Zoom lens system

Abstract

A zoom lens system of the invention is double telephoto type zoom lens system having a positive/negative/positive/negative refractive power arrangement. During zooming from the wide angle side toward the telephoto side, the first through fourth lens units move such that the distance between the first and second lens units increases and the distance between the third and fourth lens units decreases. The refractive power of the first lens unit and the refractive power of the fourth lens unit are appropriately prescribed so that an adequate back focal lens distance is preserved.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to a zoom lens system, and more particularly, to a high-power zoom lens system suitable as a photo-taking lens used in a camera.

2. Description of the Related Art

Conventionally proposed four-component zoom lens systems having a positive lens unit, a negative lens unit, a positive lens unit and a negative lens unit in that order have approximately a .times.3 zoom ratio.

However, with said conventional model, if the zoom ratio is further increased, an adequate back focal distance cannot be preserved. In addition, if the zoom ratio is made .times.3 or larger, transverse chromatic aberrations increase when the zoom lens system is in the telephoto condition.

SUMMARY OF THE INVENTION

The first object of the present invention is to provide a high-power zoom lens system in which an adequate back focal distance is preserved.

The second object of the present invention is to provide a high-power zoom lens system in which transverse chromatic aberrations are kept small throughout the entire zoom range.

In order to achieve these and other objects, the zoom lens system of the present invention comprises, from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a negative refractive power, wherein the first through fourth lens units move such that the distance between the first and second lens units increases and the distance between the third and fourth lens units decreases during zooming from the wide angle toward the telephoto end of the zoom range and wherein the refractive power of the first lens unit and the refractive power of the fourth lens unit are appropriately prescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings.

FIG. 1 is a cross-sectional view showing the construction of a lens system of a first embodiment.

FIG. 2 is a cross-sectional view showing the construction of a lens system of a second embodiment.

FIG. 3 is a cross-sectional view showing the construction of a lens system of a third embodiment.

FIG. 4 is a cross-sectional view showing the construction of a lens system of a fourth embodiment.

FIG. 5 is a cross-sectional view showing the construction of a lens system of a fifth embodiment.

FIG. 6 is a cross-sectional view showing the construction of a lens system of a sixth embodiment.

FIGS. 7A-7C show aberrations in the first embodiment in the shortest focal length condition.

FIGS. 8A-8C show aberrations in the first embodiment in the middle focal length condition.

FIGS. 9A-9C show aberrations in the first embodiment in the longest focal length condition.

FIGS. 10A-10C show aberrations in the second embodiment in the shortest focal length condition.

FIGS. 11A-11C show aberrations in the second embodiment in the middle focal length condition.

FIGS. 12A-12C show aberrations in the second embodiment in the longest focal length condition.

FIGS. 13A-13C show aberrations in the third embodiment in the shortest focal length condition.

FIGS. 14A-14C show aberrations in the third embodiment in the middle focal length condition.

FIGS. 15A-15C show aberrations in the third embodiment in the longest focal length condition.

FIGS. 16A-16C show aberrations in the fourth embodiment in the shortest focal length condition.

FIGS. 17A-17C show aberrations in the fourth embodiment in the middle focal length condition.

FIGS. 18A-18C show aberrations in the fourth embodiment in the longest focal length condition.

FIGS. 19A-19C show aberrations in the fifth embodiment in the shortest focal length condition.

FIGS. 20A-20C show aberrations in the fifth embodiment in the middle focal length condition.

FIGS. 21A-21C show aberrations in the fifth embodiment in the longest focal length condition.

FIGS. 22A-22C show aberrations in the sixth embodiment in the shortest focal length condition.

FIGS. 23A-23C show aberrations in the sixth embodiment in the middle focal length condition.

FIGS. 24A-24C show aberrations in the sixth embodiment in the longest focal length condition.

FIGS. 25A-25C show transverse chromatic aberrations in the first embodiment.

FIGS. 26A-26C show transverse chromatic aberrations in the second embodiment.

FIGS. 27A-27C show transverse chromatic aberrations in the sixth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The first through sixth embodiments of the zoom lens system of the present invention are shown in Tables 1 through 6.

