JPH04161914A - Small zoom lens - Google Patents

Abstract

PURPOSE:To reduce the production cost by constituting a zoom lens with the first lens group having positive refracting power and the second lens group having negative refracting power in sequence from the object side, containing at least one a spherical lens in both lens groups, and using a plastic lens with weak refracting power for one of them. CONSTITUTION:This zoom lens is constituted of the first lens group with positive refracting power and the second lens group with negative refracting power in sequence from the object side. The first lens group is constituted of a negative-meniscus lens with the convex face faced to the object side, a negative- meniscus lens with the concave face provided on the object side, a plastic lens having an aspherical face with weak refracting power and a lens with positive refracting power in sequence from the object side. The zoom lens has a variable power ratio of about two from the quasi-wide angle to quasi-telescope, and the production cost can be reduced while maintaining good image forming performance.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention is suitable for small zoom lenses, especially lens-shutter type cameras with few restrictions on back focus, and is suitable for use in zoom lenses that can change the angle by about twice from semi-wide angle to semi-telephoto. The present invention relates to a zoom lens having a magnification ratio.

(Prior Art) In recent years, lens-shutter compact cameras equipped with zoom lenses have become mainstream. - Unlike zoom lenses for reflex cameras, this type of zoom lens does not require a long back focus and is strongly desired to be compact, so the first lens group has a positive refractive power. A two-group zoom lens with a telephoto type refractive power arrangement including a second lens group with negative refractive power has been put into practical use.

As an example of using this type of two-group zoom lens and making it more compact, for example, Japanese Patent Application Laid-Open No. 64-5211
Examples include the zoom lens of Example 12 of No. 1.

This zoom lens is a 2x zoom lens with a focal length range of approximately 35 to 70+o+n using a 35nn version film, and is made compact by increasing the refractive power of the first and second lens groups. . In particular, since the second lens group has a strong refractive power, the amount of movement of the second lens group is shortened, which is advantageous for making the lens barrel more compact in terms of structure.

In this zoom lens, aberration deterioration due to the strong refractive power of each group is compensated for by using aspheric surfaces in each of the first and second lens groups.

However, since the two aspherical lenses in this zoom lens are both made of glass material, the manufacturing cost is high.

(Problem to be Solved) This invention aims to provide a zoom lens having a zoom ratio of approximately 2x from semi-wide angle to semi-telephoto, having good imaging performance, being inexpensive, and having a short overall lens length. That is. In particular, the objective is to obtain a telephoto ratio of 1.0 or more at the telephoto end.

(Means for Solving the Problems) The zoom lens of the present invention is composed of, in order from the object side, a first lens group having a positive refractive power and a second lens group having a negative refractive power. In a two-group zoom lens that changes magnification by changing the distance between the first lens group and the second lens group, both the first lens group and the second lens group have at least one aspherical lens, and at least one of them has a refractive power. It is characterized by a weak plastic lens.

When an aspherical plastic lens is provided in the first lens group, the aspherical plastic lens satisfies the following conditions.

l fp, 11 / fy > 1, 7 ・
...(1) However, fPL: Focal length of the plastic lens in the first lens group fT: Focal length of the entire system at the telephoto end Specifically, this first lens group has a convex surface in order from the object side to the object side. Negative meniscus lens 1-2, lens 1-2 with weak refractive power
3. It is preferably composed of lenses 1-4 having positive refractive power, and the lenses 1-3 are preferably aspherical plastic lenses.

When an aspherical plastic lens is provided in the second lens group, it is desirable that the aspherical plastic lens has a weak refractive power and satisfies the following conditions.

1fpL□l/fy>0.9 ・(
2) However, fpt2'' focal length of the plastic lens in the second lens group.More specifically, the second lens group consists of, in order from the object side, a positive refractive power lens 2 having a surface with strong refractive power on the image side. -1,
It is composed of a meniscus-shaped lens 2-2 having a convex surface on the image side, and a meniscus-shaped lens 2-3 having a convex surface on the image side, and either the 2-2 lens or the 2-3 lens has an aspherical surface. Use a weak plastic lens.

