Observation optical system and microscope including the observation optical system

1. An observation optical system comprising, in order from a sample side:

an infinity-corrected objective;

a first lens group having a positive power;

a second lens group having a positive power; and

a tube lens having a positive power,

wherein an intermediate image is formed between the first lens group and the second lens group, at a front side focal position of the second lens group,

wherein a first space along an optical axis between the objective and the first lens group is configured to be changeable,

wherein a second space along the optical axis between the second lens group and the tube lens is configured to be changeable, and

wherein if f 1 and f 3 are respectively defined to be a focal length of the first lens group and a focal length of the tube lens, a following condition is satisfied: 0.45<f 1 /f 3 <0.80.

2. The observation optical system according to claim 1 , wherein if an amount of change from a minimum space of the first space between the objective and the first lens group is defined to be dD 1 , an amount of change from a minimum space of the second space between the second lens group and the tube lens is defined to be dD 2 , and a sum of the amount of change dD 1 and the amount of change dD 2 is defined to be a total amount of change dD, a following condition is satisfied: 0<dD/f 3 <1.45.

3. The observation optical system according to claim 1 , further comprising four or more reflection surfaces on the optical axis,

wherein a normal to reflection surfaces arranged before and after the intermediate image is arranged to be nearly orthogonal to the optical axis of the objective.

4. The observation optical system according to claim 1 , further comprising four or more reflection surfaces on the optical axis,

wherein the four or more reflection surfaces configure an inversion optical system for inverting an image.

5. The observation optical system according to claim 1 , further comprising, in order from the sample side and on the optical axis, a first reflection surface, a second reflection surface, a third reflection surface, a fourth reflection surface, a fifth reflection surface, and a sixth reflection surface, each of which is intended to deflect the optical axis.

6. The observation optical system according to claim 5 , wherein a position of the intermediate image is between the first reflection surface and the third reflection surface, and

wherein a portion of the second space that is changeable is sandwiched between the fourth reflection surface and the sixth reflection surface.

7. The observation optical system according to claim 6 , wherein a first direction along the optical axis at the position of the intermediate image and a second direction along the optical axis at the portion of the second space that is changeable are nearly parallel to each other.

8. The observation optical system according to claim 7 , wherein the first reflection surface, the second reflection surface, the third reflection surface, the fourth reflection surface, the fifth reflection surface, and the sixth reflection surface respectively deflect the optical axis in nearly orthogonal directions.

9. The observation optical system according to claim 5 , wherein a position of the intermediate image is between the first reflection surface and the fifth reflection surface.

10. The observation optical system according to claim 5 , wherein a position of the intermediate image is between the second reflection surface and the third reflection surface.

11. The observation optical system according to claim 5 , wherein the sixth reflection surface is arranged between the second lens group and the tube lens, and

wherein a third space between the second lens group and the sixth reflection surface, and a fourth space between the sixth reflection surface and the tube lens are changeable in mutually different directions.

12. A microscope comprising the observation optical system according to claim 1 .

13. The microscope according to claim 12 , wherein the observation optical system further includes an eyepiece lens, and

wherein the microscope further comprises a turning mechanism for changing an angle formed between the optical axis of the tube lens and the optical axis of the eyepiece lens.

14. An observation optical system comprising, in order from a sample side:

an infinity-corrected objective;

a first lens group having a positive power;

a second lens group having a positive power; and

a tube lens having a positive power,

wherein an intermediate image is formed between the first lens group and the second lens group, at a front side focal position of the second lens group,

wherein a first space along an optical axis between the objective and the first lens group is configured to be changeable,

wherein a second space along the optical axis between the second lens group and the tube lens is configured to be changeable, and

wherein if f 1 and f 2 are respectively defined to be a focal length of the first lens group and a focal length of the second lens group, a following condition is satisfied:

0.95 <f 1 /f 2<1.05.

15. The observation optical system according to claim 14 , further comprising four or more reflection surfaces on the optical axis,

wherein a normal to reflection surfaces arranged before and after the intermediate image is arranged to be nearly orthogonal to the optical axis of the objective.

16. The observation optical system according to claim 14 , further comprising four or more reflection surfaces on the optical axis,

wherein the four or more reflection surfaces configure an inversion optical system for inverting an image.

17. The observation optical system according to claim 14 , further comprising, in order from the sample side and on the optical axis, a first reflection surface, a second reflection surface, a third reflection surface, a fourth reflection surface, a fifth reflection surface, and a sixth reflection surface, each of which is intended to deflect the optical axis.

18. The observation optical system according to claim 17 , wherein a position of the intermediate image is between the first reflection surface and the third reflection surface, and

wherein a portion of the second space that is changeable is between the fourth reflection surface and the sixth reflection surface.

19. The observation optical system according to claim 18 , wherein a first direction along the optical axis at the position of the intermediate image and a second direction along the optical axis at the portion of the second space that is changeable, are nearly parallel to each other.

20. The observation optical stem according to claim 19 , wherein the first reflection surface, the second reflection surface, the third reflection surface, the fourth reflection surface, the fifth reflection surface, and the sixth reflection surface respectively deflect the optical axis in nearly orthogonal directions.

21. The observation optical system according to claim 17 , wherein a position of the intermediate image is between the first reflection surface and the fifth reflection surface.

22. The observation optical system according to claim 17 , wherein a position of the intermediate image is between the second reflection surface and the third reflection surface.

23. The observation optical system according to claim 17 , wherein the sixth reflection surface is arranged between the second lens group and the tube lens, and

wherein a third space between the second lens group and the sixth reflection surface, and a fourth space between the sixth reflection surface and the tube lens are changeable in mutually different directions.

24. A microscope comprising the observation optical system according to claim 14 .

25. The microscope according to claim 24 , wherein the observation optical system includes an eyepiece lens, and

wherein the microscope further comprises a turning mechanism for changing an angle formed between the optical axis of the tube lens and the optical axis of the eyepiece lens.