Patent ID: 7978423

Claim:
An objective optical system comprising, in order from an object side: a first unit having two lenses with negative refracting power and positive refracting power; a stop; and a second unit with positive refracting power, wherein the objective optical system satisfies the following condition: 3.0 <fb/fL where fb is a back focus of the objective optical system in air and fL is a focal length of the entire objective optical system, and wherein the objective optical system further satisfies at least one of the following conditions: 10 <LTL/fL 8 <Σd/fL 0.3 <df/dr <1.4 1.2 <D 12 /fL <3 where LTL is an overall length of the objective optical system, Σd is a distance from a most object-side lens surface to a most image-side lens surface in the objective optical system, df is spacing between an image-side surface of a most image-side lens in the first unit and the stop, dr is spacing between the stop and an object-side surface of a most object-side lens in the second unit, and D 12 is spacing between the first unit and the second unit.