Imaging optical system

1. An imaging optical system comprising:

a negative first lens group, an aperture diaphragm, and a positive second lens group, in that order from the object side,

wherein said first lens group comprises a positive lens element provided closest to the object side,

wherein said positive lens element that is provided closest to the object side within said first lens group includes an aspherical surface on the object side thereof, said aspherical surface including a paraxial convex surface convexing toward the object side, and

wherein said aspherical surface on the object side of said positive lens element that is provided closest to the object side within said first lens group has a curvature that is positive along a meridional cross-section at the paraxial portion thereof, and the curvature of the meridional cross-section changes from a positive value to a negative value in an area within 75% of an effective aperture from the paraxial portion toward the periphery of the aspherical surface.

2. The imaging optical system according to claim 1 , wherein said aspherical surface on the object side of said positive lens element that is provided closest to the object side within said first lens group has a curvature that is positive along a meridional cross-section at the paraxial portion thereof, and the curvature of the meridional cross-section changes from a positive value to a negative value in an area within 50% to 75% of an effective aperture from the paraxial portion toward the periphery of the aspherical surface.

3. The imaging optical system according to claim 1 , wherein the following conditions (1), (2), (3) and (4) are satisfied:

R 1/ f< 1.4  (1),

D 1/ f> 0.2  (2),

f 1/ f< 20  (3), and

V> 56  (4), wherein

f designates the focal length of the entire imaging optical system,

f1 designates the focal length of said positive lens element that is provided closest to the object side within said first lens group,

R1 designates the paraxial radius of curvature of the surface on the object side of said positive lens element provided closest to the object side within said first lens group,

D1 designates the thickness along the optical axis of said positive lens element provided closest to the object side within said first lens group, and

V designates the Abbe number with respect to the d-line of the lens element that is provided closest to the aperture diaphragm within said second lens group.

4. The imaging optical system according to claim 1 , wherein the following condition (5) is satisfied:

−2< f 1/ fg 1<1  (5), wherein

f1 designates the focal length of said positive lens element that is provided closest to the object side within said first lens group, and

fg1 designates the focal length of said first lens group.

5. The imaging optical system according to claim 1 , wherein the following condition (6) is satisfied:

−0.20<( R 1− R 2)/( R 1+ R 2)<0.55  (6), wherein

R1 designates the paraxial radius of curvature of the surface on the object side of the positive lens element that is provided closest to the object side within said first lens group, and

R2 designates the paraxial radius of curvature of the surface on the image side of the positive lens element that is provided closest to the object side within said first lens group.

6. The imaging optical system according to claim 1 , wherein said first lens group comprises a negative lens element provided immediately behind said positive lens element that is provided closest to the object side.

7. The imaging optical system according to claim 6 , wherein said negative lens element, which is provided immediately behind said positive lens element that is provided closest to the object side, comprises a concave surface on the image side at the paraxial portion thereof.

8. The imaging optical system according to claim 6 , wherein a difference in Abbe number with respect to the d-line between said positive lens element that is provided closest to the object side and said negative lens element, provided immediately behind said positive lens element that is provided closest to the object side, is 10 or greater.

9. The imaging optical system according to claim 1 , wherein said second lens group comprises at least one positive lens element that has an Abbe number with respect to the d-line of 70 or greater.

10. The imaging optical system according to claim 1 , wherein said second lens group comprises at least one negative lens element that has an Abbe number with respect to the d-line of 30 or less.

11. The imaging optical system according to claim 1 , wherein a value of the focal length of the entire optical system divided by the maximum image height is greater than or equal to 2.9.

12. An imaging optical system comprising:

a negative first lens group, an aperture diaphragm, and a positive second lens group, in that order from the object side,

wherein said first lens group comprises a lens element provided closest to the object side,

wherein said lens element that is provided closest to the object side within said first lens group includes an aspherical surface on the object side thereof, said aspherical surface including a paraxial convex surface convexing toward the object side,

wherein said aspherical surface on the object side of said lens element that is provided closest to the object side within said first lens group has a curvature that is positive along a meridional cross-section at the paraxial portion thereof, and the curvature of the meridional cross-section changes from a positive value to a negative value in an area within 75% of an effective aperture from the paraxial portion toward the periphery of the aspherical surface, and

wherein a value of the focal length of the entire optical system divided by the maximum image height is greater than or equal to 2.9.