Patent Publication Number: US-2020278202-A1

Title: Ranging apparatus and moving object capable of high-accuracy ranging

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a ranging apparatus and a moving object, and more particularly to a ranging apparatus and a moving object which are capable of performing high-accuracy ranging by an imaging plane phase difference method. 
     Description of the Related Art 
     To assist the movement of a moving object, such as a vehicle, a drone, or a robot, it is required to measure a distance between a moving object, and an object, such as an obstacle, around the moving object. Particularly, to assist the movement of the moving object e.g. for avoiding collision or for tracking another object, it is also required to perform recognition processing using images of an obstacle, and hence, a camera as an image pickup apparatus is often used in the moving object, for measuring a distance to the obstacle. As a method of acquiring not only images but also distances using a camera, there has been known an imaging plane phase difference method (Japanese Laid-Open Patent Publication (Kokai) No. 2013-190622). In the imaging plane phase difference method, parallax between a pair of images formed by light fluxes having passed through two different regions (partial pupils) of an exit pupil of an optical system of the camera is determined, and distance information (phase difference of image signals) of an object is measured from the parallax based on the principle of the triangulation. In the camera using the imaging plane phase difference method, each pixel of an image pickup device has two photoelectric conversion sections (photodiodes), for example. In the imaging plane phase difference method, while parallax between a pair of images is determined from electric signals (hereinafter referred to as “image signals”) converted from images (optical images) formed by light fluxes entering the respective photoelectric conversion sections, an object image is acquired by adding up the image signals of the photoelectric conversion sections. 
     On the other hand, in a case where the camera is applied to a moving object, downsizing and durability of the camera are required, and hence normally, the camera is not provided with an auto focus function, but focus of the camera is fixed, whereby an in-focus position is set in the center of a measurable distance range (hereinafter referred to as a “distance measurement range”). 
     However, since in the triangulation, the accuracy of measurement of distance (hereinafter referred to as the “ranging accuracy”) is reduced in proportion to approximately the square of a distance to an object, there is a fear that the ranging accuracy is lowered at a long distance end of the distance measurement range. 
     SUMMARY OF THE INVENTION 
     The present invention provides a ranging apparatus and a moving object which are capable of suppressing the reduction of ranging accuracy at a long distance end of a distance measurement range, thereby making it possible to perform high-accuracy ranging over a wide distance range. 
     In a first aspect of the present invention, there is provided an ranging apparatus comprising an optical system that is a fixed focus optical system, an image pickup device that receives light fluxes from the optical system, and a distance information acquisition unit that is configured to acquire distance information based on image signals from the image pickup device, wherein the distance information acquisition unit acquires the distance information on a object, based on parallax between a first image based on a light flux from an object, having passed through a first region of an exit pupil of the optical system, and a second image based on a light flux from the object, having passed through a second region of the exit pupil, and wherein the optical system is configured such that the parallax of an object existing at a distance of 100 m from the ranging apparatus is smaller than the parallax of an object existing at a distance of 1 m from the ranging apparatus. 
     In a second aspect of the present invention, there is provided a ranging apparatus comprising an optical system that is a fixed focus optical system, an image pickup device that receives light fluxes from the optical system, and a distance information acquisition unit that is configured to acquire distance information based on image signals from the image pickup device, wherein the distance information acquisition unit acquires the distance information on a object, based on parallax between a first image based on a light flux from an object, having passed through a first region of an exit pupil of the optical system, and a second image based on a light flux from the object, having passed through a second region of the exit pupil, and wherein the optical system is configured such that the parallax of an object existing at a long distance end of a distance measurement range of the ranging apparatus is smaller than the parallax of an object existing at a short distance end of the distance measurement range of the ranging apparatus. 
     In a third aspect of the present invention, there is provided a moving object including a ranging apparatus, and a controller that controls the moving object based on a result of ranging by the ranging apparatus, wherein the ranging apparatus comprises an optical system that is a fixed focus optical system, an image pickup device that receives light fluxes from the optical system, and a distance information acquisition unit that is configured to acquire distance information based on image signals from the image pickup device, wherein the distance information acquisition unit acquires the distance information on a object, based on parallax between a first image based on a light flux from an object, having passed through a first region of an exit pupil of the optical system, and a second image based on a light flux from the object, having passed through a second region of the exit pupil, and wherein the optical system is configured such that the parallax of an object existing at a long distance end of a distance measurement range of the ranging apparatus is smaller than the parallax of an object existing at a short distance end of the distance measurement range of the ranging apparatus. 
     According to the present invention, it is possible to suppress the reduction of ranging accuracy at the long distance end of the distance measurement range, thereby making it possible to perform high-accuracy ranging over a wide distance range. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a ranging apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a schematic front view of an image pickup device appearing in  FIG. 1 . 
         FIGS. 3A to 3C  are diagrams useful in explaining the principle of distance measurement (ranging) by an imaging plane phase difference method. 
         FIGS. 4A to 4C  are diagrams useful in explaining the ranging accuracy of the ranging apparatus using the imaging plane phase difference method. 
         FIGS. 5A to 5C  are diagrams useful in explaining the relationship between a parallax  0  position and a distance measurement range in the first embodiment. 
         FIG. 6  is a diagram useful in explaining a case where the parallax  0  position displaced toward a long distance end of the distance measurement range coincides with an in-focus position. 
         FIG. 7  is a diagram useful in explaining a case where the parallax  0  position displaced toward the long distance end of the distance measurement range does not coincide with an in-focus position. 
         FIGS. 8A to 8C  are diagrams of an exit pupil of an optical system according to the first embodiment. 
         FIGS. 9A and 9B  are diagrams useful in explaining the arrangement of PDs in imaging and ranging pixels. 
         FIGS. 10A and 10B  are diagrams useful in explaining a variation of the exit pupil of the optical system, and a variation of the arrangement of PDs in an imaging and ranging pixel. 
         FIG. 11  is a schematic diagram showing a state in which a driving assistance system is installed on an automotive vehicle as a moving object according to a second embodiment of the present invention. 
         FIG. 12  is a diagram of the driving assistance system for the automotive vehicle as the moving object according to the second embodiment. 
         FIG. 13  is a flowchart of a collision avoidance process performed by the driving assistance system for the automotive vehicle as the moving object according to the second embodiment. 
         FIGS. 14A to 14D  are diagrams useful in explaining a distance corresponding to two pixels as a maximum value of parallax in a case where an object exists at a long distance end. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail below with reference to the accompanying drawings showing embodiments thereof. The component elements described in the embodiment are only described by way of example, and are by no means intended to limit the scope of the present invention to them alone. 
