Patent ID: 9709796
Date: 2017-07-18
CPC Classifications: G02B

Claim:
1. An objective lens for endoscopes consisting of, in order from the object side to the image side: a front group; an aperture stop; and a rear group having a positive refractive power as a whole; the front group consisting of; in order from the object side to the image side, a first lens having a negative refractive power, in which the absolute value of the radius of curvature of the surface toward the image side thereof is less than the absolute value of the radius of curvature of the surface toward the object side thereof, and at least one plane parallel plate; the rear group consisting of, in order from the object side to the image side, a second lens having a positive refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power; the third lens and the fourth lens being cemented together to constitute a cemented lens; the coupling surface of the cemented lens being convex toward the image side; and all of Conditional Formulae (1) through (3) below being satisfied: wherein f is the focal length of the entire lens system, k is the total number of plane parallel plates within the front group, i is a natural number within a range from 1 to k, ti is the thickness of an ith plane parallel plate from the object side within the front group, Ndi is the refractive index with respect to the d line of an ith plane parallel plate from the object side within the front group, vdi is the Abbe's number with respect to the d line of an ith plane parallel plate from the object side within the front group, fl is the focal length of the first lens, Dsc is the distance along the optical axis from the aperture stop to the coupling surface of the cemented lens, and RC is the radius of curvature of the coupling surface of the cemented lens.