In each embodiment, ri (i=1, 2, 3, . . .) represents the radius of curvature of the ith lens surface from the object side; di (i=1, 2, 3, . . .) represents the ith axial distance from the object side; and Ni (i=1, 2, 3, . . .) and .nu.i (i=1, 2, 3, . . .) represent the refractive power and the Abbe number, to the d-line, of the ith lens from the object side, respectively. In addition, focal length f and F-number FNO of the entire system are also indicated for the shortest focal length condition (W), the middle focal length condition (M) and the longest focal length condition (T).

TABLE 1______________________________________f = 102.6.about.200.1.about.390.4, FNO = 4.60.about.6.30.about.6.60radius of axial refractivecurvature distance index Abbe number______________________________________r1 157.857 d1 3.300 N1 1.74950 .nu.1 35.04r2 88.700 d2 8.500 N2 1.49310 .nu.2 83.58r3 -341.041 d3 0.200r4 91.443 d4 4.998 N3 1.49310 .nu.3 83.58r5 295.776 d5 4.000.about.43.092.about.82.234r6 -107.238 d6 1.800 N4 1.71300 .nu.4 53.93r7 66.677 d7 5.207r8 93.472 d8 2.989 N5 1.75520 .nu.5 27.51r9 -622.960 d9 57.822.about.28.879.about.4.201r10 -66.172 d10 1.700 N6 1.71300 .nu.6 53.93r11 52.843 d11 2.566 N7 1.58144 .nu.7 40.89r12 105.077 d12 2.695r13 1495.908 d13 2.800 N8 1.48749 .nu.8 70.44r14 -46.707 d14 0.200r15 75.764 d15 1.700 N9 1.84666 .nu.9 23.82r16 37.959 d16 4.400 N10 1.51680 .nu.10 64.20r17 -91.581 d17 0.200r18 41.790 d18 4.311 N11 1.51680 .nu.11 64.20r19 -91.503 d19 25.612.about.15.463.about.0.999r20 135.341 d20 1.700 N12 1.75450 .nu.12 51.57r21 31.936 d21 4.291r22 -147.557 d22 4.500 N13 1.67339 .nu.13 29.25r23 -21.200 d23 1.800 N14 1.71300 .nu.14 53.93r24 316.660.SIGMA.d = 147.290.about.147.291.about.147.290______________________________________ TABLE 2______________________________________f = 102.5.about.200.0.about.390.1, FNO = 4.60.about.6.30.about.6.62radius of axial refractivecurvature distance index Abbe number______________________________________r1 157.213 d1 3.300 N1 1.74950 .nu.1 35.04r2 88.700 d2 9.090 N2 1.49310 .nu.2 83.58r3 -302.925 d3 0.200r4 87.864 d4 4.989 N3 1.49310 .nu.3 83.58r5 235.117 d5 3.863.about.42.578.about.80.664r6 -104.353 d6 1.800 N4 1.71300 .nu.4 53.93r7 71.982 d7 5.202r8 104.848 d8 3.092 N5 1.75520 .nu.5 27.51r9 -486.232 d9 56.870.about.28.353.about.4.600r10 -93.851 d10 1.700 N6 1.71300 .nu.6 53.93r11 60.019 d11 1.982r12 63.072 d12 2.988 N7 1.61293 .nu.7 36.96r13 113.513 d13 2.683r14 250.350 d14 3.188 N8 1.48749 .nu.8 70.44r15 -58.125 d15 0.200r16 69.535 d16 1.700 N9 1.84666 .nu.9 23.82r17 36.860 d17 4.791 N10 1.51680 .nu.10 64.20r18 -96.135 d18 0.200r19 44.043 d19 3.692 N11 1.48749 .nu.11 70.44r20 -150.605 d20 25.573.about.15.355.about.0.999r21 142.369 d21 1.700 N12 1.75450 .nu.12 51.57r22 32.415 d22 4.278r23 -154.466 d23 4.486 N13 1.67339 .nu.13 29.25r24 -21.300 d24 1.800 N14 1.71300 .nu.l4 53.93r25 283.830.SIGMA.d = 149.369.about.149.348.about.149.326______________________________________ TABLE 3______________________________________f = 102.5.about.200.1.about.390.3, FNO = 4.60.about.6.20.about.6.54radius of axial refractivecurvature distance index Abbe number______________________________________r1 142.710 d1 3.296 N1 1.74950 .nu.1 35.04r2 84.932 d2 8.300 N2 1.49310 .nu.2 83.58r3 -494.257 d3 0.200r4 102.571 d4 4.700 N3 1.49310 .nu.3 83.58r5 355.074 d5 4.000.about.43.258.about.88.212r6 -97.704 d6 1.400 N4 1.71300 .nu.4 53.93r7 60.846 d7 5.167r8 79.186 d8 2.000 N5 1.75520 .nu.5 27.51r9 -1716.767 d9 58.602.about.30.789.about.3.801r10 878.472 d10 2.410 N6 1.51680 .nu.6 64.20r11 -109.926 d11 1.536r12 -148.680 d12 1.480 N7 1.80741 .nu.7 31.59r13 59.466 d13 2.785r14 145.246 d14 2.589 N8 1.48749 .nu.8 70.44r15 -96.642 d15 0.100r16 