Patent Description:
A quadrilateral image formed by a lens that does not conform to a central projection style may collapse. When a quadrilateral imaging element is used, an optical image and the imaging element do not fully overlap with each other, increasing an unused region on a photosensitive surface.

Expanding a subject at a central portion around an optical axis and forming an image on an imaging element in terms of detection and recognition of the subject has been demanded, but has not yet been fully achieved so far.

PTL <NUM> discloses a method for capturing a panoramic image using an image sensor having an oblong shape. PTL <NUM> discloses that a toric lens is used as a fisheye objective lens to convert a circular image into a quadrilateral image to allow a quadrilateral imaging element to form the image. PTL <NUM> discloses a camera apparatus capable of easily coping with separate magnification and reduction factors or the like in two different directions and setting them. The camera apparatus comprises a lens system including a cylindrical lens, wherein a first cylindrical and recessed lens face is formed around a first axis along a horizontal direction in one area and a second cylindrical and recessed lens face is formed around a second axis along a vertical direction in the other area.

PTL3 discloses a wide-angle lens comprising a front group and a rear group, and an aperture stop positioned between both groups. At least one surface of the front group is rotationally asymmetric, and a surface of the most image side lens of the rear group is aspherical.

A lens system capable of effectively utilizing a region of a photosensitive surface of a quadrilateral imaging element, and expanding a subject at a central portion around an optical axis, and a camera system and an imaging system including the lens system are provided.

A lens system according to the present disclosure is a lens system suitable to be used with an imaging element having a quadrilateral shape with shorter sides and longer sides and disposed on an optical axis of the lens system, and comprising, in order from an object side to an image surface side, a first plurality of lens elements, an aperture diaphragm, and a second plurality of lens elements, all configured to form an approximately quadrilateral image on an image surface of the imaging element, wherein the first plurality of lens elements comprises at least a first free-curved lens that is asymmetrical with respect to the optical axis, and a free-curved surface of the first free-curved lens has negative refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio, ranging from <NUM>% to <NUM>% inclusive, with respect to a minimum image height and a first surface passing through the optical axis and parallel to the longer sides of the imaging element, and positive refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having the predetermined ratio with respect to the minimum image height and a second surface passing through the optical axis and parallel to the shorter sides of the imaging element, and a portion where positive and negative inversion occurs in refractive power of the free-curved surface, wherein the minimum image height is in the shorter side direction of imaging element and corresponds to a shortest one among distances on an image surface, from an image point by a ray entering perpendicularly onto said imaging element to an end of an image circle formed by the lens system, the second plurality of lens elements comprises at least a second free-curved lens that is disposed closest to the image surface side and has two surfaces that are free-curved surfaces facing the object side and the image surface side respectively.

A camera system according to the present disclosure includes the lens system according to the present disclosure, described above, and a quadrilateral imaging element disposed at a position at which the lens system forms an image on the optical axis.

An imaging system according to the present disclosure includes the lens system according to the present disclosure, described above, a quadrilateral imaging element disposed at a position at which the lens system forms an image on the optical axis, and an image processor configured to process the image generated by the imaging element.

The present invention can achieve a lens system configured to form an approximately quadrilateral image, and to expand a subject at a central portion around an optical axis, and a camera system and an imaging system including the lens system.

Hereinafter, exemplary embodiments will be described in detail with reference to the drawings as appropriate. However, detailed description more than necessary may be omitted. For example, detailed description of well-known matters and redundant description of structures that are substantially the same may be omitted. These omissions are made to avoid unnecessary redundancy of the following description and to facilitate the understanding of those skilled in the art.

Note that the attached drawings and the following description are provided for those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter as described in the appended claims.

<FIG> is a layout diagram of a lens system according to a first exemplary embodiment, illustrating an infinity focusing state.

Part (a) of <FIG> is a YZ cross-section, and part (b) of <FIG> is an XZ cross-section, each illustrating lens system <NUM> including eight lens elements, and quadrilateral imaging element <NUM> having shorter sides and longer sides. An X direction is a direction parallel to the longer sides of imaging element <NUM>. A Y direction is a direction parallel to a shorter side direction of imaging element <NUM>. A Z direction is a direction parallel to an optical axis. The YZ cross-section includes the optical axis, and is a plane parallel to the Y direction and the Z direction. The XZ cross-section includes the optical axis, and is a plane parallel to the X direction and the Z direction.

