Microscope tube lens, microscope apparatus and image pickup optical system

1. A microscope tube lens that forms an enlarged image of an object by being combined with an infinity-corrected objective, the tube lens comprising in an order from an object side:

a first lens group that has positive power and that includes a cemented lens;

a second lens group that has negative power; and

a third lens group that has positive power, wherein

a following condition expression is satisfied, where NA is a numerical aperture on an image side of the tube lens, FN is a field number of the tube lens, and ϵ is an airy disk diameter with respect to d line (588 nm) of the tube lens:

0.04< NA ; and

1700≤ FN/ϵ.

2. A tube lens according to claim 1 , wherein:

the third lens group consists of a plurality of lenses, each having positive power, the third lens group having positive power as a whole, and

a following condition expression is satisfied, where FL is a focal length of the tube lens and D2 is a distance from a lens surface that is closest to the object in the tube lens to a position of an entrance pupil of the tube lens:

0.3< D 2/ FL< 1.3.

3. The tube lens according to claim 1 , wherein:

following condition expressions are satisfied, where FLG1 is a focal length of the first lens group, FLG2 is a focal length of the second lens group, D0 is a distance from a lens surface that is closest to the object in the tube lens to an imaging plane, on which the enlarged image is formed, and D1 is a distance from a lens surface that is closest to the object in the tube lens to a lens surface that is closest to the imaging plane in the tube lens:

0.3< FLG 1/ FL< 3;

−4< FLG 2/ FL<− 0.05; and

0.3< D 1/ D 0<0.8.

4. The tube lens according to claim 1 , wherein:

the first lens group includes a first lens having a concave surface facing toward an image side, and

the second lens group includes a second lens having a concave surface facing toward an object side.

5. The tube lens according to claim 4 , wherein:

the first lens is a meniscus lens that is arranged closest to an image plane in the first lens group, that has a concave surface facing toward an image side, and that includes a single lens or a cemented lens, and

the second lens is a meniscus lens that is arranged closest to the object in the second lens group, that has a concave surface facing toward an object side, and that includes a single lens or a cemented lens.

6. The tube lens according to claim 5 , wherein:

the first lens and the second lens include at least four lenses in total, and

at least one of the first lens and the second lens includes a cemented lens.

7. The tube lens according to claim 6 , wherein:

the first lens and the second lens are cemented lenses, respectively.

8. The tube lens according to claim 5 , wherein:

at least one of the first lens and the second lens is a cemented lens including a lens having positive power and a lens having negative power, and

a following condition expression is satisfied, where νl is a highest Abbe number among Abbe numbers of lenses with positive power included in the cemented lens and νs is a lowest Abbe number among Abbe numbers of lenses with negative power included in the cemented lens:

30<ν l−νs.

9. The tube lens according to claim 7 , wherein:

the first lens and the second lens are cemented lenses each including a lens having positive power and a lens having negative power, and

following condition expressions are satisfied, where νl1 is a highest Abbe number among Abbe numbers of lenses with positive power included in a first cemented lens, which is one of the first lens and the second lens, νs1 is a lowest Abbe number among Abbe numbers of lenses with negative power included in the first cemented lens, νl2 is a highest Abbe number among Abbe numbers of lenses with positive power included in a second cemented lens, which is the other one of the first lens and the second lens, and νs2 is a lowest Abbe number among Abbe numbers of lenses with negative power included in the second cemented lens:

30<ν l 1−ν s 1; and

25<ν l 2−ν s 2.

10. The tube lens according to claim 4 , wherein:

following condition expressions are satisfied, where RG1 is a curvature radius of the concave surface of the first lens facing toward an image side, RG2 is a curvature radius of the concave surface of the second lens facing toward an object side, NdG2 is a refractive index with respect to d line of the second lens or a refractive index with respect to d line of a lens closest to the object among lenses constituting the second lens, and νdG1 is a highest Abbe number among Abbe numbers of lenses with positive power included in the first lens group:

0<| RG 2/ RG 1|<3;

1.5< NdG 2; and

70<ν dG 1.

11. The tube lens according to claim 1 , wherein:

the third lens group includes a meniscus lens with positive power having a concave surface facing toward an object side.

12. The tube lens according to claim 1 , wherein:

the third lens group comprises:

a third lens with positive power that is arranged closest to the object; and

a fourth lens with positive power that is arranged closest to an image plane, and

a following condition expression is satisfied, where FLG3a is a focal length of the third lens and FLG3b is a focal length of the fourth lens:

0.2< FLG 3 b/FLG 3 a< 5.

13. The tube lens according to claim 1 , wherein:

following condition expressions are satisfied, where NdG3 is a highest refractive index among refractive indexes of lenses included in the third lens group and νdG3 is a lowest Abbe number among Abbe numbers of lenses included in the third lens group:

1.7< NdG 3; and

ν dG 3<45.

14. The tube lens according to claim 1 , wherein:

a following condition expression is satisfied, where Ha is a height of an off-axis principal ray when the off-axis principal ray is incident on a lens surface closest to the object in the tube lens and Hi is a height of the off-axis principal ray when the off-axis principal ray is incident on an imaging plane:

0.3< Ha/Hi< 1.

15. The tube lens according to claim 1 , wherein:

a following condition expression is satisfied, where Ea is a maximum effective diameter of a lens included in the first lens group and Ec is a maximum effective diameter of a lens included in the third lens group:

−0.04<( Ea−Ec )/ FL< 0.03.

16. A microscope apparatus comprising:

a tube lens according to claim 1 .

17. A microscope apparatus according to claim 16 , further comprising:

an image pickup element that picks up the enlarged image of the object formed by the tube lens, wherein

a following condition expression is satisfied, where L is a pixel size of the image pickup element:

1 μm≤ L≤ 17 μm.