JPH03181908A - Small-sized zoom lens compensated for influence of environmental change - Google Patents

Abstract

PURPOSE:To obtain an about twice variable magnification ratio and to use the zoom lens without using any special correcting means in the case of environmental changes in temperature and humidity by arranging a lens made of a moisture absorptive material between moisture-nonabsorptive lenses, and sealing the moisture absorptive lens with a lens holding member and eliminating a vapor exchange with the outside field. CONSTITUTION:Acryl-based plastic 2 is used for a 3rd and a 6th lens. The acryl-based plastic is easily affected by humidity variation and when the lens is used, an image is deteriorated greatly unless temperature variation and humidity variation are both compensated. For the purpose, by forming a sealed structure with the lens holding member 1, only the temperature variation needs to be compensated. Aluminum or plastic is used preferably for the lens holding member 1. Namely, the moisture nonabsorptive lenses are arranged at both ends in the optical axis direction and the moisture absorptive lens 2 arranged in the intermediate section is sealed hermetically with the moisture nonabsorptive lenses at both ends and the lens holding member 1. Consequently, the influence of the humidity variation is eliminated and the temperature variation is also compensated excellently to obtain the low-cost, compact zoom lens which displays excellent performance even in various environments.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a compact zoom lens, and particularly to a zoom lens that has few restrictions on back focus and is suitable for lens-shutter type compact cameras.

- (Prior Art) In recent years, lens shutter type compact foot cameras equipped with zoom lenses have become mainstream. Unlike zoom lenses for -eye reflex cameras, this type of zoom lens has no restrictions on back focus, but is required to be smaller and lower in cost.

Conventionally, a zoom lens for such a compact camera with a variable power ratio of about 2x was developed by Japanese Patent Application Laid-open No. 60-1.
70826, JP 64-52111, JP 1-193808, and JP 1-193.
One disclosed in Japanese Patent No. 807 is known.

(Problem that this invention is trying to solve) Japanese Patent Publication No. 1983
The zoom lenses described in JP-A-170826, JP-A-64-52111, and JP-A-1-193808 often use expensive glass materials or aspherical lenses made of glass, making them less expensive. As a photographic lens for a compact camera, there was a slight problem in terms of cost.

−4= Ways to reduce the cost of lenses include (a) reducing the number of lenses in the lens system, (b) lowering material costs.
(c) Lowering processing costs is an option, but it is difficult to achieve both. For example, if you forcibly reduce the number of sheets, you may have to increase the material cost and
On the other hand, the cost may increase. Using a plastic lens achieves both (b) and (c), and has a great effect on cost reduction. On the other hand, plastic lenses have the following drawbacks. (a) There are fewer materials than glass and there are no materials with high refractive index.

(b) The refractive index changes significantly with temperature changes, and in the case of acrylic and polycarbonate plastics, the refractive index decreases by more than 1X10-' per degree of temperature rise. (C) Plastic lenses have water absorption properties, and their water absorption rate and refractive index change with changes in humidity.Moreover, the change in water absorption rate is slow in response to changes in humidity, and an equilibrium state is reached. During the transition period, a refractive index distribution occurs within the lens, and aberrations deteriorate significantly. This property is remarkable for acrylic plastics, and the change in refractive index at 100% water absorption is
It increases by approximately 1×10−3.

Due to these drawbacks, conventional plastic lenses are rarely used in zoom lenses with a variable power ratio of 2x or more, and when they are used, a single non-power lens is used, mainly due to aberrations caused by aspherical surfaces. Most of them aim only for correction, and there are few that attempt to reduce costs by using plastic lenses.

The lens described in JP-A-1-193807 uses many acrylic or polycarbonate plastic lenses and is relatively inexpensive;
For lenses with a variable magnification ratio of about 1:2, when the temperature rises by 30 degrees, the focus position changes by 0.4 m + at the wide-angle end and 1.4 m + at the telephoto end, and acrylic plastic lenses are often used, and humidity increases. Since the influence of changes is large, some kind of correction means must be taken in practical use.

In particular, this correction is an important issue for cameras that perform focus adjustment without passing through the photographic lens.

