Lens system, imaging device, and imaging system
A lens system includes a plurality of lens elements arranged from an object side to an image plane side, and a diaphragm arranged between the plurality of lens elements. The plurality of lens elements include a plurality of freeform lenses each having a freeform surface that is asymmetrical with respect to a first direction and a second direction which cross with each other. At least one freeform lens is placed on the image plane side of the diaphragm. With a lens thickness T1 in the first direction at a maximum height of an axial ray and a lens thickness T2 in the second direction at the maximum height of the axial ray, at least one freeform lens satisfying 0.3<T1/T2<0.76 is placed on the object side of the diaphragm.
1. A lens system comprising:
a plurality of lens elements arranged from an object side to an image plane side; and
a diaphragm arranged between the plurality of lens elements,
wherein the plurality of lens elements include a plurality of freeform lenses each having a freeform surface that is asymmetrical with respect to a first direction and a second direction which cross with each other,
at least one freeform lens is placed on the image plane side of the diaphragm, and
at least one freeform lens satisfying a conditional expression (1) given below is placed on the object side of the diaphragm;
0.3< T 1/ T 2<0.76 (1)
where
T1: a lens thickness in the first direction at a maximum height of an axial ray that passes through the lens system along with an optical axis of the lens system, and
T2: a lens thickness in the second direction at the maximum height of the axial ray that passes through the lens system along with the optical axis of the lens system.
2. The lens system according to claim 1 , satisfying a conditional expression (2) given below;
2<AXRh/ Y 1<5 (2)
where
AXRh: a maximum height of the axial ray, and
Y1: an image height in the first direction.
3. The lens system according to claim 1 , wherein a freeform lens located closest to the object side among the plurality of freeform lenses has a freeform surface on the object side, and satisfies conditional expression (3) given below;
0.005<(FSAG1−FSAG2)/AXRh<0.015 (3)
where
FSAG1: a sag amount in the first direction at the height AXRh,
FSAG2: a sag amount in the second direction at the height AXRh, and
AXRh: a maximum height of the axial ray.
4. The lens system according to claim 1 , satisfying conditional expression (4) given below;
1<( Y 1/ Y 2)×|tan θ2/tan θ1|<4 (4)
where
Y1: an image height in the first direction,
Y2: an image height in the second direction,
θ1: an angle of view in the first direction, and
θ2: an angle of view in the second direction.
5. The lens system according to claim 1 , satisfying conditional expression (5) given below;
0.050<AXRh/ OAL< 0.350 (5)
where
AXRh: a maximum height of the axial ray, and
OAL: an overall optical length of the lens system.
6. The lens system according to claim 1 , satisfying conditional expression (6) given below;
0.30< FFN/FBN< 3.00 (6)
where
FFN: a number of freeform lenses on the object side of the diaphragm, and
FBN: a number of freeform lenses on the image plane side of the diaphragm.
7. The lens system according to claim 1 , satisfying conditional expression (7) given below, based on sum for freeform surfaces of the freeform lenses located on the object side of the diaphragm;
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5.000
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where
N: a total number of the freeform surfaces of freeform lenses located on the object side of the diaphragm,
k: a number indicating a freeform surface among the total N freeform surfaces,
sag1 k : a sag amount at a position where the height of the k-th freeform surface in the first direction is 40% of AXRh,
sag2 k : a sag amount at a position where the height of the k-th freeform surface in the second direction is 40% of AXRh,
AXRh: a maximum height of the axial ray, and
Δnd k : a difference resulting from subtracting the refractive index on the object side of the k-th freeform surface from the refractive index on the image plane side of the k-th freeform surface.
8. The lens system according to claim 1 , wherein a freeform lens closest to the object side satisfies the conditional expression (1).
9. The lens system according to claim 1 , wherein a freeform lens arranged on the image plane side of the diaphragm has a freeform surface in which a sag amount at a height of 50% of a maximum height of the axial ray has an extreme value in a phase direction different from the first and second directions among phase directions around the optical axis.
10. The lens system according to claim 1 , wherein a number of the plurality of lens elements is five or more.
11. The lens system according to claim 1 , wherein an F value of the diaphragm is smaller than 2.8.
12. The lens system according to claim 1 , wherein the plurality of freeform lenses include a freeform lens having positive power in the first direction and negative power in the second direction.
13. The lens system according to claim 1 , configured to form an image by light having a wavelength of 900 nm or more.
14. The lens system according to claim 1 , wherein each freeform surface in the plurality of freeform lenses has line symmetry with respect to a first axis and a second axis, the first axis corresponding to the first direction and the second axis corresponding to the second direction.
15. The lens system according to claim 1 , wherein a lens element located closest to the object side or a second closest lens element among the plurality of lens elements satisfies the conditional expression (1).
16. The lens system according to claim 15 , wherein the lens element closest to the object side has a positive power in the first direction and a negative power in the second direction.
17. The lens system according to claim 1 , wherein the plurality of freeform lenses have a lens element having a negative power stronger in the first direction than in the second direction.
18. An imaging device comprising:
the lens system according to claim 1 ; and
an imaging element that captures an image formed by the lens system.
19. The imaging device according to claim 18 , wherein the imaging element has a short side corresponding to the first direction and a long side corresponding to the second direction.
20. An imaging system comprising:
the imaging device according to claim 18 ; and
an image processor that executes image processing on the image captured by the imaging element of the imaging device.