Zoom lens

Claim 2

A zoom lens comprising, in order from the object, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power and a fourth lens group having a positive refractive power, said first through fourth lens group being independently moved in an axial direction to vary the overall focal length, said zoom lens meeting the following conditions: (1) 0.25F W <F I <0.45F W , (2) 1.65<n IV , and (3) 1.2f BW <f BT <0.95f BM where F W is the overall focal length at the minimum end, F I is the focal length of the first lens group, n IV is the average refractive index, at d-line, of the lens elements constituting the fourth lens group, f BW and f BT are the backfocuses at the minimum and maximum ends of the overall focal length, respectively, f BM is the backfocus at an intermediate value of said overall focal length between said minimum and maximum ends.

Claim 3

The zoom lens according to claim 1, wherein said first lens group is composed of a first lens component of a negative meniscus lens, a second lens component of a positive lens and a third component of a positive lens, said second lens group is composed of a fourth lens component of a negative lens, a fifth lens component of a negative lens, a sixth lens component of a positive lens and a seventh lens component of a negative lens, said third lens group is composed of an eighth lens component of a negative lens and a ninth lens component of a positive lens, and said fourth lens group is composed of at least two positive lenses and at least two negative lenses, one of the positive lenses of the fourth lens group being located at the frontmost position of the fourth lens group and one of the negative lenses of the fourth lens group being located at the rearmost position of the fourth lens group, the zoom lens further meeting the following conditions: (4) (n 2 +n 3 )/2>1.75, (5) ν 7 -ν 6 >10, and (6) n L >1.8 where n 2 and n 3 are the refractive indexes of said second and third lens components at d-line, respectively, ν 6 and ν 7 are the Abbe numbers of said sixth and seventh lens components at d-line, respectively, and n L is the refractive index, at d-line, of the negative lens located at the rearmost position of the fourth lens group.

Claim 4

The zoom lens according to claim 1, wherein said fourth lens group is composed of, in order from the object, at least one positive lens, a single negative lens having a surface of a smaller radius of curvature facing the object, a positive len component including at least one positive lens, a single negative lens having a surface of a smaller radius of curvature facing an image, at least two positive lenses, and a single negative lens having a surface of a smaller radius of curvature facing the object, said zoom lens meeting the following condition: (7) 1.8<n IVn where n IVn is the average refractive index, at d-line, of the three negative lenses of the fourth lens group.

Claim 5

The zoom lens according to claim 2, meeting the following conditions: (1') 0.28F W <F I <0.4F W , (4') (n 2 +n 3 )/2>1.77, and (6') n L >1.85.

Claim 6

The zoom lens according to claim 3, meeting the following conditions: (1') 0.28F W <F I <0.4F W , and (7') 1.82<n IVn .

Claim 7

The zoom lens according to claim 5 further meeting the following chart: ______________________________________ F = 28.805 - 132.342 No. r d n ν ______________________________________ 1 505.700 2.00 1.80518 25.4 2 69.120 1.88 3 71.400 9.84 1.72916 54.7 4 -444.818 0.15 5 66.300 5.69 1.88300 40.8 6 175.741 variable 7 184.399 1.20 1.88300 40.8 8 18.240 5.70 9 -119.280 1.30 1.81600 46.6 10 57.091 0.10 11 28.002 7.41 1.75520 27.5 12 -28.002 1.20 1.81600 46.6 13 75.000 variable 14 26.470 2.00 1.75520 27.5 15 22.339 2.00 16 33.299 3.30 1.77250 49.7 17 154.400 variable 18 31.120 4.50 1.61800 63.4 19 -206.849 2.22 20 -32.889 1.20 1.80518 25.4 21 -181.168 0.20 22 32.889 9.39 1.62041 60.3 23 -52.500 0.30 24 260.000 1.30 1.83481 42.7 25 24.157 2.30 26 35.862 4.84 1.48749 70.1 27 -61.411 0.20 28 104.599 5.06 1.66680 33.0 29 -36.800 3.21 30 -19.380 1.30 1.88300 40.8 31 -88.200 ______________________________________ variable F distance 28.806 50.037 103.062 132.342 ______________________________________ d6 1.200 15.420 34.470 44.580 d13 18.580 11.540 5.360 3.200 d17 17.820 10.030 2.540 0.800 ______________________________________ F.sub.I = 0.31F.sub.W n.sub.IV = 1.702 f.sub.BT = 0.91f.sub.BM = 1.54f.sub.BW (n.sub.2 + n.sub.3)/2 = 1.806 ν.sub.7 -v.sub.6 = 19.1 n.sub.L = 1.883 n.sub.IVn = 1.841 ______________________________________

