Zoom lens system

Claim 2

A zoom lens system that comprises, in order from the object side, a first lens group that includes at least three lens elements and that has a negative overall power and a second lens group that includes at least four lens elements, said second lens group being composed of, in order from the object side, a positive lens unit, a cemented lens having a positive lens element and a negative lens element, and a positive lens element, said second lens group element having a positive overall power, said zoom lens system being capable of changing its focal length by adjusting the overall distance between the first and the second lens groups to achieve a zoom ratio greater than 2.5, and said zoom lens system satisfying the following conditions (a) and (b):

Claim 3

05<n W -n P <0.25 (a) 10<ν.sub.P -ν.sub.N <35 (b) where n P is the refractive index at the d-line of the positive lens element of the cemented lens in the second lens group, n W is the refractive index at the d-line of the negative lens element of the cemented lens in the second lens group, ν P is the Abbe number of the cemented positive lens element at the d-line, and ν N is the Abbe number of the cemented negative lens element at the d-line.

Claim 4

05<n W -n P <0.25 (a) 10<ν.sub.P -ν.sub.N <35 (b) where n P is the refractive index at the d-line of the positive lens element of the cemented lens in the second lens group, n W is the refractive index at the d-line of the negative lens element of the cemented lens in the second lens group, ν P is the Abbe number of the cemented positive lens element at the d-line, and ν N is the Abbe number of the cemented negative lens element at the d-line.

Claim 5

The zoom lens system according to claim 1 which further satisfies the following condition (c): 0.5<|r.sub.C /f2|<10 (c) where f2 is the focal length of the second lens group and r C is the radius of curvature of the interface between the positive and negative lens elements of the cemented lens in the second lens group.

Claim 6

The zoom lens system according to claim 1 wherein the first lens group comprises, in order from the object side, a negative meniscus lens element having a convex surface directed toward the object, a negative lens element and a positive lens element, said second lens group comprising, in order from the object side close to the first lens group, a positive lens unit, a cemented lens consisting of a positive and a negative lens element, and a positive lens element, said zoom lens system further satisfying the following conditions (d) and (e) 1.2<|f1/f.sub.W |<1.9 (n1+n2)/2<1.7 where f W is the focal length of the overall system at the wide-angle end, f1 is the focal length of the first lens group, and n1 and n2 are the refractive indices at the d-line of the two negative lens elements in the first lens group, with n1 referring to the lens element which is the closer to the object.

Claim 7

The zoom lens system according to claim 1 wherein the cemented lens of said second lens group is composed of a positive and a negative element and has a negative focal length.