Zoom lens

A zoom lens formed of no more than four lens groups having refractive power of, in order from the object side, positive, negative, positive, and positive. The first and third lens groups are fixed in position, whereas the position of the second lens group along the optical axis is varied to change the focal length during zooming, and the position of the fourth lens group is adjusted along the optical axis so as to compensate for what otherwise would be movement of the image surface with zooming and distance to the object. The second lens group includes a negative lens element having a concave surface or a planar surface on its object side, a combined lens which includes a biconcave lens element joined to a positive meniscus lens element with its convex surface on the object side. The third lens group consists of a single lens element with at least one surface thereof being aspherical, and the fourth lens group includes a lens element having an aspherical surface. Various conditions are satisfied in order to ensure the overall length of the lens is short and that various aberrations, especially axial chromatic aberration at the telephoto end, are favorably corrected.

BACKGROUND OF THE INVENTION
 Recently, the image detector size in new video cameras being sold has been
 decreasing from about 1/3 of an inch in linear dimension to about 1/4 of
 an inch in linear dimension. This has placed demands on the development of
 new zoom lenses for such cameras which are simple in design and yet have a
 sufficiently high optical performance. As such a zoom lens, rear-focus
 type lenses have been known in which the entire zoom lens is formed of
 four lens groups, with the first lens group and the third lens group being
 fixed in position. Power is varied by moving the second lens group along
 the optical axis and the image plane is made to remain at a fixed location
 as the power is varied by moving the fourth lens group along the optical
 axis. Moreover, lenses with a high variable power ratio, as described in
 Japanese Laid Open Patent Applications Nos. H8-005916 and H11-194269 have
 been known for use as zoom lenses for video cameras.
 More recently, handy electronic still cameras, so-called digital cameras,
 have rapidly become popular, and zoom lenses of high variable power ratio
 have also been used in such electronic still cameras. However, the
 required degree of compactness is different among video cameras and
 digital cameras. Therefore, the overall length of zoom lenses for these
 two applications is also different. Namely, the size of the digital camera
 is much smaller, as it is desired that such cameras be capable of being
 carried in a breast pocket of a garment. Thus, the overall length of a
 zoom lens for a digital camera must be very small. Indeed, the desired
 overall length of a zoom lens for a digital camera is so small that none
 of the above-discussed prior art zoom lenses used in video cameras is
 sufficiently small. Therefore a zoom lens having a shorter overall length
 has been desired.
 In digital cameras, the resolution required of the zoom lens employed has
 rapidly increased as the pixel size has decreased in linear dimension to
 about 3-4 .mu.m. This has made the axial chromatic aberration at the
 telephoto end increases to the point that it is no longer of little
 concern.
 BRIEF SUMMARY OF THE INVENTION
 A first object of the present invention is to provide a zoom lens having a
 very short overall length so that the zoom lens can be used with a digital
 camera, while maintaining a high variable power ratio and excellent
 optical imaging properties. More particularly, a second object of the
 present invention is to provide a zoom lens having reduced axial chromatic
 aberration at the telephoto end.

DETAILED DESCRIPTION
 This invention relates to a rear-focus type zoom lens having four lens
 groups, particularly to a zoom lens which can achieve a variable power of
 about 5-6 while having a small overall length that is suitable for use in
 electronic still cameras. In order from the object side, the four lens
 groups are of positive, negative, positive, and positive refractive power,
 respectively. The third lens group consists of a single lens element, at
 least one surface of which is aspherical, and the fourth lens group also
 includes a lens element having at least one surface that is aspherical.
 The first lens group and the third lens group are fixed in position, the
 focal length of the zoom lens is changed by moving the second lens group
 along the optical axis, and what would otherwise be fluctuations of the
 imaging position due to zooming (i.e., for imaging objects at different
 object distances) are compensated by moving the fourth lens group along
 the optical axis. The second lens group includes, in order from the object
 side, a negative lens element having a planar or concave surface on the
 object side, and an optical element formed of a biconcave lens element
 that is joined to a positive meniscus lens element having its convex
 surface on the object side.
