Patent ID: 8824067

Claim:
An endoscope objective optical system comprising, in order from an object side: a first group; an aperture stop; and a second group, wherein the first group has positive power and is constituted of a negative first lens whose surface on the object side is flat and a positive second lens; the second group has positive power and is constituted of a combined lens formed of a positive third lens and a negative fourth lens; and the following Conditional Expressions (1) to (3) are satisfied: 1.2 <f 31 /f< 1.55, (1) −2.8 <f 32 /f<− 1.98, and (2) 0.38 <|R 4 +R 3 |/|R 4 −R 3|<0.77, (3) where f 31 is a focal length of the third lens, f 32 is a focal length of the fourth lens, f is a focal length of the entire system, R 3 is a radius of curvature of the object-side surface of the second lens, and R 4 is a radius of curvature of the image-side surface of the second lens.