Patent ID: 7307799

Claim:
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.