 Moreover, it is preferable that the fourth lens group includes: (1) a
 combined lens formed of a negative meniscus lens element having its
 concave surface on the image side joined to a lens element having a convex
 surface on the object side, and (2) a single lens element that has at
 least one surface that is aspherical.
 Furthermore, it is preferable that the following Conditions (1) and (2) are
 both satisfied:
EQU .nu.1.sub.d &gt;50 Condition (1)
EQU .nu.2.sub.d &gt;50 Condition (2)
 where
 .nu.1.sub.d is the Abbe number, at the d line, of the biconcave lens
 element in the second lens group, and
 .nu.2.sub.d is the Abbe number, at the d line, of the single lens element
 in the fourth lens group.
 Additionally, it is also preferred that the following Condition (3) be
 satisfied:
EQU 2.5&lt;f4/fw&lt;3.1 Condition (3)
 where
 f4 is the focal length of the fourth lens group, and
 fw is the focal length of the zoom lens at the wide-angle end.
 The zoom lens of the present invention achieves a variable power ratio of
 over 5 as well as a wide-angle of view by having the second lens group
 formed of a negative lens element having a planar or concave surface on
 the object side, and a combined lens formed of a biconcave lens element
 that is joined to a positive meniscus lens element that has its convex
 surface on the object side.
 Moreover, the zoom lens of this invention maintains a fixed image plane
 during zooming and focusing. Since the first lens group, in order from the
 object side, is fixed in position, the overall length of the zoom lens, as
 measured from the vertex of the object side of the first lens group to the
 image-forming surface does not change when zooming or focusing. Further,
 the zoom lens is compact enough to be used in a digital camera by having
 the fourth lens group composed of: (1) a combined lens (a negative
 meniscus lens element with its convex surface on the object side that is
 joined to a biconvex lens element), and (2) a single lens element having
 at least one aspherical surface.
 Furthermore, the zoom lens of the present invention enables lateral color
 aberrations to be favorable, especially at the wide-angle end, by
 satisfying the above Condition (1) while compensating for axial color
 aberration, especially on the telephoto side, by satisfying Condition (2).
 In addition, the zoom lens of the present invention reduces fluctuations
 of the image plane position which otherwise would occur with zooming by
 satisfying Condition (3).
 As shown in FIG. 1, the zoom lens of each embodiment comprises a first lens
 group G.sub.1 of positive refractive power, a second lens group G.sub.2 of
 negative refractive power, a third lens group G.sub.3 of positive
 refractive power, and a fourth lens group G.sub.4 of positive refractive
 power. Various embodiments of the invention will now be described in
 detail.
 Embodiment 1
 The first lens group G.sub.1 is formed of, in order from the object side, a
 first lens element L.sub.1 of negative meniscus shape with its convex
 surface on the object side, a second lens element L.sub.2 of biconvex
 shape having surfaces of different radii of curvature, with the surface of
 smaller radius of curvature on the object side, and a third lens element
 L.sub.3 of positive meniscus shape with its convex surface on the object
 side. The first lens element L.sub.1 and second lens element L.sub.2 are
 joined to form a combined lens.
 The second lens group G.sub.2 is formed of, in order from the object side,
 a fourth lens element L.sub.4 of plano-concave shape with its concave
 surface on the image side, a fifth lens element L.sub.5 of biconcave shape
 with surfaces of different radii of curvature, with the surface of smaller
 radius of curvature on the image side, and a sixth lens element L.sub.6 of
 positive meniscus shape with its convex surface on the object side. The
 fifth lens element L.sub.5 and sixth lens element L.sub.6 are joined to
 form a combined lens.
 The third lens group G.sub.3 is formed of a seventh lens element L.sub.7
 having a plano-convex shape, with its convex surface on the object side
 being aspherical.
