Lens system and camera system including the lens system

1. A lens system which includes a lens group including at least one lens element and forms an image on an image sensor which is rectangular with a first side and a second side greater than or equal to the first side in length, the lens system comprising:

in order from an object side to an image surface side,

a front lens group;

a diaphragm; and

a rear lens group, wherein

at least one of the front lens group and the rear lens group includes a freeform surface lens asymmetric about an optical axis, and

the lens system satisfies Condition (1) and Condition (2) below when an axis passing through the optical axis and parallel to the first side is a vertical axis, an axis passing through the optical axis and parallel to the second side is a horizontal axis, and an axis passing through the optical axis and parallel to a line diagonally joining opposite corners of the image sensor is a diagonal axis,

1< A/ ( B−C )<10000   (1)

1< A/ ( D−E )<10000   (2)

where:

A is a value represented by [Math. 1];

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i is a number, indicating a position, of a freeform surface lens when freeform surface lenses are counted in order from the object side to the image surface side;

m is a number of freeform surface lenses located between an object and the diaphragm;

n is a total number of freeform surface lenses;

Lcci is a distance in an i-th freeform surface lens travelled by a principal ray forming an image at a center of the image sensor;

Lhvi is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Yv along the horizontal axis of the image sensor;

Lvvi is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Yv along the vertical axis of the image sensor;

Ldhi is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Yh along the diagonal axis of the image sensor;

Lhei is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Ye along the horizontal axis of the image sensor;

ndi is a refractive index of the i-th freeform surface lens with respect to d line;

Yv is a distance from the optical axis to the second side of the image sensor;

Yh is a distance from the optical axis to the first side of the image sensor; and

Ye =√(( Yv 2 +Yh 2 )/2).

2. The lens system according to claim 1 , wherein

the lens system satisfies Condition (3) below

1< A/ ( F−G )<10000   (3)

where:

F is a value represented by [Math. 6];

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and

Lhhi is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Yh along the horizontal axis of the image sensor.

3. The lens system according to claim 1 , comprising

at least two lens elements which are axially symmetric about the optical axis.

4. The lens system according to claim 1 , wherein

each of the front lens group and the rear lens group includes at least one freeform surface lens.

5. The lens system according to claim 1 , wherein

the freeform surface lens has a shape symmetric about the horizontal axis and the vertical axis.

6. The lens system according to claim 1 , wherein

the front lens group includes a freeform surface lens, and

a principal curvature of the freeform surface lens included in the front lens group has negative power.

7. The lens system according to claim 1 , wherein

the rear lens group includes a freeform surface lens, and

a principal curvature of the freeform surface lens included in the rear lens group has positive power.

8. The lens system according to claim 1 , wherein

the front lens group includes, in order from the object side to the image surface side, a meniscus-shaped first lens element having negative power and having a convex shape on the object side, and a meniscus-shaped second lens element having negative power and having a convex shape on the object side.

9. The lens system according to claim 1 , wherein

the lens system satisfies Condition (4) below

0.1< Lsi/Ltt< 0.7   (4)

where:

Lsi is a distance from the diaphragm to an image surface; and

Ltt is a distance from a lens surface located closest to the object to the image surface.

10. The lens system according to claim 1 , wherein

both surfaces of a freeform surface lens located closest to an image surface and having a principal curvature with positive power are freeform surfaces.

11. The lens system according to claim 1 , wherein

the lens system is a fisheye lens system.

12. A camera system comprising:

a lens system which includes a lens group including at least one lens element; and

an image sensor which is rectangular with a first side and a second side greater than or equal to the first side in length, wherein

the lens system includes:

in order from an object side to an image surface side,

a front lens group;

a diaphragm; and

a rear lens group,

at least one of the front lens group and the rear lens group includes a freeform surface lens asymmetric about an optical axis, and

the lens system satisfies Condition (1) and Condition (2) below when an axis passing through the optical axis and parallel to the first side is a vertical axis, an axis passing through the optical axis and parallel to the second side is a horizontal axis, and an axis passing through the optical axis and parallel to a line diagonally joining opposite corners of the image sensor is a diagonal axis,

1< A/ ( B−C )<10000   (1)

1< A/ ( D−E )<10000   (2)

where:

A is a value represented by [Math. 1];

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i

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n

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·

Lcci

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B is a value represented by [Math. 2];

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C is a value represented by [Math. 3];

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D is a value represented by [Math. 4];

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i is a number, indicating a position, of a freeform surface lens when freeform surface lenses are counted in order from the object side to the image surface side;

m is a number of freeform surface lenses located between an object and the diaphragm;

n is a total number of freeform surface lenses;

Lcci is a distance in an i-th freeform surface lens travelled by a principal ray forming an image at a center of the image sensor;

Lhvi is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Yv along the horizontal axis of the image sensor;

Lvvi is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Yv along the vertical axis of the image sensor;

Ldhi is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Yh along the diagonal axis of the image sensor;

Lhei is a distance in the i-th freeform surface lens travelled by a principal ray forming an image at a position away from the optical axis by Ye along the horizontal axis of the image sensor;

ndi is a refractive index of the i-th freeform surface lens with respect to d line;

Yv is a distance from the optical axis to the second side of the image sensor;

Yh is a distance from the optical axis to the first side of the image sensor; and

Ye=√(( Yv 2 +Yh 2 )/2).