     First, a description will be given of a first embodiment of the present invention.  FIG. 1  is a schematic block diagram of a ranging apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the ranging apparatus is comprised of a camera  10  which includes an optical system  11  as a fixed focus optical system and an image pickup device  12  having a large number of pixels arranged therein, an image analysis section  13 , and a distance information acquisition section  14 . The optical system  11  includes, for example, two lenses  11   a  and  11   b  arranged along the optical axis, and forms an image of an object on the image pickup device  12 . AS shown in  FIG. 2 , the plurality of pixels included in the image pickup device  12  are classified into a plurality of image pickup pixels  12   a  and a plurality of ranging pixels  12   b . Note that in  FIG. 2 , to eliminate troublesomeness, only how a plurality of pixels are arranged at an upper left portion of the image pickup device  12  is shown, thereby omitting illustration of the arrangement of all pixels on the whole surface of the image pickup device  12 . Each of the image pickup pixels  12   a  and the ranging pixels  12   b  includes a photoelectric conversion element, such as a photodiode (hereinafter referred to as the “PD”), as a photoelectric conversion portion. Each image pickup pixel  12   a  receives a light flux having passed through a partial area of an exit pupil of the optical system  11  (hereinafter referred to as a “partial pupil”), and forms an image signal of the object. Further, each ranging pixel  12   b  receives one of light fluxes having passed through two different ranging partial pupils of the exit pupil of the optical system  11 . In the image pickup device  12 , for example, according to a Bayer array, the image pickup pixels  12   a  having a spectral sensitivity to G (green) are arranged as two pixels, diagonally opposite to each other, of four pixels in two rows and two columns, and the image pickup pixels  12   a  having spectral sensitivities to R (red) and B (blue), respectively, are arranged as the other two pixels of the four pixels. The spectral sensitivities to specific colors, which the respective image pickup pixels  12   a  have, are added by primary color filters provided in the respective image pickup pixels  12   a . Further, in the image pickup device  12 , in some of the pixels in two rows and two columns, the two image pickup pixels  12   a  diagonally opposite to each other and having the spectral sensitivity to G are left as they are, and the image pickup pixels  12   a  having the spectral sensitivities to R and B are replaced by the ranging pixels  12   b . In the image pickup device  12 , in some of the pixels in two rows and two columns, the two ranging pixels  12   b  diagonally opposite to each other receive light fluxes having passed through the ranging partial pupils, respectively, to thereby output a pair of image signals of the object. The image analysis section  13  performs image processing on the output image signals, and further, analyzes the image signals to thereby acquire feature information on the object contained in the images. The distance information acquisition section  14  calculates parallax between a pair of images from the image signals subjected to the image processing, and further, calculates a distance to the object based on the calculated parallax. That is, the ranging apparatus measures distance information of the object by an imaging plane phase difference method. Note that in the present specification, hereinafter, the distance information of the object is defined as information on a position of the object, such as the distance to the object, a defocus amount, and parallax (an image shift amount and a phase difference). To calculate the parallax between the pair of images from the image signals, it is not necessarily required to perform the image processing on the image signals output from the image pickup device  12 . For example, the image pickup device  12  may output the image signals to the distance information acquisition section  14  without via the image analysis section  13 , and the distance information acquisition section  14  may generate the distance information from the image signals. 
     Although in the above-described image pickup device  12 , the two ranging pixels  12   b  receive light fluxes having passed through the ranging partial pupils, respectively, to form a pair of image signals, one imaging and ranging pixel  12   c , described hereinafter, may receive the light fluxes having passed through the ranging partial pupils, respectively, to form a pair of image signals. In this case, the imaging and ranging pixel  12   c  includes at least two PDs, and the PDs receive the light fluxes having passed through the ranging partial pupils, respectively. Further, the imaging and ranging pixel  12   c  combines the light fluxes received by the PDs after having passed through the ranging partial pupils to form an image signal of the object. Therefore, the imaging and ranging pixels  12   c  may be arranged in almost the whole area of the image pickup device  12 . Note that in the present embodiment, the lenses  11   a  and  11   b  of the optical system  11  of the camera  10  are fixed, and a so-called auto focus function is omitted. 
       FIGS. 3A to 3C  are diagrams useful in explaining the principle of distance measurement (ranging) by the imaging plane phase difference method. More specifically,  FIG. 3A  shows a case where the distance measurement is performed using image signals respectively formed by a pair of the two ranging pixels  12   b .  FIG. 3B  shows a plurality of pixels appearing in  FIG. 3A  as viewed from the direction of the optical axis.  FIG. 3C  shows a case where the distance measurement is performed using a pair of image signals formed by the one imaging and ranging pixel  12   c . Note that in  FIGS. 3A to 3C , the image pickup pixel  12   a , each ranging pixel  12   b , and the imaging and ranging pixel  12   c  are illustrated in a state as viewed from a side. 
     First, in  FIG. 3A , the exit pupil  30  of the optical system  11  includes two ranging partial pupils (hereinafter referred to as the “ranging pupils”)  31  and  32  (a first region and a second region) which are positioned close to opposite ends of the exit pupil  30  in a horizontal direction (lateral direction as viewed in  FIG. 3A , hereinafter referred to as the “parallax direction”), respectively. Further, the exit pupil  30  includes an image pickup partial pupil (hereinafter referred to as the “image pickup pupil”)  33  (a third region) which is positioned in a substantially central portion of the exit pupil  30  in the parallax direction so as to be sandwiched between the ranging pupils  31  and  32 . Ranging light fluxes  34  and  35  are emitted from the respective ranging pupils  31  and  32 , and enter the pair of the ranging pixels  12   b , respectively. Further, an image pickup light flux  36  is emitted from the image pickup pupil  33 , and enters the image pickup pixel  12   a . Each ranging pixel  12   b  includes a micro lens  37  and a PD  38  opposed to the exit pupil  30  via the micro lens  37 . Furthermore, each ranging pixel  12   b  includes a light shielding film  40  that is arranged between the micro lens  37  and the PD  38 , and has an opening  39  to cause the PD  38  to partially face the micro lens  37 . Further, the image pickup pixel  12   a  includes a micro lens  41  and a PD  42  opposed to the exit pupil  30  via the micro lens  41 . Furthermore, the image pickup pixel  12   a  includes a light shielding film  66  that is arranged between the micro lens  41  and the PD  42 , and has an opening  65  to cause the PD  42  to partially face the micro lens  41 . 