82.480 d16 2.901 N9 1.48749 .nu.9 70.44r17 -132.272 d17 0.100r18 51.542 d18 3.750 N10 1.48749 .nu.10 70.44r19 -104.191 d19 30.410.about.18.966.about.0.999r20 76.034 d20 1.400 N11 1.75450 .nu.11 51.57r21 30.217 d21 3.599r22 -108.973 d22 4.090 N12 1.67339 .nu.12 29.25r23 -22.122 d23 1.500 N13 1.71300 .nu.13 53.93r24 480.277.SIGMA.d = 146.314.about.146.315.about.146.314______________________________________ TABLE 4______________________________________f = 102.6.about.200.3.about.391.1, FNO = 4.60.about.5.80.about.6.90radius of axial refractivecurvature distance index Abbe number______________________________________r1 188.993 d1 2.500 N1 1.85000 .nu.1 40.04r2 88.006 d2 8.000 N2 1.49310 .nu.2 83.58r3 -210.439 d3 0.200r4 79.791 d4 4.600 N3 1.48749 .nu.3 70.44r5 276.324 d5 4.000.about.40.951.about.83.231r6 -84.282 d6 1.400 N4 1.71300 .nu.4 53.93r7 65.658 d7 4.350r8 86.940 d8 2.400 N5 1.75520 .nu.5 27.51r9 -352.847 d9 52.349.about.27.963.about.3.801r10 -554.394 d10 1.500 N6 1.84666 .nu.6 23.83r11 48.833 d11 2.700r12 256.480 d12 3.000 N7 1.59270 .nu.7 35.45r13 -81.803 d13 1.400r14 80.485 d14 2.400 N8 1.51680 .nu.8 64.20r15 -253.109 d15 0.100r16 50.413 d16 4.500 N9 1.48749 .nu.9 70.44r17 -60.077 d17 31.682.about.19.117.about.0.999r18 195.848 d18 1.200 N10 1.75450 .nu.10 51.57r19 33.636 d19 3.300r20 -222.323 d20 3.200 N11 1.67339 .nu.11 29.25r21 -26.697 d21 1.200 N12 1.69680 .nu.12 56.47r22 429.991.SIGMA.d = 135.981.about.135.981.about.135.981______________________________________ TABLE 5______________________________________f = 102.6.about.200.3.about.391.4, FNO = 4.60.about.6.80.about.6.90radius of axial refractivecurvature distance index Abbe number______________________________________r1 169.174 d1 2.500 N1 1.85000 .nu.1 40.04r2 80.693 d2 7.800 N2 1.49310 .nu.2 83.58r3 -272.468 d3 0.200r4 76.426 d4 5.700 N3 1.48749 .nu.3 70.44r5 553.897 d5 3.800.about.38.346.about.75.758r6 -90.962 d6 1.400 N4 1.80420 .nu.4 46.50r7 47.050 d7 3.400r8 53.036 d8 2.700 N5 1.74000 .nu.5 28.26r9 -297.497 d9 49.121.about.25.332.about.3.801r10 -246.833 d10 1.500 N6 1.84666 .nu.6 23.83r11 45.971 d11 2.700r12 170.154 d12 2.700 N7 1.59270 .nu.7 35.45r13 -75.384 d13 1.400r14 80.001 d14 2.700 N8 1.51680 .nu.8 64.20r15 -141.281 d15 0.100r16 46.799 d16 4.500 N9 1.48749 .nu.9 70.44r17 -58.861 d17 27.638.about.16.880.about.0.999r18 198.793 d18 1.200 N10 1.80420 .nu.10 46.50r19 31.120 d19 3.300r20 -222.323 d20 4.300 N11 1.67339 .nu.11 29.25r21 -21.349 d21 1.200 N12 1.69680 .nu.12 56.47r22 298.186.SIGMA.d = 129.859.about.129.858.about.129.858______________________________________ TABLE 6______________________________________f = 76.9.about.154.4.about.292.5, FNO = 4.60.about.5.87.about.5.87radius of axial refractivecurvature distance index Abbe number______________________________________r1 72.631 d1 5.700 N1 1.48749 .nu.1 70.44r2 720.269 d2 0.100r3 77.246 d3 2.500 N2 1.72342 .nu.2 37.99r4 39.588 d4 8.680 N3 1.48749 .nu.3 70.44r5 359.905 d5 2.310.about.34.360.about.63.275r6 -134.793 d6 1.400 N4 1.77250 .nu.4 49.62r7 32.710 d7 6.000r8 44.493 d8 2.550 N5 1.75000 .nu.5 25.14r9 248.917 d9 32.751.about.14.826.about.5.029r10 -38.615 d10 1.400 N6 1.69680 .nu.6 55.46r11 48.306 d11 2.900 N7 1.59551 .nu.7 39.22r12 -96.873 d12 6.600r13 -206.323 d13 2.700 N8 1.51823 .nu.8 58.96r14 -42.442 d14 0.200r15 .infin. d15 1.400 N9 1.75000 .nu.9 25.14r16 35.900 d16 4.700 N10 1.51823 .nu.10 58.96r17 -61.261 d17 0.200r18 45.763 d18 3.910 N11 1.51823 .nu.11 58.96r19 -75.456 d19 33.954.about.19.829.about.0.711r20 3594.536 d20 2.700 N12 1.75000 .nu.12 25.14r21 -54.749 d21 1.370r22 -51.853 d22 1.400 N13 1.77250 .nu.13 49.62r23 42.152.SIGMA.d = 125.425.about.125.425.about.125.425______________________________________

