Objective lens system for endoscope

Abstract

An objective lens system for an endoscope includes first to fifth lens elements L.sub.1 to L.sub.5 arranged in this order from the front to the rear. An aperture stop is disposed between the second and third lens elements L.sub.2 and L.sub.3. The first lens element L.sub.1 is a negative lens concave to the rear, the second lens element L.sub.2 is a positive lens having a flat surface facing rearward, the third lens element L.sub.3 is a positive lens having a flat surface facing forward, the fourth lens element L.sub.4 is a positive lens having a flat surface facing forward and the fifth lens element L.sub.5 is a negative meniscus lens. The fourth and fifth lens elements L.sub.4 and L.sub.5 are cemented together to form a cemented lens having a positive refracting power. The following formulae (1) to (3) are satisfied, 0.75<.vertline.f.sub.1 .times.Bf/f.sup.2 .vertline.<1.45 (1) D.sub.i <.vertline.R.sub.i .vertline. (2) .upsilon..sub.2 <45 (3) wherein f.sub.1 represents the focal length of the first lens element L.sub.1, Bf represents the back focal length of the overall objective lens system, f represents the combined focal length of the overall objective lens system, D.sub.i represents the central thickness of an lens element L.sub.i having a convex face, R.sub.i represents the radius of curvature of the convex face of an lens element L.sub.i having a convex face and .upsilon..sub.2 represents the Abbe's number of the second lens element L.sub.2.

Dependencies are inferred from the claim text.