     In the pair of the ranging pixels  12   b , the micro lenses  37  are disposed close to the image surface of the optical system  11 , and condense the ranging light fluxes  34  and  35  onto associated ones of the light shielding films  40  (associated ones of the openings  39 ), respectively. The optical system  11  and each micro lens  37  are configured such that the exit pupil  30  and the associated light shielding film  40  (the associated opening  39 ) are optically conjugate to each other. Therefore, the micro lenses  37  cause the shapes of the openings  39  of the respective light shielding films  40  to be projected onto the ranging pupils  31  and  32  of the exit pupil  30 . That is, the arrangements (positions and sizes (areas)) of the ranging pupils  31  and  32  are defined by the positions and sizes of the openings  39  of the light shielding films  40  associated therewith. Further, in the image pickup pixel  12   a , the micro lens  41  is disposed close to the image surface of the optical system  11 , and condenses the image pickup light flux  36  onto the light shielding film  66  (the opening  65 ). Further, in the image pickup pixel  12   a , the optical system  11  and the micro lens  41  are configured such that the exit pupil  30  and the light shielding film  66  (the opening  65 ) are optically conjugate to each other. Therefore, the micro lens  41  causes the shape of the opening  65  to be projected onto the image pickup pupil  33  of the exit pupil  30 . That is, the arrangement (position and size (area)) of the image pickup pupil  33  is defined by the position and size of the opening  65  of the light shielding film  66 . The PDs  38  of the pair of the ranging pixels  12   b  output image signals obtained by photoelectrically converting images formed from the ranging light fluxes  34  and  35  having passed through the ranging pupils  31  and  32 , by the respective micro lenses  37 , respectively. In the present embodiment, the parallax between a pair of images (hereinafter referred to as the “first image” and the “second image”, respectively) is calculated by performing image shift detection arithmetic processing (correlation processing and phase difference detection processing) and the like on the output image signals from the PDs  38 . Further, a defocus amount of the object and a distance thereto are calculated from the parallax based on the principle of the triangulation (see e.g. Publication of US Patent Application No. 2015/0092988). Further, the PD  42  of the image pickup pixel  12   a  outputs an image signal obtained by photoelectrically converting an image formed from the image pickup light flux  36  having passed through the image pickup pupil  33  by the micro lens  41 , and a picked-up image (hereinafter also referred to as the “third image”) of the object is formed from the image signal. Although in  FIGS. 3A and 3B , the light shielding films  40  and the light shielding film  66  are provided, these light shielding films may be omitted. In this case, by making the positions and sizes of the PDs  38  and the PD  42  the same as the positions and sizes of the openings  39  and the opening  65 , it is possible to define the above-described arrangements of the ranging pupils  31  and  32  and the image pickup pupil  33 . 
     Further, in  FIG. 3C , the ranging light fluxes  34  and  35  are emitted from the respective ranging pupils  31  and  32 , and enter the imaging and ranging pixel  12   c . The image pickup light flux  36  is emitted from the image pickup pupil  33 , and enters the imaging and ranging pixel  12   c . The imaging and ranging pixel  12   c  includes a micro lens  43 , and PDs  44  to  46  opposed to the exit pupil  30  via the micro lens  43 . In the imaging and ranging pixel  12   c , the micro lens  43  is disposed close to the image surface of the optical system  11 . The micro lens  43  condenses the ranging light fluxes  34  and  35  and the image pickup light flux  36  onto the PDs  44  to  46 , respectively. The optical system  11  and the micro lens  43  are configured such that the exit pupil  30  and the PDs  44  to  46  are optically conjugate to each other. Therefore, the micro lens  43  causes the shape of the PD  44  to be projected onto the ranging pupil  31  of the exit pupil  30 . Further, the micro lens  43  causes the shape of the PD  45  to be projected onto the ranging pupil  32  of the exit pupil  30 . Furthermore, the micro lens  43  causes the shape of the PD  46  to be projected onto the image pickup pupil  33  of the exit pupil  30 . That is, the arrangements (positions and sizes) of the ranging pupils  31  and  32  and the image pickup pupil  33  are defined by the positions and sizes of the PDs  44  to  46 . The PDs  44  and  45  of the imaging and ranging pixel  12   c  output image signals obtained by photoelectrically converting images formed from the ranging light fluxes  34  and  35  having passed through the ranging pupils  31  and  32 , by the micro lens  43 , respectively. Similar to the case of the PDs  38  of the pair of the ranging pixels  12   b , the parallax between a pair of images (first and second images) is calculated by performing the image shift detection arithmetic processing (correlation processing and phase difference detection processing) on the output image signals from the PDs  44  and  45 . Further, a defocus amount of the object and a distance thereto are calculated from the parallax based on the principle of the triangulation. Further, the PD  46  of the imaging and ranging pixel  12   c  outputs an image signal obtained by photoelectrically converting an image formed from the image pickup light flux  36  having passed through the image pickup pupil  33  by the micro lens  43 , and an image (third image) of the object is formed from the image signal. 
       FIGS. 4A to 4C  are diagrams useful in explaining the ranging accuracy of the ranging apparatus using the imaging plane phase difference method. In  FIGS. 4A to 4C , only centerlines of the ranging light fluxes  34  and  35  having passed through the respective ranging pupils  31  and  32  are drawn for clarity. Further, only the lens  11   a  is drawn as a representative of the optical system  11 , and it is assumed for convenience&#39; sake that the whole surface of the lens  11   a  forms the exit pupil  30 . 
     Normally, in the ranging apparatus that includes the fixed focus optical system without the auto focus function, a range of distance measurement is set in advance. Conventionally, the optical system  11  is configured such that a position at which the parallax between a pair of images formed from the respective ranging light fluxes  34  and  35  becomes 0 (hereinafter referred to as the “parallax  0  position D 0 ”) is located about a midpoint between a long distance end E 1  and a short distance end E 2  of the distance measurement range ( FIG. 4A ). Specifically, the optical system  11  is configured such that the parallax  0  position D is located slightly toward the short distance end E 2  from the midpoint. That is, to use a picked-up image for recognition processing, it is desirable that the amount of blur of the picked-up image is small within the distance measurement range, and hence an in-focus position is fixed such that image blur sizes at the opposite ends of the distance measurement range are equal to each other. At this time, an image  51  of the object, formed from the ranging light flux  34  at the parallax  0  position Do, and an image  52  of the object, formed from the ranging light flux  35  at the parallax  0  position D 0  coincide with each other. Note that in  FIG. 4A , the image  51  and the image  52  are each shown as an intensity distribution with respect to incident angles. In this ranging apparatus, a distance from the lens  11   a  to the parallax  0  position D 0  is also set in advance, and a distance from the lens  11   a  to the object (distance information) is acquired based on the distance to the parallax  0  position D 0 , and the parallax between the pair of image signals of the object, using the principle of the triangulation. 