Claim 8

The zoom lens according to claim 5, further meeting the following chart: ______________________________________ F = 28.801 - 132.316 No. r d n ν ______________________________________ 1 270.359 2.00 1.80518 25.4 2 66.762 1.40 3 68.597 9.39 1.72916 54.7 4 -1277.450 0.15 5 66.300 6.00 1.81600 46.6 6 218.700 variable 7 242.198 1.20 1.88300 40.8 8 18.768 5.70 9 -117.780 1.30 1.83481 42.7 10 50.845 0.10 11 28.598 7.41 1.80518 25.4 12 -28.598 1.20 1.88300 40.8 13 79.199 variable 14 27.300 2.00 1 80518 25.4 15 23.370 2.00 16 35.000 3.30 1.77250 49.7 17 349.704 variable 18 29.989 4.50 1.61800 63.4 19 6450.000 2.69 20 -31.514 1.20 1.80518 25.4 21 -226.050 0.20 22 34.871 8.80 1.56873 63.2 23 -54.300 0.30 24 118.902 1.30 1.83481 42.7 25 25.970 2.30 26 39.900 4.88 1.48749 70.1 27 -59.800 0.20 28 136.110 5.06 1.67270 32.1 29 -37.200 4.42 30 -19.600 1.30 1.88300 40.8 31 -65.243 ______________________________________ variable F distance 28.801 49.996 103.041 132.316 ______________________________________ d6 1.200 14.850 31.880 42.180 d13 19.210 11.910 5.270 3.200 d17 17.820 9.580 2.420 0.800 ______________________________________ F.sub.I = 0.32F.sub.W n.sub.IV = 1.696 f.sub.BT = 0.89f.sub.BM = 1.58f.sub.BW (n.sub.2 + n.sub.3)/2 = 1.773 ν.sub.7 - ν.sub.6 = 15.4 n.sub.L = 1.883 n.sub.IVn = 1.841 ______________________________________

Claim 9

The zoom lens according to claim 4 further meeting the following chart: ______________________________________ F = 28.799 - 132.000 No. r d n ν ______________________________________ 1 442.230 2.00 1.84666 23.9 2 64.265 0.66 3 67.343 8.80 1.88300 40.8 4 2219.557 0.15 5 62.922 6.30 1.81600 46.6 6 215.088 variable 7 79.830 1.60 1.88300 40.8 8 17.527 5.96 9 -132.071 2.00 1.83481 42.7 10 35.927 0.10 11 25.661 7.41 1.71736 29.5 12 -17.565 1.50 1.83481 42.7 13 134.848 variable 14 29.523 1.50 1.84666 23.9 15 27.524 2.00 16 47.276 2.23 1.83481 42.7 17 464.510 variable 18 25.241 7.44 1.72916 54.7 19 -59.424 1.60 20 -31.754 8.43 1.84666 23.9 21 -14.981 1.30 1.76180 27.1 22 39.641 2.20 23 -181.678 3.23 1.83481 42.7 24 -39.108 0.20 25 47.277 10.06 1.48749 70.1 26 -23.660 1.00 27 -18.441 1.60 1.88300 40.8 28 -54.267 ______________________________________ variable F distance 28.799 50.000 103.000 132.000 ______________________________________ d6 0.804 15.060 28.638 43.895 d13 15.195 8.750 2.795 1.999 d17 16.603 10.798 3.736 0.800 ______________________________________ F.sub.I = 0.31F.sub.W n.sub.IV = 1.757 f.sub.BT = 0.75f.sub.BM = 1.43f.sub.BW (n.sub.2 + n.sub.3)/2 = 1.806 ν.sub.7 - ν.sub.6 = 14.6 n.sub.L = 1.883 ______________________________________