As illustrated in <FIG>, lens system <NUM> according to the first exemplary embodiment includes, in order from an object side to an image surface side, five lens elements L1 to L5, aperture diaphragm A, and three lens elements L6 to L8. A position at which lens system <NUM> forms an image corresponds to an image surface of imaging element <NUM>. In part (b) of <FIG>, reference marks are omitted.

Lens system <NUM> will further be described in detail. Lens system <NUM> includes, in order from the object side to the image surface side, lens element L1 having a negative meniscus shape where a convex surface faces the object side, lens element L2 having both surfaces formed into concave shapes, lens element L3 having both surfaces formed into concave shapes, lens element L4 having both surfaces formed into convex shapes, lens element L5 having a positive meniscus shape where both surfaces are formed into aspherical shapes and a convex surface faces the object side, aperture diaphragm A, lens element L6 having both surfaces formed into convex shapes, lens element L7 having a negative meniscus shape where a convex surface faces the image surface side, and lens element L8 having a positive meniscus shape where a convex surface faces the object side. Lens element L6 and lens element L7 are joined with each other. In here, lens element L1 is an example of a first lens element, and lens element L2 is an example of a second lens element.

In lens system <NUM>, lens element L3 and lens element L8 each have both surfaces respectively facing the object side and the image surface side and being XY-polynomial, free-curved surfaces. In <FIG>, the free-curved surfaces are each added with an asterisk "*". In here, lens element L3 is an example of a first free-curved lens, and lens element L8 is an example of a second free-curved lens.

The free-curved surface, facing the image surface side, of lens element L3 has negative refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio with respect to a minimum image height and an XZ plane (first surface) passing through the optical axis and parallel to the longer sides of the imaging element, and positive refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio with respect to the minimum image height and a YZ plane (second surface) passing through the optical axis and parallel to the shorter sides of the imaging element. In here, in the present exemplary embodiment, all ratios correspond to the predetermined ratio with respect to the minimum image height. The free-curved surface, facing the image surface side, of lens element L3 has negative refractive power with respect to a ray parallel to the optical axis at all intersection points with the XZ plane, and positive refractive power with respect to a ray parallel to the optical axis at all intersection points with the YZ plane. Surface data of the lens elements will be described later. Shapes of surfaces of free-curved lenses and aspherical lenses correspond to shapes around the optical axis (peak) in the Y direction.

<FIG> is a layout diagram of a lens system according to a second exemplary embodiment. Part (a) of <FIG> is a YZ cross-section, and part (b) of <FIG> is an XZ cross-section, each illustrating lens system <NUM> including eight lens elements, and quadrilateral imaging element <NUM> having shorter sides and longer sides. In part (b) of <FIG>, reference marks are omitted. Compared with lens system <NUM> according to the first exemplary embodiment, lens system <NUM> according to the second exemplary embodiment is identical in number, kinds, and a disposition order of lens elements, but differs in surface data of lens system elements L1 to L8. Differences in surface data will be described later. In lens system <NUM>, lens element L1 is an example of a first lens element, lens element L2 is an example of a second lens element, lens element L3 is an example of a first free-curved lens, and lens element L8 is an example of a second free-curved lens.

Even in lens system <NUM>, the free-curved surface, facing the image surface side, of lens element L3 has negative refractive power with respect to a ray parallel to the optical axis at all intersection points with the XZ plane, and positive refractive power with respect to a ray parallel to the optical axis at all intersection points with the YZ plane.

<FIG> is a layout diagram of a lens system according to a third exemplary embodiment. Part (a) of <FIG> is a YZ cross-section, and part (b) of <FIG> is an XZ cross-section, each illustrating lens system <NUM> including eight lens elements, and quadrilateral imaging element <NUM> having shorter sides and longer sides. In part (b) of <FIG>, reference marks are omitted. Compared with lens system <NUM> according to the first exemplary embodiment, lens system <NUM> according to the third exemplary embodiment is identical in number of lens elements, but differs in kind of lens element L2 and surface data of lens system elements L1 to L8. Lens element L2 has a negative meniscus shape where a convex surface faces the object side. Differences in surface data will be described later. In lens system <NUM>, lens element L1 is an example of a first lens element, lens element L2 is an example of a second lens element, lens element L3 is an example of a first free-curved lens, and lens element L8 is an example of a second free-curved lens.