     Here, on a side closer to the long distance end E 1  than the parallax  0  position D 0  (hereinafter simply referred to as the “long distance end E 1  side”), when the object has moved toward the long distance end E 1  by a distance I (for clarity, it is illustrated such that the object is moved to the long distance end E 1 ), as shown in  FIG. 4B , a difference (parallax) between the center of gravity of the image  51  and the center of gravity of the image  52  is changed from d a  to d b . On the other hand, on a side closer to the short distance end E 2  than the parallax  0  position D 0  (hereinafter simply referred to as the “short distance end E 2  side”), when the object has moved toward the short distance end E 2  by the distance I (for clarity, it is illustrated such that the object is moved to the short distance end E 2 ), as shown in  FIG. 4C , the parallax is changed from d c  to d d . Here, in a case where the object has moved by the same distance I, the amount of change in the incident angles of the ranging light fluxes  34  and  35  on the long distance end E 1  side is smaller than the amount of change in the incident angles of the ranging light fluxes  34  and  35  on the short distance end E 2  side. Therefore, in the case where the object has moved by the same distance I, the amount |d a −d b | of change in the parallax on the long distance end E 1  side is smaller than the amount |d c −d d | of change in the parallax on the short distance end E 2  side. That is, on the long distance end E 1  side, the amount of change in the parallax is smaller even when the object has moved, so that the ranging accuracy becomes lower than on the short distance end E 2  side in the imaging plane phase difference method using the principle of the triangulation. In a generalized expression, the amount of change in the parallax is proportional to the reciprocal of approximately the square of the distance from the lens  11   a  to the object. In other words, as the distance from the lens  11   a  to the object is larger, the amount of change in the parallax becomes smaller, and the ranging accuracy becomes lower in the imaging plane phase difference method using the principle of the triangulation. In the present embodiment, to cope with this, the parallax  0  position D 0  is displaced toward the long distance end E 1 . 
       FIGS. 5A to 5C  are diagrams useful in explaining the relationship between the parallax  0  position and the distance measurement range in the present embodiment. Also in  FIGS. 5A to 5C , only the centerlines of the ranging light fluxes  34  and  35  having passed through the respective ranging pupils  31  and  32  are drawn for clarity. Further, only the lens  11   a  is drawn as a representative of the optical system  11 , and it is assumed for convenience&#39; sake that the whole surface of the lens  11   a  forms the exit pupil  30 . 
     In the ranging apparatus according to the present embodiment, the long distance end E 1  is set e.g. 100 meters or 50 meters away from the lens  11   a , and the short distance end E 2  is set e.g. 1 meter or 2 meters away from the lens  11   a . Further, in the ranging apparatus, the optical system  11  is configured such that the parallax  0  position D 0  is located a position shifted toward the long distance end E 1  ( FIG. 5A ). This makes it possible to define the distance measurement range such that the long distance end E 1  side where the ranging accuracy is lowered is made shorter than the short distance end E 2  side. That is, it is possible to reduce a range where the ranging accuracy is lowered, and hence it is possible to improve average reliability of the ranging accuracy in the whole distance measurement range. 
     Further, in general, in the triangulation, when an object moves away from the parallax  0  position Do, an image of the object is blurred to reduce signal intensity of an image signal of the object in the image pickup device  12 , and further, the image itself of the object is deformed, which makes it difficult to determine the center of gravity of the image of the object. As a consequence, the ranging accuracy is lowered. On the other hand, when the object exists close to the parallax  0  position Do, the image of the object is difficult to be blurred, whereby the signal intensity of the image signal of the object is not reduced, and also the image of the object is difficult to be deformed, which makes it easy to determine the center of gravity of the image of the object. As a consequence, the ranging accuracy is improved. In the present embodiment, since the parallax  0  position D 0  is located at a position shifted toward the long distance end E 1 , the distance from the parallax  0  position D 0  to the long distance end E 1  of the distance measurement range becomes relatively shorter, and it is difficult for the object to move away on the long distance end E 1  side from the parallax  0  position Do. As a consequence, it is possible to prevent the ranging accuracy from being lowered on the long distance end E 1  side. 
     On the other hand, in the present embodiment, the distance from the parallax  0  position Do to the short distance end E 2  becomes relatively longer, and it is easy for the object to move away on the short distance end E 2  side from the parallax  0  position Do, and hence there is a fear of lowering of the ranging accuracy at the short distance end E 2 . However, as described hereinabove, the ranging accuracy is lowered as the distance from the lens  11   a  to the object is increased. In other words, the ranging accuracy is improved as the distance from the lens  11   a  to the object is reduced. Therefore, in the present embodiment, on the short distance end E 2  side, the lowering of the ranging accuracy due to the object moving away from the parallax  0  position D 0  is compensated for by the improvement in the ranging accuracy due to the reduction of the distance from the lens  11   a  to the object. This makes it possible to prevent the ranging accuracy from being lowered on the short distance end E 2  side. 
     In the present embodiment, as described above, since the parallax  0  position D 0  is located at the location shifted toward the long distance end E 1 , parallax d 1  ( FIG. 5B ) in a case where the object exists at the long distance end E 1  becomes shorter than parallax d 2  ( FIG. 5C ) in a case where the object exists at the short distance end E 2 . At this time, if the parallax  0  position D 0  is set such that the parallax d 2  becomes 1.2 or more times larger than the parallax d 1 , it is possible to suppress lowering of the ranging accuracy without being affected by manufacturing errors and the like. In particular, by setting the parallax  0  position D 0  such that the parallax d 2  becomes 2.0 or more times larger than the parallax d 1 , it is possible to attain the effect of suppression of lowering of the ranging accuracy for both the long distance end E 1  and the short distance end E 2  at the same time. 
       FIG. 6  is a diagram useful in explaining a case where the parallax  0  position displaced toward the long distance end of the distance measurement range coincides with the in-focus position. Note that in  FIG. 6 , the ranging light fluxes  34  and  35  (indicated by solid lines in  FIG. 6 ) having passed through the ranging pupils  31  and  32 , respectively, and the image pickup light flux  36  (indicated by broken lines in  FIG. 6 ) having passed through the image pickup pupil  33  are drawn as they are, for clarity. Further, a position on the optical axis which is optically conjugate to the image pickup device  12  with respect to the image pickup pupil  33  is set as the in-focus position F 0  of the camera  10 . Furthermore, only the lens  11   a  is drawn as a representative of the optical system  11 , and it is assumed for convenience&#39; sake that the whole surface of the lens  11   a  forms the exit pupil  30 . 
