Objective lens system for endoscope

An objective lens system for an endoscope comprising from the subject end to the image end a meniscus lens element of negative optical power having a concave image side surface, a plano-convex lens element of positive optical power having a convex subject side surface, an aperture element, a plano-convex lens element of positive optical power having a convex image side surface and a biconvex lens element of positive optical power, said objective lens systems satisfying the following relations:

• Bf'>1.5f'
• 1.80<D/R2<2.40
• .mu..sub.2 >45.0
• .mu..sub.3 >50.0
• .mu..sub.4 >50.0
• where Bf' designates the back focal length of the objective lens system, f' designates the overall focal length of the objective lens system, D designates the value of d.sub.23 +(d.sub.34 /n.sub.2)+d.sub.4S, where d.sub.23, d.sub.34 and d.sub.4S are the axial distance between the second and third lens surfaces, the third and fourth lens surfaces and the fourth lens surface and the image side surface of the aperture element, respectively, and n.sub.2 is the index of refraction of the second lens element, R2 is the radius of curvature of the second lens surface, and .mu..sub.2, .mu..sub.3 and .mu..sub.4 are the dispersion of the second, third and fourth lens elements as measured by the Abbe number, respectively.