(Problems to be solved by this invention) This invention has two problems.
The object of the present invention is to provide an inexpensive zoom lens that has a variable power ratio of about 200 yen and can be used without special correction means even when the temperature changes in the temperature environment.

(Means for Solving the Problems) In order to achieve the above two objects, the zoom lens of the present invention has at least two movable lens groups for zooming, in which one of the movable lens groups is At least one lens is made of a material whose refractive index changes by absorbing moisture in the air (hereinafter referred to as a hygroscopic lens);
It consists of a lens made of a material that does not absorb moisture (hereinafter referred to as a non-hygroscopic lens) and a lens holding member for fixing these lenses.The non-hygroscopic lens is placed at both ends in the optical axis direction, and the lens is placed in the middle. The hygroscopic lens is sealed by the non-hygroscopic lenses at both ends and the lens holding member.

More specifically, it is composed of a first lens group with a positive focal length and a second lens group with a negative focal length, and by shortening the distance between the first lens group and the second lens group, In a two-group zoom lens whose magnification is changed from to a long focal length end, the first lens group has, from the object side, a negative a lens unit consisting of a positive meniscus first lens and a biconcave second lens; It is constituted by a positive b lens unit consisting of a third lens and a biconvex fourth lens, and the second lens group includes, in order from the object side, a positive meniscus fifth lens with its convex surface facing the image side, and a negative sixth lens. The lens is composed of a negative meniscus seventh lens with a convex surface facing the image side, and the third lens,
The sixth lens is a plastic lens, at least one of which is a moisture absorbing lens, each lens in the first and second lens groups is hermetically fixed by a lens holding member that does not allow water vapor to pass through, and , the sixth lens is isolated from the outside world, eliminating the exchange of water vapor.

In addition, the focal lengths of the above-mentioned lens groups are F1, 3rd, and 6th.
The focal lengths of the lenses are f3, f6, and 8, respectively, and the distance between the second principal point of lens unit a and the first principal point of lens unit b is calculated, and the focal length of lens units b and a is fa (fa<O). When , the following conditional expression is satisfied.

3.0< f3/F, <8.0...■
0.5<1f61/f3<0.7...■
0 < D, b/ 11 < 0.2 ,,,
(2) Furthermore, the third lens is characterized by using an aspherical surface.

Polymethyl methacrylate or styrene acrylonitrile is used as the material for the moisture absorbing lens.

(Function) By placing a lens made of a hygroscopic material between non-hygroscopic lenses and sealing the hygroscopic lenses with a lens holding member, eliminating the exchange of water vapor with the outside world, the humidity of the hygroscopic lenses is reduced. It is possible to eliminate the influence of As a result, if there are other influences due to temperature changes, it is only necessary to compensate for them, and good compensation can be achieved. Generally, plastic materials such as polymethyl methacrylate and styrene acrylonitrile, which have good moldability and are ideal for optical materials, have such hygroscopic properties, but according to the structure of the present invention, this drawback can be avoided when used. The aspherical bottom shape is also simple and low cost. Furthermore, these materials are lightweight, and by using them for the movable lens group, the motive force for the lens group can be reduced, making it possible to make the sliding mechanism more compact.

In addition, by using a telephoto type two-group zoom lens with a positive first lens group and a negative second lens group, the overall length of the lens, especially at the wide-angle end, can be shortened, making it possible to use a compact camera. This makes it ideal for compact zoom lenses for use in other applications.

In the zoom lens of the present invention, the third and sixth lenses are made of plastic lenses, making it lightweight and low cost. Moreover, the plastic lens is sealed by the lens holding member and the glass lenses before and after the plastic lens, eliminating the exchange of water vapor with the outside world, so even if the outside humidity changes, there is no change in the internal humidity, and the internal humidity due to temperature carbonization. The only change in humidity can be caused by a change in saturated water vapor pressure, and since the volume of the internal space is small, this effect is negligibly small. Therefore, the water absorption rate of the internal plastic lens does not change, and it is possible to avoid changes in back focus due to changes in humidity, that is, focal shift and aberration deterioration. Furthermore, since the surface of the plastic lens cannot be touched directly after assembly, there is no chance of damage caused by accidental touching, and there is no need for a protective coat, resulting in further cost reduction.