     In  FIG. 6 , the optical system  11  is configured such that the parallax  0  position D 0  coincides with the in-focus position F 0 . At this time, the in-focus position F 0  as well is displaced toward the long distance end E 1 , as a consequence. With this, a blur size (circle of confusion) Φ 1  of an image formed from the image pickup light flux  36  of the object existing at the long distance end E 1  becomes smaller than a blur size Φ 2  of an image formed from the image pickup light flux  36  of the object existing at the short distance end E 2 . Note that the blur size Φ 1  of the image at the long distance end E 1  is set such that it is smaller than a size of two pixels of the image pickup device  12 . At this time, the size of the image pickup pupil  33  is determined such that the blur size Φ 2  of the image at the short distance end E 2  becomes not larger than a maximum blur size allowable in the recognition processing. The image of an object appears larger at a short distance than at a long distance, and hence the allowance of the blur size in the recognition processing is larger on the short distance side. Therefore, even when the blur size Φ 2  of the image at the short distance end E 2  becomes larger than the blur size Φ 1  of the image at the long distance end E 1 , it is possible to suppress the lowering of the accuracy of the recognition processing. 
       FIG. 7  is a diagram useful in explaining a case where the parallax  0  position displaced toward the long distance end of the distance measurement range does not coincide with the in-focus position. Note that also in  FIG. 7 , the ranging light fluxes  34  and  35  (indicated by solid lines in  FIG. 7 ) having passed through the ranging pupils  31  and  32 , respectively, and the image pickup light flux  36  (indicated by broken lines in  FIG. 7 ) having passed through the image pickup pupil  33  are drawn as they are, for clarity. Here as well, a position on the optical axis which is optically conjugate to the image pickup device  12  with respect to the image pickup pupil  33  is set as the in-focus position F 0  of the camera  10 . Furthermore, only the lens  11   a  is drawn as a representative of the optical system  11 , and it is assumed for convenience&#39; sake that the whole surface of the lens  11   a  forms the exit pupil  30 . 
     In  FIG. 7 , the optical system  11  is configured such that the in-focus position F 0  does not coincide with the parallax  0  position D 0 , but is located about the midpoint between the long distance end E 1  and the short distance end E 2  of the distance measurement range. The non-coincidence between the in-focus position F 0  and the parallax  0  position D 0  is realized by intentionally changing the aberration of each of the lenses  11   a  and  11   b . This makes it possible to set the blur size Φ 1  of the image formed from the image pickup light flux  36  of the object existing at the long distance end E 1  to the same degree as the blur size Φ 2  of the image formed from the image pickup light flux  36  of the object existing at the short distance end E 2 . Further, the optical system  11  is set such that a ratio between image blur sizes at the opposite ends E 1  and E 2  of the distance measurement range becomes smaller than a ratio (parallax ratio) between the parallaxes d 1  and d 2  in respective cases where the object exists at the respective ends E 1  and E 2  of the distance measurement range. More specifically, the optical system  11  is set such that the blur size Φ 1  of the image at the long distance end Eland the blur size Φ 2  of the image at the short distance end E 2  satisfy the following expression: 
         d   2   /d   1 &gt;Max(Φ 1 ,Φ 2 )/Min(Φ 1 ,Φ 2 )≥1
 
     wherein Max and Min represent a maximum value and a minimum value in parentheses, respectively. 
     As a consequence, it is possible to prevent the quality of an image of an object obtained in the distance measurement range from being largely reduced in one of the case where the object exists toward the long distance end E 1  and the case where the object exists toward the short distance end E 2 . Further, the size of the image pickup pupil  33  can be made larger than in the case shown in  FIG. 6 , so that it is possible to obtain a picked-up image which is brighter and has less noise. 
       FIGS. 8A to 8C  are diagrams of the exit pupil of the optical system according to the first embodiment. Note that a lateral direction as viewed in  FIGS. 8A to 8C  corresponds to the parallax direction. 
     In  FIG. 8A , the exit pupil  30  includes the two elliptical ranging pupils  31  and  32  which are located opposite (preferably symmetrical) to each other in the parallax direction with respect to the center of the exit pupil  30  (the optical axis of the optical system  11 ) and also are located close to the opposite ends of the exit pupil  30  in the parallax direction, respectively. Further, the exit pupil  30  includes the image pickup pupil  33  having a perfect circular shape, which is located in a substantially central portion of the exit pupil  30  so as to be sandwiched between the ranging pupils  31  and  32  in the parallax direction, and also includes the optical axis of the optical system  11 . A ratio of a distance L between the centers of gravity of the ranging pupils  31  and  32  in the parallax direction to an exit pupil width W as the length (diameter) of the exit pupil  30  in the parallax direction is not smaller than 0.6 and also not larger than 0.9. Further, a ratio of a distance D from the center of gravity of the image pickup pupil  33  to the optical axis of the optical system  11  in the parallax direction to the exit pupil width W is not smaller than 0 and also not larger than 0.05. 
     This makes it possible to cause the ranging pupil  31  and the ranging pupil  32  to be sufficiently spaced from each other in the parallax direction, thereby making it possible to increase a base line length. As a consequence, it is possible to enhance the accuracy of distance information of the object measured using the imaging plane phase difference method. Further, since the distance D is not larger than 0.05, it is possible to reduce displacement between the center of the image pickup light flux  36  and the optical axis. As a consequence, it is possible to make natural the blurring of a picked-up image obtained from the image pickup light flux  36 , thereby making it possible to enhance the accuracy of image analysis and the like. 
     Further, a ratio of each of ranging pupil widths W 1  and W 2  (partial pupil widths) as the lengths of the respective ranging pupils  31  and  32  in the parallax direction to the exit pupil width W is not smaller than 0.1 and also not larger than 0.4. Furthermore, a ratio of an image pickup pupil width W 3  as the length of the image pickup pupil  33  in the parallax direction to the exit pupil width W is not smaller than 0.05 and also not larger than 0.4. 
