JP3389255B2 - Lens system using heterogeneous medium - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lens system used in a camera or the like, which lens system uses an inhomogeneous medium lens.

[0002]

2. Description of the Related Art A Gauss type lens system is well known as a lens system used in a silver halide camera or the like and having a large aperture with an F number of about 2 or less. This Gauss type lens system has a large number of 6 to 7 elements, which is disadvantageous in that the cost is high and the lens system is large.

A triplet type lens system is known as a lens system having a small number of lens components. However, the triplet type lens system has an F number of 2.
It has a drawback that it is as dark as about 8 or more and has a large field curvature.

It is possible to use an aspherical lens in order to solve the above-mentioned drawbacks. However, it is difficult to significantly reduce the number of lenses by using an aspherical surface in a Gauss type lens system. This is because spherical aberration, coma, distortion, etc. can be corrected well on an aspherical surface, but Petzval sum and chromatic aberration cannot be corrected. Even if an aspherical surface is used for the triplet type lens system, the field curvature cannot be corrected.

Recently, it has been considered to introduce a so-called inhomogeneous lens whose refractive index changes depending on the location. In particular, the radial type inhomogeneous lens can correct spherical aberration and the like in a favorable manner as well as using an aspherical surface, and can also correct Petzval sum and chromatic aberration.

As an example of a lens system using a radial type inhomogeneous lens, Applied Optics, Volume 21, 99.
There is a lens system shown on page 3-. This is the configuration shown in FIG. 5, in which a radial type inhomogeneous lens having a concave shape and a medium having a positive refractive index is symmetrically arranged with respect to the diaphragm to achieve good aberration correction and a conventional homogeneous spherical system. Compared to this, the number of lenses is less than 1/3 and the number of lenses is significantly reduced.

As another conventional example, Japanese Patent Laid-Open No. 61-401
There is one described in Japanese Patent No. It is a lens system composed of a total of 2 to 3 pieces of radial type inhomogeneous lens and homogeneous lens .

[0008]

The above-mentioned lens system comprising two radial type inhomogeneous lenses has a small number of lens components, but since it uses two expensive radial type inhomogeneous lenses, the cost is high. It has the drawback that In addition, since the radial type inhomogeneous lens has a difference between the designed refractive index distribution and the actual refractive index distribution inevitably in manufacturing,
When assembled as a lens system, it is effective to slightly change the curvature of the surface of the lens system to correct the above deviation. However, when two radial type inhomogeneous lenses are used, if the distributions of both inhomogeneous lenses are displaced, the degree of freedom is small and it is difficult to correct the displacement of the distribution only by changing the curvatures of the four surfaces. As a result, the tolerance for the distribution shape of the refractive index must be made very strict.

In addition, the total number of the radial type inhomogeneous lens and the homogeneous lens described in Japanese Patent Laid-Open No. 61-4012 is 2-3.
The lens system configured as described above has a drawback that a large amount of coma is generated and it is difficult to increase the aperture.

An object of the present invention is to provide a bright and high-performance lens system with a very small number of lenses.

[0011]

A lens system of the present invention comprises, in order from the object side, a first lens having a positive refractive power, a second lens having a negative refractive power, and a radial lens having a positive refractive power. It is characterized in that the radial type inhomogeneous lens comprises the third lens of the inhomogeneous type lens and satisfies the following conditions. (1) N ₁ <0 (2) −0.5 <φ _S / φ _M <0 (3) v ₁ > 40 (4) N ₂ · f ⁴ > 1 where N ₁ and N ₂ are refractive index distributions , Φ _S ,
φ _M is the refractive power of the surface of the radial type inhomogeneous lens and the medium, v ₁ is a coefficient representing the Abbe number of the medium, and f is the focal length of the entire system.

The lens system of the present invention is based on a so-called tesser type lens system in order to achieve a bright lens system with a small number of lenses and an F number of about 2 or less.

The tesser type lens system includes a positive first lens, a negative second lens, and a third lens which is a cemented lens in which a negative lens and a positive lens are cemented. this is,
The third positive lens of the triplet type lens system including the positive lens, the negative lens, and the positive lens is a cemented lens. This tesser type lens system reduces the chromatic aberration and Petzval sum by the cemented lens, and improves the image plane property which is a defect of the triplet type. This is because the cemented lens functions equivalently as a lens using glass having a high refractive index and low dispersion.

