JPH0139563B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a zoom lens made of a so-called plastic lens made of organic glass, and particularly to an improvement of a zoom lens system having a variable power section and a master section. In various lens systems, if all the lenses as constituent elements are replaced with organic glass lenses called plastic lenses, the weight will be reduced by 1/1.
The weight can be reduced to 3 to 1/2, resulting in a significant weight reduction and lower manufacturing costs. However, plastic has a larger coefficient of expansion than glass, and its refractive index changes greatly with temperature, so it has the disadvantage of having so-called temperature aberration, which causes the focal plane of the lens to shift due to changes in temperature. In order to improve this drawback, in a fixed focal length lens with a triplet configuration, one of the positive lenses is made of glass, the other one is made of low dispersion plastic, and the negative lens is made of high dispersion plastic to correct chromatic aberration while changing temperature. A technique for erasing is known from Japanese Patent Application Laid-Open No. 143518/1983, filed by the same applicant as the present application. In this Japanese Patent Application Laid-Open No. 55-143518, for a fixed focus lens system consisting of N lenses including a plastic lens, when the composite focal length of the lens system is f, the i-th lens is Li, The focal length of Li at temperature t is fi, and the height from the object side is f.
The incident height of the paraxial ray to the lens Li is hi,
Let the refractive index of the lens Li at the temperature t be n _i (t),
The temperature dispersion number iwi of the material constituting the lens is defined as w _i = n _i (t) - 1/n _i (t ₁ ) - n _i (t ₂ ), where t ₁ < t < t ₂ , and each lens The temperature aberration coefficient h _i ² /(f _i w _i ) for the entire system, that is, | _N 〓 ⁱ⁼¹ h _i ² /f _i w _i |=0 is the “temperature extinction condition”. And so. As a result, the amount of focal shift due to temperature becomes zero for the two points t ₁ and t ₂ . However, this is limited to fixed focus lenses; for zoom lenses, the focal position changes significantly between the shortest and best focal length states (hereinafter referred to as temperature aberration fluctuations). . For example, if the piston is set at the longest focal length and zoomed to the shortest focal length, even if the object is in focus at room temperature, it will be out of focus at high or low temperatures. An object of the present invention is to provide a zoom lens system using a so-called plastic lens made of organic glass, which has the advantages of a plastic lens.
Further, it is an object of the present invention to provide a zoom lens system having a relatively simple configuration, in which the focal position is practically sufficiently corrected in the entire magnification range by zooming even when there is a large temperature change while maintaining a practically sufficient correction state of chromatic aberration. It is in. A basic proposal for a zoom lens system that achieved the above objectives was previously filed by the applicant (Japanese Patent Application No. 56-59685 (Japanese Patent Laid-open No. 57-176015), but it is mainly used when changing the magnification. For the purpose of correcting changes in the focal plane of a zoom lens, in at least one of the lens groups forming the variable power system of the zoom lens, at least one lens among the lens elements in the lens group is made of inorganic glass. This is effective for a zoom lens with a large zoom magnification of about 6, but for a zoom lens with a small zoom magnification of about 3, the focal point changes due to temperature changes occurring in the variable magnification section. In some cases, it may not be so large, and in such cases, correction can be made by omitting the correction of the variable magnification system and controlling only the DC component of the master section to successfully approach 0, or by distributing the positive and negative components. In this case, the focal plane may change during zooming depending on the operating temperature, but there is no actual harm if the amount is kept within the depth of focus.The present invention has a variable power section and a master section. At least one of the positive lenses that form the master section in a zoom lens is made of inorganic glass, so that temperature correction is performed only in the master section.In order to balance focal fluctuations. , the master lens section must be configured to cancel the DC component generated in the variable magnification section.Of course, it is necessary to properly correct not only temperature aberration but also chromatic aberration, and while correcting this chromatic aberration, The most important point of the present invention is to control temperature aberration.It is also necessary to maintain the power of the master section.The focal length of each lens that makes up the master section is set to f ₁ , f ₂ , ... f _i ...f _k , the temperature dispersion number of each lens is w ₁ , w ₂ , ... w _i , w _k , the height of the paraxial ray in each lens is h ₁ , h ₂ , ... h _i
...h _k , the Atsube number of each lens is ν ₁ , ν ₂ ... ν _i ...
