JPS60189722A - Zoom lens - Google Patents

Abstract

PURPOSE:To compose the zoom lens almost of plastic lenses while decreasing the number of glass lens element and to perform temperature compensation sufficiently by performing the temperature compensation while regarding a magnification variation system consisting of plural movable lens groups as on lens group. CONSTITUTION:The 1st lens group consists of >= lenses including at least one plastic lens and one glass lens. The 2nd lens group consists of only plural plastic lenses. The 3rd lens group is compsed of all plastic lenses. The temperature compensation of the magnification variation system is so performed that power values phiI, phiIOTAIOTA, phiIII of the 1st, the 2nd, and the 3rd, lens groups satisfy inequalities I respectively, thereby obtaining the lightweight, low-cost zoom lens while decreasing the number of lens elements. In the inequalities, C is the coordinate position of an image of a body formed through the magnification variation system, nuP is the Abbe number of the plastic lens constituting the 1st group, and nuG is the Abbe number of the glass lens of the 1st lens group.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a zoom lens using a plastic lens, and particularly relates to a zoom lens that compensates for movement of a formed image due to temperature changes. ] Lenses using plastic as optical lens material,
In other words, plastic lenses have a specific gravity that is 1/3 to 1/4 that of glass, so they are much lighter than glass lenses, and their material costs are low and they can be made into any desired shape by injection molding. Since lenses with a lens surface shape can be easily obtained, they can be produced at low cost, and optical lenses can be significantly reduced in weight and cost. In particular, in zoom lenses that consist of many optical lenses, the use of plastic lenses makes them lighter and lighter.
This is extremely useful in promoting cost reduction.

However, plastic lenses have extremely large changes in refractive index and linear expansion coefficient due to temperature changes compared to glass lenses, which are 100 times and 10 times that of glass lenses, respectively, so the imaging plane moves significantly due to temperature changes. However, the image becomes blurred and the image quality deteriorates. The allowable amount of movement of the imaging plane due to temperature changes is determined by the depth of focus of the lens.

For example, in the case of a video camera using a 215-inch size photographing element, the permissible circle of confusion is about 4 Qltm, and in the case of a lens of about 15 inches, the depth of focus is about 200 μm. However, when it comes to a large aperture lens of about Fl, 4, the depth of focus is about 50 μm. On the other hand, in the case of a plastic lens, assuming that the practical temperature change range is 130° C., a movement of the formed image occurs by ±600 to ±500 μm.

Furthermore, in the case of a zoom lens, multiple lens groups change their positions to change the focal length, so the amount of movement of the image plane due to temperature changes varies depending on the position of each lens group.
Therefore, the amount of movement of the imaging plane due to temperature (hereinafter referred to as zoom fluctuation amount) changes depending on the amount of magnification change. The amount of zoom variation depends on the amount of focal length change during zooming, but
200μIB even at double zoom, 400μIB at 6x zoom
It is about μm.

In this way, when a plastic lens is used as a large-diameter zoom lens, the movement of the imaging plane due to temperature changes and the amount of zoom fluctuation increase the depth of focus.

This results in a blurred image.

Therefore, in order to eliminate image blur, it is necessary to correct the movement of the imaging plane due to temperature changes and the amount of zoom fluctuation, but it is particularly difficult to correct the amount of zoom fluctuation mechanically. , compensation in optical design is required.

Temperature compensation in optical design can be performed in line 5 as follows.

Now, if the rate of change in refractive index due to temperature change of a plastic lens is βj, and the coefficient of linear expansion is γ, then for a single lens with a refractive index of R1, a radius of curvature on both surfaces R,, R, and a focal length of f, the temperature is The amount of change Δf in the focal length when the focal length changes by ΔT is expressed by the following formula.

Here, -Δf when the practical temperature change ΔT is 30°C
/fc-1/νT) is the focal length change rate, and for the t-th lens or lens group of a lens system consisting of a plurality of lenses, the height from the optical axis of the incident light is 1. The focal length is f=, the rate of change in focal length due to temperature change is 1/, and the focal length of the entire lens system 1 is f. Then, the amount of movement of the imaging plane "tA" due to temperature changes in the lens system is expressed by the following formula, and this formula (2)
Therefore, in order to compensate for the movement of the image plane due to temperature changes (hereinafter referred to as temperature compensation), A, = 0...
... Song... Hit... It should be f31. Formula (2)
is the same as the condition for chromatic aberration correction, where ν is the Abbe number, and when considered in the same way as chromatic aberration correction, A t A can be made zero by combining lenses with different ν.