FIGS. 1 through 6 show the first through sixth embodiments in the shortest focal length condition. The first through sixth embodiments each comprises, from the object side, first lens unit Gr1 having a positive refractive power, second lens unit Gr2 having a negative refractive power, third lens unit Gr3 having a positive refractive power and fourth lens unit Gr4 having a negative refractive power. Third lens unit Gr3 comprises, from the object side, front lens unit Gr3F having a negative refractive power and rear lens unit Gr3R having a positive refractive power.

Loci m1 through m4 in FIGS. 1 through 6 show the movement of first lens unit Gr1, second lens unit Gr2, third lens unit Gr3 and fourth lens unit Gr4, respectively, during zooming from the shortest focal length condition (W) to the longest focal length condition (T). As shown in each drawing, during zooming from the shortest focal length condition (W) to the longest focal length condition (T), first through fourth lens units Grl through Gr4 move such that the distance between first lens unit Gr1 and second lens unit Gr2 increases while the distance between second lens unit Gr2 and third lens unit Gr3 and the distance between third lens unit Gr3 and fourth lens unit Gr4 decrease.

In the first embodiment, first lens unit Gr1 comprises (1) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconvex lens, and (2) a positive meniscus lens having a convex surface on the object side; second lens unit Gr2 comprises (1) a biconcave lens and (2) a biconvex lens; third lens unit Gr3 comprises (1) front lens unit Gr3F, comprising a cemented lens of (i) a biconcave lens and (ii) a positive meniscus lens having a convex surface on the object side, and (2) rear lens unit Gr3R, comprising (i) a biconvex lens, (ii) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconvex lens, and (iii) a biconvex lens; and fourth lens unit Gr4 comprises (1) a negative meniscus lens having a concave surface on the image side and (2) a cemented lens of a positive meniscus lens having a convex surface on the image side and a biconcave lens.

In the second embodiment, first lens unit Gr1 comprises (1) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconvex lens, and (2) a positive meniscus lens having a convex surface on the object side; second lens unit Gr2 comprises (1) a biconcave lens and (2) a biconvex lens; third lens unit Gr3 comprises (1) front lens unit Gr3F comprising (i) a biconcave lens and (ii) a positive meniscus lens having a convex surface on the object side, and (2) rear lens unit Gr3R comprising (i) a biconvex lens, (ii) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconcave lens and (iii) a biconvex; and fourth lens unit Gr4 comprises (1) a negative meniscus lens having a concave surface on the image side and (2) a cemented lens of a positive meniscus lens having a convex surface on the image side and a biconcave lens.