Let ν _k be the temperature aberration coefficient for the master part.
Tm, chromatic aberration Cm, and power (refracting power) Pm are _k 〓 ⁱ⁼¹ k _i ² ／f _i w _i ＝Tm (1) _k 〓 ⁱ⁼¹ k _i ² ／f _i n _i ＝Cm (2) It is expressed as _k 〓 ⁱ⁼¹ −h _i /f _i =Pm (3). It is necessary to perform corrections that balance the chromatic aberration Cm and the temperature aberration coefficient Tm while maintaining the power Pm of the master section at a certain value. It is necessary to balance it so that it does not fluctuate. Regarding the temperature aberration coefficient T _V of the variable power section, for example, the applicant's previous application (Japanese Patent Application No. 1983-59685)
In the 3x zoom lens shown as the first example, the reference temperature t = 20°C, the low temperature t ₁ = -10°C and the high temperature t ₂
= 50℃ and at short focus end T _V (W) = 0.00759,
At the long focus end, a variation of T _V (T) = 0.02413 and a difference of 0.01654 was observed. By changing various variables of the master lens (curvature, interplanar spacing, power arrangement, etc.), it is not possible to eliminate the fluctuation component, but by approaching it to 0 or dividing it into positive and negative values,
Substantial harm can be reduced. This balance is different from the balance performed with fixed focus lenses, and at the short focal length end, the Tm value may be pushed in the negative direction. However, in this case, the positive and negative values are balanced when looking at the zooming as a whole. In the present invention, the low temperature t ₁ = -10°C and the high temperature t ₂
= 50℃, assuming the reference temperature is 20℃,
It is desirable that the temperature aberration coefficient Tm of the master part is in the range -0.02≦Tm≦0.01 (4), and in this case,
The temperature aberration coefficient Ttot of the entire lens system is set to be ｜T _tpt (W)｜≦0.01 (5) at the shortest focal length, and 0.005≦T _tpt (T) ≦0.03 (6) at the longest focal length. It is desirable to configure The conditional expressions (4), (5), and (6) above provide clear guidelines at the optical design stage in order to properly correct fluctuations in temperature aberrations associated with zooming, and are extremely effective. It is a condition. If all lens elements of a lens system are made of organic glass, the temperature aberration coefficient of the entire system will inevitably have a positive value since the entire system has positive refractive power, and if it is a zoom lens, a certain positive DC It has a value that includes a variable component due to scaling. The temperature aberration coefficient of each organic glass lens constituting the master section of the zoom lens is compared with the DC component of the temperature aberration coefficient of the entire system, and the coefficient value of one or more lenses in the master section is calculated as a whole. If the lens element that is almost equal to the DC component of the system is replaced with an inorganic glass, the DC component can be corrected, but the resulting value of the temperature aberration coefficient Tm of the master section is (4)
By setting the value within the range of the formula, it becomes possible to satisfactorily correct temperature aberrations of the entire zoom lens system. If the lower limit of equation (4) is exceeded, the DC component will be overcorrected, and if the upper limit is exceeded, the correction will be insufficient, making it difficult to correct temperature aberration fluctuations due to zooming to a practically sufficient value. And the temperature aberration coefficient T _tpt of the entire system is (5)
As expressed by the condition in equation (6), it can be a negative or positive value around zero in the shortest focal length state, and it can be a slightly larger positive value in the longest focal length state, as expressed in the condition in equation (6). , ideal for correcting temperature aberrations in zoom lenses. If these conditions are not satisfied, the balance of temperature aberrations will be disrupted and deterioration of the image will inevitably occur. Furthermore, in the present invention, it is desirable that the Abbe number νd of the inorganic glass lens provided in the master portion be >50 in order to satisfactorily correct the chromatic aberration as shown in equation (2) above. Also,
Organic glass lenses with larger refractive power have larger inherent temperature aberration coefficients, so in order to correct the positive DC component of the temperature aberration coefficient that occurs in the entire system, it is necessary to use a positive lens element with a larger refractive power in the master section. Advantageously, it is made of inorganic glass. Examples of the present invention will be described below. The first embodiment of the invention basically consists of the so-called 4
It is a group zoom lens, and its configuration, as shown in Figure 1, consists of, in order from the object side, a positive first group G ₁ as a focusing group, a negative second group G ₃ as a variator, and a positive first group as a compensator. Group 3
This is a zoom lens system with a zoom ratio of 3 and an F number of 1.8, which includes G ₃ and a positive fourth group G ₄ as a master section. Here, the first group, second group, and third group constitute a variable power section. The first group G ₁ is composed of a negative lens L ₁ made of polystyrene P ₈ , a positive lens L ₂ made of acrylic (PMMA) bonded to this, and a positive lens L ₃ made of acrylic.
