Patent ID: 8896940

Claim:
An objective optical system, comprising: a first lens group having a negative refractive power; an aperture stop; and a second lens group having a positive refractive power, provided in this order from an object side; the first lens group comprising a negative single lens and a cemented lens formed by cementing a positive lens and a negative lens together, provided in this order from the object side; the second lens group comprising a positive single lens and a cemented lens formed by cementing a positive lens and a negative lens together, provided in this order from the object side; and the objective optical system satisfying Conditional Formulae (1) and (5b) below: 15.0 <νd ( RN )<18.6 (1) −1.4 <f 1 /f<− 1.2 (5b) wherein νd(RN) is the Abbe's number of the negative lens in the cemented lens within the second lens group with respect to the d line, f1 is the focal length of the lens provided most toward the object side, and f is the focal length of the entire lens system.