In the third embodiment, first lens unit Gr1 comprises (1) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconvex lens and (2) a positive meniscus lens having a convex surface on the object side; second lens unit Gr2 comprises (1) a biconcave lens and (2) a biconvex lens; third lens unit Gr3 comprises (1) front lens unit Gr3F comprising (i) a biconvex lens and (ii) a biconcave lens and (2) rear lens unit Gr3R comprising three biconvex lenses; and fourth lens unit Gr4 comprises (1) a negative meniscus lens having a concave surface on the image side and (2) a cemented lens of a positive meniscus lens having a convex surface on the image side and a biconcave lens.

In the fourth embodiment, first lens unit Grl comprises (1) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconvex lens and (2) a positive meniscus lens having a convex surface on the object side; second lens unit Gr2 comprises (1) a biconcave lens and (2) a biconvex lens; third lens unit Gr3 comprises (1) front lens unit Gr3F comprising (i) a biconcave lens and (ii) a biconvex lens and (2) a rear lens unit Gr3R comprising two biconvex lenses; and fourth lens unit Gr4 comprises (1) a negative meniscus lens having a concave surface on the image side and (2) a cemented lens of a positive meniscus lens having a convex surface on the image side and a biconcave lens.

In the fifth embodiment, first lens unit Gr1 comprises (1) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconvex lens and (2) a positive meniscus lens having a convex surface on the object side; second lens unit Gr2 comprises (1) a biconcave lens and (2) a biconvex lens; third lens unit Gr3 comprises (1) front lens unit Gr3F comprising a biconcave lens and (2) rear lens unit Gr3R comprising three biconvex lenses; and fourth lens unit Gr4 comprises (1) a negative meniscus lens having a concave surface on the image side and (2) a cemented lens of a positive meniscus lens having a convex surface on the image side and a biconcave lens.

In the sixth embodiment, first lens unit Gr1 comprises (1) a positive meniscus lens having a convex surface on the object side and (2) a cemented of a negative meniscus lens having a concave surface on the image side and a positive meniscus lens having a convex surface on the object side; second lens unit Gr2 comprises (1) a biconcave lens and (2) a positive meniscus lens having a convex surface on the object side; third lens unit Gr3 comprises (1) front lens unit Gr3F comprising a cemented lens of a biconcave lens and a biconvex lens and (2) rear lens unit Gr3R comprising (i) a positive meniscus lens having a convex surface on the image side (ii) a cemented lens of a negative meniscus lens having a concave surface on the image side and a biconvex lens and (iii) a biconvex lens; and fourth lens unit Gr4 comprises (1) a biconvex lens and (2) a biconcave lens.

FIGS. 7A through 7C, FIGS. 10A through 10C, FIGS. 13A through 13C, FIGS. 16A through 16C, FIGS. 19A through 19C and FIGS. 22A through 22C show aberrations in the first through sixth embodiments in the shortest focal length condition. FIGS. 8A through 8C, FIGS. 11A through 11C, FIGS. 14A through 14C, FIGS. 17A through 17C, FIGS. 20A through 20C and FIGS. 23A through 23C show aberrations in the first through sixth embodiments in the middle focal length condition. FIGS. 9A through 9C, FIGS. 12A through 12C, FIGS. 15A through 15C, FIGS. 18A through 18C FIGS. 21A through 21C and FIGS. 24A through 24C show aberrations in the first to sixth embodiments in the longest focal length condition.

In the drawings, solid line (d) represents an aberration with regard to the d-line while the dashed line (g) represents an aberration with regard to the g-line and the two-dot chain line (c) represents an aberration with regard to the c-line. The dotted line (SC) represents the sine condition. The dotted line (DM) and solid line (DS) represent astigmatisms on the meridional surface and the sagittal surface, respectively.

The preferred construction of the zoom lens system of the present invention will now be explained. The first through sixth embodiments meet each of the conditions explained below, and accordingly, various effects may be produced.

The first through sixth embodiments are all double-telephoto type zoom lens systems having a positive/negative/positive/negative refractive power arrangement and a .times.3 or larger zoom ratio.

In a zoom lens system of such a construction, it is desirable that the following condition (1) be met.

-0.3<.phi.1/.phi.4<-0.1 (1) where, .phi.1: the refractive power of the first lens unit, and .phi.4: the refractive power of the fourth lens unit.

