US Patent 7773318 Objective optical system for endoscopes

An objective optical system for endoscopes is characterized by comprising, in order from its object side, a stop, a first lens and a second lens. The first lens comprises a positive meniscus lens concave on its object side and the second lens comprises a positive lens convex on its object side. The following conditions (1), (2), (3) and (4) are satisfied:−1.2<r1/f<−0.8  (1)−1.0<r2/f<−0.6  (2)2.1<f2/f<4.2  (3)nd1>1.65  (4)where r1 is the radius of curvature of the object-side surface of the first lens, r2 is the radius of curvature of the image-side surface of the first lens, f is the focal length of the whole optical system, f2 is the focal length of the second lens, and nd1 is the d-line refractive index of the first lens.