Patent ID: 8715170

Claim:
An endoscope objective optical system comprising, in order from an object side, a first lens comprising a negative single lens, a second lens comprising a positive single lens, an aperture stop, a third lens comprising a positive single lens, and a fourth lens comprising a positive combined lens, wherein the first lens has a flat surface facing the object side, the second lens has a flat surface facing an image side, and the third lens has a meniscus shape having a convex surface facing the image side, the endoscope objective optical system satisfying the following conditions: 0.3 <Df/f< 1.15 (1) n 1<1.79 (2) n 2 >n 1 (3) 0.6 <IH/f< 0.83 (4) wherein Df is the distance from a surface of the first lens facing the object side to the aperture stop, f is the focal length of the entire system, n1 is the refractive index of the first lens (d-line), n2 is the refractive index of the second lens (d-line), IH is the maximum image height of the entire system.