Optical system and optical instrument, image pickup apparatus, and image pickup system using the same

1. An optical system comprising in order from an object side,

a lens unit Gf having a positive refractive power,

a stop, and

a lens unit Gr having a positive refractive power, and

the optical system includes no lens unit between the lens unit Gf and the lens unit Gr, and the following conditional expressions (4-1), (5), (9-1), and (13) are satisfied:

0.08< NA, 0.08< NA′   (4-1)

−2<β<−0.5  (5)

0< d 1 /Σd< 0.2  (9-1)

−20<Δ f cd /εd< 20  (13)

where,

NA denotes a numerical aperture on the object side of the optical system,

NA′ denotes a numerical aperture on an image side of the optical system,

β denotes a projection magnification of the optical system,

d 1 denotes a distance on an optical axis from a surface positioned nearest to the image side of the lens unit Gf up to a surface positioned nearest to the object side of the lens unit Gr,

Σd denotes a sum total of lens thickness on the optical axis of an overall optical system,

εd denotes an Airy disc radius for a d-line which is determined by the numerical aperture on the image side of the optical system, and

Δf cd denotes a difference in a focal position on a C-line and a focal position on the d-line, which is a difference in positions at which light is focused when parallel light is made to be incident on the lens unit Gr from the stop side.

2. The optical system according to claim 1 , wherein the following conditional expression (6) is satisfied:

0.5< f OB /f TL <2  (6)

where,

f OB denotes a focal length of the lens unit Gf, and

f TL denotes a focal length of the lens unit Gr.

3. The optical system according to claim 1 , wherein the following conditional expression (14) is satisfied:

0.7< d SHOB /d SHTL <1.3  (14)

where,

d SHOB denotes a distance on the optical axis from a front principal point of the lens unit Gf up to the stop, and

d SHTL denotes a distance on the optical axis from the stop up to a rear principal point of the lens unit Gr.

4. The optical system according to claim 1 , wherein a positive lens Lf 1 is disposed nearest to an image in the lens unit Gf.

5. The optical system according to claim 1 , wherein the lens unit Gf includes a lens Lfe which is disposed nearest to the object, and at least one lens surface of the lens Lfe has a shape which has an inflection point.

6. The optical system according to claim 1 , wherein the lens unit Gr includes a lens Lre which is disposed nearest to the image, and at least one lens surface of the lens Lre has a shape which has an inflection point.

7. The optical system according to claim 1 , wherein the following conditional expressions (7-1) and (8-1) are satisfied:

40%≤MTF OB   (7-1)

40%≤MTF TL   (8-1)

where,

MTF OB denotes an MTF on an axis of the lens unit Gf, and is an MTF with respect to a spatial frequency of fc′/4, where

MTF TL denotes an MTF on an axis of the lens unit Gr, and is an MTF with respect to a spatial frequency of fc′/4, where

fc denotes a cut-off frequency with respect to the numerical aperture on the object side of the optical system, and

fc′ denotes a cut-off frequency with respect to the numerical aperture on the image side of the optical system, and both MTF OB and MTF TL are MTFs at positions at which, light is focused when parallel light of an e-line is made to be incident from a direction of the stop side, respectively.

8. The optical system according to claim 1 , wherein a positive lens Lr 1 is disposed nearest to the object in the lens unit Gr.

9. The optical system according to claim 4 , wherein a negative lens Lf 2 is disposed on the object side of the positive lens Lf 1 such that, the negative lens Lf 2 is adjacent to the positive lens Lf 1 .

10. The optical system according to claim 8 , wherein a negative lens Lr 2 is disposed on the image side of the positive lens Lr 1 such that, the negative lens Lr 2 is adjacent to the positive lens Lr 1 .

11. The optical system according to claim 9 , wherein an object-side surface of the negative lens Lf 2 is concave toward the object side.

12. The optical system according to claim 10 , wherein an image-side surface of the negative lens Lr 2 is concave toward the image side.

13. The optical system according to claim 5 , wherein the lens Lfe has a negative refractive power.

14. The optical system according to claim 6 , wherein the lens Lre has a negative refractive power.

15. The optical system according to claim 1 , wherein

the optical system includes at least one pair of lenses which satisfies the following conditional expressions (1), (2), and (3), and

one lens in the pair of lenses is included in the lens unit Gf, and

the other lens in the pair of lenses is included in the lens unit Gr,

−1.1< r OBr <−0.9  (1)

−1.1 <r OBr /r TLF <−0.9  (2)

−0.1<( d OB −d TL )/( d OB +d TL )<0.1  (3)

where,

r OBf denotes a paraxial radius of curvature of an object-side surface of the one lens in the pair of lenses,

r OBr denotes a paraxial radius of curvature of an image-side surface of the one lens in the pair of lenses,

r TLf denotes a paraxial radius of curvature of an object-side surface of the other lens in the pair of lenses,

r TLr denotes a paraxial radius of curvature of an image-side surface of the other lens in the pair of lenses,

d OB denotes a thickness on the optical axis of the one lens in the pair of lenses, and

d TL denotes a thickness on the optical axis of the other lens in the pair of lenses.

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

−10°<θ o <30°  (12-1)

where,

θ o denotes an angle made by a normal of a plane perpendicular to the optical axis with a principal ray on the object side.

17. An optical instrument comprising:

an optical system according to claim 1 ; and

an image pickup element.