Patent Publication Number: US-2019199897-A1

Title: Image capturing apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image capturing apparatus (or imaging apparatus) configured to capture an image in all directions (or azimuths) using a plurality of cameras. 
     Description of the Related Art 
     Japanese Patent Laid-open No. (“JP”) 2016-118742 discloses an image capturing apparatus capable of capturing an image in all directions including horizontal 360° around it and positions directly above and below from the position of the image capturing apparatus, or an omnidirectional image. This image capturing apparatus includes two fisheye lenses facing sides opposite to each other, combines two images obtained through imaging with these two cameras, and generates an omnidirectional image. U.S. Pat. No. 8,902,322 discloses an image capturing apparatus that includes four cameras facing four vertices of a regular tetrahedron surrounding these cameras, combine four images captured by these four cameras with one another, and generates an omnidirectional image. 
     The image capturing apparatus disclosed in JP 2016-118742 exposes lens surfaces closest to the object in the fisheye lenses at both sides. Therefore, when the image capturing apparatus falls down and the lens surface of one of the fisheye lenses collide with the ground, the lens surface gets scratched. On the other hand, the image capturing apparatus disclosed in U.S. Pat. No. 8,902,322 arranges the lens surface vertices of the respective cameras deeper than a mechanical member as part of the image capturing apparatus and the lens surface is unlikely to contact the ground. However, the lens surface may get damaged by any convex portions, such as pebbles, on the ground. 
     SUMMARY OF THE INVENTION 
     The present invention provides an omnidirectional image capturing apparatus that is less likely to get scratched on a lens surface. 
     An image capturing apparatus according to one aspect of the present invention includes first, second, third, and fourth image capturers each including an optical system and an image sensor having a rectangular image capturing surface orthogonal to an optical axis in the optical system. First and second optical axes in the first and second image capturers are line-symmetrical with respect to a reference axis in a first plane, and a short side direction of the image capturing surface in each of the first and second image capturers is orthogonal to the first plane. Third and fourth optical axes in the third and fourth image capturers are line-symmetric with respect to the reference axis in a second plane orthogonal to the first plane, and a short side direction of the image capturing surface of each of the third and fourth image capturers is orthogonal to the second plane. The first and second optical axes incline to a third plane orthogonal to the reference axis on one side of the third plane. The third and fourth optical axes incline to the third plane on the other side of the third plane. A protector is a portion projecting to an object side from a vertex of an optical surface closest to the object side of each of the two optical systems between visual fields of the two image capturers among the first to fourth image capturers. The protector is provided between visual fields of the first and third image capturers, between visual fields of the third and second image capturers, between visual fields of the second and fourth image capturers, and between visual fields of the first and fourth image capturer. No protector is provided between the visual fields of the first and second image capturers and between the visual fields of the third and fourth image capturers. 
     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 an external perspective view of an omnidirectional image capturing apparatus according to a first embodiment of the present invention when viewed from an upper oblique direction. 
         FIGS. 2A to 2D  are a top view, a bottom view, a front view, and a side view of the image capturing apparatus according to the first embodiment. 
         FIG. 3  is an external perspective view of the image capturing apparatus according to the first embodiment when viewed from a lower oblique direction. 
         FIG. 4  illustrates a configuration of a camera used in the image capturing apparatus according to the first embodiment. 
         FIGS. 5A and 5B  illustrate an arrangement of four cameras in the image capturing apparatus according to the first embodiment. 
         FIGS. 6A to 6C  illustrate directions and arrangement of the optical axes in the four cameras in the image capturing apparatus according to the first embodiment. 
         FIG. 7  is a spherical view illustrating all directions of the optical axes in the four cameras in the image capturing apparatus according to the first embodiment. 
         FIGS. 8A to 8C  illustrate a solid angle range imaged on image sensors in the four cameras in the image capturing apparatus according to the first embodiment. 
         FIGS. 9A to 9C  illustrate a relationship among visual fields of four cameras in the image capturing apparatus according to the first embodiment. 
         FIGS. 10A and 10B  illustrate view angle allocations on an equatorial plane and a meridional direction of four cameras in the image capturing apparatus according to the first embodiment. 
         FIGS. 11A to 11C  illustrate a solid angle range imaged on image sensors in four cameras in an image capturing apparatus according to a comparative example 1. 
         FIGS. 12A and 12B  illustrate a superimposed portion among visual fields of the four cameras in the image capturing apparatus according to the comparative example 1. 
         FIGS. 13A to 13C  illustrate a solid angle range imaged on image sensors in four cameras in an image capturing apparatus according to a comparative example 2. 
         FIGS. 14A and 14B  illustrate a superimposed portion among visual fields of four cameras in the image capturing apparatus according to the comparative example 2. 