The lens systems according to the first to third exemplary embodiments each include a plurality of lens elements to form an image on quadrilateral imaging element <NUM> having the shorter sides and the longer sides. The lens systems each include, as lens elements, the free-curved lenses that are rotational asymmetrical with respect to the optical axis. That is, the lens systems each include, in order from the object side to the image surface side, a plurality of the lens elements, an aperture diaphragm, and a plurality of the lens elements. With this configuration, an approximately quadrilateral image that is almost quadrilateral can be formed.

The lens systems according to the first to third exemplary embodiments each have such a configuration that includes at least three or more lens elements that are rotational symmetrical with respect to the optical axis. With this configuration, free-curved lenses have been reduced in number, minimizing unevenness in capability due to directions. Furthermore, the lens systems according to the first to third exemplary embodiments can advantageously shorten calculation periods during designing.

All the free-curved surfaces of the free-curved lenses configuring the lens systems according to the first to third exemplary embodiments, each having a shape that is symmetrical with respect to the XZ plane and the YZ plane, are advantageous in terms of that centers of the free-curved surfaces can be determined, allowing easy management on shapes during manufacturing.

The lens systems according to the first to third exemplary embodiments each have a configuration including, in order from the object side, lens element L1 being a meniscus having a convex shape facing the object side and negative power, and lens element L2 having negative power. This configuration is advantageous in terms of that light entering at a wider angle can be collected, a lens system with a wide field of view can be easily achieved, respective power can be reduced through the two negative lenses arranged in series, and shapes that are easily manufactured can be achieved.

The lens systems according to the first to third exemplary embodiments, each having a configuration where the free-curved lens disposed at a position most adjacent to the image surface side has both surfaces respectively being free-curved surfaces and respectively facing the object side and the image surface side, are advantageous in terms of that position control for image heights and aberration reductions can be easily performed.

The lens systems according to the first to third exemplary embodiments each include the fisheye lens having a half angle of view of <NUM>° or wider. A wider angle of view can thus be covered. Fisheye lenses generally face difficulty in forming an image around diagonal positions of an imaging element. However, by using the free-curved lenses according to the present disclosure, an image can be formed even around the diagonal positions of an imaging element.

A lens system configured to form an image on a quadrilateral imaging element disposed on an optical axis, as can be seen in the lens systems according to the first to third exemplary embodiments, includes a first free-curved lens being asymmetrical with respect to the optical axis. A free-curved surface of the first free-curved lens has negative refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio with respect to a minimum image height and an XZ plane passing through the optical axis and parallel to the longer sides of the imaging element, and positive refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio with respect to the minimum image height and a YZ plane passing through the optical axis and parallel to the shorter sides of the imaging element (hereinafter, the configuration is referred to as a basic configuration of the exemplary embodiments). In here, the predetermined ratio with respect to the minimum image height ranges from <NUM>% to <NUM>% inclusive, and preferably is <NUM>%.

In here, the minimum image height refers to a shortest one among distances on an image surface, from an image point by a ray entering perpendicularly onto imaging element <NUM> to an end of an image circle formed by a lens system. The lens systems according to the first to third exemplary embodiments each have a minimum image height in the shorter side direction of imaging element <NUM>.

The free-curved surface as can be seen on the first free-curved lens allows expanding an image at a central portion of an image circle around an optical axis, and capturing a subject present around the optical axis in an enlarged manner, leading to a higher detection and recognition rate. Furthermore, an image circle of a fisheye lens, which is normally circular, can be expanded in the longer side direction in particular. With a portion where positive and negative inversion occurs in refractive power of a free-curved surface, fine image-expanding effects can be achieved. With the portion where positive and negative inversion occurs in refractive power, which is separated from the optical axis by a length that ranges from <NUM>% to <NUM>% inclusive (more preferably, that is <NUM>%) of a minimum image height in a radial direction, fine image-expanding effects can further be achieved.

