Variable power copying lens system

Claim 1

A variable power copying lens system comprising, in order from the object side, a first lens group having a negative focal length, a second lens group having a positive focal length and a third lens group having a negative focal length, a distance between said first and second lens groups and a distance said second and third lens groups being variable while said lens system is moved to thereby maintain the distance between an object surface and an image surface constant, said first and third lens groups mainly functioning to maintain said distance between said object surface and said image surface constant, and movement of said second lens group mainly providing a variable power effect, said first lens group comprising, in order from the object side, a negative meniscus lens convex on the object side and a positive meniscus lens convex on the object side, said second lens group comprising substantially a lens type used in a fixed focus copying lens and said second lens having a stop diaphragm therein, said second lens comprising, in order from said stop diaphragm on the object side, a positive meniscus lens concave toward said stop diaphragm and a cemented lens including a negative meniscus lens concave toward said stop diaphragm and a positive meniscus lens concave toward said stop diaphragm, and in order from said stop diaphragm on the image side, a positive meniscus lens concave toward said stop diaphragm and a cemented lens including a negative meniscus lens concave toward said stop diaphragm and a positive meniscus lens concave toward said stop diaphragm, said third lens group comprising, in order from the image side, a negative meniscus lens convex on the image side and a positive meniscus lens convex on the image side, said lens system satisfying: ______________________________________ F = 1:5.7 f = 245.3 ˜ 256.0 ˜ 248.0 ω = 13.8° ˜ 16.3° ˜ 14.1° NA = 0.053 ˜ 0.044 ˜ 0.036 m = -0.67 ˜ -1 ˜ -1.414 H.sub.max = Y.sub.max = 150 ______________________________________ lens no. r d N ν ______________________________________ 1 107.969 4.00 1.51633 64.1 2 54.495 7.07 3 54.739 8.00 1.51823 59.0 4 80.535 3.00 5 51.496 9.10 1.69680 55.5 6 114.028 9.36 1.60342 38.0 7 40.394 5.00 8 65.366 4.50 1.65160 58.6 9 121.506 30.10 10 -149.350 4.50 1.65160 58.6 11 -74.670 5.00 12 -42.391 11.05 1.57501 41.5 13 -136.193 9.10 1.67000 57.4 14 -56.934 3.00 15 -97.001 8.00 1.51823 59.0 16 -54.974 3.10 17 -54.650 4.00 1.55963 61.2 18 -114.084 ______________________________________ magnifi- cation variable distance -1 -0.67 -1.414 ______________________________________ d.sub.4 3.00 17.997 14.174 d.sub.14 3.00 17.997 14.174 ______________________________________ (1) M.sub. max /M.sub.min = 2.11 (2) f.sub.max /f.sub.I = -0.335 (3) f.sub.max /f.sub.III = -0.253 (4) ΔD.sub.I,II /f.sub.max = 0.0586 (5) ΔD.sub.II,III /f.sub. max = 0.0586 (6) r.sub.IP /f.sub.max = r.sub.3 /f.sub.max = 0.214 (7) |r.sub.IIIP |/f.sub.max = |r.sub.1 6 |/f.sub.max = 0.215 ______________________________________ wherein: f is an overall focal lens of said lens system, ω is a half viewing angle of a primary light ray, F is a F-number at an infinite object distance, m is a magnification factor, M max /M min is the variable power ratio, and NA is an aperture number defined by NA=1/[2F(1+|m|)], H max is a maximum height of an object, y max is a maximum height of an image, r i is a radius of curvature of an i-th lens surface, d i is a lens thickness or a space between adjacent lenses, N is a refractive power at an α-line and ν is an Abbe number.