 The fourth lens group G.sub.4 is formed of, in order from the object side,
 an eighth lens element L.sub.8 having a meniscus shape with its convex
 surface on the object side, a ninth lens element L.sub.9 having a biconvex
 shape with surfaces of different radii of curvature, with the surface of
 smaller radius of curvature on the object side, and a tenth lens element
 L.sub.10 having a positive meniscus shape with its convex surface on the
 image side. The eighth lens element L.sub.8 and ninth lens element L.sub.9
 are joined to form a combined lens, and the image-side surface of the
 tenth lens element L.sub.10 is aspherical. Moreover, this zoom lens
 satisfies each of the above Conditions (1)-(3).
 Furthermore, a diaphragm 1 is arranged on the object side of the third lens
 group G.sub.3, and a CCD array that includes a cover glass 2 with a
 low-pass filter (not shown) is arranged at the image surface. Thus, a
 light beam that is incident from the object side along the optical axis X
 is imaged onto a detecting surface 3 of the CCD array.
 Table 1 below lists the surface number # in order from the object side, the
 radius of curvature R (in mm) of each surface near the optical axis, the
 on-axis spacing D (in mm) between surfaces, as well as the index of
 refraction N.sub.d and the Abbe Number .nu..sub.d (at the d line) of each
 lens element of Embodiment 1. Further, the focal length f of the zoom
 lens, the F number F.sub.NO and the range of picture angles 2.omega. (from
 the wide-angle end to the telephoto end) are given in the lower portion of
 the table.
 TABLE 1
 # R D N.sub.d .nu..sub.d
 1 67.0393 1.50000 1.84666 23.8
 2 32.1650 6.47000 1.62041 60.3
 3 -274.0495 0.13000
 4 27.9582 4.20000 1.71300 53.9
 5 93.8790 D5 (variable)
 6 .infin. 1.00000 1.88300 40.8
 7 8.7967 3.63300
 8 -22.0199 1.66000 1.48749 70.4
 9 10.5890 2.93000 1.84666 23.8
 10 36.9291 D10 (variable)
 11 stop 1.12500
 12* 16.5772 1.95000 1.51760 63.5
 13 .infin. D13 (variable)
 14 12.1536 1.80000 1.84666 23.8
 15 7.0410 3.50000 1.48749 70.4
 16 -24.6606 1.08700
 17 -7.0389 1.85000 1.51760 63.5
 18* -6.7477 D18 (variable)
 19 .infin. 1.20000 1.51680 64.2
 20 .infin.
 f = 8.08 - 44.82 F.sub.NO = 2.88 - 3.12 2.omega. = 66.2 - 12.2.degree.
 The overall length of the zoom lens of this embodiment (from the vertex of
 the object side of the first lens group to the image plane) is 80.0 mm.
 Those surfaces with a * to the right of the surface number in Table 1 are
 aspherical surfaces, and the aspherical surface shape is expressed by
 Equation (A) below.
EQU Z=CY.sup.2 /{1+(1-KC.sup.2 Y.sup.2).sup.1/2 }+A.sub.4 Y.sup.4 +A.sub.6
 Y.sup.6 +A.sub.8 Y.sup.8 +A.sub.10 Y.sup.10 (Equation A)
 where
 Z is the length (in mm) of a line drawn from a point on the aspherical
 surface at distance Y from the optical axis to the tangential plane of the
 aspherical surface vertex,
 C (=1/R) is the curvature of the aspherical surface near the optical axis,
 Y is the distance (in mm) from the optical axis,
 K is the eccentricity, and
 A.sub.4, A.sub.6, A.sub.8, and A.sub.10 are the 4th, 6th, 8th, and 10th
 aspherical coefficients.