         FIG. 15  illustrates a relationship between the image sensor and the angle of view according to prior art 1. 
         FIG. 16  illustrates a relationship between the image sensor and the angle of view according to prior art 1. 
         FIG. 17  illustrates a visual field according to prior art 2. 
         FIG. 18  illustrates the image capturing apparatus according to the first embodiment that has fallen. 
         FIG. 19  illustrates an angle of view in a sensor long side direction in the image capturing apparatus according to the first embodiment. 
         FIG. 20  illustrates an all-round barrier. 
         FIG. 21  illustrates an angle of view in a sensor short side direction in the image capturing apparatus according to the first embodiment. 
         FIG. 22  illustrates an arrangement relationship between a close object and two cameras in the image capturing apparatus according to the first embodiment. 
         FIGS. 23A to 23C  illustrate the positions of the protector in the image capturing apparatus according to the first embodiment. 
         FIG. 24  is an external view of an image capturing apparatus according to a second embodiment of the present invention. 
         FIG. 25  illustrates a relationship between a visual field in each camera and the position of the protector in the image capturing apparatus according to the first embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the accompanying drawings, a description will now be given of embodiments according to the present invention. 
     First Embodiment 
       FIG. 1  illustrates an external appearance of an omnidirectional image capturing apparatus (simply referred to as an “image capturing apparatus” hereinafter)  100  according to a first embodiment of the present invention when viewed from an upper oblique direction.  FIGS. 2A to 2D  are a top view, a bottom view, a front view, and a side view of the image capturing apparatus  100 , respectively. 
     The image capturing apparatus  100  includes four cameras, i.e., a first camera (a first image capturer) C 1 , a second camera (a second image capturer) C 2 , a third camera (a third image capturer) C 3 , and a fourth camera (a fourth image capturer) C 4 , a holding member  2 , and an exterior member  3 . The four cameras C 1  to C 4  are integrally held by the holding member  2 . The exterior member  3  is fixed onto the holding member  2  so as to cover the four cameras C 1  to C 4  while exposing the front end lens surface (the optical surface closest to the object) of each camera and the surface of the holding member  2  around it. 
     Each of the cameras C 1  to C 4  includes a lens system (optical system) and an image sensor (image pickup element) having a rectangular image capturing surface orthogonal to an optical axis AXL in the lens system, and captures a directionally divided image in all directions. 
     Each of terms “orthogonal” and “parallel” in this embodiment is not limited to its strict meaning but may shift from the strict state within a range (e.g., within 5°) that permits the manufacturing error or does not impair the function the image capturing apparatus according to this embodiment. 
     The following description will simply refer to the first to fourth cameras C 1  to C 4  as cameras C 1  to C 4 . In  FIGS. 2A and 2B , a reference axis X is set to an axis perpendicular to the paper plane of the figure and parallel to the paper surface of the figure in  FIGS. 2C and 2D . 
     As illustrated in  FIG. 2A , the cameras C 1  and C 2  are arranged so that optical axes A 1  and A 2  are line-symmetric with respect to the reference axis X in a first plane P 1  that includes the reference axis X and optical axes A 1  and A 2  as illustrated in  FIG. 2A . The cameras C 1  and C 2  are arranged so that the first plane P 1  is orthogonal to short side directions S 1  and S 2  of the image capturing surfaces in the cameras C 1  and C 2 . The following description will refer to the short side direction of the image capturing surface as a sensor short side direction, and the long side direction of the image capturing surface as a sensor long side direction. 
     The cameras C 3  and C 4  are arranged so that their optical axes A 3  and A 4  are line-symmetric with respect to the reference axis X in a second plane P 2  that includes the reference axis X and the optical axes A 3  and A 4 . The cameras C 3  and C 4  are arranged so that the second plane P 2  is orthogonal to sensor short side directions S 3  and S 4  of the cameras C 3  and C 4 . 
     Each of terms “line symmetry” or “rotational symmetry” as used in this embodiment is not limited to the strict meaning but may shift from the strict state within a range that permits the manufacturing error or does not impair the function of the image capturing apparatus according to this embodiment. 
     The optical axes A 1  and A 2  of the cameras C 1  and C 2  located on a first side (the one side or the upper side in  FIG. 2C ) incline to a third plane P 3  orthogonal to the reference axis X by the same first angle α. The optical axes A 3  and A 4  of the cameras C 3  and C 4  located on a second side (the other side or the lower side in  FIG. 2C ) opposite to the first side incline to the third plane P 3  by the same second angle β. This embodiment set both α and β to 22.5°. 