As for temperature changes, since the focus movement caused by the temperature change occurring in the third lens can be canceled out by the focus movement occurring in the sixth lens, the focus movement caused by temperature change can also be reduced in the entire zoom range.

In the zoom lens of the present invention, the distribution of refractive power between the third lens and the sixth lens is made appropriate so that when the temperature rises, the back focus is extended at the wide-angle end and the focal point is shifted positively, and the focal point is negative at the telephoto end. The absolute value of the focal point movement is made small by 11 so that the focal point movement is as follows. Furthermore, since the camera body, lens barrel, etc. expand due to temperature rise, taking this effect into consideration, it is desirable that the absolute value of negative focus movement when temperature rises be smaller than the absolute value of positive focus movement.

In the zoom lens of the present invention, the influence of temperature change in the first lens group is magnified by the magnification of the second lens group and contributes to focus movement, so in order to reduce the absolute value of focus movement, it is necessary to It is desirable to have appropriate refractive power and lens configuration.

The focal length of the first lens group is Fl, the focal length of lens unit a is fM, the focal length of lens unit b is fb, a
If the distance between the second principal point of the lens unit and the first principal point of the b lens unit is Dab, the following equation holds true.

Fl-"=fi-1+fb-'Dabfa-"fb
-' (2) Also, assuming that the b lens unit is made up of a third lens with a focal length f3 and a fourth lens with a focal length f4 in close contact with each other, the following equation holds true.

f b-" = f 3-' + f 4-'
(2) When (2) is differentiated with respect to the temperature T, it follows that only fb, especially f3 among them, changes in temperature.

Similarly, by differentiating ■, ■, ■Therefore, the focal length change due to temperature rise of △T is ΔF,
If △f3, then it can be written as.

On the other hand, if the refractive index of the third lens is nl, the change in ΔT with temperature rise is Δn3, and the linear expansion coefficient with temperature rise of ΔT is α3, the following relationship generally holds: Δf3=(Ayu and +α3) f3 ・3-1 ■ For plastics used as lens materials such as polycarbonate and acrylic, (-Δn3/(
n, -1) + a3) is about 0.01.

From ■, ■ to ΔF□=0.01 (1-dan■) [phase] f3
fa Therefore, in order to decrease ΔF1, it is necessary to increase f3 and decrease Dab/1fal.

The conditional expression is a condition regarding the refractive power distribution of the third lens, and when the lower limit of (2) is below, the focus shift due to temperature changes becomes large, and when the upper limit is exceeded, the refractive power of the b lens unit is concentrated on the fourth lens. , and spherical aberration and comatic aberration are more likely to occur.

Conditional expression (2) is a condition regarding the distribution of refractive power between the third lens and the sixth lens, and when the lower limit of (2) is below, the cancellation of the focal shift by the sixth lens becomes small, and the focal shift at the wide-angle end becomes large. If the upper limit of (2) is exceeded, the amount of cancellation becomes too large, and the negative focus movement at the telephoto end becomes large.

If the upper limit of conditional expression
This is not desirable because the outer diameter of the lenses in the lens group also increases.

If the lower limit of condition (2) is not reached, pincushion distortion at the wide-angle end becomes large and correction becomes difficult.

Moreover, if an aspherical surface is used for the third lens, spherical aberration can be corrected well.

More preferably, if an aspheric surface is also used for the sixth lens, off-axis aberrations such as distortion, astigmatism, and aberration can be corrected well. Since these lenses are made of plastic, they can be easily mass-produced, and unlike glass lenses, they do not require high manufacturing costs even if they are aspherical.

(Example) Examples of the present invention that satisfy each of the above conditions are shown below. 1st
The figure also shows the structure of the lens barrel, but in the figure only the outline of the lens holding member 1 is shown, which is not made of only one piece of material but is made of a plurality of members. In each embodiment, the third and sixth lenses are made of acrylic plastic 2. As mentioned above, acrylic plastics are easily affected by changes in humidity, and unless both temperature and humidity changes are corrected during use, the image will deteriorate dramatically depending on the conditions of use. In the present invention, as shown in FIG. 1, since the lens holding member is used to form a sealed structure, it is only necessary to consider correction of temperature changes. Aluminum and plastic are suitable for the lens holding member.

