Patent ID: 8422150

Claim:
An objective optical system having a field angle of 140° or larger and comprising a front group, an aperture stop, and a positive rear group arranged in order from an object side, wherein the front group has at least one aspherical surface and includes a negative first lens disposed closest to the object side and satisfying the following conditional expression (1), (4), and (8): 0≦ r 2 /r 1<0.24 (1) 0.1 <|g 2 /g 1|<2.2 (4) −1.8 <f 1 /fl<− 0.8 (8) where r 1 denotes a radius of curvature of the object side of the first lens, r 2 denotes a radius of curvature of an image side of the first lens, g 1 denotes the focal length of the front group, g 2 denotes the focal length of the rear group, f 1 denotes a focal length of the first lens, and fl denotes the focal length of the entire system.