When, different from the basic configuration of the present exemplary embodiments, a first free-curved lens does not have a free-curved surface having negative refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio with respect to a minimum image height and an XZ plane, and positive refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio with respect to the minimum image height and a YZ plane, an image cannot be fully expanded at a central region of a photosensitive surface of a quadrilateral imaging element. Otherwise, lens elements may increase in number, expanding a lens system in size. Such a lens having a free-curved surface, as described above, which has been difficult to manufacture so far, has become possible to manufacture as processing and molding techniques have been advancing in recent years.

It is preferable that a lens system having the basic configuration of the present exemplary embodiments, as can be seen in the lens systems according to the first to third exemplary embodiments, for example, satisfy the following condition (<NUM>). <MAT> where,.

By further satisfying at least either of the following conditions (<NUM>)' and (<NUM>)", the effects described above can further be achieved. <MAT> <MAT>.

It is preferable that a lens system having the basic configuration of the present exemplary embodiments, as can be seen in the lens systems according to the first to third exemplary embodiments, for example, satisfy the following condition (<NUM>). <MAT> where,.

By further satisfying the following condition (<NUM>)', the effects described above can further be achieved.

It is preferable that a lens system having the basic configuration of the present exemplary embodiments, as can be seen in the lens systems according to the first to third exemplary embodiments, for example, include aperture diaphragm A between an object and an imaging element, and satisfy the following condition (<NUM>). <MAT> where,.

A lens system having the basic configuration of the present exemplary embodiments, as can be seen in the lens systems according to the first to third exemplary embodiments, for example, include aperture diaphragm A between the object and imaging element <NUM>, at least one first free-curved lens, that is asymmetrical with respect to the optical axis, and loser to the object than aperture diaphragm A is, and at least one second free-curved lens closest to the image surface. Adopting the configuration described above is advantageous in terms of that curvatures of image surface can be reduced in any directions including the longer side direction, the shorter side direction, and a diagonal direction.

It is preferable that, in a lens system having the basic configuration of the present exemplary embodiments, as can be seen in the lens systems according to the first to third exemplary embodiments, for example, imaging element <NUM> do not include an image circle of the lens system. In particular, in a lens system using a free-curved lens, securing resolution around ends of an image circle is difficult in terms of designing and manufacturing. By allowing imaging element <NUM> to not include an image circle, a fine image forming capability can be secured on imaging element <NUM>.

The lens elements configuring the lens systems according to the first to third exemplary embodiments are refraction-type lens elements only (i.e., such a type of lens elements that achieve deflection on an interface between media having different refraction factors) that deflect an incident ray through refraction. However, the present disclosure is not limited to use such lens elements. The lens systems may include, for example, one or any of diffraction-type lens elements that deflect an incident ray through diffraction, refraction and diffraction hybrid-type lens elements that deflect an incident ray through a combination of a diffraction effect and a refraction effect, and refractive index distribution-type lens elements that deflect an incident ray through refractive index distribution in a medium. In particular, when a diffraction structure is formed on an interface of media having different refraction factors in a refraction and diffraction hybrid-type lens element, wavelength dependency of diffraction efficiency is preferably improved.

The lenses configuring the lens systems according to the first to third exemplary embodiments have symmetrical surfaces with respect to the longer sides or the shorter sides of imaging element <NUM>. However, even when asymmetrical surfaces are used, enough effects can be achieved, as long as the basic configuration of the present exemplary embodiments and the conditions are satisfied.

<FIG> is a schematic block diagram of a camera system according to a fourth exemplary embodiment. Camera system <NUM> according to the fourth exemplary embodiment includes lens system <NUM>, imaging element <NUM> configured to receive an optical image formed by lens system <NUM> and to convert the received optical image into an electrical image signal, and camera main body <NUM>. The lens system of the fourth exemplary embodiment can be one of the lens systems according to the first to third exemplary embodiments. <FIG> illustrates a case where lens system <NUM> according to the first exemplary embodiment is used as a lens system.

