Variable-focus optical system

1. A variable-focus optical system that is used in a microscope system including an objective optical system configured to convert light coming from a specimen to a substantially collimated beam and an imaging optical system configured to image, at a predetermined position, the light converted to the substantially collimated beam by the objective optical system, that is disposed between the objective optical system and the imaging optical system, and that varies a working distance of the objective optical system, the variable-focus optical system comprising:

a front-group optical system having positive power on the whole and a rear-group optical system having positive power substantially the same as that of the front-group optical system on the whole, the optical systems being disposed in this order from the objective optical system side,

an optical-system driving unit configured to drive at least one of the front-group optical system and the rear-group optical system in an optical axis direction to relatively change the distance between the front-group optical system and the rear-group optical system in the optical axis direction,

wherein

the front-group optical system consists of, in order from the objective optical system side, a first-group optical system having positive power on the whole, a second-group optical system having positive power on the whole, and a third-group optical system,

the first-group optical system consists of a single joined lens formed by joining, at a joining surface having negative power, a lens made of a low-dispersion material with a small refractive index and a lens made of a high-dispersion material with a large refractive index,

the second-group optical system consists of a positive-power lens disposed closest to the objective optical system such that a convex surface faces the objective optical system,

the third-group optical system consists of two lenses, of which the lens disposed closest to the objective optical system has a concave surface facing the objective optical system,

the rear-group optical system consists of, in order from the objective optical system side, a fourth-group optical system having positive power on the whole, a fifth-group optical system, and a sixth-group optical system having positive power on the whole,

the fourth-group optical system consists of, in this order from the objective optical system side, two lenses, that is, a positive-power lens and a negative-power lens disposed such that the negative-power lens disposed closest to the imaging optical system has a concave surface facing the imaging optical system,

the fifth-group optical system consists of a positive-power lens disposed closest to the imaging optical system such that a convex surface facing the imaging optical system, and

the sixth-group optical system consists of a single joined lens formed by joining, at a joining surface having negative power, a lens made of a low-dispersion material with a small refractive index and a lens made of a high-dispersion material with a large refractive index.

2. The variable-focus optical system according to claim 1 , wherein the following Conditional Expressions (1) to (6) are satisfied:

35<ν 1L −ν 1H <60  (1)

0.6 <|F 2 /Fa|< 0.9  (2)

0.1 <|R G3 /Fa|< 0.3  (3)

0.1 <|R G4 /Fb|< 0.2  (4)

0.9 <|F 5 /Fb|< 1.2  (5)

40<ν 6L −ν 6H <60  (6)

where ν 1L represents the d-line Abbe number of the lens of the joined lens in the first-group optical system made of a low-dispersion material with a small refractive index, ν 1H represents the d-line Abbe number of the lens of the joined lens in the first-group optical system made of a high-dispersion material with a large refractive index, Fa represents the focal length of the entire front-group optical system, Fb represents the focal length of the entire rear-group optical system, F 2 represents the focal length of the second-group optical system, R G3 represents the radius of curvature of the concave surface of the lens disposed closest to the objective optical system in the third-group optical system, R G4 represents the radius of curvature of the concave surface of the negative-power lens disposed closest to the imaging optical system in the fourth-group optical system, F 5 represents the focal length of the fifth-group optical system, ν 6L represents the d-line Abbe number of the lens of the joined lens in the sixth-group optical system made of a low-dispersion material with a small refractive index, and ν 6H represents the d-line Abbe number of the lens of the joined lens in the sixth-group optical system made of a high-dispersion material with a large refractive index.

3. The variable-focus optical system according to claim 2 , wherein a value of the front focus position of the front-group optical system and a value of the rear focus position of the rear-group optical system are both negative.

4. The variable-focus optical system according to claim 3 , wherein the following Conditional Expressions (7) and (8) are satisfied:

−1.8 <Fa/F 3<0.2  (7)

1.5 <Fb/F 4<2.2  (8)

where F 3 represents the focal length of the third-group optical system, and F 4 represents the focal length of the fourth-group optical system.

5. The variable-focus optical system according to claim 2 , wherein the following Conditional Expression (9) is satisfied:

0.1 <Fa/Ft 1<0.2  (9)

where Ft 1 represents the focal length of the imaging optical system.