Claim 10

The zoom lens according to claim 4 further meeting the following chart: ______________________________________ F = 28.792 - 131.983 No. r d n ν ______________________________________ 1 257.656 2.00 1.84666 23.9 2 63.576 9.10 1.72916 54.7 3 -555.009 0.15 4 57.585 5.80 1.88300 40.8 5 158.336 variable 6 194.661 1.20 1.88300 40.8 7 17.157 5.96 8 -111.536 1.30 1.88300 40.8 9 59.238 0.10 10 26.088 7.41 1.78470 26.2 11 -20.752 1.20 1.88300 40.8 12 66.129 variable 13 27.122 1.50 1.84666 23.9 14 24.218 2.00 15 38.908 2.80 1.61800 63.4 16 -278.161 variable 17 29.183 6.95 1.80400 46.6 18 -72.819 1.60 19 -38.303 13.22 1.80518 25.4 20 38.079 2.20 21 137.720 3.23 1.48749 70.1 22 -35.268 0.20 23 80.076 5.80 1.60342 38.0 24 -23.995 2.46 25 -19.152 1.30 1.88300 40.8 26 -60.514 ______________________________________ variable F distance 28.792 49.991 102.978 131.983 ______________________________________ d5 1.197 14.609 28.195 36.892 d12 17.381 11.386 5.101 3.197 d16 17.059 10.276 3.007 0.794 ______________________________________ F.sub.I = 0.37F.sub.W n.sub.IV = 1.717 f.sub.BT = 0.85f.sub.BM = 1.40f.sub.BW (n.sub.2 + n.sub.3)/2 = 1.806 ν.sub.7 - ν.sub.6 = 14.6 n.sub.L = 1.883 ______________________________________

Claim 11

The zoom lens according to claim 5 further meeting the following chart: ______________________________________ F = 28.820 - 132.448 No. r d n ν ______________________________________ 1 377.370 2.00 1.80518 25.4 2 69.147 1.20 3 70.160 9.77 1.72916 54.7 4 -533.185 0.15 5 69.120 5.39 1.88300 40.8 6 178.352 variable 7 183.798 1.20 1.88300 40.8 8 18.372 5.70 9 -116.421 1.30 1.83481 42.7 10 57.600 0.10 11 27.915 1.00 1.80440 39.6 12 21.000 6.61 1.75520 27.5 13 -29.210 1.00 1.81554 44.4 14 76.700 variable 15 26.946 2.00 1.75520 27.5 16 22.606 2.00 17 33.306 3.30 1.75520 49.6 18 173.952 variable 19 31.800 4.50 1.61800 63.4 20 -289.338 2.36 21 -32.000 1.20 1.80518 25.4 22 -195.000 0.20 23 34.040 7.12 1.67000 57.4 24 -30.134 2.00 1.56873 63.1 25 -96.900 0.30 26 165.100 1.30 1.88300 40.8 27 25.064 2.30 28 37.980 5.08 1.48749 70.1 29 -51.300 0.20 30 193.485 5.06 1.66680 33.0 31 -36.395 4.32 32 -19.080 1.30 1.88300 40.8 33 -56.405 ______________________________________ variable F distance 28.820 50.020 103.075 132.448 ______________________________________ d6 1.187 13.367 35.417 44.753 d14 18.624 11.032 5.300 3.207 d18 17.831 9.590 2.630 0.781 ______________________________________ F.sub.I = 0.30F.sub.W n.sub.IV = 1.698 f.sub.BT = 0.94f.sub.BM = 1.59f.sub.BW n.sub.IVn = 1.857 ______________________________________

Claim 12

The zoom lens according to claim 1, wherein f BM is the backfocus at 0.78 times the maximum overall focal lenth.