As can be seen from condition (1), the refractive power of the first lens unit is characteristically much smaller than that of the fourth lens unit in a zoom lens system having a positive/negative/positive/negative refractive power arrangement. If the lower limit of condition (1) above is not met, or in other words, if the positive refractive power of the first lens unit increases or the negative refractive power of the fourth lens unit decreases, the system's refractive power arrangement deviates from the telephoto type refractive power arrangement when the system is in the telephoto condition. Consequently, the entire length of the system increases. In addition, if the refractive power of the first lens unit increases, compensation for chromatic aberration and spherical aberration in the telephoto condition becomes difficult. Conversely, if the upper limit of condition (1) is not met, or in other words, if the negative refractive power of the fourth lens unit increases, the system's refractive power arrangement becomes close to that of a telephoto type system. Consequently, the back focal distance in the wide angle condition becomes too short, making it difficult to preserve adequate back focal distance. In a conventional model, if the zoom ratio is increased to .times.3 or larger, it becomes impossible to preserve adequate back focal distance in the wide angle condition, which, in the embodiments of the present invention, is resolved by meeting condition (1) described above. Namely, by making the refractive power .phi.1 of first lens unit Gr1 relatively weaker than refractive power .phi.4 of fourth lens unit Gr4 as compared to the conventional model, adequate back focal distance can be preserved in the wide angle condition. On the other hand, in the telephoto condition, chromatic aberration and spherical aberration can be well compensated for. Further, because the refractive power arrangement is close to the telephoto type system's refractive power arrangement, the entire length of the system may be reduced.

In order to attain good balance over the system's entire length, as well as aberration compensation and back focal distance, it is preferred that the following condition (2) be met.

-20<(.phi.2/fW).times.100,000<-2 (2) where, .phi.2: the refractive power of second lens unit Gr2, and fW: the focal length of the entire system in the shortest focal length condition (W).

If the lower limit of condition (2) is not met, the system's construction becomes quite similar to the retro-focus type construction, and the system's length in the telephoto condition becomes too large. On the other hand, since the height of light entering third lens unit Gr3 becomes high in the wide angle condition, third lens unit Gr3 must have a strong refractive power. However, if the refractive power of third lens unit Gr3 is increased, spherical aberration compensation becomes difficult. Conversely, if the upper limit of condition (2) is not met, the system's construction becomes quite dissimilar to the retro-focus type construction, and the back focal distance becomes too short in the shortest focal length condition.

Further, it is preferred that the following condition (2') be met.

-8<(.phi.2/fW).times.100,000<-2 (2')

If condition (2') is met, spherical aberration compensation becomes easier.

Second lens unit Gr2 and third lens unit Gr3 move during zooming from the wide angle side toward the telephoto side of the zoom range, such that the distance between them decreases. In the zoom lens systems of the present invention, because second lens unit Gr2 has a negative refractive power while third lens unit Gr3 has a positive refractive power, when the distance between second lens unit Gr2 and third lens unit Gr3 decreases during zooming from the wide angle toward the telephoto end of the zoom range as described above, the refractive power arrangement for second lens unit Gr2 and third lens unit Gr3 becomes the retro-focus type in the wide angle condition, thereby preserving adequate back focal distance. On the other hand, since second lens unit Gr2 and third lens unit Gr3 come close to one another in the telephoto condition, the refractive power arrangement for second lens unit Gr2 and third lens unit Gr3 deviates from the retro-focus type, thereby reducing the system's length.

The refractive power arrangement for third lens unit Gr3 is a negative/positive retro-focus type arrangement, said lens unit comprising, from the object side, front lens unit Gr3F having a negative refractive power and rear lens unit Gr3R having a positive refractive power. This prevents the back focal distance in the shortest focal length condition from becoming short as the zoom ratio increases.

Further, by meeting the following condition (3), adequate back focal distance may be preserved while good optical performance is maintained.

1<T3 (3) where, T3: the distance between front lens unit Gr3F and rear lens unit Gr3R of third lens unit Gr3.

If distance T3 between front lens unit Gr3F and rear lens unit Gr3R becomes smaller than the range of condition (3), it becomes necessary to increase the negative refractive power of front lens unit Gr3F and the positive refractive power of rear lens unit Gr3R in order to preserve adequate back focal distance. However, it would then be difficult to compensate for off-axial coma aberration in the wide angle condition and spherical aberration in the telephoto condition.

