Telescopic lens system having small telephoto ratio

Claim 1

A telescopic lens system comprising a front group of lenses and a rear group of lenses separated from said front group of lenses by a large air space, said front group of lenses consisting of a first biconvex lens, a second biconcave lens spaced apart from said first biconvex lens, a thirD biconvex lens spaced apart from said second biconcave lens by a small air space, and a fourth biconcave lens, said third and fourth lenses being substantially in contact, said rear group of lenses consisting of a fifth biconcave lens and a sixth biconcave lens, said fifth and sixth lenses being in contact or spaced apart by a small air space, the whole lens system being constructed such that the following five conditions 1. f234 < OR = -f, 2. r3 is about -0.57f, 3. 0.03f < OR = d2 < OR = 0.1f, 4. n3 - n4 < OR = 0.03 and ( theta 4 - theta 3/ Nu 3 Nu 4) < OR = 0.0015, and 5. n1 < OR = 1.55 and Nu 1 > OR = 60 are satisfied, where f234 is the composite focal length of the second, third and fourth lenses, f the composite focal length of the whole lens system ri, di, ni, theta i and vi (i 1, 2, 3. . . ) the radius of curvature of the ith lens surface, thickness of the ith lens or air space between the ith and (i +1)th lenses, refractive index, partial dispersion ratio (ng-nf/nF-nc) where ng . . . refractive index for g-line, nF . . . refractive index for F-line and nc . . . refractive index for c-line), and Abbe''s number of the ith lens, respectively.