By the way, the shift of plastic lenses is almost the same regardless of the material, so in order to perform temperature compensation,
It is necessary to combine glass lenses with different focal length change rates with plastic lenses, and in fact, in the case of single focal length plastic lenses, temperature compensation is performed by incorporating a glass lens.

However, in the case of a zoom lens, since the height of the incident light from the optical axis (hereinafter referred to as the height of the incident light) changes depending on the zooming position (i.e. zoom magnification), the amount of zoom fluctuation is corrected. For this reason, temperature compensation was performed by making each term on the right-hand side of equation (2) sufficiently small for each lens group, like a separate apochromat in the correction of chromatic aberration. .

Therefore, in order to perform temperature compensation for each lens group, a glass lens with a very large ν is required for each lens group, and in order to also correct chromatic aberration,
A glass lens is also required for this purpose, and in the end, a lens group using plastic lenses requires at least three lenses.

FIG. 1 is a configuration diagram showing an example of a conventional zoom lens that performs such temperature-compensated nine-chromatic aberration correction.

In the figure, the first lens group, which is a focus lens, consists of lenses 1, 2, and 3, of which lens 3 is a glass lens. The ■th lens group determines the amount of magnification, and among the ■th lens group, the ■th lens group has three glass lenses 4 and 5.
．． The 2nd lens group consists of 6 lenses, and the 2nd lens group consists of one glass lens 7. The 2nd lens group directs the image from the 2nd lens group and the 2nd lens group onto a predetermined screen (for example, the imaging plane of a video camera) 9. A diaphragm 8 is provided to form an image.

In this way, in conventional zoom lenses, even if plastic lenses are used, each lens group has at least one
It is necessary to incorporate several glass lenses, and as the number of lenses increases, the number of glass lenses also increases (,
The disadvantage was that it was not possible to make sufficient plastic lenses, and the advantages of using plastic lenses could not be fully utilized.

[Purpose of the invention]

It is an object of the present invention to eliminate the drawbacks of the above-mentioned prior art, make it possible to significantly reduce the number of glass lenses for temperature compensation, greatly improve the use of plastic, reduce the number of lenses in each lens group, and thereby reduce weight and cost. The goal is to provide a zoom lens with a wide range of features.

[Summary of the invention]

In order to achieve this object, the present invention configures a variable magnification system with lens groups ①, ①, and ① arranged sequentially from the object side, and are respectively ψ1. ψ,.

When it is 9m, ψ■〉0. ψ■〈0 and each power ψ1. ψ, , ψ, are set so that they each satisfy G〉O, the first lens group I and the The feature is that the temperature of the system is compensated.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a configuration diagram showing one embodiment of the zoom lens according to the present invention. It consists of four lens groups: 1st, 2nd, and 2nd lens groups. The 1st and 2nd lens groups constitute a variable power system. ■The lens group is concave lens 1
．． This lens group consists of two lenses, ie, a convex lens t2 and a convex lens t2, and adjusts focus according to the amount of magnification change. The 2nd lens group consists of two lenses, a concave lens t and a convex lens t4, and is a lens group for changing the amount of magnification. The 2nd lens group consists of two lenses, a concave lens L and a convex lens t6, and is a lens group for always keeping the position of the image plane in the variable magnification system constant regardless of the amount of magnification. ■Is the lens group made up of multiple lenses? +'B r t9, which is a lens group for forming an image of the imaging plane of the variable magnification system on a screen 9, and is provided with an aperture 8.

In this embodiment, with the above configuration, instead of performing temperature compensation for each lens group, the variable power system consisting of the first (2) and (2) lens groups is temperature compensated all at once, and the fixed lens group (2) is The lens group is designed to perform temperature compensation independently.

Here, FIG. 3 schematically shows the optical path of incident light in this embodiment. In the same figure, 10 is an incident ray, 11
．． 12. 13 and 14 are the principal surfaces of the 1st, 2nd, 2nd, and 2nd lens groups, respectively (for convenience, the object side principal surface and image side principal surface are made to coincide), 15 is the optical axis, and 16 is Portions corresponding to those in FIG. 2, which are the imaging points of the variable magnification system, are given the same reference numerals.

In FIGS. 2 and 6, the focal lengths of lens groups 2, 2, and 2 are now iff and f.
Let ν of each lens group be ν , , ν and ν□, and the T TI
TII Tl1 When the light ray 10 parallel to the optical axis 15 is incident on the l lens group at a height of l from the optical axis 15, the ■, ■, and ■th lens groups i2. The incident light 100 optical axis 15 at ia, i4
Assuming that the heights from .
Therefore, the following equation (4) must be satisfied.

・・・・・・・・・・・・・・・・・・(4) (However,
(fo: focal length of the entire system) Here, assuming that lens group (2) performs its own temperature compensation since it is fixed, it is set as follows in equation (4).