     This embodiment does not limit “the same angle” to the strict meaning but can shift within a range that permits the manufacturing error or does not impair the function of the image capturing apparatus according to this embodiment. 
     Each of two lens exposing surfaces  3   a   1  and  3   a   2  formed as bevel planes exposes the front end lens surface of a corresponding one of the cameras C 1  and C 2  of the exterior member  3  inclines by 22.5° relative to the third plane P 3  facing the first side (upper oblique side). Each of two lens exposing surfaces  3   a   3  and  3   a   4  formed as bevel planes exposes the front end lens surface of a corresponding one of the cameras C 3  and C 4  of the exterior member  3  inclines by 22.5° relative to the third plane P 3  facing the second side (lower oblique side). 
     A protector  3   b   13  as a portion protruding to the object side from the vertices of the front end lens surfaces of the cameras C 1  and C 3  is provided between the lens exposing surfaces  3   a   1  and  3   a   3  in the exterior member  3  or between the visual fields of the cameras C 1  and C 3 . The visual field will be described later. Similarly, a protector  3   b   23  protruding to the object side from the vertices of the front end lens surfaces of the cameras C 2  and C 3  is provided between the lens exposing surfaces  3   a   2  and  3   a   3  (or between the visual fields of the cameras C 2  and C 3 ). A protector  3   b   24  protruding to the object side from the vertices of the front end lens surfaces of the cameras C 2  and C 4  is provided between the lens exposing surfaces  3   a   2  and  3   a   4  (or between the visual fields of the cameras C 2  and C 4 ). A protector  3   b   14  protruding to the object side from the vertices of the front end lens surfaces of the cameras C 1  and C 4  is provided between the lens exposing surfaces  3   a   1  and  3   a   4  (or between the visual fields of the cameras C 1  and C 4 ). 
     No protector is provided to a portion of the exterior member  3  between the lens exposing surfaces  3   a   1  and  3   a   2  (or between the visual fields of the cameras C 1  and C 2 ) and between the lens exposing surfaces  3   a   3  and  3   a   4  (or between the visual fields of the cameras C 3  and C 4 ). Specific shapes of the protectors  3   b   13 ,  3   b   23 ,  3   b   24 , and  3   b   14  will be described later. 
     As described above, the image capturing apparatus  100  according to this embodiment is configured rotationally symmetrical by 180° around the reference axis X. 
       FIG. 3  illustrates the external appearance of the image capturing apparatus  100  when viewed from a lower oblique direction. A leg fixing screw hole portion  4  is provided to the bottom surface of the exterior member  3  along the reference axis X and used to fix the image capturing apparatus  100  onto a leg member by fastening a male screw provided on the leg member, such as an unillustrated single leg or a tripod. 
       FIG. 4  illustrates the same structure of the cameras C 1  to C 4 . The camera  1  includes an image capturing lens (lens system)  5 , an image sensor  6 , a substrate  7 , and a body  8 . The image capturing lens  5  includes a concave meniscus lens having the largest diameter at the front end (closest to the object side) as a wide-angle lens that performs an f-θ projection. The image capturing lens  5  according to this embodiment has an angle of view 2ω of 135°. 
     The image sensor  6  is a photoelectric conversion element, such as a CCD sensor or a CMOS sensor, and is mounted on the substrate  7 . The image capturing surface of the image sensor  6  is formed in a rectangular shape with a ratio of the short side length to the long side length is 2:3. The body  8  holds the substrate  7  mounted with the image sensor  6  and the image capturing lens  5 . The optical axis A in the image capturing lens  5  passes through the center of the image capturing surface of the image sensor  6  and is orthogonal to the image capturing surface. 
       FIGS. 5A and 5B  illustrate the arrangement of the four cameras C 1  to C 4 .  FIG. 5A  illustrates the arrangement viewed from the front, and  FIG. 5B  illustrates the arrangement viewed from the side. As illustrated in  FIG. 5A , the two or left and right cameras C 1  and C 2  are arranged so that their optical axes A 1  and A 2  obliquely upwardly extend.  FIG. 6B  illustrates a relationship between the optical axes A 1  and A 2  and the reference axis X. The optical axes A 1  and A 2  and the reference axis X are included in the first plane P 1  as the same plane. The optical axes A 1  and A 2  are arranged line-symmetrically with respect to the reference axis X and incline upwardly by 22.5° to the third plane P 3  orthogonal to the reference axis X. As illustrated in  FIG. 5B , the image sensors  6  in the cameras C 1  and C 2  are arranged such that their short side directions S 1  and S 2  are perpendicular to the first plane P 1 . 
