JPH0320732B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a viewfinder optical system for a camera.
More specifically, the present invention relates to an inverted Galilean finder optical system that introduces an aspherical surface into the objective lens of the optical system.

In an inverted Galilean finder optical system that has an objective lens and an eyepiece in order from the object side,
If each lens is composed of only spherical surfaces, distortion and mismatch between the central diopter and the peripheral diopter will occur.
In order to improve this, it may be possible to increase the number of lenses by dividing the constituent lenses, but this would impair the characteristic of the inverted Galilean finder optical system, which is the simple construction. Here, it is desired to introduce an aspheric surface into the lens surface.

Therefore, an object of the present invention is to study the relationship between the shape of the aspherical surface, distortion, and diopter when introducing an aspherical surface to the lens surface in an inverted Galilean finder optical system, and to improve the distortion and improve the surrounding area. An object of the present invention is to provide an inverted Galilean type finder optical system whose diopter is made to match the center diopter well.

The present invention will be explained in more detail below.
In the present invention, the shape of an aspherical surface is defined by a rotational curved surface given by the following equation.

X=CoY ² /1+(1−εCo ² Y ² )1/2+ _∞ 〓 ⁱ⁼⁰ C _i Y ²ⁱ …(1) Here, X is the distance taken from the vertex in the optical axis direction in the direction in which the ray travels, Y is height from optical axis, ε is rotation 2
Co is the curvature of the reference sphere (curvature at the vertex), and Ci is the 2i-order aspherical curvature.

In equation (1), the first term on the right side represents the reference rotation quadratic surface. In other words, when ε=0, it represents a spherical surface with radius 1/Co, when ε>0, it represents a spheroidal surface, and when ε<0
Each represents a hyperboloid of rotation. The second term on the right side is a displacement term, and is added to the first term to deform the reference rotation quadratic surface. radius of curvature r
To obtain the desired aspherical shape from the reference surface of Co
=1/r, ε, Ci(i=2,3,
4,...), give an appropriate value. As mentioned above, ε determines the shape of the reference rotation quadratic curved surface. Next, by selecting the value of Ci, this reference rotation 2
Next, we will examine what happens to the distortion and diopter when deforming the curved surface.

First, when the second-order aspherical curvature C ₁ in equation (1) takes a value other than zero, the curvature of the apex of the surface is C ₀ +2C ₁
It has the effect of Therefore, selecting the value of C ₁ is equivalent to ultimately setting the curvature of the reference sphere to C ₀ +2C ₁ . That is, the C ₁ Y ² term changes only the paraxial characteristics of the finder optical system.

On the other hand, the term C _i Y ²ⁱ (i≧2) affects the curvature of the lens surface off-axis without changing the curvature of the apex, and contributes to improving distortion and diopter mismatch.
When the second surface of the objective lens is an aspherical surface, when Ci is negative, a deformation that weakens the refractive power off the axis of the objective lens can be applied to the reference rotation quadratic curved surface. In addition, the influence of surface deformation due to Ci is
The larger i is, the larger the distance from the center is. For example, if C ₂ acts mainly between the center and the periphery, C ₃ acts mainly on the periphery. Also
The smaller C ₀ is, the more the effect range of C ₂ is on the periphery. When the above-described surface deformation due to Ci acts to weaken the refractive power of the objective lens, the distortion in the peripheral area and the diopter in the peripheral area are generally moved from the minus side to the plus side. In an inverted Galilean finder optical system using a spherical surface, the distortion is generally largely biased toward the negative side, and the peripheral diopter is also generally biased greatly toward the negative side, so the above will contribute to these improvements. In consideration of the above, distortion and diopter can be improved by selecting a value of Ci that makes both the characteristics of distortion and diopter depending on the angle of view as flat as possible.

Therefore, the present invention introduces an aspherical surface into the second surface of the inverted Galilean finder optical system, and when the shape of the aspherical surface is expressed by equation (1), C ₂ <0...() C ₃ ≧0, C ₄ ≦0 or C ₃ ≦0, C ₄ ≧0
…() 0.001<|f ³ C ₂ |<10 …() It is characterized by satisfying the following conditions. However, f is the focal length of the objective lens, _C2 is the fourth-order aspherical curvature,
C ₃ is a 6th order aspherical curvature, and C ₄ is an 8th order aspherical curvature.