Furthermore, at least one aspherical surface in the first lens group is
The lens has an aspherical displacement that makes the lens thicker at a position away from the optical axis than a spherical surface with the same paraxial radius of curvature, and at least one aspherical surface in the second lens group has the same paraxial radius of curvature. It is desirable that the lens has an aspherical displacement such that the lens becomes thinner at a position farther from the optical axis than a certain spherical surface, and that the following conditions are satisfied.

2XIO-s<l△x□l /fw<2XIO-'
(3) 4X10-'<l△x21/f w<9X
IO-' (4)1,. 2<f, /fT<1.
6 (5) 0.8 < l f21/f1
< 1.1 (6) However, △ Displacement amount ΔX2: Displacement amount f2 of the above aspherical surface in the second lens group from a spherical surface with the same paraxial radius of curvature at a position away from the optical axis by a distance half the effective radius: Wide-angle end Focal length f1 of the entire system: Focal length f2 of the first lens group: Focal length of the second lens group Further, the zoom lens of the present invention preferably satisfies the following conditions.

45 < 92-1 < 65 (7)
48<v, -s<62 (8) However, shoe 0: 2-i Atsube number of lens Vz-N: Abbe number (effect) of the lens with the largest negative refractive power in the second lens group Positive refractive power In order to design a compact two-group zoom lens consisting of a first lens group and a second lens group with negative refractive power, it is necessary to strengthen the refractive power of each of the first and second lens groups, and to increase the refractive power of each lens group. It is also important to shorten the lens length of the group. Furthermore, in order to reduce the amount of zoom movement of the second lens group, it is necessary to particularly reduce the power of the second lens group.

In the zoom lens of the present invention, the first lens group, the second L/
By using an aspherical surface in each lens group, aberration deterioration caused by strong refractive power is corrected, and by making at least one aspherical lens from a plastic material, manufacturing costs are reduced. In addition, plastic lenses are generally susceptible to temperature changes because plastic has a large change in refractive index and coefficient of linear expansion due to temperature changes, but in this invention, by setting the refractive power of the plastic lens to be weak, , the movement of the focal point position due to temperature changes is reduced.

The meaning of the conditional expressions will be explained below using Examples described below.

Embodiment 1 is an example in which an aspherical plastic lens is used in the first lens group. As shown in the cross-sectional view of the lens in FIG. Negative meniscus lens 1-1 with the concave surface facing the object side, negative meniscus lens 1-2 with the concave surface facing the object side, lens 1 with weak refractive power
-3 and lenses 1-4 with positive refractive power, and lenses 1-:3 are plastic lenses having aspherical surfaces.

Conditional expression (1) is a condition that defines the focal length of the lenses 1-3 so that when the first lens group has the above configuration, the change in the focal position with respect to the temperature change Lm does not pose a practical problem. Assuming that lenses 1-3 are thin lenses, the lens technician -
:3 refractive index, refractive index change with respect to temperature change △1'', and paraxial ray heights are rl, △n, and h, respectively, and temperature ν value is v.
7=(n ], )/Δr), the focus shift ΔS caused by the lens 1-3 with respect to a temperature change of ΔT is expressed by the following equation.

"' = "fptG" In the above equation, for a plastic such as acrylic polycarbonate, the shift value is 100 for a temperature change of ΔT=30°. Further, in the first lens group, h is approximately equal to the focal length of the entire system. Therefore, when considering the telephoto end, which has a large effect, if the focal length of the telephoto end is fT=70, assuming that the focal length of the telephoto end is a semi-telephoto of the 35th Cabinet version film, then h=70. If 1△St<0.4 is allowed for a temperature change of 30°, substitute these into ``l fpcll >], 20
get.

Therefore, lfr+l/f engineering>1.7 was set.