     On the other hand, as illustrated in  FIG. 5B , the two or front and rear cameras C 3  and C 4  are arranged so that their optical axes A 3  and A 4  obliquely downwardly extend.  FIG. 6C  illustrates a relationship between the optical axes A 3  and A 4  and the reference axis X. The optical axes A 3  and A 4  and the reference axis X are included in the second plane P 2  as the same plane. The optical axes A 3  and A 4  are arranged line-symmetrically with respect to the reference axis X and incline to the third plane P 3  downwardly by 22.5°. As illustrated in  FIG. 6A , the first plane P 1  and the second plane P 2  are orthogonal to each other on the reference axis X. As illustrated in  FIG. 5A , the image sensors  6  of the cameras C 3  and C 4  are arranged such that their short side directions S 3  and S 4  are orthogonal to the second plane P 2 . 
       FIG. 7(A)  to (D) spherically illustrates all directions. Now assume that an axis corresponding to the reference axis X is called an earth axis of the sphere, a plane including a great circle that passes through the center of the sphere and is orthogonal to the earth axis is called an equatorial plane, and an outer circumference of the great circle is called the equator. The top end of the sphere is called a north pole and the bottom end is called the south pole. Angles of an east longitude and a west longitude are expressed based on a meridian intersecting the optical axis A 1  of the camera C 1  as a reference (0°) among the plurality of meridians drawn based on the sphere, and angles of the north latitude and the south latitude are expressed based on the equator as a reference (0°). 
     As illustrated in  FIG. 7(D) ,  FIG. 7(A)  illustrates a sphere viewed from the top (north pole side),  FIG. 7(B)  illustrates a sphere viewed from the side (+90° direction), and  FIG. 7(C)  illustrates a sphere viewed from the front (0° direction), respectively. Illustrated on the sphere are intersections with the optical axes A 1  to A 4  in the cameras C 1  to C 4  and the directions of the image sensors  6  (in which the short side and the long side of 2:3 extend). 
       FIGS. 8A to 8C  illustrate the visual fields of the cameras C 1  to C 4  on the same sphere as in  FIG. 7  or the angular ranges of the object imaged on the image sensors  6 . The image capturing lenses  5  in the cameras C 1  to C 4  have focal lengths of 24 mm in the sensor short side direction and 36 mm in the sensor long side direction. 
     The visual field of the camera C 1  includes a north pole at the top, and extends to the bottom around 45° S (south latitude) and from around 45° E (east longitude) to around 45° W (west longitude) via 0° E (W) on the equator. As illustrated in  FIG. 8A , the top of the visual field of the camera C 1  extends from around 120° E to around 120° W via 0° E (W) around the north pole. 
     The visual field of the camera C 2  is line-symmetric with respect to the image capturing range of the camera C 1  and the earth axis or surface-symmetrical with respect to a plane including the earth axis and meridians of 90° E and 90° W. The top of the visual field of the camera C 2  includes the north pole, and its lower part extends to around 45° S and from the 135° E to 135° W via the 180° E (W) on the equator. The top of the visual field of the camera C 2  extends from around 60° E to around 60° W via 180° E (W) around the north pole. 
     The lower part of the visual field of the camera C 3  includes a south pole, and the upper part extends to around 45° N (north latitude) and from around 45° W to 135° W via 90° W on the equator. The lower part of the visual field of the camera C 3  extends from around 30° E to around 150° E via 90° W around the south pole point. 
     The visual field of the camera C 4  has a range line-symmetrical with respect to the image capturing range of the camera C 3  and the earth axis or surface-symmetrical with respect to a plane including the earth axis and the meridian of 0° E (W). The lower part of the visual field of the camera C 4  includes a south pole point, and the upper part extends to around 45° N and from 45° E to 135° E via 90° E on the equator. The lower part of the visual field of the camera C 4  extends from around 30° W to around 150° W via 90° W around the south pole. 
       FIGS. 9A to 9C  illustrate superimposed visual fields of the cameras C 1  to C 4  illustrated in  FIGS. 8A to 8C . The visual field of the camera C 1  and the visual field of the camera C 2  illustrated by C 1 +C 2  in the figure are superimposed on each other near the north pole point enclosed by a bold circle  9  in  FIG. 9A . Each of these visual fields covers a range from around 45° E to around 45° W on the equator, a range from around 135° E to around 135° W on the equator, and a range up to around 45° S. 
     The visual field of the camera C 3  and the visual field of the camera C 4  illustrated by C 3 +C 4  superimpose on each other around the south pole point. Each of these visual fields covers a range from around 45° W to around 135° W on the equator, a range from around 45° E to around 135° E on the equator, and a range up to around 45° N. 