In each embodiment, a sufficient air gap is provided between the a lens unit and the b lens unit, and b is made large. 1st
In the second embodiment, an aspherical surface is used on the image-side surface of the third lens to effectively correct spherical aberration, and in the third embodiment, an aspherical surface is also used on the object-side surface of the sixth lens. is used to effectively correct astigmatism.

In the embodiment of the present invention, the diaphragm (shutter) 3 is placed at a position of 1 mm immediately after the first lens group, so that the outer diameter of the second lens group and the outer diameter of the first lens are balanced. This is good, and the camera body won't have to become larger because of one or the other. Further, when focusing is performed by moving only the first lens group without moving the shutter, the weight becomes lighter and the burden on the focusing drive mechanism 6- is also reduced. In particular, when a plastic lens is used in the first lens group as in the present invention, the weight becomes lighter, so the focusing drive motor and the like can be downsized.

In addition, each symbol in the table is as follows: R is the radius of curvature of each refractive surface, D is the distance between the refractive surfaces, N is the refractive index of the lens material, y2 is the Abbe number, f is the focal length of the entire lens system, ω is half angle of view, F
indicates the F number and FB indicates the back focus.

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.
When the axis is taken, the traveling direction of light is positive, and K, A1, and A2 are aspherical coefficients, it is expressed by the following equation.

Example 1 F=36.01-68.00 F Nα=4.2
4~8.01ω=31.06~17.6°RD Na2O,8792,401,58+44 40.771
．． 098 2.40 27.029 1.00 1.72825 28
．． 568.78] 5.00 *-141,,5622,001,4920057,0
50,8211,20 30,1152,501,5163364,120,2
], 3 variable 31.527 3.10 1.53172 48
．． 9-17.499 1.50 * -32,3361,501,4920057,
0-106,5344,80 13,6340,801,7440044,8-50,
758 *mark indicates a plastic lens.

F D8 Fe12, OL
13.46 6.9749.49 7.41
20.3068.00 3.00 38.6
2 6th surface aspheric coefficient K = 1.77613 A1 = 4.41500X10-' A2 = 4.07027xlO-7 f3/F, = 5.62 1f, l/f3 = 0.5D, l
, / fa = 0.030 Focus shift when temperature rises by 30 degrees Wide-angle end: 0.044 Telephoto end knee 0.061 Example 2 F = 35.99 to 68.01 F No = 4.00
~7.56ω=3], 0°~17.6°* 22.074 61.180 27.958 565.004 -111,650 -44.691 2.40 1.80 1.00 6.50 2. 00 0.5O N, 1 1.60342 1.78472 1.49200 38.0 25.7 57°0 9- 30.087 2.50 1.51633 64
．． 1-24.987 Variable 30.681 2.80 1.53172 48
．． 9-17.205 1.50 * -32,1341,501,4920057,
0-100,0194,60 13,4230,801,7440044,849,3
43 Honjin shows a plastic lens.