Next, Example 2 is an example in which an aspherical plastic lens is arranged in the second lens group, and in this way, the second lens group is arranged in order from the object side to the image side to form a positive refractive surface with a strong refractive power. It consists of a power lens 2-1, a meniscus-shaped lens 2-2 with a convex surface on the image side, and a meniscus-shaped lens 2-3 with a convex surface on the image side, and one of the lenses 2-2 and 2-3 is a non-convex lens. By using a plastic lens with a spherical surface and weak refractive power, it maintains a compact shape as shown in the cross-sectional view of the lens in Figure 3, and even when it has strong refractive power, astigmatism is less likely to become excessive. It is structured as follows. Conditional expression (2) is a condition that defines the focal length of the plastic lens in the second lens group so that when the second lens group has the above configuration, changes in the focal position due to temperature changes do not pose a practical problem. .

In the same equation as in the case of condition 0), for lens 2-2.2-3, h is 7 antacids after the focal length of the entire system, so assume that h = 0.71゛, = 49, Assuming other conditions are the same as above, lf, L21>
Get 60. Therefore l f PL21/ fT〉0.9
It was set.

Conditional expression (3) and conditional expression (4) define the aspherical displacement in the first lens group and the second lens group, respectively, and reduce coma aberration,
This is a condition for satisfactorily correcting distortion aberration. In this type of zoom lens, it is appropriate for aberration correction to provide the shutter diaphragm immediately behind the final surface of the first lens group, or at the rear of the first lens group. However, if the refractive power of the first and second lens groups is strengthened in order to make it more compact, positive refractive power will be distributed strongly to the front side of the diaphragm, and positive refractive power will be distributed strongly to the rear side of the diaphragm. A problem arises in which coma aberration and positive distortion become large. In this invention,
By providing an aspherical displacement in the first lens group that becomes thicker at the periphery, it is possible to correct the extroverted coma that tends to occur especially in the upper luminous flux in the second lens group. By providing an aspherical displacement that reduces the thickness,
This corrects the positive distortion that is noticeable at the wide-angle end.

At this time, regarding spherical aberration and astigmatism.

Since the effect of one of the aspherical displacements is canceled, the aberration correction effect described above can be obtained without excessively affecting these aberrations.

If the lower limits of conditional expression (3) and conditional expression (4) are exceeded, and the amount of aspherical displacement is small, it is difficult to obtain a sufficient aberration correction effect. On the other hand, if each upper limit is exceeded, the aspherical surface displacement is large, so the sensitivity to eccentricity of the lens becomes large.
This creates manufacturing difficulties.

It should be noted that, in addition to the aspherical surface described above, it is also possible in this invention to add an aspherical surface with a displacement opposite to that described above in the first lens group or in the second lens group to perform even more subtle aberration correction. It is not contrary to the purpose.

Conditional expression (5) is a condition that provides appropriate refractive power to the first lens group, and if this lower limit is exceeded and the refractive power of the first lens group becomes weak, the pack focus at the wide-angle end becomes too short, and as a result, This increases the diameter of the second lens group, which is not desirable for making the camera more compact. On the other hand, if the refractive power becomes strong beyond the upper limit, the magnification of the second lens group will inevitably increase, so the aberrations generated in the first lens group will be excessively magnified, making it difficult to correct the aberrations.

Conditional expression (6) is mainly a condition for giving appropriate refractive power to the second lens group. Since the zoom movement amount of the 2nd L lens group is expressed as 1f21・(fT-fw)/fT,
By providing a strong refractive power to the second lens group, the amount of zoom movement of the second lens group can be reduced, which is advantageous in making the camera more compact in view of the structure of the lens barrel.

However, if the lower limit of this conditional expression is exceeded and the refractive power of the second lens group becomes strong, difficulties arise in correcting aberrations such as increased distortion at the wide-angle end. If the upper limit is exceeded and the refractive power of the second lens group becomes weak, it becomes difficult to obtain a compact camera as described above.

Conditional expression (7) and conditional expression (8) relate to correction of chromatic aberration. Generally, with this type of zoom lens, lateral chromatic aberration is
Since short-wavelength images tend to become smaller on the wide-angle side and in the opposite direction on the telephoto side, in this invention, the second
This tendency is reduced by using a relatively large number of lenses in the lens group. If the lower limit of these conditional expressions is exceeded, the change in chromatic aberration of magnification due to zooming becomes large. If the upper limit of conditional expression (7) is exceeded, the axial chromatic aberration will be greatly exceeded on the telephoto side. Further, if the upper limit of conditional expression (8) is exceeded, it is good for correcting chromatic aberration, but the refractive index of these negative lenses also decreases, and the image plane tends to be overlapping.

