Patent ID: 7466490

Claim:
An objective optical system for an endoscope having an object side and an image side and comprising, arranged along an optical axis in order from the object side, as follows: a front lens group having negative refractive power and consisting of a first lens component; a middle lens group having positive refractive power and including at least two lens components, each of said at least two lens components having positive refractive power; and a rear lens group having positive refractive power and consisting of one lens component; wherein only one lens component of said middle lens group moves in the direction of the optical axis during focusing; a stop for controlling image brightness is positioned within said middle lens group or between said front lens group and said middle lens group; and the following conditions are satisfied: 2 <|f UM /f TF |<10 −1.25 <f U1 /f TF <−0.6 where f UM is the focal length of said only one lens component; f TF is the focal length of the entire objective optical system for an endoscope focused at the far point on the object side; and f U1 is the focal length of said first lens component.