In the fourth exemplary embodiment, one of the lens systems according to the first to third exemplary embodiments is used. Imaging element <NUM> can thus form an approximately quadrilateral image. Camera system <NUM> capable of effectively utilizing the region of the photosensitive surface of imaging element <NUM> having a quadrilateral shape to obtain an image can be achieved.

<FIG> is a schematic block diagram of an imaging system according to a fifth exemplary embodiment. As can be seen in camera system <NUM> according to the fourth exemplary embodiment, imaging optical system <NUM> used in imaging system <NUM> according to the fifth exemplary embodiment includes one of the lens systems according to the first to third exemplary embodiments. By allowing image processor <NUM> to process an image obtained by imaging optical system <NUM>, the image can be modified and processed into an image applicable in various applications. Image processor <NUM> may be provided inside or outside of camera main body <NUM> (see <FIG>).

A first numerical value example in which lens system <NUM> according to the first exemplary embodiment has been specifically implemented will be described herein. In the first numerical value example, a unit of length is "mm", and a unit of angle of view is "°" in the drawings and tables. In the first numerical value example, radius of curvature r, surface spacing d, refraction factor nd with respect to the d-line, and Abbe's number vd with respect to the d-line are illustrated. Sag amounts z of surfaces parallel to a Z axis of aspherical and free-curved surfaces are respectively defined by mathematical expression <NUM> and mathematical expression <NUM>. <MAT>
where,.

<FIG> is a spherical aberration diagram and astigmatism diagrams in an infinity focusing state of lens system <NUM> according to the first numerical value example. <FIG> illustrates, in order from left, spherical aberration (SA) in the shorter side direction of imaging element <NUM>, astigmatism (AST-V), astigmatism (ASTH) in the longer side direction of imaging element <NUM>, and astigmatism (AST-D) in the diagonal direction. In the spherical aberration diagram, a horizontal axis illustrates spherical aberrations, whereas a perpendicular axis illustrates pupil heights. A solid line illustrates a characteristic of the d-line. A short dashed line illustrates a characteristic of the C-line. A long dashed line illustrates a characteristic of the F-line. In the astigmatism diagrams, a horizontal axis illustrates astigmatism, whereas a perpendicular axis illustrates angles of view. A solid line illustrates a characteristic of a YZ plane (in the diagram, y direction). A dashed line illustrates a characteristic of an XZ plane (in the diagram, x direction).

The first exemplary embodiment uses only even-number terms, i.e., x and y, in the XY polynomial. The first exemplary embodiment is therefore symmetrical with respect to the x axis and the y axis. Astigmatism AST-D in the diagonal direction thus becomes identical in any directions.

<FIG> is a diagram illustrating a relationship between an angle of view and an image point in the infinity focusing state of lens system <NUM> according to the first numerical value example. <FIG> uses the optical axis as an origin (<NUM>,<NUM>), and plots image points per an angle of view of <NUM>° in the first quadrant of the image surface. Other quadrants each have such a relationship, with the first quadrant, that each quadrant is line symmetrical with respect to the perpendicular axis and the horizontal axis. Compared with a normal rotational symmetrical lens, it can be seen that a shape of the image surface is expanded, and the region of the photosensitive surface of imaging element <NUM> having a quadrilateral shape is effectively utilized. Furthermore, as it is noticeable in particular in an X-image height direction, it can be seen that an image at a central portion around the optical axis is expanded greater than an image in a peripheral area separated from the optical axis.

Lens system <NUM> according to the first numerical value example corresponds to the first exemplary embodiment illustrated in <FIG>. Surface data of lens system <NUM> according to the first numerical value example is illustrated in Table <NUM>. Various kinds of data is illustrated in Table <NUM>. Aspherical and free-curved surface data of a fifth surface, a sixth surface, a ninth surface, a tenth surface, a fifteenth surface, and a sixteenth surface is illustrated in Table <NUM>, Table <NUM>, Table <NUM>, Table <NUM>, Table <NUM>, and Table <NUM>, respectively.

