Finite distance zoom lens system

Claim 1

In a finite distance zoom lens system composed, in order from the object side, of a first lens group having a negative focal length and a second lens group having a positive focal length, said first and second lens groups being mechanically moved to thereby vary the focal length while maintaining constant the distance between the object surface and the image surface, a finite distance zoom lens system comprising an improvement wherein said first lens group mainly serves to maintain constant the distance between the object surface and the image surface and said second lens group mainly serves to vary the magnification, said first lens group consisting, in order from the object side, of a first positive lens component, a first negative lens component and a second positive lens component, said first positive lens component including a positive lens having a surface on the image side which is convex to the image side, said first negative lens component including a single negative lens, said second positive lens component including a positive meniscus lens having a convex surface on the object side, said second lens group consisting of a third positive lens component, a second negative lens component and a fourth positive lens component, said third positive lens component including two positive lenses, each convex surface of which is directed to the object, said second negative lens component including a negative lens having on the image side a surface concave to the image side, said fourth positive lens component including two positive lenses, said finite distance zoom lens system satisfying the following conditions: ______________________________________ F.sub.∞ = 4.8˜4.5 f = 32.1˜50.2 ω = 22.6°˜13.8° NA = 0.1 L = 436.80 M = -0.096˜-0.158 ψ = 1.32˜0.98 Surface No. r d N ν ______________________________________ 1 -1322.943 4.50 1.72916 54.7 2 -140.878 5.51 First 3 1475.589 2.00 1.83400 37.2 Lens Group 4 29.209 5.80 5 33.591 4.50 1.80518 25.4 6 66.896 50.33˜3.00 7 25.748 2.70 1.80610 40.9 8 84.754 0.10 9 16.542 2.70 1.77250 49.6 10 49.702 1.68 Second 11 386.228 1.33 1.80518 25.4 Lens Group 12 13.520 4.68 13 -79.226 3.83 1.77250 49.6 14 -34.093 0.2 15 -372.101 2.50 1.80610 40.9 16 -76.432 ______________________________________ where, Mmax/Mmin is the variable power ratio; Mmax is the magnification at the high magnification side in the variable magnification range; Mmin is the magnification at the low magnification side in the variable magnification range; fmax is the overall focal length at the high magnification side; f I is the focal length of the first lens group; Δl I ,II is the amount of variation between the distance between the first and second lens groups; f 2 is the focal length of the image side surface of the first positive lens of the first lens component of the first lens group (f 2 =r 2 /1-n 1 ); r 2 is the radius of curvature of the image side surface of the first lens of the first positive lens component of the first lens group; n 1 is the refractive index, at the d-line, of the first positive lens of the first lens component of the first lens group; n II1 is the refractive index, at the d-line, of the first positive lens of the third lens component of the second lens group; and n II2 is the refractive index, at the d-line, of the image side lens of the fourth positive component of the second lens group Mmax/Mmin=1.621 fmax/f.sub.I =-0.465 Δl.sub.I,II /fmax=0.943 fmax/f.sub.2 =0.26 (n.sub.III +n.sub.II2)/2=1.789 where, r is the radius of curvature d is the lens thickness or the air gap, N is the refractive index at d-line ν is the Abbe number f is the overall focal length F.sub.∞ is the F-number with respect to the infinite object M is the magnification ω is half the view angle L is the distance between the object and the image NA is the numerical aperture and ψ is the pupil magnification.