Inner focus lens, interchangeable lens apparatus and camera system

1. An inner focus lens, in order from an object side to an image side, comprising:

a first lens unit having positive optical power;

a second lens unit having negative optical power; and

a third lens unit having positive optical power, wherein

the second lens unit is moved along an optical axis so that focusing from an infinite-distance object side to a short-distance object side is achieved,

the first lens unit includes a bi-convex air lens, and

the following conditions (1) and (2) are satisfied:

0.65<| f 2 /f|< 5.00   (1)

0.5< f 23 /f 1 <9.0   (2)

where,

f 2 is a focal length of the second lens unit,

f is a focal length of the entire system in an infinity in-focus condition,

f 23 is a composite focal length of the second lens unit and the third lens unit in an infinity in-focus condition, and

f 1 is a focal length of the first lens unit.

2. The inner focus lens as claimed in claim 1 , wherein the following condition (3) is satisfied:

−0.5< SF< 0.5   (3)

where,

SF =( Ra+Rb )/( Rb−Ra ),

Ra is a radius of curvature of an object side surface of the bi-convex air lens included in the first lens unit, and

Rb is a radius of curvature of an image side surface of the bi-convex air lens included in the first lens unit.

3. The inner focus lens as claimed in claim 1 , wherein the second lens unit is composed of a single unit having negative optical power.

4. The inner focus lens as claimed in claim 3 , wherein the single unit having negative optical power is composed of one single lens element having negative optical power.

5. The inner focus lens as claimed in claim 1 , wherein the following condition (4) is satisfied:

0.02< D 2 /{f ×tan(ω)}<0.30   (4)

where,

D 2 is an optical axial thickness of the second lens unit,

f is a focal length of the entire system in an infinity in-focus condition, and

ω is a half view angle (°) of the entire system in an infinity in-focus condition.

6. The inner focus lens as claimed in claim 1 , having

an aperture diaphragm for restricting axial light beam, wherein

the following condition (5) is satisfied:

0.5< f sb /f< 3.0   (5)

where,

f sb is a composite focal length of lens elements located on the image side relative to the aperture diaphragm in an infinity in-focus condition, and

f is a focal length of the entire system in an infinity in-focus condition.

7. The inner focus lens as claimed in claim 1 , wherein the following condition (6) is satisfied:

0.2<|(1−β 2 )×β 3 |<0.9   (6)

where,

β 2 is a lateral magnification of the second lens unit in an infinity in-focus condition, and

β 3 is a lateral magnification of the third lens unit in an infinity in-focus condition.

8. An interchangeable lens apparatus comprising:

an inner focus lens as claimed in claim 1 ; and

a lens mount section which is connectable to a camera body including an image sensor for receiving an optical image formed by the inner focus lens and converting the optical image into an electric image signal.

9. A camera system comprising:

an interchangeable lens apparatus including an inner focus lens as claimed in claim 1 ; and

a camera body which is detachably connected to the interchangeable lens apparatus via a camera mount section, and includes an image sensor for receiving an optical image formed by the inner focus lens and converting the optical image into an electric image signal.