 The values of each of the constants K and A.sub.4 -A.sub.10 of the
 aspherical surfaces indicated in Table 1 are shown in the top portion of
 Table 2. The values of D5, D10, D13, D18 at positions resulting in the
 zoom lens having focal lengths of f=8.08 (i.e., fw), f=20.2, f=44.8) are
 shown in the middle portion of Table 2, and the value of f4/fw is shown in
 the lower portion of the table.
 TABLE 2
 Aspherical coefficients Aspherical coefficients
 of Surface #12 of Surface #18
 K = 0.7742461766 K = 0.7057989793
 A.sub.2 = -0.3154268352 .times. 10.sup.-4 A.sub.2 = 0.4984741710
 .times. 10.sup.-4
 A.sub.3 = -0.6214297832 .times. 10.sup.-6 A.sub.3 = -0.1395366239
 .times. 10.sup.-5
 A.sub.4 = 0.3820617076 .times. 10.sup.-7 A.sub.4 = 0.1254094025
 .times. 10.sup.-7
 A.sub.5 = -0.1057579273 .times. 10.sup.-8 A.sub.5 = -0.9724176338
 .times. 10.sup.-9
 f D5 D10 D13 D18
 8.08 1.03 19.375 9.029 16.476
 20.2 11.699 8.707 5.297 20.208
 44.8 18.727 1.678 4.926 20.579
 f4/fw = 2.799
 Conditions (1), (2) and (3) are satisfied for this embodiment, since the
 value of .nu.1.sub.d is 70.4 (see Table 1), the value of .nu.2.sub.d is
 63.5 (see Table 1), and the value of the ratio f4/fw is 2.799 (as given in
 Table 2).
 Embodiment 2
 The zoom lens of Embodiment 2 has nearly the same lens element
 configuration as that of Embodiment 1, but it is different in that the
 seventh lens element L.sub.7 in this embodiment is a biconvex lens having
 surfaces of different radii of curvature, with the surface of smaller
 radius of curvature on the object side.
 Table 3 below lists the surface number # in order from the object side, the
 radius of curvature R (in mm) of each surface near the optical axis, the
 on-axis spacing D (in mm) between surfaces, as well as the index of
 refraction N.sub.d and the Abbe Number .nu..sub.d (at the d line) of each
 lens element of Embodiment 2. Further, the focal length f of the zoom
 lens, the F number F.sub.NO and the range of picture angles 2.omega. (from
 the wide-angle end to the telephoto end) are given in the lower portion of
 the table.
 TABLE 3
 # R D N.sub.d .nu..sub.d
 1 63.7836 1.50000 1.84666 23.8
 2 30.4741 6.26378 1.71300 53.9
 3 -334.4146 0.10000
 4 24.5563 4.48721 1.56384 60.7
 5 73.1850 D5 (variable)
 6 .infin. 1.00000 1.88300 40.8
 7 8.6943 4.28294
 8 -20.1897 1.01000 1.48749 70.4
 9 10.7502 2.80068 1.84666 23.8
 10 41.0292 D10 (variable)
 11 stop 1.50000
 12* 17.3999 2.13607 1.51760 63.5
 13 -135.4775 D13 (variable)
 14 13.7563 1.80000 1.84666 23.8
 15 7.6818 3.32979 1.48749 70.4
 16 -24.1785 1.24776
 17 -8.2249 1.87795 1.51760 63.5
 18* -7.7694 D18 (variable)
 19 .infin. 1.20000 1.51680 64.2
 20 .infin.
 F = 8.71 - 48.3 F.sub.NO = 2.89 - 3.05 2.omega. = 62.2.degree. -
 11.0.degree.
 The overall length of the zoom lens of this embodiment (from the vertex of
 the object side of the first lens group to the image plane) is 80.0 mm.