     The upper end line of the visual field of the camera C 4  contacts the intersection between the visual field of the camera C 1  and the visual field of the camera C 2  in a region indicated by a bold circle  10  in  FIG. 9A  in C 1 +C 2 +C 3 +C 4  illustrating the superimposed visual fields of the four cameras C 1  to C 4 . Thereby, the superimposed visual fields of the cameras C 1  to C 3  can be minimized. A bold circle  11  in  FIG. 9B  illustrates the superimposed visual fields of the cameras C 1  and C 4  on the equator. These visual fields superimpose on each other by several degrees. Thus, the visual fields of the cameras C 1  to C 3  may be superimposed on each other at three areas indicated by the bold circles  9  to  11 . 
     The portions indicated by the bold circles  9  and  11  need a superimposition area to some extent, in jointing images as described later. Since the visual fields of the cameras C 1 , C 2 , and C 4  are superimposed on the point in the bold circle  10 , the superimposed area may be the minimum necessary. This also applies to the superimposed visual fields of the cameras C 1 , C 2 , and C 4 . In order to arrange the three portions in a well-balanced manner, a ratio of the short side length to the long side length on the image capturing surface in the image sensor  6  may be 2:3, and an angle between the equatorial plane and each camera may be around 22.5°. 
     The reasons will be explained below.  FIGS. 10A and 10B  illustrate the visual field allocation (in other words, effective angle of view, simply referred to as an “angle of view” hereinafter) of the cameras C 1  to C 4  on the equatorial plane and the meridional direction. The angle of view corresponding to the sensor short side direction of each camera on the equatorial plane has a minimum angle of view of 90° obtained by quadrisecting 360°, as illustrated in  FIG. 10A . In the meridional direction, as illustrated in  FIG. 10B , one extreme side of the north pole and the south pole has an angle of view corresponding to the sensor short side direction, and the other extreme side has an angle of view corresponding to the sensor long side direction, and these two cover 360°. Thus, 135° is required for the angle of view corresponding to the sensor long side direction. In minimizing the superimposed visual fields, the ratio of the angle of view corresponding to the sensor short side direction to the angle of view corresponding to the sensor long side direction is 2:3 and this ratio can maximize the utilization efficiency of the image sensor  6 . 
     The four images acquired through image capturing by the four cameras C 1  to C 4  are combined (combined) through image processing. Then, a superimposed portion is necessary to some extent among the four images. In order to increase the angle of the superimposed portion, it is necessary to widen the angle of view of the image capturing lens  5  to the wide-angle side. However, the combined image never exceeds the range of 360°, and the total of angles of view used as the combined image among the images acquired by the respective cameras C 1  to C 4  is 360°. In other words, even if there is a superimposed portion, the ratio of 2:3 maximizes the utilization efficiency of the image sensor  6  since the angle of view of each camera is defined at the boundary between the combined images. 
     The image capturing apparatus disclosed in JP 2016-118742 requires the image capturing lens in each camera to be an all-around fisheye lens so as to capture an omnidirectional image with two image sensors. A general image sensor has a rectangular image capturing surface with a typical aspect ratio of 3:4, 2:3, or 9:16. Even when the image sensor having the aspect ratio of 3:4 closest to 1:1 among them is used for the image capturing apparatus disclosed in JP 2016-118742, the effective image capturing area of 58.9% is actually used to capture an image in the image capturing surface, as illustrated in  FIG. 15 , and the utilization efficiency is low. 
     In the image capturing apparatus disclosed in U.S. Pat. No. 8,902,322, the four cameras face the four highly symmetrical vertexes in the regular tetrahedron and thus it appears that equally dividing all directions in capturing an image is efficient. However, when four image sensors having rectangular image capturing surfaces of the same shapes are used, the utilization efficiency of the image sensors is low.  FIG. 16  illustrates an effective image capturing area in one image sensor in the image capturing apparatus disclosed in U.S. Pat. No. 8,902,322. Since the four image sensors are arranged at mutually symmetrical positions, the visual field allocated to one image sensor corresponds to an angular range when one regular triangle plane is viewed from the center of the regular tetrahedron.  FIG. 17  illustrates an isometric projection of the angular range on the image capturing surface on the image sensor. In this case, as illustrated in  FIG. 16 , the effective image capturing area is as low as about 60% of the area of the image capturing surface in the image sensor. 
       FIG. 18  illustrates the image capturing apparatus  100  falling down on the ground (asphalt pavement surface). The image capturing apparatus  100  is often fixed and lifted on the ground with a pod member such as a monopod. The general asphalt pavement uses crushed stone as the aggregate, and its particle diameter is 13 to 15 mm. Pebbles on the pavement surface are mainly caused by this aggregate. 
