organic photoelectric conversion film  230  which is covered by the lower partial electrode  232   d.    FIG. 18E  shows the fifth photoelectric conversion area  255 A. The fifth photoelectric conversion area  255 A corresponds to the area of the organic photoelectric conversion film  230  where the upper partial electrode  231   d  and the lower partial electrode  232   c  overlap. 
     It is to be noted that the pixels  210  described earlier and the pixels  210 A may be disposed at alternate positions along the column direction in the first image sensor  21 , as shown in  FIGS. 19A and 19B .  FIG. 19A  schematically illustrates the positions of the first and second partial electrodes  231   a,    231   b,    231   c  and  231   d  in the individual pixels  210  and  210 A in the first image sensor  21 , viewed from the subject side.  FIG. 19B  schematically illustrates the positions of the third and fourth partial electrodes  232   a,    232   b,    232   c  and  232   d  in the individual pixels  210  and  210 A in the first image sensor  21 , viewed from the subject side.  FIG. 19C  schematically illustrates the positions of the first and second photoelectric conversion units  220   a  and  220   b  in the pixels  220  at the second image sensor  22 , viewed from the subject side. It is to be noted that  FIGS. 19A to 19C  only show pixels  210  and  210 A disposed in the first image sensor  21  and pixels  220  disposed in the second image sensor  22  over a range of 5 rows×6 columns in order to simplify the illustrations. The pixels  220  in the second image sensor  22  shown in  FIG. 19C  are identical to the pixels  220  in the embodiment described earlier. 
       FIGS. 20A and 20B  show shapes of various partial electrodes, viewed from the subject side, in schematic illustrations of a pixel  210  in the first image sensor  21  achieved in another variation. As in the pixel  210 B in this variation, first and second partial electrodes  231   e  and  231   f  in the first image sensor  21  may be formed in shapes achieved by dividing a large rectangle asymmetrically into two portions along a direction running parallel to a diagonal of the rectangle, as shown in  FIG. 20A , and third and fourth partial electrodes  232   e  and  232   f  in the first image sensor  21  may be formed in shapes achieved by dividing a large rectangle asymmetrically into two portions along a direction running parallel to a diagonal of the rectangle, as shown in  FIG. 20B . The position of the boundary between the first and second partial electrodes  231   e  and  231   f,  i.e., the position at which they are separated from each other, and the position of the boundary between the third and fourth partial electrodes  232   e  and  232   f,  i.e., the position at which they are separated from each other, do not match. 
     In addition, the first image sensor  21  may include pixels  210 C each having upper partial electrodes asymmetrically separated from each other both along the row direction and also along the column direction, as shown in  FIG. 21 . The pixel  210 C may also include lower partial electrodes asymmetrically separated from each other both along the row direction and also along the column direction. In the example presented in  FIG. 21 , upper partial electrodes  231   g,    231   h,    231   i  and  231   j  are formed at the upper surface of the organic photoelectric conversion film  230 , i.e., at the surface of the organic photoelectric conversion film  230  located on the subject side, and lower partial electrodes  232   g,    232   h,    232   i  and  232   j  are formed at the lower surface of the organic photoelectric conversion film  230 . It is to be noted that the row direction extends along the left/right direction in  FIG. 21 . 
     The position at which the upper partial electrodes  231   g  and  231   i  are separated from the upper partial electrodes  231   h  and  231   j  along the row direction is different from the position at which the lower partial electrodes  232   g  and  232   i  are separated from the lower partial electrodes  232   h  and  232   j  along the row direction. Likewise, the position at which the upper partial electrodes  231   g  and  231   h  are separated from the upper partial electrodes  231   i  and  231   j  along the column direction is different from the position at which the lower partial electrodes  232   g  and  232   h  are separated from the lower partial electrodes  232   i  and  232   j  along the column direction. 
     In other words, upper partial electrodes separated from one another along the row direction and along the column direction, and lower partial electrodes separated from one another along the row direction and along the column direction may be formed, with the boundary positions at which the individual upper partial electrodes are separated from one another set differently from the boundary positions at which the individual lower partial electrodes are separated from one another. 
     It is to be noted that while the length W 1  of the first partial electrode  231   a,  measured along the row direction, is set equal to the length W 1  of the fourth partial electrode  232   b  measured along the row direction, and the length W 2  of the second partial electrode  231   b  measured along the row direction is set equal to the length W 2  of the third partial electrode  232   a  measured along the row direction in the embodiment described earlier, the length W 1  of the first partial electrode  231   a  measured along the row direction may be different from the length W 1  of the fourth partial electrode  232   b  measured along the row direction and the length W 2  of the second partial electrode  231   b  measured along the row direction may be different from the length W 2  of the third partial electrode  232   a  measured along the row direction instead. 
     Likewise, while the length W 3  of the first partial electrode  231   c,  measured along the column direction, is set equal to the length W 3  of the fourth partial electrode  232   d  measured along the column direction, and the length W 4  of the second partial electrode  231   d  measured along the column direction is set equal to the length W 4  of the third partial electrode  232   c  measured along the column direction in the variation described earlier, the length W 3  of the first partial electrode  231   c  measured along the column direction may be different from the length W 3  of the fourth partial electrode  232   d  measured along the row direction and the length W 4  of the second partial electrode  231   d  measured along the column direction may be different from the length W 4  of the third partial electrode  232   c  measured along the column direction instead. 
     It is to be noted that the embodiment described above may be adopted in combination with any of the variations described above. 
     While the present invention has been described in reference to an embodiment and variations thereof, the present invention is in no way limited to the particulars of these examples. Any mode conceivable within the scope of the technical teaching of the present invention is also within the scope of the present invention. 
     The disclosure of the following priority application is herein Incorporated by reference: 
     Japanese Patent Application No. 2016-2729 filed Jan. 8, 2016 
     REFERENCE SIGNS LIST 
       1 ; digital camera,  10 ; photographic optical system,  11 ; image-capturing unit,  12 ; control unit,  12   a;  focus detection unit,  21 ; first image sensor,  22 ; second image sensor,  210 ,  210 A,  210 B,  210 C,  220 : pixel,  230 ; organic photoelectric conversion film,  231   a,    231   c,    231   e;  first partial electrode,  231   b,    231   d,    231   f;  second partial electrode,  232   a,    232   c,    232   e;  third partial electrode,  232   b,    232   d,    232   f;  fourth partial electrode,  251 ; first photoelectric conversion area,  252 ; second photoelectric conversion area,  253 ; third photoelectric conversion area,  254 ; fourth photoelectric conversion area,  255 ; fifth photoelectric conversion area,  301 - 308 ; electrode selector transistor,  311 ,  312 ; reset transistor,  313 ,  314 ; output transistor,  315 ,  316 ; row selector transistor