 Those surfaces with a * to the right of the surface number in Table 3 are
 aspherical surfaces, and the aspherical surface shape is expressed by
 Equation (A) above. The values of each of the constants K and A.sub.4
 -A.sub.10 of the aspherical surfaces indicated in Table 3 are shown in the
 top portion of Table 4. The values of D5, D10, D 13, D18 at positions
 resulting in the zoom lens having focal lengths of f=8.71 (i.e., fw),
 f=21.8, and f=48.3 are shown in the middle portion of Table 4, and the
 value of the ratio f4/fw is shown in the lower portion of the table.
 TABLE 4
 Aspherical coefficients Aspherical coefficients
 of surface #12 of surface #18
 K = 0.6532030275 K = 0.7513747068
 A.sub.2 = -0.3506050826 .times. 10.sup.-4 A.sub.2 = 0.4953224677
 .times. 10.sup.-4
 A.sub.3 = -0.4251705791 .times. 10.sup.-6 A.sub.3 = -0.7859310777
 .times. 10.sup.-5
 A.sub.4 = 0.2094701593 .times. 10.sup.-7 A.sub.4 = 0.6897117528
 .times. 10.sup.-8
 A.sub.5 = -0.4742574727 .times. 10.sup.-9 A.sub.5 = -0.4599956246
 .times. 10.sup.-9
 f D5 D10 D13 D18
 8.71 1.018 19.000 8.382 17.047
 21.8 11.283 8.735 4.610 20.819
 48.3 18.104 1.913 5.179 20.250
 f4/fw = 2.842
 Conditions (1), (2) and(3) are satisfied for this embodiment, since the
 value of .nu.1.sub.d is 70.4 (see Table 3), the value of .nu.2.sub.d is
 63.5 (see Table 3), and the value of the ratio f4/fw is 2.842 (as given in
 Table 4).
 Embodiment 3
 The zoom lens of Embodiment 3 has nearly the same lens element
 configuration as that of Embodiment 2, but it is different in that the
 fourth lens element L.sub.4 is biconcave having surfaces of different
 radii of curvature, with the surface of larger radius of curvature being
 on the object side, and the seventh lens element L.sub.7 in this
 embodiment is a biconvex lens element having surfaces of different radii
 of curvature, with the surface of smaller radius of curvature on the
 object side.
 Table 5 below lists the surface number # in order from the object side, the
 radius of curvature R (in mm) of each surface near the optical axis, the
 on-axis spacing D (in mm) between surfaces, as well as the index of
 refraction N.sub.d and the Abbe Number .nu..sub.d (at the d line) of each
 lens element of Embodiment 3. Further, the focal length f of the zoom
 lens, the F number F.sub.NO and the range of picture angles 2.omega. (from
 the wide-angle end to the telephoto end) are given in the lower portion of
 the table.
 TABLE 5
 # R D N.sub.d .nu..sub.d
 1 52.0657 1.49999 1.84666 23.8
 2 30.5464 6.74895 1.49700 81.5
 3 -211.7404 0.10000
 4 26.7405 4.37914 1.71300 53.9
 5 95.7874 D5 (variable)
 6 -9103.4885 1.00000 1.88300 40.8
 7 8.4282 3.44255
 8 -21.6421 1.46263 1.52249 59.8
 9 10.0336 3.10282 1.84666 23.8
 10 46.1903 D10 (variable)
 11 (stop) 1.50000
 12* 16.4229 2.08166 1.51760 63.5
 13 -400.1927 D13 (variable)
 14 13.1863 1.80000 1.84666 23.8
 15 7.3701 3.20712 1.48749 70.4
 16 -21.1520 1.20435
 17 -6.5727 1.84345 1.51760 63.5
 18* -6.4671 D18 (variable)
 19 .infin. 1.20000 1.51680 64.2
 20 .infin.
 f = 8.11 - 45.00 F.sub.NO = 2.88 - 3.08 2.omega. = 66.0.degree. -
 12.2.degree.
 The overall length of the zoom lens of this embodiment (from the vertex of
 the object side of the first lens group to the image plane) is 80.0 mm.