<FIG> is a spherical aberration diagram and astigmatism diagrams in an infinity focusing state of lens system <NUM> according to the second numerical value example. <FIG> is a diagram illustrating a relationship between an angle of view and an image point in the infinity focusing state of lens system <NUM> according to the second numerical value example. Even in a case of lens system <NUM> illustrated in <FIG>, compared with a normal rotational symmetrical lens, it can be seen that a shape of the image surface is expanded, and the region of the photosensitive surface of imaging element <NUM> having a quadrilateral shape is effectively utilized. As it is noticeable in particular in the X'image height direction, it can be seen that an image at a central portion around the optical axis is expanded greater than an image in a peripheral area separated from the optical axis. Lens system <NUM> according to the second numerical value example corresponds to the second exemplary embodiment illustrated in <FIG>. Surface data of lens system <NUM> according to the second numerical value example is illustrated in Table <NUM>. Various kinds of data is illustrated in Table <NUM>. Aspherical and free-curved surface data of a fifth surface, a sixth surface, a ninth surface, a tenth surface, a fifteenth surface, and a sixteenth surface is illustrated in Table <NUM>, Table <NUM>, Table <NUM>, Table <NUM>, Table <NUM>, and Table <NUM>, respectively.

<FIG> is a spherical aberration diagram and astigmatism diagrams in an infinity focusing state of lens system <NUM> according to a third numerical value example. <FIG> is a diagram illustrating a relationship between an angle of view and an image point in the infinity focusing state of lens system <NUM> according to the third numerical value example. Even in a case of lens system <NUM> illustrated in <FIG>, compared with a normal rotational symmetrical lens, it can be seen that a shape of the image surface is expanded, and the region of the photosensitive surface of imaging element <NUM> having a quadrilateral shape is effectively utilized. As it is noticeable in particular in the X-image height direction, it can be seen that an image at a central portion around the optical axis is expanded greater than an image in a peripheral area separated from the optical axis. Lens system <NUM> according to the third numerical value example corresponds to the third exemplary embodiment illustrated in <FIG>. Surface data of lens system <NUM> according to the third numerical value example is illustrated in Table <NUM>. Various kinds of data is illustrated in Table <NUM>. Aspherical and free-curved surface data of a fifth surface, a sixth surface, a ninth surface, a tenth surface, a fifteenth surface, and a sixteenth surface is illustrated in Table <NUM>, Table <NUM>, Table <NUM>, Table <NUM>, Table <NUM>, and Table <NUM>, respectively.

Table <NUM> described below illustrates corresponding values to the conditions in the lens systems according to the numerical value examples.

The lens systems according to the implementations are applicable to digital still cameras, digital video cameras, cameras of cellular phones, cameras of personal digital assistances (PDAs), monitoring cameras of monitoring systems, Web cameras, and on-vehicle cameras, for example. In particular, the lens systems according to the implementations are preferable for photographing optical systems for which high image quality is required, such as digital still camera systems and digital video camera systems.

Claim 1:
A lens system suitable to be used with an imaging element having a quadrilateral shape with shorter sides and longer sides and disposed on an optical axis of the lens system, and comprising,
in order from an object side to an image surface side, a first plurality of lens elements (L1, L2, L3, L4, L5), an aperture diaphragm (A), and a second plurality of lens elements (L6, L7, L8), all configured to form an approximately quadrilateral image on an image surface of the imaging element (<NUM>),
wherein
the first plurality of lens elements comprises at least a first free-curved lens (L3) that is asymmetrical with respect to the optical axis, and a free-curved surface of the first free-curved lens (L3) has
negative refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having a predetermined ratio, ranging from <NUM>% to <NUM>% inclusive, with respect to a minimum image height and a first surface passing through the optical axis and parallel to the longer sides of the imaging element, and
positive refractive power with respect to a ray parallel to the optical axis at an intersection point between a circle separated from the optical axis by a length having the predetermined ratio with respect to the minimum image height and a second surface passing through the optical axis and parallel to the shorter sides of the imaging element, and
a portion where positive and negative inversion occurs in refractive power of the free-curved surface,
wherein the minimum image height is in the shorter side direction of imaging element and corresponds to a shortest one among distances on an image surface, from an image point by a ray entering perpendicularly onto said imaging element (<NUM>) to an end of an image circle formed by the lens system, the second plurality of lens elements comprises at least a second free-curved lens (L8) that is disposed closest to the image surface side and has two surfaces that are free-curved surfaces facing the object side and the image surface side respectively.