Inner focus lens

1. An inner focus lens comprising sequentially from an object side:

a first lens group having a positive refractive power;

a second lens group having a negative refractive power; and

a third lens group having a negative refractive power, and consisting of, sequentially from the object side:

a front sub-lens group having a positive refractive power, and

a rear sub-lens group having a negative refractive power; and

an axial air gap that is widest in the third lens group is formed between the front sub-lens group and the rear sub-lens group,

the second lens group is configured to move along an optical axis, whereby focusing from a focus state for an object at infinity to a focus state for a minimum object distance is performed, and

the inner focus lens satisfies a conditional expression (1) −29.0≦f3/f≦−5.4, where f3 is a focal length of the third lens group at the focus state for an object at infinity and f is a focal length of the optical system overall of the inner focus lens at the focus state for an object at infinity.

2. The inner focus lens according to claim 1 , wherein

the inner focus lens satisfies a conditional expression (2) 0.18≦f1/f≦0.99, where f1 is a focal length of the first lens group at the focus state for an object at infinity.

3. The inner focus lens according to claim 1 , wherein

the inner focus lens satisfies a conditional expression (3) 0.51≦βinf/β mod≦2.07, where βinf is paraxial magnification of the second lens group at the focus state for an object at infinity and β mod is paraxial magnification of the second lens group at the focus state for the minimum object distance.

4. The inner focus lens according to claim 1 , wherein

the third lens group includes a simple lens element having a negative refractive power and disposed farthest on an image side, and

the inner focus lens satisfies a conditional expression (4) (R1+R2)/(R1−R2)≦0.0, where R1 is radius of curvature of a surface at an air interface on the object side of the simple lens element having a negative refractive power and R2 is radius of curvature of a surface at an air interface on the image side of the simple lens element having a negative refractive power.

5. The inner focus lens according to claim 4 , wherein

the simple lens element having a negative refractive power and disposed farthest on the image side of the third lens group is configured by a glass material, and

the inner focus lens satisfies a conditional expression (8) 30≦νen, where νen is an Abbe number for e-line of the simple lens element having a negative refractive power and disposed farthest on the image side of the third lens group.

6. The inner focus lens according to claim 1 , wherein

the inner focus lens satisfies a conditional expression (5) 0.01≦L1s/L≦0.53, where L1s is an axial distance from a surface farthest on the object side of the first lens group to an aperture stop and L is an overall optical system length of the inner focus lens.

7. The inner focus lens according to claim 6 , wherein the overall optical system length is an air-conversion optical path length from an apex of a lens surface farthest on the object side to an imaging plane.

8. The inner focus lens according to claim 1 , wherein

the second lens group is configured by a simple lens element having a negative refractive power.

9. The inner focus lens according to claim 1 , wherein

the inner focus lens satisfies a conditional expression (6) −2.12≦f2/f≦−0.18, where f2 is a focal length of the second lens group at the focus state for an object at infinity.