Imaging optical system

1. An imaging optical system comprises an aperture stop, a positive first lens group, and a positive or negative second lens group, in this order from an object,

wherein said positive first lens group comprises a positive first lens element and a negative second lens element, in this order from the object, said positive first lens element having its most image-side surface as a convex surface facing toward an image side, and said negative second lens element having its most image-side surface as a concave surface facing toward the image side;

wherein said second lens group comprises a meniscus third lens element;

wherein said meniscus third lens element comprises an aspherical surface on an object-side surface and an image-side surface thereof, and has the convex surface facing toward the object in the vicinity of the optical axis;

wherein said positive first lens element of said positive first lens group is provided with an aspherical surface on at least the object-side surface thereof;

wherein said aspherical surface of said positive first lens element is formed so that the higher a height from the optical axis in a radial direction becomes, the more an aspherical surface at the height is displaced toward the object, compared with a paraxial spherical surface; and

wherein said imaging optical system satisfies the following conditions:

0.6<| f 1 /f 2 |<1.0

1.0< f 12 /f< 1.6

0.301≦| R 2 /f|< 0.7

wherein

f designates the focal length of said entire imaging optical system;

f 1 designates the focal length of said positive first lens element;

f 2 designates the focal length of said negative second lens element;

f 12 designates the focal length of said positive first lens group; and

R 2 designates the radius of curvature of the image-side surface of said positive first lens element.

2. The imaging optical system according to claim 1 , wherein said negative second lens element of said positive first lens group is provided with an aspherical surface on at least the image-side surface thereof;

wherein said aspherical surface of said image-side surface of said negative second lens element is formed so that the higher a height from the optical axis in a radial direction becomes, the more an aspherical surface at the height is displaced toward the object, compared with a paraxial spherical surface; and

wherein said imaging optical system satisfies the following conditions:

0.2< R 4 /f< 0.7

0.15< D/f< 0.4

wherein

R 4 designates the radius of curvature of the image-side surface of said negative second lens element; and

D designates the distance between said positive first lens group and said second lens group.

3. The imaging optical system according to claim 1 , further satisfying the following condition:

−0.1< f 12 /f 3 <0.7

wherein

f 12 designates the focal length of said positive first lens group; and

f 3 designates the focal length of said meniscus third lens element.

4. The imaging optical system according to claim 1 , wherein said meniscus third lens element of said second lens group is made of a resin lens material;

wherein said object-side aspherical surface of said meniscus third lens element is formed so that the higher a height from the optical axis in a radial direction becomes the weaker the positive refractive power is, compared with a paraxial spherical surface;

wherein said image-side aspherical surface of said meniscus third lens element is formed so that the higher a height from the optical axis in a radial direction becomes the weaker the negative refractive power is, compared with a paraxial spherical surface; and the peripheral area of the image-side surface of said meniscus third lens element is arranged to generate a positive refractive power instead of the negative refractive power; and

wherein said meniscus third lens element of said second lens group satisfies the following condition:

0.4< R 5 /f< 1.0

wherein

R 5 designates the radius of curvature of the paraxial spherical surface on the object-side surface of said meniscus third lens element; and

f designates the focal length of said entire imaging optical system.

5. An imaging optical system comprising: an aperture stop; a positive first lens group; and a positive or negative second lens group, in this order from an object,

wherein said positive first lens group comprises a positive first lens element and a negative second lens element, in this order from the object, said negative second lens element provided with an aspherical surface on at least the image-side surface of said negative second lens element;

wherein said second lens group comprises a meniscus third lens element;

wherein said meniscus third lens element comprises an aspherical surface on an object-side surface and an image-side lens surface of said third lens element, and has the convex surface facing toward the object in the vicinity of the optical axis;

wherein said positive first lens element of said positive first lens group is provided with an aspherical surface on at least the object-side surface thereof;

wherein said aspherical surface of said positive first lens element is formed such that the higher a height from the optical axis in a radial direction becomes, the more an aspherical surface at the height is displaced toward the object, compared with a paraxial spherical surface; and

wherein said imaging optical system satisfies the following conditions:

0.6<| f 1 /f 2 |<1.0

1.0< f 12 /f< 1.6

0.301≦| R 2 /f |<0.7

0.2< R 4 /f <0.7

0.15< D/f <0.4

wherein

f designates the focal length of said entire imaging optical system;

f 1 designates the focal length of said positive first lens element;

f 2 designates the focal length of said negative second lens element;

f 12 designates the focal length of said positive first lens group;

R 2 designates the radius of curvature of the image-side surface of said positive first lens element;

R 4 designates the radius of curvature of the image-side surface of said negative second lens element; and

D designates the distance between said positive first lens group and said second lens group.

6. The imaging optical system according to claim 5 , further satisfying the following condition:

−0.1< f 12 /f 3 <0.7

wherein f 12 designates the focal length of said positive first lens group; and f 3 designates the focal length of said meniscus third lens element.

7. The imaging optical system according to claim 5 , wherein said meniscus third lens element of said second lens group is made of a resin lens material;

wherein said object-side aspherical surface of said meniscus third lens element is formed so that the higher a height from the optical axis in a radial direction becomes the weaker the positive refractive power is, compared with a paraxial spherical surface;

wherein said image-side aspherical surface of said meniscus third lens element is formed so that the higher a height from the optical axis in a radial direction becomes the weaker the negative refractive power is, compared with a paraxial spherical surface; and

the peripheral area of the image-side surface of said meniscus third lens element is arranged to generate a positive refractive power instead of the negative refractive power; and

wherein said meniscus third lens element of said second lens group satisfies the following condition:

0.4< R 5 /f <1.0

wherein R 5 designates the radius of curvature of the paraxial spherical surface on the object-side surface of said meniscus third lens element; and

f designates the focal length of said entire imaging optical system.