     As illustrated, the front end lens surface of the camera  1  is retracted to the rear (opposite to the object) from the surrounding exterior member  3 , and thereby protected from hitting of the pebbles or the like on the pavement surface. Based on the particle size of the aggregate, it is necessary to retract the front lens surface from the exterior member  3  by at least 10 mm, or by 15 mm or more, or by 17 mm or more. 
       FIG. 19  illustrates a region where the protector is to be provided to the exterior member  3  in the sensor long side direction. A half field angle ω is necessary on one side of the optical axis A from the entrance pupil position of the image capturing lens  5 , where 2ω is a total angle of view in the sensor long side direction in the camera  1 .  FIG. 19  illustrates the angle of view 2ω of 135° in the sensor long side direction. 
     When there is a mechanical member at least within this angle of view, a necessary light flux is shielded by the mechanical member. In order to retract the vertex of the front end lens surface from the exterior member  3  by d 0  without shielding the necessary light flux, a protector as a mechanical member is necessary in a region enclosed by a bold line in the drawing. 
     The shortest distance R from the optical axis A in the region in which the protector can be disposed is given by the following expression (1). 
         R= ( t 1 +d 0)×tan 67.5°  (1)
 
     For example, where the entrance pupil position t 1  of the image capturing lens  5  is 7 mm and a retraction amount of the vertex of the front end lens surface is 15 mm, the shortest distance R from the optical axis A in the area in which the barrier or protector can be disposed is 53.1 mm. 
       FIG. 20  illustrates a protector  3   b  provided over the entire circumference of the front end lens surface.  FIG. 20  illustrates a diameter difference of the protector  3   b  when the protector  3   b  is provided at substantially the same height as the vertex of the front end lens surface so that d 0  is equal to 0 mm and when the protector  3   b  is provided so that d 0  is equal to 15 mm. Where the field angle 2ωL is 135°, the former diameter is 33.8 mm and the latter diameter is φ106.2 mm. Hence, if the protector is provided over the entire circumference of the front end lens surface, the image capturing apparatus  100  becomes larger due to the increased diameter. 
       FIG. 21  illustrates a region where the protector is to be provided to the exterior member  3  in the sensor short side direction. A half angle of view ω is necessary on one side of the optical axis A from the position of the entrance pupil in the image capturing lens  5 , where 2ωS is a field angle in the sensor short side direction of the camera  1 .  FIG. 19  illustrates the angle of view 2ωL of 135° in the sensor long side direction. 
       FIG. 21  illustrates a region where the protector is to be provided to the exterior member  3 . Due to the calculation similar to the sensor long side direction which sets the total angle of view 2ωS of the camera  1  in the sensor short side direction to 90°, the protector has a diameter of φ14 mm when the protector  3   b  is provided at substantially the same height as the vertex of the front end lens surface so that d 0  is equal to 0 mm. On the other hand, the protector has a diameter of φ44 mm so that d 0  is equal to 15 mm. Hence, the image capturing apparatus  100  can be prevented from becoming larger by providing the protector  3   b  in the sensor short side direction. 
     In using a wide-angle lens, a flare cutting hood may be used so as to prevent unnecessary light from entering the camera. Then, the image capturing apparatus  100  that arranges the four cameras adjacent to one another has a problem of mechanical interference between the protectors provided between the cameras and the hoods. It is thus necessary to place the protector away from each camera. 
     While the above description assumes the infinite object distance, an actual product needs to consider a close or shot-distance object. More specifically, it is necessary to consider the close object with an object distance of 1 m or less, and to combine images obtained by imaging the close object with at least the object distance of about 50 cm using two cameras. As illustrated in  FIG. 22 , where the distance is 10 cm between the entrance pupil positions in the image capturing lenses in the two adjacent cameras, the half angle of view of the image capturing lens requires a margin amount θ of about 5.7° or more in order to combine images obtained by imaging a close object  16  having an object distance L of 50 cm. As a result, it is necessary to dispose the protector at a more distant position, and the image capturing apparatus  100  becomes larger. 
     Where the intervals between the entrance pupil positions in the image capturing lenses in the four cameras are all the same, the margin amount of the angle of view determined by the shortest image capturing distance is about 5.7° or higher regardless of the direction, such as the sensor short side direction and the sensor long side direction. However, when the angle of view is widened by the same margin amount of 5.7°, a change amount in the size of the protector in the sensor long side direction is about twice as large as that in the sensor short side direction. In order to minimize the size of the image capturing apparatus  100  including the protector, it is necessary to provide a protector only between the visual fields in the sensor short side direction as in this embodiment, and it is effective not to provide a protector between the visual fields in the sensor long side direction. 
       FIGS. 23A to 23C  illustrate positions on the sphere illustrated in  FIG. 7  of the protectors  3   b   13 ,  3   b   23 ,  3   b   24 , and  3   b   14  illustrated in  FIGS. 1 to 3 . 
