US20010017734A1

US20010017734A1 - Zoom lens

Info

Publication number: US20010017734A1
Application number: US09/766,581
Authority: US
Inventors: Tetsuya Ori
Original assignee: Individual
Current assignee: Fujinon Corp
Priority date: 2000-02-04
Filing date: 2001-01-23
Publication date: 2001-08-30
Anticipated expiration: 2021-01-23
Also published as: JP4581042B2; JP2001215408A; US6317272B2

Abstract

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.

Description

BACKGROUND OF THE INVENTION

Recently, the image detector size in new video cameras being sold has been decreasing from about ⅓ of an inch in linear dimension to about ¼ 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 μm. 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.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, wherein: [0005]
FIG. 1 shows the basic lens element configuration for all embodiments of the present invention, [0006]
FIGS. [0007] 2A-2D show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the wide-angle end for Embodiment 1,
FIGS. [0008] 2E-2H show the spherical aberration, astigmatism, distortion and lateral color, respectively, at a mid-position for Embodiment 1,
FIGS. [0009] 2I-2L show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the telephoto end for Embodiment 1,
FIGS. [0010] 3A-3D show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the wide-angle end for Embodiment 2,
FIGS. [0011] 3E-3H show the spherical aberration, astigmatism, distortion and lateral color, respectively, at a mid-position for Embodiment 2,
FIGS. [0012] 3I-3L show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the telephoto end for Embodiment 2,
FIGS. [0013] 4A-4D show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the wide-angle end for Embodiment 3,
FIGS. [0014] 4E-4H show the spherical aberration, astigmatism, distortion and lateral color, respectively, at a mid-position for Embodiment 3, and
FIGS. [0015] 4I-4L show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the telephoto end for Embodiment 3.

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. [0016]
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. [0017]
Furthermore, it is preferable that the following Conditions (1) and (2) are both satisfied: [0018]
υ1_d>50 Condition (1)
υ2_d>50 Condition (2)
where [0019]
υ1[0020] _dis the Abbe number, at the d line, of the biconcave lens element in the second lens group, and
υ2[0021] _dis 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: [0022]
2.5<f4/fw<3.1 Condition (3)
where [0023]
f4 is the focal length of the fourth lens group, and [0024]
fw is the focal length of the zoom lens at the wide-angle end. [0025]
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. [0026]
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. [0027]
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). [0028]
As shown in FIG. 1, the zoom lens of each embodiment comprises a first lens group G[0029] ₁of positive refractive power, a second lens group G₂of negative refractive power, a third lens group G₃of positive refractive power, and a fourth lens group G₄of positive refractive power. Various embodiments of the invention will now be described in detail.

Embodiment 1

The first lens group G[0030] ₁is formed of, in order from the object side, a first lens element L₁of negative meniscus shape with its convex surface on the object side, a second lens element L₂of 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 L₃of positive meniscus shape with its convex surface on the object side. The first lens element L₁, and second lens element L₂are joined to form a combined lens.
The second lens group G[0031] ₂is formed of, in order from the object side, a fourth lens element L₄of plano-concave shape with its concave surface on the image side, a fifth lens element L₅of 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 L₆of positive meniscus shape with its convex surface on the object side. The fifth lens element L₅and sixth lens element L₆are joined to form a combined lens.
The third lens group G[0032] ₃is formed of a seventh lens element L₇having a plano-convex shape, with its convex surface on the object side being aspherical.
The fourth lens group G[0033] ₄is formed of, in order from the object side, an eighth lens element L₈having a meniscus shape with its convex surface on the object side, a ninth lens element L₉having 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 L₁₀having a positive meniscus shape with its convex surface on the image side. The eighth lens element L₈and ninth lens element L₉are joined to form a combined lens, and the image-side surface of the tenth lens element L₁₀is aspherical. Moreover, this zoom lens satisfies each of the above Conditions (1)-(3).
Furthermore, a [0034] diaphragm 1 is arranged on the object side of the third lens group G₃, 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 _dand the Abbe Number υ_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_NOand the range of picture angles 2ω (from the wide-angle end to the telephoto end) are given in the lower portion of the table.

TABLE 1


#	R	D	N_d	U_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	∞	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	∞	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	∞	1.20000	1.51680	64.2
20	∞

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. [0036]
Z=CY ²/{1+(1−KC ² Y ²)^½ }+A ₄ Y ⁴ +A ₆ Y ⁶ +A ₈ Y ⁸ +A ₁₀ Y ¹⁰ (Equation A)
where [0037]
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, [0038]
C(=1/R) is the curvature of the aspherical surface near the optical axis, [0039]
Y is the distance (in mm) from the optical axis, [0040]
K is the eccentricity, and [0041]
A[0042] ₄, A₆, A₈, and A₁₀are the 4th, 6th, 8th, and 10th aspherical coefficients.

The values of each of the constants K and A ₄-A₁₀of 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 of Surface	Aspherical coefficients of Surface
#
12	#18

K = 0.7742461766	K = 0.7057989793
A2 = −0.3154268352 × 10⁻⁴	A2 = 0.4984741710 × 10⁻⁴
A3 = −0.6214297832 × 10⁻⁶	A3 = −0.1395366239 × 10⁻⁵
A4 = 0.3820617076 × 10⁻⁷	A4 = 0.1254094025 × 10⁻⁷
A5 = −0.1057579273 × 10⁻⁸	A5 = −0.9724176338 × 10⁻⁹

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

Conditions (1), (2) and (3) are satisfied for this embodiment, since the value of υ1[0044] _dis 70.4 (see Table 1), the value of υ2_dis 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 [0045] Embodiment 2 has nearly the same lens element configuration as that of Embodiment 1, but it is different in that the seventh lens element L₇in 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 _dand the Abbe Number υ_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_NOand the range of picture angles 2ω (from the wide-angle end to the telephoto end) are given in the lower portion of the table.

