Patent ID: 8107170

Claim:
An objective optical system comprising, in sequence from an object side: a first group having positive refractive power; a second group having positive refractive power; a third group having negative refractive power; a fourth group having positive refractive power; and a fifth group having positive refractive power, the first group including a plano-convex lens with a convex surface facing towards an image side, the second group including a lens whose extreme-object-side lens surface is a convex surface facing towards the object side, the third group including a lens whose extreme-object-side lens surface is a convex surface facing towards the object side, the fourth group including a lens disposed on the extreme object side, whose image-side lens surface is a convex surface facing towards the image side, and a lens disposed on the extreme image side, whose object-side lens surface is a convex surface facing towards the object side, the fifth group including a combined lens formed by joining a convex lens and a concave lens, a joined surface thereof having negative refractive power, wherein the objective optical system is designed to focus at infinity and has an intermediate image plane between the fourth group and the fifth group.