     The protectors  3   b   13 ,  3   b   23 ,  3   b   24 , and  3   b   14  are respectively disposed between the cameras C 1  and C 3 , between the cameras C 2  and C 3 , between the cameras C 2  and C 4 , and between the cameras C 1  and C 4  in the sensor short side direction so as to cross the equator in the vertical direction. The protectors  3   b   13 ,  3   b   23 ,  3   b   24 , and  3   b   14  are arranged at the positions such that they do not appear in the respective cameras (so that the light beams toward the respective cameras are not shielded). On the other hand, no protector is arranged around the north pole between the cameras C 1  and C 2  in the sensor long side direction, and around the south pole between the cameras C 3  and C 4 . 
     More specifically, the four protectors  3   b   13 ,  3   b   23 ,  3   b   24 , and  3   b   14  are provided by cutting off four grooves from a cube and by forming the exterior member  3  in a shape with the lens exposing surfaces  3   a   1 ,  3   a   2 ,  3   a   3 , and  3   a   4  as bottom surfaces described with reference to  FIGS. 1 to 3 . 
       FIG. 25  illustrates a relationship between visual fields V 1  to V 4  of the cameras C 1  to C 4  and the protectors  3   b   13 ,  3   b   23 ,  3   b   24  and  3   b   14 . The visual fields V 1  to V 4  intersect (superimpose) at positions distant from the corresponding cameras C 1  to C 4 . The protector  3   b   13  is provided at a position closer to the cameras C 1  and C 3  than a position where the visual field V 1  of the camera C 1  and the visual field V 3  of the camera C  3  intersect each other, or between the visual field V 1  and the visual field V 3 . Similarly, the protector  3   b   23  is provided between the visual field V 2  of the camera C 2  and the visual field V 3  of the camera C 3 , and the protector  3   b   24  is provided between the visual field V 2  of the camera C 2  and the visual field V 4  of the camera C 4 . The protector  3   b   14  is provided between the visual field V 1  of the camera C 1  and the visual field V 4  of the camera C 4 . 
     The shape of the exterior member  3  described above forms an opening of 180° to each camera in the sensor long side direction. In the sensor short side direction, a portion protruding to the object side from the vertex of the front end lens surface of each camera serves as the protectors  3   b   13 ,  3   b   23 ,  3   b   24 , and  3   b   14 . 
     The exterior member  3  having such protectors  3   b   13 ,  3   b   23 ,  3   b   24 , and  3   b   14  separates the vertex of the front end lens surface from the ground by a sufficient height, even if the image sensor  100  is placed on the ground (even if the image capturing apparatus  100  falls down) so that any one of cameras faces down. Any pebbles or the like on the ground never contact the front end lens surface or are prevented from scratching the front end lens surface. 
     The following conditions are necessary to prevent the image capturing light flux from being shielded by the protector provided in the short side direction. 
         R ≥( t 1 +d 0)×tan 45°
 
     Since the required minimum angle of view in the short side direction is 90°, it is necessary to provide the protector so as not to shield at least the light flux. 
     The angle of view in each lens system needs to satisfy the following two conditions. 
       2ω S× 4≥360°
 
     On the equatorial plane, the field angle in the sensor short side direction of the four cameras needs to cover the entire circumference angle of view of 360° and thus satisfy the above conditions. 
       2ω S× 2+2ω L≥ 360°
 
     On the other hand, the angle of view in the sensor long side direction of the two cameras and the angle of view in the sensor short side direction of one camera need to cover 360° in the meridional direction. The minimum angles of view of each camera that satisfies these conditions are 2ωL of 135° and 2ωS of 90°. Then, 2ωS:2ωL=2:3 is satisfied. Since the mutually superimposed portions are actually required in combining images, each angle of view becomes accordingly larger. It is ideal to increase the angle of view while the ratio of the angle of view in the sensor short side direction to the angle of view in the sensor long side direction is maintained to be 2:3. In changing this ratio, it is better to increase the margin in the sensor short side direction. 
     The angle of view in the sensor long side direction may be 1.8 times as large as the angle of view in the sensor short side or less as in the following condition, and the angle of view in the sensor long side direction larger than this value optically undesirably wastes the margin and the angle of view of the lens system. 
       2ω L≤ 1.8×2ω S  
 
     In other words, the specification of the optical system can be made simpler since the angle of view 2ωD in the diagonal direction of the image sensor is equal to or smaller than the diagonal length of the rectangle formed by the short side (2ωS) and the long side (2ωL). Since the angle of view in the sensor long side direction is necessary, the lower limit value is 2ωL but the size of the entire optical system can be suppressed by reducing the margin of the angle of view in the diagonal direction. 
