Patent ID: 7706085

Claim:
An imaging optical system, comprising: an aperture stop for limiting an axial light beam, an object-side lens group that is located on an object side of said aperture stop and has negative refracting power, and an image-side lens group that is located on an image side of said aperture stop and has positive refracting power, wherein: said object-side lens group comprises, in order from its object side, an object-side positive meniscus lens that is convex on its object side and has positive refracting power and an object-side negative meniscus lens that is convex on its object side and has negative refracting power, said image-side lens group comprises, in order from its object side, an image-side negative lens, an image-side first positive lens and an image-side second positive lens, said image-side negative lens and said image-side first positive lens adjacent thereto form together a cemented doublet, and said imaging optical system has a total of five lenses, with satisfaction of the following conditions (1), (2) and (3): 2.3≦| f 1/ f|≦ 5.3 (1) 0.8≦ f 2/ f≦ 1.0 (2) 0.55≦ L/f≦ 0.85 (3) where f is a focal length of the whole imaging optical system, f 1 is a focal length of the object-side lens group in said imaging optical system, f 2 is a focal length of the image-side lens group in said imaging optical system, and L is an axial distance from a vertex of the lens surface located in, and nearest to the object side of, said object-side lens group to a vertex of the lens surface located in, and nearest to the object side of, said image-side lens group.