(Example) Examples of the present invention that satisfy each of the above conditions will be shown below.

Each symbol in the table is as follows: R is the radius of curvature of each refractive surface, D is the distance between refractive surfaces Nd is the refractive index of the lens material, Y is the Atsube number, f is the focal length of the entire lens system, and ω is the half angle of view. , F indicates the F number, and FB indicates the back focus. Also, this seal has a plastic lens.

The shape of the aspherical surface is the X axis in the optical axis direction and the Y axis in the perpendicular direction to the optical axis.
Take the axis and take the direction of light travel as positive, K, A4, A, ,A
, is the aspherical coefficient, and is expressed by the following equation.

1st example f=36.2~68.6 F=3.7~7.0ω=
30.2''~17.4''' Na RD N.

1, 14.531 1.60 1.53172
48.92 22.707 2.80 3 -15.692 0.70 1.84666
23.84 -35.991 2.42 5 Car -24,4311,601,492
0057,06-23,0230,20 747,7502,601,5174252,48-1
,3,71,7 variable 9 -53.073 3.40 1.53172
48.910 -19.065 3.32 1], -1,6,5081,001,7725
049,6+2 -177.191 4.00
i3 -17.000 1.20 1.7
2916 54.714 -28.406 Aspheric coefficient 6iii on the 10th surface = -9,01
39=-3,2464A,,=8.8402xlO
-' A4=-6,7741xlO-5A S=
1.1521X] 0-6 ノ\6 =
-4,5584X 10-'A, = -2.4974X
10-9A, = -2,8838X10-” variable spacing
f D , F B36.2 12.
52 6.0 49.6 7.41 1.8.7 68.6 3.60 36.6 1 f PL□l/fT=8.6 Focus shift by plastic lens at 30° temperature rise Wide-angle end: 0 .03 Telephoto end: 0.091△x
11/f w= 1.3 X 10-' △X
1〉.

l△X21/fw=1. lX1. o-' △X2
〈Ofyu/f 工=],, 35 1fi/fT= 0
．． 94 Telephoto ratio at the telephoto end: 0.95 The cross-sectional view of the lens of the second embodiment is shown in Fig. 1, and the aberration diagram is shown in Fig. 2 (
Shown in A) to (C).

Second example f=36.1-68. ．． 3F=3.7~7.
0ω=30.5'~17.4' \ORD Nd 1 15.492 1.80 +, 51633
64. ,]2 27.682 2.30 3 -16.786 0.70. 1.84666
23.84 -36.1.11 2.60 5 -66.817 2.40 1.58913
61.26 -37.306 0.20 7 1.76.947 1.90 1.53172
48.98 -14.629 Variable 9 -37.445 2.00 1.58913
61.210 -20.945 1.00 11 Car -19,75El 2.00
1.49200 57.01.2 -20
．． 272 3.801.3 -10.960 1.
．． 00 1.69680 55.51,,I
-63,927 41 Spherical coefficient 5th city 1 11th surface=-8,7848X 10 K=-3,,229
6A,=-1,,1421xlo-4A,=9.40
32xlO-', 8, = 9.3751xlO-9
A,,=7.171.7X10-7A,=-6,
5+99X10-' A, = 7.4658X10
-”Variable interval f D, FFl3
6.1. 12.70 8.3 49.3 7.40 21. ．． 0 68.3 3.40 39.2 1 f pL, l/ f T=81.930°Plastic lens when temperature rises [This causes focal shift at wide-angle end: -0.02 Telephoto end 20.101△X 11
/f w=7.2x 1.0-S Δx, <
01△x21 /fw=6,2x1.0- △x 2>
OL,,/fi=1.33 1f21/f,=0.
96 Telephoto ratio at the telephoto end: 0.94 The cross-sectional view of the lens of this example is shown in Figure 3. The aberration diagram is shown in Figure 1.
Shown in Figures (A,) to (C).