Examples based on this condition ()()() will be described below. FIG. 1 is a sectional view showing a general inverted Galilean type finder optical system. In FIG. 1, light enters from the left. G ₁ is an objective lens with negative refractive power, the radius of curvature of the first surface is r ₁ , the radius of curvature of the second surface is r ₂ , and the refractive index is n ₁
shall be. _G2 is an eyepiece having positive refractive power, the radius of curvature of the first surface is _r3 , the radius of curvature of the second surface is _r4 , and the refractive index is _n3 . n ₂ is the refractive index between lenses G ₁ and G ₂ . Furthermore, d ₁ , d ₂ , and d ₃ represent the axial spacing between the surfaces r ₁ , r ₂ , r ₃ , and r ₄ as shown in the figure.

Conditions () and () are conditions for weakening the refractive power around the aspherical surface.

Condition () defines the 6th and 8th order aspherical curvatures. By satisfying this condition (), it is possible to weaken the peripheral refractive power slightly excessively with C ₂ and compensate for this with C ₃ or C ₄ , and to improve distortion and diopter mismatch. Can be corrected.

The following table shows the configuration of an embodiment in which an aspherical surface is introduced on the second surface of the objective lens based on the present invention.

Table 1 Magnification m = 0.53 r ₁ = ∞ d ₁ = 1.5 n ₁ = 1.491 r ₂ = 12.8 d 2 = 22.0 _{n 2 =} 1.0 r ₃ = 26.1 d ₃ = 2.0 n ₃ = 1.5168 r ₄ = ∞ C ₀ = 1 /r ₂ ε=1.0 C ₁ =0, C ₂ =0.00007 C ₃ , C ₄ ,...C _∞ =0 |f ³ C ₂ |=1.24 The distortion and diopter of Example 1 are shown in Figure 2. show.

Table 2 Example 2 Magnification m = 0.50 r ₁ = -100.0 d ₁ = 2.0 n ₁ = 1.491 r ₂ = 13.3 d ₂ = 22.5 n ₂ = 1.0 r ₃ = 24.0 d ₃ = 3.0 n ₃ = 1.491 r ₄ = ∞ C ₀ = 1/r ₂ ε = 1.0 C ₁ = 0, C ₂ = −0.00008 C ₃ , C ₄ ...C _∞ = 0 | f ³ C ₂ | = 1.074 Distortion and diopter of Example 2 It is shown in Figure 3.

Table 3 Example 3 Magnification m = 0.53 r ₁ = ∞ d ₁ = 1.5 n ₁ = 1.491 r ₂ = 12.8 d ₂ = 22.0 n ₂ = 1.0 r ₃ = 26.1 d ₃ = 2.0 n ₃ = 1.5168 r ₄ = ∞ C ₀ = 1/r ₂ ε = 1.0 C ₁ = 0, C ₂ = -0.00004 C ₃ = -0.0000003 C ₄ , C ₅ ,...C _∞ = 0 | f ³ C ₂ | = 0.709 Example 3 The distortion and diopter are shown in FIG.

Table 4 Example 4 Magnification m = 0.48 r ₁ = 140.0 d ₁ = 1.5 n ₁ = 1.491 r ₂ = 9.5 d ₂ = 22.0 n ₂ = 1.0 r ₃ = 22.0 d ₃ = 2.5 n ₃ = 1.491 r ₄ = ∞ C ₀ = 1/r ₂ ε = 0.73 C ₁ = 0, C ₂ = -0.000018 C ₃ = -0.000001 C ₄ , C ₅ ,...C _∞ = 0 | f ³ C ₂ | = 0.163 Example 4 The distortion and diopter are shown in FIG. In this embodiment, the reference rotational quadric surface is a spheroidal ellipsoid, and this is modified. As shown in Example 4, one way to obtain good results is to make both C ₂ and C ₃ negative and compensate for each other while weakening the refractive power in the peripheral area.

