Lens system, camera system, and imaging system

1. A lens system configured to form an image on an imaging element having a quadrilateral shape disposed on an optical axis, the lens system comprising:

a first free-curved lens being asymmetrical with respect to the optical axis, wherein a free-curved surface of the first free-curved lens has

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 a minimum image height and a first surface passing through the optical axis and parallel to longer sides of the imaging element, and

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 the predetermined ratio with respect to the minimum image height and a second surface passing through the optical axis and parallel to shorter sides of the imaging element.

2. The lens system according to claim 1 , wherein the predetermined ratio ranges from 60% to 100% inclusive.

3. The lens system according to claim 1 , further comprising an aperture diaphragm, wherein the first free-curved lens is disposed closer to an object than the aperture diaphragm is.

4. The lens system according to claim 3 , further comprising a second free-curved lens disposed closer to an image surface than the aperture diaphragm is.

5. The lens system according to claim 4 , wherein the second free-curved lens is disposed closest to the image surface, and both surfaces respectively facing the object and the image surface are free-curved surfaces.

6. The lens system according to claim 1 , further comprising, in order from the object:

a first lens element that is a meniscus having a convex shape facing the object and having negative power; and

a second lens element having negative power.

7. The lens system according to claim 6 , wherein the first free-curved lens is closer to the image surface than the second lens element, and has a free-curved surface at least on a side closer to the object.

8. The lens system according to claim 1 , further comprising

at least three or more lens elements being rotational symmetrical with respect to the optical axis.

9. The lens system according to claim 1 , wherein a condition (1) below is satisfied:

ω LONG >60°  (1)

where,

ω LONG is a maximum half angle of view in a longer side direction of the imaging element.

10. The lens system according to claim 1 , wherein a condition (2) below is satisfied:

1< D LSHORT /D SSHORT   (2)

where,

D LSHORT is a maximum distance in a longer side direction between an image point with respect to incident light in the longer side direction of the imaging element and an image point with respect to incident light perpendicular to the imaging element, within an angle of view equivalent to a maximum half angle of view in a shorter side direction of the imaging element, and

D SSHORT is a maximum distance in the shorter side direction between an image point with respect to incident light and an image point with respect to incident light perpendicular to the imaging element, within the maximum half angle of view in the shorter side direction of the imaging element.

11. The lens system according to claim 1 , wherein a condition (3) below is satisfied:

0.5< D SSHORT×ωLONG ( D LLONG×ωSHORT )<1  (3)

where,

ω LONG is a maximum half angle of view in a longer side direction of the imaging element,

ω SHORT is a maximum half angle of view in a shorter side direction of the imaging element,

D LLONG is a maximum distance in the longer side direction between an image point with respect to incident light and an image point with respect to incident light perpendicular to the imaging element, within the maximum half angle of view in the longer side direction of the imaging element, and

D SSHORT is a maximum distance in the shorter side direction between an image point with respect to incident light and an image point with respect to incident light perpendicular to the imaging element, within the maximum half angle of view in the shorter side direction of the imaging element.

12. The lens system according to claim 1 , wherein a condition (4) below is satisfied:

ω LONG−ωSHORT >0  (4)

where,

ω LONG is a maximum half angle of view in a longer side direction of the imaging element, and

ω SHORT is a maximum half angle of view in a shorter side direction of the imaging element.

13. The lens system according to claim 1 , wherein a condition (5) below is satisfied:

L×Fno./D LLONG <40  (5)

where,

L is an optical total length of the lens system,

Fno. is an F value of the lens system, and

D LLONG is a maximum distance in a longer side direction between an image point with respect to incident light and an image point with respect to incident light perpendicular to the imaging element, within a maximum half angle of view in the longer side direction of the imaging element.

14. The lens system according to claim 1 , wherein a condition (6) below is satisfied:

n FREE <1.7  (6)

where,

n FREE is a refraction factor with respect to a d-line of the first free-curved lens or a second free-curved lens.

15. The lens system according to claim 1 , further comprising:

lens elements; and

an aperture diaphragm, wherein a condition (7) below is satisfied:

−3≤ N o −N i ≤3  (7)

where,

N 0 is a number of the lens elements including the first free-curved lens and disposed closer to an object than the aperture diaphragm is, and

N i is a number of the lens elements disposed closer to an image surface than the aperture diaphragm is.

16. The lens system according to claim 1 , wherein the imaging element does not include an image circle of the lens system.

17. A camera system comprising:

the lens system according to claim 1 ; and

the imaging element having a quadrilateral shape and disposed at a position at which the lens system forms an image on the optical axis.

18. An imaging system comprising:

the lens system according to claim 1 ;

the imaging element having a quadrilateral shape and 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.