F D 8 F B5.99
13.46 6.999.50 7.40
20.318.01 3.00 38.57
6th surface aspheric coefficient K = 7.75604 Ai = 3.77216X10-' A2 = 3.52266X10-' L/F□=5.27 1f, l/f3=o, 6Dab/
Ifal" 0.018 Focus shift 0 when temperature rises by 30 degrees - Wide angle @: 0.053 Telephoto end knee 0.014 Example 3 F=36.01~68.00 ω=31.0'~17,6 ″ 20.733 80.380 -28.171 61.804 * -440,468 -52,149 30,013 -22,675 35,034 17,365 * -27,562 -201,406 -13,951 - 46,597 2,40 2,00 1,00 6,00 2,00 0,80 2,50 Variable 3.10 1.50 1.50 4.80 0.80 F Nn =4.00-7.56 1.58144 40.7 1.72825 28.5 1.49200 57.0 1.51633 64.1 1.54072 47.2 1.49200 57.0 1.74400 44.8 F D 8 F
e12.01 13.38 6.974
9.50 7.33 20.2968.0
0 3.00 38.56 6th surface aspheric coefficient K = 8.37602 A1 = 3.85735X10-5A2 = 3
．． 57089X10-7 11th surface aspheric coefficient K = 2.84891 A1 = 9.89538xlO, -'A2 = 2.
34603xlO-” f3/F engineering=4.25 f,/f,=0.540a
b/fa = 0.035 Focus shift when temperature increases by 30 degrees Wide-angle end: 0.064 Telephoto end knee 0.066 (Effects of the invention) The compact zoom lens of the present invention is shown in the examples and each aberration diagram. As you can see, by sealing the plastic lens in the appropriate position, it eliminates the effects of humidity changes and also compensates well for temperature changes, making it a low-cost and compact product that exhibits good performance in a variety of environments. This makes it possible to create a zoom lens with excellent performance.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a lens and a lens holding member showing the concept of the structure of the zoom lens of the present invention, and FIG. 2 is a cross-sectional view of a first embodiment that also shows the method of varying the magnification of the zoom lens of the present invention. , and FIGS. 4 and 6 are cross-sectional views of the lenses of the second and third embodiments, with the lens holding members omitted, and the third, fifth, and third embodiments, respectively.
FIG. 7 is an aberration diagram of the first to third embodiments. In each aberration diagram, (A) is an aberration diagram at the wide-angle end, (B) is an aberration diagram at the black dot, and (C) is an aberration diagram at the telephoto end. 1: Lens holding member 2 Niblastic lens 3: Aperture (shutter)

Claims (1)

[Claims] (1) In a zoom lens having at least two movable lens groups for zooming, at least one of the movable lens groups is made of a material whose refractive index changes by absorbing moisture in the air. It consists of a lens made of a material that does not absorb moisture (hereinafter referred to as a hygroscopic lens), a lens made of a material that does not absorb moisture (hereinafter referred to as a non-hygroscopic lens), and a lens holding member for fixing these lenses. (2) A compact zoom lens characterized in that a non-hygroscopic lens is disposed in the middle, and the moisture-absorbing lens disposed in the middle is sealed by the non-hygroscopic lenses at both ends and a lens holding member. A two-group zoom lens that is composed of a first lens group and a second lens group with a negative focal length, and that changes magnification from a short focal length end to a long focal length end by shortening the distance between the first lens group and the second lens group. , the first lens group includes, from the object side, a negative a lens unit consisting of a positive meniscus first lens and a biconcave second lens, a positive third lens unit consisting of a positive third lens, and a biconvex fourth lens. b lens unit, and the second lens group includes, in order from the object side, a positive meniscus fifth lens with a convex surface facing the image side, a negative sixth lens, and a negative meniscus lens with a convex surface facing the image side. 7 lenses, the third and sixth lenses are plastic lenses, and at least one of them is a moisture-absorbing lens, each lens in the first and second lens groups is fixed by a lens holding member, and the third 2. The compact zoom lens according to claim 1, wherein the sixth lens is isolated from the outside world. (3) The focal length of the first lens group is F_1, the focal lengths of the third and sixth lenses are f_3 and f_6, respectively, and a
When the distance between the second principal point of the lens unit and the first principal point of the b lens unit is D_a_b, and the focal length of the a lens unit is f_a (f_a<0), the following conditional expression is satisfied. The compact zoom lens according to claim 2. 3.0<f_3/F_1<8.0 0.5<|f_6|/f_3<0.7 0<D_a_b/|f_a|<0.2 (4) Using an aspherical surface for the third lens A compact zoom lens according to claim 3, characterized in that: (5) A small zoom lens according to any one of claims 1 to 4, characterized in that polymethyl methacrylate or styrene acrylonitrile is used as a material for the moisture absorbing lens. (6) Photographing the small zoom lens according to claim 2. A small lens-shutter type camera that is characterized by being equipped with a lens for personal use.