Endoscope objective optical system

1. 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 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)

| r 1|−| r 2|+ d 1>1.8  (5)

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,

r1 is the radius of curvature of a surface of the third lens facing the object side,

r2 is the radius of curvature of the surface of the third lens facing the image side, and

d1 is the center thickness of the third lens.

2. The endoscope objective optical system according to claim 1 , wherein the endoscope objective optical system satisfies the following condition:

n 1<1.70.  (2′)