Relay optical system, and optical system for rigid endoscope and rigid endoscope using the same

1. A relay optical system, comprising:

a cemented lens in which a first lens having a positive refractive power, a second lens having a positive refractive power, and a third lens having a negative refractive power are cemented,

wherein:

the cemented lens is disposed in an optical path of the relay optical system, which is formed by an object-side optical path and an image-side optical path, and

the first lens is a meniscus lens which is adjacent to the third lens,

a dispersion and a partial dispersion ratio differ for the first lens and the third lens,

in a rectangular coordinate system in which a horizontal axis is set to be νd LA and a vertical axis is set to be θgF LA ,

when a straight line expressed by θgF LA =α×νd LA +β LA is set, where α=−0.00163,

θgF LA and νd LA of a medium of the first lens are included in an area determined by the following conditional expression (1) and conditional expression (2), and

the following conditional expression (3) is satisfied:

0.67≤β LA   (1)

ν d LA <50  (2)

−1.4< mg<− 0.6  (3)

where,

θgF LA denotes a partial dispersion ratio (ng LA −nF LA )/(nF LA −nC LA ) of the medium of the first lens,

νd LA denotes Abbe number (nd LA −1)/(nF LA −nC LA ) for the medium of the first lens,

nd LA , nC LA , nF LA , and ng LA are refractive indices of the medium of the first lens for a d-line, a C-line, an F-line, and a g-line respectively,

mg denotes a magnification of the relay optical system,

the object-side optical path is an optical path positioned on an object side of a center of the relay optical system, and

the image-side optical path is an optical path positioned on an image side of the center of the relay optical system.

2. The relay optical system according to claim 1 , wherein the first lens is positioned between the second lens and the third lens.

3. The relay optical system according to claim 1 , wherein the third lens is positioned between the second lens and the first lens.

4. The relay optical system according to claim 1 , wherein the first lens is a resin lens.

5. The relay optical system according to claim 1 , wherein the following conditional expression (4) is satisfied:

0<|( R 1− R 2)/( R 1+ R 2)|<3  (4)

where,

R1 denotes a radius of curvature of an object side of the first lens, and

R2 denotes a radius of curvature of an image side of the first lens.

6. The relay optical system according to claim 1 , wherein the following conditional expressions (5) and (6) are satisfied:

1.4< nd LB <1.6  (5)

50<ν d LB <100  (6)

where,

nd LB denotes a refractive index of a medium of the second lens for the d-line,

νd LB denotes Abbe number (nd LB −1)/(nF LB −nC LB ) for the medium of the second lens, and

nd LB , nC LB , nF LB , and ng LB are refractive indices of the medium of the second lens for the d-line, the C-line, the F-line, and the g-line.

7. The relay optical system according to claim 1 , wherein the cemented lens is disposed in at least one of the object-side optical path and the image-side optical path.

8. The relay optical system according to claim 1 , wherein the cemented lens is disposed in each of the object-side optical path and the image-side optical path.

9. The relay optical system according to claim 1 , wherein the relay optical system includes at least one aspheric surface.

10. The relay optical system according to claim 1 , comprising:

an object-side lens which is disposed in the object-side optical path; and

an image-side lens which is disposed in the image-side optical path,

wherein:

the object-side lens has a positive refractive power, and is disposed such that a convex surface is directed toward the object side,

the image-side lens has a positive refractive power, and is disposed such that a convex surface is directed toward the image side, and

the cemented lens is disposed between the object-side lens and the image-side lens.

11. The relay optical system according to claim 1 , wherein:

a lens surface positioned nearest to image in the object-side optical path is a surface which is convex toward the image side, and

a lens surface positioned nearest to object in the image-side optical path is a surface which is convex toward the object side.

12. The relay optical system according to claim 1 , wherein the following conditional expression (7) is satisfied:

0.2<(OBH+IH)/Φ ce <1.8  (7)

where,

Φce denotes a light-ray effective diameter of the cemented lens,

OBH denotes a maximum object height, and

IH denotes a maximum image height.

13. An image relay unit, comprising:

a plurality of relay optical systems,

wherein at least one relay optical system of the plurality of relay optical systems is the relay optical system according to claim 1 .

14. An optical system for rigid endoscope, comprising:

an objective optical system; and

an image relay unit which is disposed on an image side of the objective optical system,

wherein the image relay unit is the image relay unit according to claim 13 .

15. The optical system for rigid endoscope according to claim 14 , comprising:

an eyepiece optical system which is disposed on an image side of the image relay unit.

16. A rigid endoscope, comprising:

the optical system for rigid endoscope according to claim 14 ; and

an image pickup element which captures an image formed by the image relay unit.

17. A rigid endoscope, comprising:

the optical system for rigid endoscope according to claim 15 ; and

an illuminating unit which illuminates an object to be observed.

18. The rigid endoscope according to claim 16 , comprising:

an illuminating unit for illuminating an object to be observed.