     With this, it is possible to increase the degree of freedom in the arrangement of the ranging pupils  31  and  32  in the parallax direction while keeping small the ranging pupil widths W 1  and W 2 . This makes it possible to locate the ranging pupils  31  and  32  close to the opposite ends of the exit pupil  30  in the parallax direction, whereby it is possible to positively increase the base line length. Further, if the ranging pupil widths W 1  and W 2  are made too small, the light amounts of the ranging light fluxes  34  and  35  are largely reduced to reduce the S/N (signal-to-noise) ratios of the obtained ranging image signals, which reduces the accuracy of a measured distance. However, as described above, since the ratio of each of the ranging pupil widths W 1  and W 2  to the exit pupil width W is not smaller than 0.1, it is possible to prevent large reduction of the light amounts of the ranging light fluxes  34  and  35 . Further, if the ranging pupil widths W 1  and W 2  are made larger, i.e. if the ranging pupils  31  and  32  are made larger, the base line length is reduced to reduce the accuracy of a measured distance. However, as described above, since the ratio of each of the ranging pupil widths W 1  and W 2  to the exit pupil width W is not larger than 0.4, it is possible to prevent reduction of the base line length. 
     Further, the elliptical ranging pupils  31  and  32  each have long sides in a vertical direction (direction perpendicular to the parallax direction), as viewed in  FIG. 8A . A ratio (hereinafter referred to as the “aspect ratio”) of a ranging pupil height H as the length of each of the ranging pupils  31  and  32  in the vertical direction as viewed in  FIG. 8A , to each of the ranging pupil widths W 1  and W 2  is not smaller than 1, and preferably is not smaller than 2. This makes it possible to increase the amounts of the ranging light fluxes  34  and  35  having passed through the respective ranging pupils  31  and  32 . As a consequence, it is possible to increase the S/N ratios of the image signals obtained from the images formed by the ranging light fluxes  34  and  35 , whereby it is possible to determine the distance information of the object with high accuracy. 
     Furthermore, the exit pupil  30  includes the image pickup pupil  33  sandwiched by the ranging pupils  31  and  32  in the parallax direction. A picked-up image of the object is formed from the image signal obtained by photoelectrically converting the image formed from the image pickup light flux  36  having passed through the image pickup pupil  33 . As described hereinabove, the ratio of the image pickup pupil width W 3  of the image pickup pupil  33  to the exit pupil width W is not larger than 0.4. With this, compared with a case where fluxes having passed through the whole area of the exit pupil  30  are used, a diaphragm can be made smaller to increase a depth of focus, thereby making it possible to obtain a picked-up image of the object, which is suitable for the recognition processing. On the other hand, since the ratio of the image pickup pupil width W 3  of the image pickup pupil  33  to the exit pupil width W is not smaller than 0.05, it is possible to increase the S/N ratio of the image signal. 
     Note that the sizes and shapes of the ranging pupils  31  and  32  are set as desired insofar as the above-described restrictions on the ranging pupil widths W 1  and W 2  and the ranging pupil height H are followed. For example, as shown in  FIG. 8B , the ranging pupils  31  and  32  may be somewhat smaller. Further, as shown in  FIG. 8C , the ranging pupils  31  and  32  and the image pickup pupil  33  may have the same shape (perfect circular shape). However, the sizes of the ranging pupils  31  and  32  are required to be large enough to increase the intensities of the image signals formed based on the ranging light fluxes  34  and  35  having passed through the respective ranging pupils  31  and  32  to such a degree that accurate distance information of the object can be acquired. In short, it is preferable that the ranging pupil height H is as long as possible, and that it is longer than an image pickup height as the length of the image pickup pupil  33  in the vertical direction as viewed in  FIG. 8A  (for example, it is more preferable that the ranging pupil height H is 1.1 or more times as long as an image pickup pupil height). 
     As described hereinabove, the arrangement of each of the ranging pupils  31  and  32  is defined by the position and size of the light shielding film  40  of an associated one of the ranging pixels  12   b , and the arrangement of the image pickup pupil  33  is defined by the position and size of the light shielding film  66  of the image pickup pixel  12   a . Alternatively, the arrangements of the ranging pupils  31  and  32  and the image pickup pupil  33  are defined by the positions and sizes of the PDs  44  to  46  of the imaging and ranging pixel  12   c . Therefore, in the imaging and ranging pixel  12   c , as shown in  FIG. 9A , the PDs  44  and  45  each have a rectangular shape having long sides in the vertical direction (direction perpendicular to the parallax direction), in association with the respective ranging pupils  31  and  32  which are vertically long. Further, the PDs  44  and  45  are arranged to be spaced away from each other in the parallax direction in association with the two ranging pupils  31  and  32  which are located close to the opposite ends of the exit pupil  30 , respectively. Further, the PD  46  has a square shape in association with the image pickup pupil  33  having the perfect circular shape, and is arranged in the substantially central portion of the imaging and ranging pixel  12   c  in association with the image pickup pupil  33  located in the substantially central portion of the exit pupil  30 . In a case where one imaging and ranging pixel  12   c  includes the PDs  44  and  45  as shown in  FIG. 9A , image signals for calculating the parallax between a pair of images can be obtained from the one imaging and ranging pixel  12   c , so that it is possible to obtain a large number of image signals. That is, it is possible to increase the resolution of the image signals. This makes it possible to enhance the image quality of a formed image. Further, it is possible to increase the resolution of distance information. 
     Note that the imaging and ranging pixel  12   c  may include only one of the PD  44  and the PD  45 . For example, as shown in  FIG. 9B , one imaging and ranging pixel  12   c  (lower one as viewed in  FIG. 9B ) includes the PD  44 , and another imaging and ranging pixel  12   c  (upper one as viewed in  FIG. 9B ) includes the PD  45 . In this case, the ranging light flux  34  having passed through the ranging pupil  31  is received by the PD  44  of the one imaging and ranging pixel  12   c , and the PD  44  outputs an image signal obtained by photoelectrically converting an image formed from the ranging light flux  34 . Also, the ranging light flux  35  having passed through the ranging pupil  32  is received by the PD  45  of the other imaging and ranging pixel  12   c , and the PD  45  outputs an image signal obtained by photoelectrically converting an image formed from the ranging light flux  35 . Further, the parallax between a pair of images is calculated from the image signals output from the PD  44  of the one imaging and ranging pixel  12   c  and the PD  45  of the other imaging and ranging pixel  12   c . As shown in  FIG. 9B , in the case where the imaging and ranging pixel  12   c  includes only one of the PD  44  and the PD  45 , it is possible to reduce the number of the PDs included in the one imaging and ranging pixel  12   c  to two. This makes it possible to leave room for arranging the PDs, whereby it is possible to increase the sizes of the PDs. As a consequence, it is possible to increase the amounts of light received by the PDs to improve the sensitivities of the PDs. This makes it possible to enhance the image quality of an image formed even in an environment where the amount of light is insufficient, whereby it is possible to improve the accuracy of calculation of the distance to the object. 