Table 5 Example 5 Magnification m = 0.53 r ₁ = 400 d ₁ = 1.5 n ₁ = 1.491 r ₂ = 12.4 d ₂ = 22.4 n ₂ = 1.0 r ₃ = 26.35 d ₃ = 2.0 n ₃ = 1.5168 r ₄ = ∞ C ₀ = 1/r ₂ ε = 1.0 C ₁ = 0, C ₂ = -0.000034 C ₃ = -0.00000048 C ₄ = 0.0000000006 C ₅ , C ₆ ,...C _∞ = 0 | f ³ C ₂ | = 0.604 The distortion and diopter of Example 5 are shown in FIG. As in this example, if C ₂ and C ₃ are negative and C ₄ is positive,
C ₂ and C ₃ slightly weaken the peripheral refractive power, and C ₄
This can be compensated for and provides a means to obtain good results.

Table 6 Example 6 Magnification m = 0.46 r ₁ = 140 d ₁ = 1.5 n ₁ = 1.491 r ₂ = 9.5 d ₂ = 22.0 n ₂ = 1.0 r ₃ = ∞ d ₃ = 2.5 n ₃ = 1.491 r ₄ = -22.85 C ₀ = 1/r ₂ ε = 1.0 C ₁ = 0, C ₂ = −0.00009 C ₃ = 0.00000018 C ₄ = −0.00000002 CE, C ₆ ,...C _∞ = 0 | f ³ C ₂ | = 0.814 The distortion and diopter of Example 6 are shown in FIG. As in this example, if C ₂ is negative, C ₃ is positive, and C ₄ is negative, C ₂ weakens the refractive power in the peripheral area somewhat excessively, C ₃ compensates for this, and C ₄ can be corrected and good results can be obtained.

In addition, when the second surface of the objective lens is made aspherical,
To achieve the objective by making C ₃ negative,
It is desirable to satisfy the following conditions.

0.01<|f ⁵ C ₃ |<100...() Condition () specifies the sixth-order aspherical curvature, and by satisfying this condition, a good You can get good results.

Note that the distortion that is often perceived by the eye is not the absolute amount of distortion at that point, but the rate of change in the degree of distortion, when paying attention to a certain point in the viewfinder field of view. Therefore, if the rate of change in the degree of distortion in the periphery can be reduced, the absolute amount of the degree of distortion can be allowed to be somewhat large as in Example 5 to prevent the peripheral diopter from being excessively corrected to the positive side. It can be prevented. Such considerations should also be taken into account when implementing the present invention, from the perspective of improving distortion and diopter.

Since the present invention is configured as described above, it is possible to improve distortion and diopter in an inverted Galilean type finder optical system. Furthermore, if the i of the aspherical surface curvature Ci differs, the portions where the deformation is greatly affected will differ, so the surface can be deformed according to the purpose by selecting i of Ci or by combining a plurality of Ci. However, as is clear from the above embodiments, if C ₂ , C ₃ , and C ₄ are all 0, a sufficient effect cannot be obtained from the aspheric surface, so such a case is naturally excluded. Furthermore, according to the present invention, an aspheric surface suitable for the purpose can be obtained even when C ₀ is small and even when C ₀ is 0 (a special case where the reference rotational quadric surface is a perfect plane). , the scope of application during design is wide.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a general configuration of an inverted Galilean finder optical system, and FIGS. 2 to 7 are graphs showing distortion and diopter of Examples 1 to 6 of the present invention, respectively. G ₁ : Objective lens, G ₂ : Eyepiece lens, r ₂ : Second surface of the objective lens.

Claims (1)

[Claims] 1. In an inverted Galilean finder optical system having an objective lens and an eyepiece in order from the object side, the second surface of the objective lens is an aspherical surface, and the shape of this aspherical surface is as follows: X=CoY ² /1+(1-εCo ² Y ² )1/2+ _∞ 〓 ⁱ⁼⁰ C _i Y ^2iwhere , , ε: rotational quadratic surface shape coefficient, Co: curvature of reference sphere (curvature at apex), An inverse Galilean finder optical system characterized by satisfying the following conditions: C ₂ <0 C ₃ ≧0, C ₄ ≦0 or C ₃ ≦0, C ₄ ≧0 0.001<|f ³ C ₂ |<10 where, f: focal length of objective lens, C ₂ : fourth-order aspheric curvature, C ₃ ; 6th order aspherical curvature; C ₄ : 8th order aspherical curvature.