 Those surfaces with a * to the right of the surface number in Table 5 are
 aspherical surfaces, and the aspherical surface shape is expressed by
 Equation (A) above. The values of each of the constants K and A.sub.4
 -A.sub.10 of the aspherical surfaces indicated in Table 5 are shown in the
 top portion of Table 6. The values of D5, D 10, D13, D18 at positions
 resulting in the zoom lens having focal lengths of f=8.11 (i.e., fw),
 f=20.3, and f=45.0 are shown in the middle portion of Table 6, and the
 value of the ratio f4/fw is shown in the lower portion of the table.
 TABLE 6
 Aspherical coefficients Aspherical coefficients
 of surface #12 of surface #18
 K = 0.9264641656 K = 0.7342702751
 A.sub.2 = -0.3802321152 .times. 10.sup.-4 A.sub.2 = 0.4041192633
 .times. 10.sup.-4
 A.sub.3 = -0.5752001227 .times. 10.sup.-6 A.sub.3 = -0.1447026660
 .times. 10.sup.-5
 A.sub.4 = 0.3949579841 .times. 10.sup.-7 A.sub.4 = 0.1485066987
 .times. 10.sup.-7
 A.sub.5 = -0.1128302302 .times. 10.sup.-8 A.sub.5 = -0.9926608602
 .times. 10.sup.-9
 f D5 D10 D13 D18
 8.11 1.000 19.000 8.721 16.712
 20.3 11.455 8.545 4.972 20.461
 45.0 18.313 1.687 4.837 20.596
 f4/fw = 2.867
 Conditions (1), (2) and(3) are satisfied for this embodiment, since the
 value of .nu.1.sub.d is 59.8 (see Table 5), the value of .nu.2.sub.d is
 63.5 (see Table 5), and the value of the ratio f4/fw is 2.867 (as given in
 Table 6).
 FIGS. 2A-2D show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at the wide-angle end for Embodiment 1,
 FIGS. 2E-2H show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at a mid-position for Embodiment 1,
 FIGS. 2I-2L show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at the telephoto end for Embodiment 1,
 FIGS. 3A-3D show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at the wide-angle end for Embodiment 2,
 FIGS. 3E-3H show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at a mid-position for Embodiment 2,
 FIGS. 3I-3L show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at the telephoto end for Embodiment 2,
 FIGS. 4A-4D show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at the wide-angle end for Embodiment 3,
 FIGS. 4E-4H show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at a mid-position for Embodiment 3, and
 FIGS. 4I-4L show the spherical aberration, astigmatism, distortion and
 lateral color, respectively, at the telephoto end for Embodiment 3.
 Moreover, the spherical aberration is shown for the d, F and C lines, and
 the astigmatism is shown for both the sagittal S image surface and
 tangential T image surface. As is evident from these figures, all
 embodiments of the invention favorably correct these aberrations. Further,
 axial color at the telephoto end is reduced as compared to prior art
 lenses.
 As described above, the zoom lens of this invention enables the zoom lens
 to achieve a variable power ratio of over 5 and have a wide angle of view
 by reason of the second lens group being composed of a negative lens
 element having a concave or planar surface on the object side, and a
 combined lens formed of a biconcave lens element joined to a positive
 meniscus lens element with its convex surface on the object side. Such a
 design enables the overall length of the zoom lens to be short enough for
 use in a digital camera.
 The invention being thus described, it will be obvious that the same may be
 varied in many ways. For example the radius of curvatures R near the
 optical axis and surface spacings D may be readily scaled to achieve a
 lens of a desired focal length. Further, in the third and fourth lens
 groups, the lens surfaces having aspherical surfaces may be formed on
 other lens surfaces. Such variations are not to be regarded as a departure
 from the spirit and scope of the invention. Rather the scope of the
 invention shall be defined as set forth in the following claims and their
 legal equivalents. All such modifications as would be obvious to one
 skilled in the art are intended to be included within the scope of the
 following claims.