Imaging optical system

1. An imaging optical system comprising:

a front lens group including at least one front lens element, and a reflector element which reflects a light bundle that exits from said front lens element, in that order from the object side; and

a rear lens group provided on the image side of said front lens group,

wherein said front lens element serves as a spherically-swingable lens group which, in an image-stabilizing operation, suppresses image shake on the imaging plane by spherically-swinging said spherically-swingable lens group about a predetermined spherical-swing center in accordance with vibrations applied to said imaging optical system, and

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

ν d> 58  (1),

and

f/y> 3  (2),

wherein

νd designates the Abbe number with respect to the d-line of at least one lens element of said spherically-swingable lens group;

f designates the focal length of said imaging optical system, in a state where the focal length is at a maximum; and

y designates the maximum image height of the imaging plane.

2. The imaging optical system according to claim 1 , wherein the following condition (3) is satisfied:

−2< SF< 0  (3),

wherein

SF=(R2+R1)/(R2−R1);

R1 designates the radius of curvature of the surface closest to the object side on said spherically-swingable lens group; and

R2 designates the radius of curvature of the surface closest to the image side on said spherically-swingable lens group.

3. The imaging optical system according to claim 1 , wherein the following condition (4) is satisfied:

−0.3<( SC−R 2)/ f 1<0  (4),

wherein

SC designates the distance from the surface closest to the image side on said spherically-swingable lens group to said predetermined spherical-swing center;

R2 designates the radius of curvature of the surface closest to the image side on said spherically-swingable lens group; and

f1 designates the focal length of said spherically-swingable lens group.

4. The imaging optical system according to claim 1 , wherein said spherically-swingable lens group comprises a single lens element.

5. The imaging optical system according to claim 4 , wherein said single lens element, which serves as said spherically-swingable lens group, has a negative refractive power.

6. The imaging optical system according to claim 5 , wherein said single lens element, which serves as said spherically-swingable lens group, has a concave surface on the image side thereof.

7. The imaging optical system according to claim 5 , wherein the surface on the object side of said single lens element, which serves as said spherically-swingable lens group, has a smaller refractive power than the surface on the image side thereof.

8. The imaging optical system according to claim 1 , wherein, in a reference state in which said image-stabilizing operation is not carried out, said predetermined spherical-swing center is positioned on an extension of an optical axis, of said spherically-swingable lens group, extending behind an underside of a reflection surface of said reflector element.

9. The imaging optical system according to claim 1 , wherein said front lens group comprises a negative single lens element, which serves as said spherically-swingable lens group, having a concave surface on the image side thereof, said reflector element, a positive single lens element having a convex surface on the image side, and a negative single lens element having a concave surface on the object side, in that order from the object side.

10. The imaging optical system according to claim 1 , wherein said front lens group comprises a negative single lens element, which serves as said spherically-swingable lens group, having a concave surface on the image side thereof, said reflector element, a biconcave single lens element, and a positive meniscus lens element having a convex surface on the object side.

11. The imaging optical system according to claim 1 , wherein said front lens group comprises a negative single lens element, which serves as said spherically-swingable lens group, having a concave surface on the image side thereof, said reflector element, and a positive meniscus lens element having a convex surface on the object side.