In order to make the back focal distance long in the shortest focal length condition by having a retro-focus type refractive power arrangement for third lens unit Gr3, it is necessary for the refractive power of front lens unit Gr3F to be negative, as described above. In addition, in order to compensate for axial chromatic aberration and lateral chromatic aberration in the shortest and middle focal length conditions under high zoom ratios, it is necessary to adequately perform chromatic compensation within front lens unit Gr3F. When the zoom ratio increases, the positions at which off-axial light beams pass through third lens unit Gr3 differ greatly between the wide angle condition and the telephoto condition. Therefore, if compensation for lateral chromatic aberration is attempted using both front lens unit Gr3F and rear lens unit Gr3R of third lens unit Gr3, the variation in lateral chromatic aberration during zooming cannot be adequately dealt with. Consequently, in order to adequately deal with the variation in lateral chromatic aberration during zooming, it is necessary that compensation for lateral chromatic aberration take place within front lens unit Gr3F and rear lens unit Gr3R only. Therefore, in the present invention, at least one positive lens and one negative lens are used in front lens unit Gr3F as described above, and condition (4), which is a condition for achromatism in front lens unit Gr3F, is met based on the fact that the refractive power of front lens unit Gr3F is negative (i.e., the refractive power of the positive lens is smaller than that of the negative lens). As a result of this construction, lateral chromatic aberration may be kept low throughout the entire zoom range even under a high zoom ratio.

.nu.+<.nu.- (4) Where, .nu.+: Abbe number of the positive lens of front lens unit Gr3F, and .nu.-: Abbe number of the negative lens of front lens unit Gr3F.

Embodiments 1 through 6 are all double telephoto type zoom lens systems having a positive/negative/positive/negative refractive power arrangement with a .times.3 or larger zoom ratio. As described above, when the zoom ratio is increased to .times.3 or larger in the conventional model having this refractive power arrangement, lateral chromatic aberration compensation becomes difficult. However, this can be resolved by meeting condition (4) above. In other words, by meeting condition (4) by virtue of a lens system having at least one positive lens and one negative lens in front lens unit Gr3F whose refractive power is negative, lateral chromatic aberration may be kept low throughout the entire zoom range even under a high zoom ratio.

FIGS. 25, 26 and 27 show lateral chromatic aberrations in the first, second and sixth embodiments, respectively. In each chart, (W) indicates an aberration in the shortest focal length condition, (M) indicates an aberration in the middle focal length condition and (T) indicates an aberration in the longest focal length condition. Solid line (g) represents an aberration as to the g-line and dotted line (c) represents an aberration as to the c-line.

It is preferable that the following condition (5) be met as well.

-6<S3/fW<-0.1 (5) Where, S3: the radius of curvature of the surface of third lens unit Gr3, which is closest to the object.

If the lower limit of condition (5) is not met, the degree of concavity of the surface of third lens unit Gr3 closest to the object becomes small and the degree of light refraction upward decreases, as a result of which the back focal distance in the shortest focal length condition (W) becomes too short. Conversely, if the upper limit of condition (4) is not met, due to aberrations occurring on this surface, compensation for off-axial coma aberration in the wide angle condition as well as for spherical aberration in the telephoto condition becomes difficult.

Table 7 shows the refractive power of each lens unit and the back focal distance in the shortest focal length condition for first through sixth embodiments. Table 8 shows the values which meet conditions (1) through (5) for the first through sixth embodiments.

TABLE 7______________________________________ .O slashed.1 .O slashed.2 .O slashed.3 .O slashed.4 LB______________________________________Embodiment 1 0.00700 -0.00720 0.02225 -0.02754 36.746Embodiment 2 0.00700 -0.00720 0.02225 -0.02754 37.697Embodiment 3 0.00661 -0.00806 0.01979 -0.02493 37.258Embodiment 4 0.00715 -0.00775 0.01991 -0.02450 36.987Embodiment 5 0.00790 -0.00800 0.02203 -0.02890 37.114Embodiment 6 0.00829 -0.01314 0.02200 -0.01869 39.032______________________________________