(Effects of the Invention) As shown in the examples, when the zoom lens of the present invention has a focal length range of about 35 to 70 m for 35 parallel film, the telephoto ratio at the telephoto end is 0.95 in both examples. This is very small, and the zoom movement amount of the second lens group is also small, about 31 m, making it possible to realize a compact camera. As seen in the aberration diagram, various aberrations are well corrected, and although not shown, good depiction performance can be obtained even when focusing at short distances by extending the first lens group.

[Brief explanation of drawings]

FIGS. 1 and 3 respectively show the zoom lens of the present invention. FIG. 3 is a cross-sectional view of a lens according to a second embodiment. FIGS. 2 and 4 are aberration diagrams of the first and second embodiments, respectively. In each aberration diagram, (A) is at the wide-angle end. (B) is an aberration diagram at the intermediate focal length (C) at the telephoto end. @Figure 1 Figure 2 (A) Spherical aberration Astigmatism φ
Curvature aberration Fig. 2 (B) Spherical aberration Astigmatism Distortion aberration Fig. 2 (C) Spherical aberration Astigmatism Distortion aberration Fig. 3 Fig. 4 (A) Spherical aberration Astigmatism Distortion aberration F 5.1 Spherical aberration R-plane aberration 4 Figure (B) Astigmatism Distortion IIy Stop 4 Figure
(C)

Claims (1)

[Claims] 1) Consisting of a first lens group having a positive refractive power and a second lens group having a negative refractive power in order from the object side, the distance between the first lens group and the second lens group In a two-group zoom lens that changes magnification by changing
A small zoom lens characterized in that at least one of the lenses is a plastic lens with a weak refractive power.2) A claim characterized in that the first lens group has an aspherical plastic lens with a weak refractive power that satisfies the following conditions. Zoom lens in item 1 | f_P_L__1/f_T>1.7 However, f_P_L__1: Focal length of the plastic lens in the first lens group f_T: Focal length of the entire system at the telephoto end 3) The first lens group is A positive meniscus lens 1-1 with a convex surface facing the object side, a negative meniscus lens 1-2 with a concave surface facing the object side, and a lens 1-3 with weak refractive power.
, and a lens 1-4 having a positive refractive power, the zoom lens according to claim 1 or 2, wherein the lens 1-3 is an aspherical plastic lens.4) The second lens group satisfies the following conditions. The zoom lens according to claim 1, characterized in that it has an aspherical plastic lens with a weak refractive index that satisfies |f_P_L__2|/f_T>0.9, where f_P_L__2: focal length of the plastic lens in the second lens group 5) second The lens groups include, in order from the object side, a positive refractive power lens 2-1 having a surface with strong refractive power on the image side, a meniscus-shaped lens 2-2 having a convex surface on the image side, and a meniscus-shaped lens 2-2 having a convex surface on the image side. 6) The zoom lens according to claim 1 or 4, wherein one of the lenses 2-2 and 2-3 is a plastic lens having an aspherical surface and a weak refractive power. One aspherical surface has an aspherical displacement such that the lens becomes thicker at a position away from the optical axis than a spherical surface with the same paraxial radius of curvature, and at least one aspherical surface in the second lens group has: 2. The 2×10 zoom lens according to claim 1, wherein the zoom lens has an aspherical displacement that makes the lens thinner at a position farther from the optical axis than a spherical surface having the same paraxial radius of curvature, and satisfies the following conditions. ＾−＾5<｜Δx_1｜/f_W<2×10＾
−＾44×10＾−＾5<|Δx_2|/f_W<9×
10^-^41.2<f_W/f_1<1.6 0.8<|f_2|/f_1<1.1 where Δx_1: Optical axis distance of half the effective radius of the aspheric surface in the first lens group Displacement amount Δx_2 from a spherical surface with the same paraxial radius of curvature at a position away from Displacement from the spherical surface with the same values f_W: Focal length of the entire system at the wide-angle end f_1: Focal length of the first lens group f_2: Focal length of the second lens group