Liquid-immersion objective optical system
The invention provides a liquid-immersion objective optical system including a first lens group and a second lens group. The first lens group includes a first lens component having positive refractive power, an object side thereof being a flat surface and a convex surface thereof facing an image-plane side; a second lens component having positive refractive power, a convex surface thereof facing the image-plane side; a third lens component having positive refractive power as a whole, formed by cementing a biconvex positive lens and a negative lens; and a fourth lens component formed by cementing a negative lens and a biconvex positive lens. The second lens group includes a fifth lens component formed by cementing a lens having a concave surface facing the object side and a lens having a convex surface facing the image-plane side; a sixth lens component having positive refractive power; and a seventh lens component having negative refractive power. The focal length F 1 of the entire first lens group, the magnification M of the objective optical system, the distance LT between the object plane and the image plane, and the distance LG between the object side of the first lens component L 1 and the image side of the fourth lens component L 4 satisfy the following conditional expressions: 0.2<| M·F 1 /LT |<0.45 0/2<| LG 1/ LT |<0.4.
1. A liquid-immersion objective optical system comprising:
a first lens group; and
a second lens group,
the first lens group including
a first lens component having positive refractive power, wherein an object side thereof is a flat surface and a convex surface thereof faces an image-plane side;
a second lens component having positive refractive power, wherein a convex surface thereof faces the image-plane side;
a third lens component having positive refractive power as a whole, formed by cementing a biconvex positive lens and a negative lens; and
a fourth lens component formed by cementing a negative lens and a biconvex positive lens;
the second lens group including
a fifth lens component formed by cementing at least a lens having a concave surface facing the object side and a lens having a convex surface facing the image-plane side;
a sixth lens component having positive refractive power; and
a seventh lens component having negative refractive power,
wherein the liquid-immersion objective optical system satisfies conditional expressions (1) and (2)
0.2 <|M·F 1 /LT |<0.45 (1)
0.2 <|LG 1 /LT |<0.4 (2)
wherein F 1 is the overall focal length of the first lens group, M is the magnification of the liquid-immersion objective optical system, LT is the distance from an object plane to the image plane, and LG 1 is the distance from the object side of the first lens component to the image side of the fourth lens component.
2. A liquid-immersion objective optical system according to claim 1 , wherein the following conditional expressions (3), (4) and (5) are satisfied
ν42−ν41>40 (3)
n 41 −n 42>0.3 (4)
0.7 <|R 4 /F 1|<1.2 (5)
wherein R 4 is the radius of curvature of a cemented surface in the fourth lens component, ν 41 and ν 42 are the d-line Abbe numbers of the negative lens and the biconvex positive lens in the fourth lens component, respectively, and n 41 and n 42 are the d-line refractive indices of the negative lens and the biconvex positive lens in the fourth lens component, respectively.
3. A liquid-immersion objective optical system according to claim 1 , wherein the following conditional expressions (6) and (7) are satisfied
ν31−ν32≧40 (6)
0.7 <|R 3 /F 1|<1.7 (7)
wherein R 3 is the radius of curvature of the cemented surface of the third lens component, and ν 31 and ν 32 are the d-line Abbe numbers of the biconvex positive lens and the negative lens, respectively, in the third lens component.
4. A liquid-immersion objective optical system according to claim 1 , wherein the following conditional expression (8) is satisfied
0.5 <|R 5 O/R 5 I |<0.8 (8)
wherein R 5 O is the radius of curvature of the concave surface at the object side of the fifth lens component, and R 5 I is the radius of curvature of the convex surface at the image-plane side of the fifth lens component.
5. A liquid-immersion objective optical system according to claim 1 , wherein the first lens component is formed by cementing a parallel flat plate and a plano-convex lens.
6. A liquid-immersion objective optical system according to claim 1 , wherein the sixth lens component includes two lenses having positive refractive power.