On the other hand, the power of a lens is expressed as the reciprocal of its focal length,
The same applies to the lens group, so the first point is as follows. If the powers of the ■ and ■ lens groups are used respectively as ψ■, ψkawa, and The ratio is ΦI1. If Φ■, the ratio of these powers is Φll+
Φ becomes as follows.

Therefore, by normalizing n=1 and transforming equation (4) using equations (51, (6), and (7)), the following equation (8) is obtained.

This equation (8) is based on hII
and hII are the conditions to be satisfied (ie, temperature compensation conditions).

In addition, in order to prevent the imaging plane of a zoom lens from moving when changing the zoom magnification, the ① lens group must also move according to the movement of the ① lens group that performs the zoom magnification change (hereinafter, this will be referred to as the zoom condition). ). In order to satisfy this zoom condition, hII and hIIl must approximately satisfy the following equation.

Here, C represents the image coordinate position of the variable magnification system, with the position of the No. 1 lens at the wide-angle end being the origin, and the direction from the object side toward the No. 1 lens group being positive.

The relationship between hII and hIll under the temperature compensation condition of equation (8), and the relationship between A■ and hIll under the zoom condition of equation (9)
When the relationship with l matches, in the variable magnification system, movement of the imaging plane due to temperature change is corrected regardless of the zoom magnification.

Therefore, if we change the power ratios Φ■ and Φm variously and find a condition under which the relationship between Φ■ and hnl matches between the temperature compensation condition (Equation (8)) and the zoom condition (Equation (9)), we can obtain this agreement. It has been found that the condition can be obtained only in the range where the power ratio Φ to Φ is expressed by the following equation.

-17,6<Φ■ku-1・・・・・・・・・・・・・・・
・・・・・・・・・α groanΦ■〉 0 ・・・・・・・・・
・・・・・・・・・・・・・・・（lυHowever・0−
LeT summer/, 1... (I3TI[ Note that using formula (61, f force, formula Ql, αυs
Qia is transformed as follows.

9m)o (11') Next, under the conditions of the above formula Ql, αυ, OJ, αken,
If νTl'', T■N, and B, then the above temperature compensation condition (
Ar in Equation (8)) and zoom condition (Equation (9))
1. It has been found that the condition for the relationship of All to match is satisfied. The reason why No. 204 is not expressed in an unspoken manner and the image coordinate position C has a range is because there is a permissible range for the amount of movement of the imaging plane due to temperature changes. In addition, formula αa is transformed from formula (6) to the following 5.Furthermore, the range of zoom magnification that can be taken while satisfying formula Q4) depends on α and ΦB, and the larger Φml for α, the better the zoom It was discovered that the ratio can be increased.

Now, if the variable magnification system is constructed entirely of plastic lenses, α=1, and 1 to 1 are 4, just like conventional glass lenses.
Even if it is about 5, the zoom ratio is limited to 25 to 6 times, and it is difficult to obtain a large zoom ratio.In order to increase the zoom ratio, 1Φ■
If 1 is further increased, the power of the ■th lens group is 91
The power ψ■ of the first lens group must be increased or the power ψ■ of the first lens group must be increased, but the former method requires a larger lens;
Ryosha's method makes it difficult to correct chromatic aberration.

Therefore, in order to obtain a large zoom ratio, it is better to increase α. In plastic lenses, ν is almost the same regardless of the type of material, so in order to make α>1, it is necessary to use a glass lens. Although it is necessary to use a glass lens in the first lens group or the second lens group, it is advantageous to use a glass lens in which it is difficult to use an aspheric surface in the second lens group, which has a weak power, from the viewpoint of correcting chromatic aberration.

That is, in FIG. 2, the lens t1 of the 1st lens group is a glass lens, the lens t1 is a plastic lens, and the 2nd lens t of the 2nd lens group.

The t4 hub is a plastic lens, and the lens tS#'@ of the ■th lens group is a plastic lens. The lens group Ⅰ has its own temperature compensation as before.
For example, let lens t7 be a glass lens.

In addition, when all of the lens group (■) is made of plastic lenses, and ν of the plastic material is denoted by C and P, the formula (
From 1 rise, the condition for α is as follows α=1νrr/v
I>1 ・・・・・・・・・・・・・・・・・・・・・
...aIP The above has been about temperature compensation, but when designing an actual zoom lens, it is also necessary to correct chromatic aberration. However, chromatic aberration correction for variable power systems must be performed for each lens group (separate apochromat).