     Although in the present embodiment, the exit pupil  30  includes the ranging pupils  31  and  32  arranged in the lateral direction as viewed in the figures, the exit pupil  30  may further include two more ranging pupils arranged in the vertical direction as viewed in the figures. For example, as shown in  FIG. 10A , the exit pupil  30  includes two elliptical ranging pupils  47  and  48  arranged in the vertical direction as viewed in  FIG. 10A  in addition to the ranging pupils  31  and  32 . The ranging pupils  47  and  48  are located close to the opposite ends of the exit pupil  30  in the vertical direction as viewed in  FIG. 10A , respectively. This makes it possible to calculate not only the parallax between the pair of images in the lateral direction as viewed in  FIG. 10A , but also the parallax between a pair of images in the vertical direction as viewed in  FIG. 10A , whereby it is possible to improve the accuracy of measurement of a distance to a lateral line or an oblique line in the object. In this case, as shown in  FIG. 10B , the imaging and ranging pixel  12   c  includes PDs  49  and  50  each having a rectangular shape having long sides in the lateral direction as viewed in  FIG. 10B  in association with the respective elliptical ranging pupils  47  and  48 . Further, the PDs  49  and  50  are arranged to be spaced away from each other in the vertical direction as viewed in  FIG. 10B  in association with the ranging pupils  47  and  48  located close to the opposite ends of the exit pupil  30  in the vertical direction, respectively. 
     Further, in the present embodiment, each image pickup pixel  12   a  has a primary color filter, and hence an image formed from the image pickup light flux  36  received by the image pickup pixel  12   a  is a color image. Note that the color filter of the image pickup pixel  12   a  may be not the primary color filter but a complementary color filter. The complementary color filter passes a larger amount of light flux therethrough than the primary color filter, and hence it is possible to improve the sensitivity of the PD  42 . On the other hand, in the ranging pixel  12   b , the light flux received by the PD  38  is limited to a light flux having passed through the opening  39 , and in the imaging and ranging pixel  12   c , the sizes of the PDs  44  and  45  are limited. However, the ranging pixel  12   b  and the imaging and ranging pixel  12   c  include no color filters or include complementary color filters. With this, the amounts of light received by the PD  38  and the PDs  44  and  45  are not largely limited, and therefore, it is possible to prevent large reduction of the sensitivities of the PD  38  and the PDs  44  and  45 . In the case where the ranging pixel  12   b  and the imaging and ranging pixel  12   c  include no color filters, a pair of images formed from the ranging light fluxes  34  and  35  received by the ranging pixel  12   b  and the imaging and ranging pixel  12   c  are monochrome images. 
     Next, a description will be given of a second embodiment of the present invention. In the second embodiment of the present invention, the above-described ranging apparatus according to the first embodiment is applied to an automotive vehicle as a moving object. 
       FIG. 11  is a schematic diagram showing a state in which a driving assistance system is installed on an automotive vehicle as a moving object according to the present embodiment, and  FIG. 12  is a diagram of the driving assistance system. 
     Referring to  FIGS. 11 and 12 , the vehicle  59  is provided with a ranging apparatus  60  including the camera  10 , the image analysis section  13 , and the distance information acquisition section  14 , and a vehicle position determination section  61 . The vehicle position determination section  61  determines a relative position of the vehicle  59  with respect to a preceding vehicle, based on a ranging result calculated by the ranging apparatus  60 , e.g. a distance to the preceding vehicle. Note that the image analysis section  13 , the distance information acquisition section  14 , and the vehicle position determination section  61  can be installed both as software (programs) and as hardware or may be installed as a combination of software and hardware. For example, processing of each section may be realized by storing a program in a memory of a computer (a microcomputer, an FPGA (field-programmable gate array) or the like) incorporated in the camera  10 , and causing the program to be executed by the computer. Further, a dedicated processor, such as an ASIC (Application Specific Integrated Circuit), for realizing all or part of the processing of each section using a logic circuit, may be provided. 
     Further, the vehicle  59  includes a vehicle information acquisition device  62  (moving object information acquisition device), a controller  63 , and a alarm device  64 . The vehicle position determination section  61  is connected to the vehicle information acquisition device  62 , the controller  63 , and the alarm device  64 . The vehicle position determination section  61  acquires at least one of a vehicle speed (speed), a yaw rate, and a rudder angle of the vehicle  59  as vehicle information (information of the moving object) from the vehicle information acquisition device  62 . The controller  63  controls the vehicle  59  based on a result of determination by the vehicle position determination section  61 . The alarm device  64  issues an alarm based on a result of determination by the vehicle position determination section  61 . The controller  63  is e.g. an ECU (Engine Control Unit). For example, in a case where there is a high possibility that the vehicle  59  collides with a preceding vehicle, as a result of determination by the vehicle position determination section  61 , the controller  63  performs vehicle control for avoiding collision or reducing damages, by braking the vehicle  59 , releasing an accelerator pedal, suppressing an engine output, or the like. Further, e.g. in the case where there is a high possibility that the vehicle  59  collides with a preceding vehicle, the alarm device  64  warns a driver e.g. by sounding an alarm, displaying alarm information on a screen of a car navigation system, and vibrating a sheet belt or the steering wheel. In the present embodiment, the camera  10  of the ranging apparatus  60  picks up an image around the vehicle  59 , e.g. forward or backward of the vehicle  59 . Note that the controller  63  may be configured to control the vehicle  59  based on not only a result of ranging by the ranging apparatus  60  but also vehicle information acquired by the vehicle information acquisition device  62 . Note that, as an engine of the vehicle  59 , there may be used an internal combustion engine that uses gasoline or light oil as fuel or a motor operated by electricity. 
       FIG. 13  is a flowchart of a collision avoidance process performed by the driving assistance system according to the present embodiment. Hereinafter, a detailed description will be given of the operations by the respective sections of the driving assistance system, by describing the collision avoidance process. 
     First, in a step S 1 , image signals of a plurality of images (e.g. the first to third mages) are acquired using the camera  10 . Next, in a step S 2 , vehicle information is acquired from the vehicle information acquisition device  62 . The vehicle information acquired in the step S 2  is information including at least one of the vehicle speed, yaw rate, and rudder angle of the vehicle  59 . Then, in a step S 3 , feature analysis (recognition processing) is performed on at least one of the plurality of acquired image signals. More specifically, the image analysis section  13  analyzes feature amounts, such as the edge amount, edge direction, density value, color, luminance value of each image signal, to thereby recognize (detect) an object (automotive vehicle, bicycle, pedestrian, traffic lane, guardrail, brake lamp, etc.). Note that the feature amount analysis may be performed on each of the plurality of image signals, or on one or some of the plurality of image signals (e.g. only the image signal of the third image). 
