Patent ID: 7733583

Claim:
A small-diameter objective optical system comprising, in order from an object plane: a first lens group with positive refractive power, including at least one plano-convex lens whose convex surface faces an image plane; a second lens group with positive refractive power, including at least one concave lens; and a third lens group including a cemented lens of which a cemented surface has negative refractive power, wherein the focal length of the third lens group is larger than the focal length of the first lens group and conditional expression (1) below is satisfied: 0.2<( F 1 ×h 3 )/( F 3 ×h 1 )<0.3 (1) where F 1 is the focal length of the first lens group, F 3 is the focal length of the third lens group, h 1 is a beam diameter of a substantially collimated beam at the first lens group side of the second lens group, and h 3 is a beam diameter of a substantially collimated beam at the third lens group side of the second lens group.