The second group G ₂ is composed of a negative lens L ₄ made of acrylic, a negative lens L ₅ made of acrylic, and a positive lens L ₆ made of polystyrene bonded to this, and the third group G ₃ is a positive lens L made of acrylic. Consists of ₇ .
The relay system of the master section is the fourth group _G4 , which includes a positive lens L ₈ made of inorganic glass, a negative lens L ₉ made of polystyrene, two positive lenses L ₁₀ made of acrylic,
Consists of L ₁₁ . In FIG. 1, lenses made of inorganic glass are indicated by diagonal lines. The specifications of the first example are shown in Table 1 below. The temperature dispersion number wi of each lens element is also listed in the table. This temperature dispersion number wi is the reference temperature t = 20℃ and the low temperature t ₁ = -10
℃, high temperature t ₂ =50℃.

[Table] In the first embodiment, before temperature aberration correction, that is, when the eighth lens _L8 is made of acrylic and all lens elements are made of organic glass, the temperature aberration coefficient of the entire system is T _tpt (W) in distance state
= 0.03981, T _tpt (T) = 0.0565 in the longest focal length state
, both of which are positively large values. Therefore, in this example, the eighth lens _L8 is made of inorganic glass, and as a result, the temperature aberration coefficient of the fourth group as the master section is changed from Tm = 0.03334 before correction to Tm
= 0.00037, and for the entire system T _tpt (W) =
0.00684, T _tpt (T) = 0.02353, and although the fluctuation component could not be eliminated, it was possible to make it a fairly small value. FIG. 2 shows the ray aberration of the first embodiment. Second
In the figure, a shows the shortest focal length state, b shows the intermediate focal length state, and c shows the longest focal length state, Sph shows spherical aberration, Ast shows astigmatism, and Dis shows distortion aberration. d
The line (λ = 587.6 mm) is used as the reference wavelength, and the g-line (λ = 435.8 mm) is used as a guide for correcting chromatic aberration. The effect of temperature aberration correction in this example is explained in the third section.
It is shown in Fig. 4. Fig. 3 is an aberration diagram of an example in which temperature aberration is corrected, and Fig. 4 is an aberration diagram of an example before temperature aberration correction, that is, L ₈ is formed of acrylic, and the fourth group G ₄ as a master section is entirely made of organic glass. It is an aberration diagram in the case of In each figure, a indicates the shortest focal length state, b indicates the intermediate focal length state, c indicates the longest focal length state, Sph indicates spherical aberration, and Ast indicates astigmatism. In addition, the reference temperature is 20℃, and -10℃
The low temperature state of 50°C is shown as t ₁ , and the high temperature state of 50°C is shown as t ₂ . From these temperature aberration diagrams, it is clear that the zoom lens of this example has little fluctuation in the focal position even with large temperature changes from -10°C to 50°C, and maintains sufficient imaging performance for practical use. It is. In the first embodiment described above, one positive lens made of inorganic glass was used in the master part, but by forming another positive lens of inorganic glass,
Temperature aberrations can be better corrected. The second embodiment is an example of a zoom lens using two inorganic glass lenses in the master portion.
In the second embodiment, as shown in FIG. 5, in addition to the eighth lens L8, which was an inorganic glass lens in the first embodiment, an eleventh lens _L11 , which is _a positive lens closest to the image side, is also provided. It is made of inorganic glass.
If the lens closest to the image side is made of inorganic glass, the internal organic glass lens can be protected. Second
The specifications of the examples are shown in Table 2 below.

[Table] In this second embodiment, the temperature aberration coefficient of the master section is
Tm = -0.01333, which is a negative value, and the temperature aberration coefficient of the entire system is T _tpt (W) = -0.00686 at the shortest focal length,
In the longest focal length state, T _tpt (T) = 0.00983. The values of each of these temperature aberration coefficients are shown in Table 3 below together with the values of the first example.