     In the following step S 4 , the parallax between a pair of images (e.g. the first and second images) picked up by the camera  10  is calculated by the distance information acquisition section  14 , whereby distance information of an object existing in the picked-up images is acquired. The acquisition of the distance information is performed by the distance information acquisition section  14 . Note that in the present embodiment, a detailed description of the method of calculating the parallax is omitted since the SSDA (Sequential Similarity Detection Algorithm) method, the area correlation method, and so forth, already exist as known techniques. Further, the steps S 2 , S 3 , and S 4  may be performed by executing the steps in the above-mentioned order or by executing the steps in parallel. Here, a distance to an object existing in the picked-up images, and a defocus amount thereof can be calculated from the parallax calculated in the step S 4 , and internal and external parameters of the camera  10 . 
     Then, in a step S 5 , it is determined whether or not the acquired distance information is within a predetermined setting, i.e. whether or not there is an obstacle within a set distance, whereby it is determined whether or not there is a possibility of a forward or backward collision. In a case where there is an obstacle within the set distance, it is determined that there is a possibility of a collision, and the controller  63  causes the vehicle  59  to perform an avoidance operation (step S 6 ). More specifically, the possibility of a collision is notified to the controller  63  and the alarm device  64 . At this time, the controller  63  controls at least one of the direction of the movement of the vehicle  59  and the speed of the movement thereof. For example, the controller  63  avoids the collision with a preceding vehicle and reduces the possibility of the collision by braking the vehicle  59 , i.e. by generating and outputting a control signal for generating a braking force on each of the wheels of the vehicle  59 , and suppressing the output of the engine. Further, the alarm device  64  notifies a user of a danger using a sound, a video or a vibration. After that, the present process is terminated. On the other hand, in a case where there is no obstacle within the set distance, it is determined that there is no possibility of a collision, followed by terminating the present process. 
     According to the collision avoidance process in  FIG. 13 , it is possible to effectively detect an obstacle. That is, it is possible to accurately detect an obstacle, to thereby avoid a collision with the obstacle and reduce damages. 
     Although in the present embodiment, the description has been given of the collision avoidance based on distance information, it is possible to apply the present invention to a vehicle which runs following a preceding vehicle, maintains the vehicle position in the center of a lane, or suppresses deviation from the lane, based on distance information. Further, the present invention can be applied not only to the driving assistance of the vehicle  59  but also to the autonomous driving of the vehicle  59 . Furthermore, the ranging apparatus  60  of the present invention can be applied not only to vehicles, such as automotive vehicles, but also to moving objects, such as boats, aircrafts, drones, or industrial robots. Further, the ranging apparatus  60  of the present invention can be applied not only to the moving objects but also to a wide range of apparatuses making use of object recognition, such as apparatuses used in intersection monitoring systems and intelligent transport systems (ITS). For example, the present invention may be applied to intersection monitoring cameras as non-moving objects in the intersection monitoring systems. 
     Although in the above-described first embodiment, the distance measurement range is set to 1 to 100 m or 2 to 50 m, it may be set e.g. to 100 to 150 m. Although in the above-described first embodiment, the distance measurement range acquires the first to third images, the ranging apparatus may be configured not to acquire the third image without being provided with the image pickup pixels  12   a  or the PDs  46 . In this case, the image analysis section  13  performs the feature amount analysis on the first and second images. 
     Further, the imaging and ranging pixel  12   c  shown in  FIG. 9A  may be configured to output the image signal from the PD  44  and the image signal from the PD  45 , respectively. Note that the image signal from the PD  44  is a signal of the first image based on the ranging light flux  34  having passed through the ranging pupil  31 , and is hereinafter simply referred to as the “first image signal”. Further, the image signal from the PD  45  is a signal of the second image based on the ranging light flux  35  having passed through the ranging pupil  32 , and is hereinafter simply referred to as the “second image signal”. Alternatively, the imaging and ranging pixel  12   c  may be configured to output the first image signal and an image signal obtained by adding the image signal from the PD  44  and the image signal from the PD  45 . The image signal obtained by adding the image signal from the PD  44  and the image signal from the PD  45  is a signal of a fourth image based on a light flux having passed through the ranging pupil  31  and a light flux having passed through the ranging pupil  32 , and is hereinafter simply referred to as the “fourth image signal”. In this case, the second image signal can be calculated by calculating a difference between the first image signal and the fourth image signal using the distance information acquisition section  14 . 
     In the above-described first embodiment, the parallax d 1  in the case where the object exists at the long distance end E 1  becomes shorter than the parallax d 2  in the case where the object exists at the short distance end E 2 . Particularly, from the viewpoint of suppressing the lowering of the ranging accuracy, it is more preferable, as the parallax d 1  in the case where the object exists at the long distance end E 1  is smaller. Therefore, it is preferable, for example, that the parallax d 1  in the case where the object exists at the long distance end E 1  is set to be smaller than a distance corresponding to two pixels in the image pickup device  12 . Here, the distance corresponding to two pixels corresponds, in a case where distance measurement is performed using image signals formed by a pair of two ranging pixels  12   b , respectively, to a center-to-center distance between the pair of two ranging pixels  12   b  sandwiching the image pickup pixel  12   a  therebetween ( FIG. 14A ). Furthermore, the method of arranging the respective ranging pixels  12   b  is not limited to the one shown in  FIG. 3B , but for example, as shown in  FIG. 14B , a method of arranging the ranging pixels  12   b  without sandwiching the image pickup pixel  12   a  therebetween can be realized. In this case, the distance corresponding to two pixels corresponds to a center-to-center distance between two ranging pixels  12   b  disposed respectively at the opposite ends of a pixel row formed by three ranging pixels  12   b  arranged in the parallax direction. Further, in a case where distance measurement is performed using a pair of image signals formed by one imaging and ranging pixel  12   c , the distance corresponding to two pixels corresponds to a center-to-center distance between two imaging and ranging pixels  12   c  disposed respectively at the opposite ends of a pixel row formed by three ranging pixels  12   c  arranged in the parallax direction ( FIGS. 14C and 14D ). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Applications No. 2016-050237 filed Mar. 14, 2016 and No. 2017-031363 filed Feb. 22, 2017, which are hereby incorporated by reference herein in their entirety.