TABLE 3


#	R	D	N_d	U_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	∞	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	∞	1.20000	1.51680	64.2
20	∞

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 ₄-A₁₀of the aspherical surfaces indicated in Table 3 are shown in the top portion of Table 4. The values of D5, D10, D13, 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 of surface	Aspherical coefficients of surface
#
12	#18

K = 0.6532030275	K = 07513747068
A₂= −0.3506050826 × 10⁻⁴	A₂= 0.4953224677 × 10⁻⁴
A₃= −0.4251705791 × 10⁻⁶	A₃= −0.7859310777 × 10⁻⁵
A₄= 0.2094701593 × 10⁻⁷	A₄= 0.6897117528 × 10⁻⁸
A₅= −0.4742574727 × 10⁻⁹	A₅= −0.4599956246 × 10⁻⁹

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

Conditions (1), (2) and(3) are satisfied for this embodiment, since the value of υ1[0048] _dis 70.4 (see Table 3), the value of υ2_dis 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 [0049] Embodiment 3 has nearly the same lens element configuration as that of Embodiment 2, but it is different in that the fourth lens element L₄is 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 L₇in 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 _dand the Abbe Number υ_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_NOand the range of picture angles 2ω (from the wide-angle end to the telephoto end) are given in the lower portion of the table.

TABLE 5


#	R	D	N_d	U_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	∞	1.20000	1.51680	64.2
20	∞

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 ₄-A₁₀of the aspherical surfaces indicated in Table 5 are shown in the top portion of Table 6. The values of D5, D10, 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 of surface	Aspherical coefficients of surface
#
12	#18

K = 0.9264641656	K = 07342702751
A₂= −0.3802321152 × 10⁻⁴	A₂= 0.4041192633 × 10⁻⁴
A₃= −0.5752001227 × 10⁻⁶	A₃= −0. 1447026660 × 10⁻⁵
A₄= 0.3949579841 × 10⁻⁷	A₄= 0.1485066987 × 10⁻⁷
A₅= −0.1128302302 × 10⁻⁸	A₅= −0.9926608602 × 10⁻⁹

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

Conditions (1), (2) and(3) are satisfied for this embodiment, since the value of υ1[0052] _dis 59.8 (see Table 5), the value of υ2_dis 63.5 (see Table 5), and the value of the ratio f4/fw is 2.867 (as given in Table 6).
FIGS. [0053] 2A-2D show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the wide-angle end for Embodiment 1,
FIGS. [0054] 2E-2H show the spherical aberration, astigmatism, distortion and lateral color, respectively, at a mid-position for Embodiment 1,
FIGS. [0055] 2I-2L show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the telephoto end for Embodiment 1,
FIGS. [0056] 3A-3D show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the wide-angle end for Embodiment 2,
FIGS. [0057] 3E-3H show the spherical aberration, astigmatism, distortion and lateral color, respectively, at a mid-position for Embodiment 2,
FIGS. [0058] 3I-3L show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the telephoto end for Embodiment 2,
FIGS. [0059] 4A-4D show the spherical aberration, astigmatism, distortion and lateral color, respectively, at the wide-angle end for Embodiment 3,
FIGS. [0060] 4E-4H show the spherical aberration, astigmatism, distortion and lateral color, respectively, at a mid-position for Embodiment 3, and
FIGS. [0061] 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. [0062]
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. [0063]
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. [0064]

Claims

What is claimed is:

1. A zoom lens formed of no more than four lens groups having refractive power, in order from the object side, as follows:

a first lens group that is fixed in position and has positive refractive power;

a second lens group having negative refractive power, the position of the second lens group being variable along the optical axis in order to change the focal length of the zoom lens, said second lens group including, in order from the object side, a negative lens element having a concave surface or a planar surface on its object side, and a combined lens which includes a biconcave lens element joined to a positive meniscus lens element with its convex surface on the object side;

a third lens group that is fixed in position and has positive refractive power, said third lens group consisting of a single lens element with at least one surface thereof being aspherical; and,

a fourth lens group having positive refractive power and including a lens element having an aspherical surface, the position of the fourth lens group being variable along the optical axis in order to compensate for what would otherwise be movement of the image plane due to changes in the amount of zoom and the object distance.

2. The zoom lens of

claim 1

, said fourth lens group including:

a combined lens that includes a negative meniscus lens element with its convex surface on the object side joined to another lens element, and

a single lens element having at least one surface that is aspherical.

3. The zoom lens of

claim 2

, wherein the following Conditions (1) and (2) are satisfied:

υ1_d>50 Condition (1) υ2_d>50 Condition (2)

where

υ1_dis the Abbe number at the d line of the biconcave lens element of the second lens group; and

υ2_dis the Abbe number at the d line of the single lens element in the fourth lens group.

4. The zoom lens of

claim 1

, wherein the following Condition (3) is also satisfied:

2.5<f4/fw<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.

5. The zoom lens of

claim 2

, wherein the following Condition (3) is also satisfied:

2.5<f4/fw<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.

6. The zoom lens of

claim 3

, wherein the following Condition (3) is also satisfied:

2.5<f4/fw<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.