[Table] FIG. 6 shows how the temperature aberration coefficient value for the entire system changes as the magnification changes. Figure 6 shows the temperature aberration coefficient T _tpt of the entire system on the vertical axis and the focal length of the entire system on the horizontal axis, with the left end representing the shortest focal length (Wide) and the right end representing the longest focal length state (Tele). represent In the figure, curve a shows the characteristics before correction, curve b shows the characteristics of the first embodiment, and curve c shows the characteristics of the second embodiment. As shown in the figure, in the first and second embodiments, although the fluctuation component due to scaling is not removed, its absolute value has become considerably small. In particular, as can be seen from curve c, in the second embodiment, the fluctuation component due to scaling is not removed. It is clear that the aberration coefficient becomes zero in the middle, and the absolute value of the aberration coefficient is corrected to be extremely small over the entire magnification range. The ray aberration of the second embodiment is shown in FIG. 7, and the temperature aberration is shown in FIG. The display of each aberration is the same as in FIGS. 2 and 3 for the first embodiment, and the temperature aberration before temperature aberration correction is as shown in FIG. 4. From a comparison of each aberration diagram, it is clear that not only ray aberration but also temperature aberration is corrected sufficiently well for practical use, even for temperature changes from -10°C to 50°C, as shown in Figure 6. This confirms the effectiveness of the characteristic curves b and c of the temperature aberration coefficient. As described above, according to the present invention, only one or several lens elements are made of inorganic glass, and all other lens elements are made of plastic (organic glass), thereby eliminating temperature aberrations inherent to organic glass. It is possible to achieve a zoom lens that is sufficiently well corrected for practical use over the entire zoom range. While maintaining many of the advantages of plastic lenses, such as light weight, ease of manufacture, and low price, not only aberrations related to the reference ray, but also chromatic aberrations were corrected in a manner favorable enough for practical use. It makes an excellent zoom lens and is extremely useful.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of the zoom lens according to the first embodiment of the present invention, and Fig. 2 is a ray aberration diagram of the first embodiment, where a is the shortest focal length state, b is the intermediate focal length state, and c is the longest focal length. state, Sph is spherical aberration, Ast
indicates astigmatism, and Dis indicates distortion. Fig. 3 is an aberration diagram of the embodiment with temperature aberration corrected, and Fig. 4 is an aberration diagram before temperature aberration correction, where a is the shortest focal length state and b
indicates an intermediate focal length state, c indicates a maximum focal length state, Sph indicates spherical aberration, and Ast indicates astigmatism. FIG. 5 is a configuration diagram of the zoom lens of the second embodiment, and FIG.
The figure shows how the temperature aberration coefficient value changes with magnification for the entire system, and Figure 7 shows the ray aberration diagram of the second example.
FIG. 8 shows a temperature aberration diagram of the second embodiment. Explanation of the symbols of the main parts, {G ₁ ... focusing group, G ₂ ... variator, G ₃ ... compensator, G ₄ ... master section} variable power section, L ₈ ... positive lens made of inorganic glass, L ₁₁ ……Positive lens made of inorganic glass.

Claims (1)

[Scope of Claims] 1. A zoom that has a variable power section and a master section, and is mainly made of lenses made of organic glass, and a part of the positive lenses of the positive lenses forming the master section are made of inorganic glass. In the lens system, the focal length of the i-th lens from the object side of the zoom lens system is f _i , and the height of the zoom lens system from the optical axis is f i
The incident height of the paraxial ray incident on the i-th lens is h _i , the refractive index of the i-th lens at temperature t is n _i , and the temperature dispersion number w of the material forming the i-th lens is _i is defined as w _i = n _i (t) - 1/n _i (t ₁ ) - n _i (t ₂ ), where t ₁ = -10°C t = 20°C t ₂ = 50°C, and each The temperature aberration coefficient for the lens, h _i ² /(f _i w _i ), is the sum of the values for the master part as Tm, and the sum of the entire system in the shortest focal length state of the zoom lens system as Ttot(W). It is characterized by satisfying the following conditions: -0.02≦Tm≦0.01 |Ttot(W)｜≦0.01 0.005≦Ttot(T)≦0.03, where Ttot(T) is the sum total for the entire system in the longest focal length state. A zoom lens system.