In the table above,

.phi.1: refractive power of first lens unit .phi.2: refractive power of second lens unit .phi.3: refractive power of third lens unit .phi.4: refractive power of fourth lens unit LB: back focal distance TABLE 8__________________________________________________________________________ Condition 2 Condition 1 (.O slashed.2/fW) Condition 3 Condition 4 Condition 5 .O slashed.1/.O slashed.4 X100,000 T3 .nu.- .nu.+ S3/fW__________________________________________________________________________Embodiment 1 -0.25 -7.02 2.695 53.93 40.89 -0.646Embodiment 2 -0.25 -7.02 2.683 53.93 36.96 -0.916Embodiment 3 -0.27 -7.86 2.785 31.59 64.20 8.570Embodiment 4 -0.29 -7.37 1.400 28.83 35.45 -5.409Embodiment 5 -0.27 -7.80 2.700 23.83 35.45 -2.408Embodiment 6 -0.44 -17.09 6.600 55.46 39.22 -0.502__________________________________________________________________________

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims

1. A zoom lens system, comprising from an object side:

a first lens unit having a positive refractive power;

a second lens unit having a negative refractive power;

a third lens unit having a positive refractive power; and

a fourth lens unit having a negative refractive power,

wherein the first through fourth lens units move such that the distance between the first and second lens units increases and the distance between the third and fourth lens units decreases during zooming from the wide angle toward the telephoto end of the zoom range, and

wherein following conditions are fulfilled:

-0.3<.phi.1/.phi.4<-0.1

where

.phi.1 represents the refractive power of the first lens unit, and

.phi.4 represents the refractive power of the fourth lens unit.

2. A zoom lens system according to claim 1, wherein a following condition is fulfilled:

-20<(.phi.2/fW).times.100,00<-2

where

.phi.2 represents the refractive power of the second lens unit, and

fW represents the focal length of the entire system in the shortest focal length condition.

3. A zoom lens system according to claim 2, wherein a following condition is fulfilled:

-8<(.phi.2/fW).times.100,000<-2.

4. A zoom lens system according to claim 1, wherein the third lens unit comprises a front lens unit having a negative refractive power and a rear lens unit having a positive refractive power.

5. A zoom lens system according to claim 4, wherein a following condition is fulfilled:

1<T3

where

T3 represents the distance between the front lens unit and the rear lens unit.

6. A zoom lens system according to claim 4, wherein the front lens unit comprises a positive lens and a negative lens.

7. A zoom lens system according to claim 6, wherein a following condition is fulfilled:

.nu.+<.nu.-

where

.nu.+ represents Abbe number of the positive lens of front lens unit, and

.nu.- represents Abbe number of the negative lens of front lens unit.

8. A zoom lens system according to claim 1, wherein a following condition is fulfilled:

-6<S3/fW<-0.1

where

S3 represents the radius of curvature of the surface of the third lens unit, which is closest to the object.

fW represents the focal length of the entire system in the shortest focal length condition.

9. A zoom lens system, comprising from an object side:

a first lens unit having a positive refractive power;

a second lens unit having a negative refractive power;

a third lens unit having a positive refractive power; and

a fourth lens unit having a negative refractive power,

wherein the first through fourth lens units move such that the distance between the first and second lens units increases and the distance between the third and fourth lens units decreases during zooming from the wide angle toward the telephoto end of the zoom range,

wherein the third lens unit consists of from the object side a front lens unit having a negative refractive power and a rear lens unit having a positive refractive power and the front lens unit includes a positive lens and a negative lens, and

wherein following conditions are fulfilled:

< T3

.nu.+<.nu.-

where

T3 represents the distance between front lens unit and the rear lens unit,

.nu.+ represents Abbe number of the positive lens of front lens unit, and

.nu.- represents Abbe number of the negative lens of front lens unit.

10. A zoom lens system according to claim 9, wherein a following condition is fulfilled:

-0.3<.phi.1/.phi.4<-0.1

where

.phi.1 represents the refractive power of the first lens unit, and

.phi.4 represents the refractive power of the fourth lens unit.

11. A zoom lens system according to claim 9, wherein a following condition is fulfilled:

-20<(.phi.2/fW).times.100,000<-2

where

.phi.2 represents the refractive power of the second lens unit, and

fW represents the focal length of the entire system in the shortest focal length condition.

12. A zoom lens system according to claim 11, wherein a following condition is fulfilled:

-8<(.phi.2/fW).times.100,000<-2.

13. A zoom lens system according to claim 9, wherein a following condition is fulfilled:

-6<S3/fW<-0.1

where

S3 represents the radius of curvature of the surface of the third lens unit, which is closest to the object, and

fW represents the focal length of the entire system in the shortest focal length condition.