       2ω D ≤√(2ω S   2 +2ω L   2 )
 
     Ideally, the ratio of the angle of view in the sensor long side direction to the angle of view in the sensor short side direction is about 1.5 as described above. The margin of the angle of view may be provided to the sensor short side direction rather than the sensor long side direction, and the condition is set wider to the image sensor with a lower aspect ratio. The image sensor with the low aspect ratio has an increased margin of the angle of view in the sensor short side direction and thus is advantageous in combining images in the equator direction. 
     In the general image capturing, a distant object to be addressed is often located in the horizontal direction. A wider margin may be advantageous in joining (combining) such object images. Hence, an image sensor having an aspect ratio satisfying the following condition, in particular, an image sensor having an aspect ratio as low as about the lower limit value may be used. 
       1.3≤2ω L/ 2ω S≤ 1.6
 
     Second Embodiment 
       FIG. 24  illustrates an image capturing apparatus  100 ′ according to a second embodiment of the present invention. The image capturing apparatus  100 ′ according to this embodiment also has four cameras, and the arrangement of these four cameras (the optical axis of the image capturing lens and the orientation of the image sensor) and the visual field are the same as those in the first embodiment. 
     In this embodiment, a first exterior member  31  has the four lens exposing surfaces corresponding to the lens exposing surfaces  3   a   1 ,  3   a   2 ,  3   a   3 , and  3   a   4  illustrated in  FIGS. 1 to 3  according to the first embodiment and is fixed onto a holding member  2  that holds the cameras C 1  to C 4 . The first exterior member  31  is also fixed onto a frame-shaped second exterior member  32  via a plurality of supporting members  13  such as wires. The second exterior member  32  serves as a protector. 
     The second exterior member  32  is a frame structured member made of lightweight metal, such as aluminum or magnesium alloy, plastic, and carbon fiber, and has eight sides out of twelve sides of a cube written in a single stroke. Eight vertices of the second exterior member  32  are connected to the first exterior component  31  by the supporting member  13 . The supporting member  13  is made of metal wire, rubber wire, or the like. 
     When the image capturing apparatus  100 ′ falls over, the second exterior member  32  serving as the protector separate the vertex of the front end lens surface from the ground by a sufficient height even if any of the cameras face down. Hence, any pebbles or the like on the ground never contact the front end lens surface or are prevented from scratching the front end lens surface. In addition, the damages to the cameras C 1  to C 4  can be reduced since the second exterior member  32  and the supporting member  13  are elastically deformed. 
     Since the supporting member  13  is detachable from the first and second exterior members  31  and  32 , the maintenance of the cameras C 1  to C 4  by the user can be facilitated. 
     Comparative Example 1 
     A comparative example 1 will be described. In this comparative example, angles of the optical axes in the four cameras relative to the equatorial plane (third plane) are 35.26° in a vertex direction of the regular tetrahedron. This comparative example also has the ratio of the short side length to the long side length of the image sensor of 2:3. The image capturing lens in each camera has a focal length off of 13.0 mm. 
       FIGS. 11A to 11C  illustrate the visual field of the camera C 1  similarly to  FIGS. 8A to 8C .  FIGS. 12A and 12B  illustrate the superimposed imaging ranges of the cameras C 1  and C 4 . This comparative example has a quite large superimposed area between the visual fields of the cameras C 1  and C 2  around the north pole point, and also a large superimposed area between the visual fields of the cameras C 1  and C 4  on the equator. Thereby, the utilization efficiency of the image sensor is considerably lower than those of the first and second embodiments. 
     Comparative Example 2 
     A comparative example 2 will be described. In this comparative example, angles of the optical axes in the four cameras relative to the equatorial plane (third plane) are 18°. In addition, this comparative example also has a ratio of the short side length to the long side length of the image sensor of 2:3. The image capturing lens in each camera has a focal length f of 14.2 mm. 
       FIGS. 13A to 13C  illustrate the visual field of the camera C 1  similarly to  FIGS. 8A to 8C .  FIGS. 14A and 14B  illustrate the superimposed imaging ranges of the cameras C 1  and C 4 . This comparative example has a considerably small superimposed area between the visual fields of the cameras C 1  and C 4  on the equator, but undesirably has a superimposed point between the visual fields of the cameras C 1  and C 2  around the north pole point. 
     Each of the above embodiments provide the protector at a proper position in the exterior member and can realize an omnidirectional image capturing apparatus in which the optical surface of each image capturing apparatus is less likely to get scratched. 
     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 Application No. 2017-250268, filed on Dec. 26, 2017, which is hereby incorporated by reference herein in its entirety.