In FIG. 2, which explains the correction of chromatic aberration of the first lens group, the power of the glass lens of the first lens group is expressed as ψ. , the Atsbe number is ν. Assume that the power of the plastic lens 1t is ψ1. Letting the Atsbe number be ν, the total power ψ■ of the Luns group is ψ! =ψ0 +ψa・・・・・・・・・・・・・・・
As mentioned earlier, as a condition for correcting chromatic aberration, in equation (2), change the shear 1 to Atsube's number, and then Since the sum of each term is zero, (however,
In equation (2), in the lens group, the lens z, l t
AI is common for t, and the focal length of the glass lens t is f. Then, the power 90 is ψ,=i
/f. Similarly, if the focal length of the plastic lens t is jp, its power ψ2 is ψ2=1/fP
De Al. ) Then, if we calculate the power of each lens from 200 and a, we get the following.

On the other hand, plastic lens T! The power of ψP shear is ν
rp, and if we calculate the amount of movement of the imaging plane due to temperature change in the first lens group from equation (2), then the glass lens 1. Since the shear of can be considered to be almost infinite, Eto=-EL νTI'TP holds true. Therefore, this formula and the formula (+1) are used. This formula shows the relationship between ν of the plastic lens and the entire ML lens group, but as mentioned above, νT of the ■th lens group consisting only of plastic lenses. is Shea P
be equivalent to. Therefore, from the formula as, H, we have.

Correction of chromatic aberration in the ① lens group can be performed without changing νTI by using plastic lenses with different Abbe numbers.

In the manner described above, temperature compensation and chromatic aberration correction of the variable magnification system are performed, and the zoom ratio can also be made sufficiently large.

Next, in the embodiment shown in FIG. 2, specific numerical values will be shown.

Here, assuming a 4x zoom lens, the focal lengths of the entire lens system and each lens group are as follows.

f(, = 11-44 m j Summer = 70 m fIl = -1ZSlI + 1 f ■ = 701E + 1 f ■ = 24 ■ In this case, ψ■/ψr = ψ■/ψ, --4.0.

Moreover, the material and Atsube number of each lens are as follows.

Material Temperature number t, = (glass) 12: Styrene (plastic) 30 t3; pMAiA (plastic) 60 t4: Styrene (plastic) 30 t,: Styrene (plastic) 60 t,: PMMA (plastic) 60
t, :, 5A5 (Glass) 61 ta: Styrene (Plastic) 60t, : pMM
A (P57 f 7ri) 6°Also, α is 1. Therefore, the zoom ratio can be increased up to 4 times.

The image position C of the variable magnification system was set at 133 m from the 1st lens group at wide-angle based on the temperature compensation conditions described above. In addition, the No. 1 lens group independently performed temperature compensation and chromatic aberration correction. In the variable power system of this embodiment, the dynamic angle of the lens group ① is relatively large, and in order to reduce the chromatic aberration of the entire system, the lens group ① is also laminated with an achromatic lens.

In this example, the amount of movement of the imaging plane when the temperature changes by 30° C. is 1004 mm at wide angle and +0.047 mm at telephoto, which is within the allowable value.

〔Effect of the invention〕

As explained above, according to the present invention, temperature compensation is performed not for each lens group independently in a variable power system consisting of a plurality of movable lens groups, but collectively for one lens group and L"r. Therefore, the number of glass lenses used can be significantly reduced, and the variable power system can be made mostly of plastic lenses.
In addition, it can be constructed with a relatively small number of variable magnification lenses, has sufficient temperature compensation, and has achieved significant weight and cost reductions. It is possible to provide a zoom lens.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram showing an example of a conventional zoom lens;
The figure is a configuration diagram showing an embodiment of the zoom lens according to the present invention, and FIG. 3 is a schematic diagram showing the optical path of the incident light in FIG. 2. 1.11. lit, IV... Lens group t
,~t9・・・・・・・・・・・・・・・ Lens ゝ-+ し）-ノ

Claims (1)

[Claims] From the object side, a first lens group having a positive power ψ■, a second lens group having a negative power ψ for setting the zoom magnification amount, and a zoom of the second lens group. A variable power system consisting of a second lens group with a power ψ that changes its position according to movement during zooming, and a fixed second lens group that forms an image of the variable power system on a predetermined screen. In the zoom lens, the first lens group consists of two or more lenses including at least one plastic lens and at least one glass lens, the (2) lens group consists only of a plurality of plastic lenses, and the (1) All of the lens groups are made of plastic lenses, and the first lens group is made of plastic lenses. Power ψ1 of each of the ■th and ■th lens groups. 5. A zoom lens characterized in that ψB and ψm each satisfy ψyo〉0, and the zoom lens system is configured to perform temperature compensation.