However, in the cemented lens of the tesser type lens system, since the cemented surface generally has a positive refractive power,
The spherical aberration cannot be corrected and the lens system cannot be brightened. Therefore, the tesser type lens system also
It is a dark lens with an F number of 2.8 or higher.

According to the present invention, the cemented lens of this tesser type lens system is replaced with a single radial type inhomogeneous lens, and chromatic aberration, Petzval sum, and spherical aberration are corrected at the same time, so that the image surface property is good and bright. I tried to achieve the system.

The distribution of the refractive index of the radial type inhomogeneous lens is such that the distance in the direction orthogonal to the optical axis is y, the refractive index at the radius y is n (y), the refractive index on the optical axis is N ₀ , and the distribution is When the coefficients are N ₁ , N ₂ , ..., _They are expressed by the following equation. n (y) = N ₀ + N ₁ y ² + N ₂ y ⁴ + ... Further, these coefficients have different values for each wavelength, and the coefficient for each wavelength is represented by a symbol representing the wavelength. And the Abbe number of the radial type inhomogeneous lens is given by the following equation. _{v 0 = (N 0d -1)} / (N 0F -N 0C) v i = N id / (N iF -N iC) (i = 1,2, ···) where N _id, N _iF, N _iC is the coefficient of N _i for the d line, the F line, and the C line, respectively.

According to the above-mentioned document (Applied Optics, Vol. 21, p. 993-), the radial type inhomogeneous lens has the following properties.

First, the Petzval sum PS of the radial type inhomogeneous lens is approximately expressed by the following equation. PS to (φ _S / N ₀ ) + (φ _M / N ₀ ² ) (a) where φ _S is the refractive power of only the surface of the radial inhomogeneous lens, and φ _M is only the medium of the radial inhomogeneous lens. Is the refractive power of.

The axial chromatic aberration PAC generated in the radial type inhomogeneous lens is approximately given by the following equation. PAC ∝ (φ _S / v ₀ ) + (φ _M / v ₁ ) (b) As is clear from the equation (a), Petzval sum, that is, field curvature, is obtained by setting φ _S and φ _M to opposite signs and It can be corrected by proper selection.

From the equation (b), it is understood that the axial chromatic aberration can be corrected by properly selecting the Abbe number v ₁ .

[0021] Still other references, journal objects
Volume 60 of Optical Society of America
As described on page 1436 and below (1970) , the third-order spherical aberration generated in the medium portion of the radial type inhomogeneous lens can be corrected by selecting the coefficient N ₂ .

As described above, it is possible to simultaneously correct Petzval sum, axial chromatic aberration, and spherical aberration with one radial type inhomogeneous lens. Therefore, by replacing the radial type inhomogeneous lens that has corrected these aberrations with a Tesser type cemented lens, the Petzval sum and spherical aberration of the entire lens system are well corrected, and a bright lens system with good image plane is achieved. You can

In the present invention, the radial type inhomogeneous lens is made to satisfy the above conditions, and the Petzval sum, axial chromatic aberration, and spherical aberration of the lens system are satisfactorily corrected.

Conditions (1) and (2) are conditions for correcting Petzval sum. Since the radial type inhomogeneous lens used in the present invention has a positive refractive power, it is necessary to give the medium a positive refractive power in order to reduce the Petzval sum PS given by the equation (a). The condition therefor is the condition (1). The relationship between the surface refracting power φ _S and the medium refracting power preferably satisfies the condition (2). If the range of these conditions is exceeded, Petzval sum cannot be satisfactorily corrected in view of formula (a), and the image surface property cannot be kept good. If these conditions are satisfied, the radial type inhomogeneous lens has a concave shape, the medium has a positive refractive power, and the entire lens system also has a positive refractive power.

The condition (3) is a condition for favorably correcting chromatic aberration. From the requirement of the condition (2), φ _S becomes smaller than φ _M , and it is necessary to increase the denominator of the second term of the formula (b) in order to correct the chromatic aberration from the formula (b). Become. If the range of this condition (3) is exceeded, chromatic aberration cannot be satisfactorily corrected, and if the chromatic aberration is forcibly corrected , it becomes impossible to maintain good image surface property.

The condition (4) is a condition for favorably correcting spherical aberration. If the range of this condition is exceeded, spherical aberration will be undercorrected, and it will not be possible to increase the diameter of the lens system.

Further, as described above, the radial type inhomogeneous lens can satisfactorily correct Petzval sum, axial chromatic aberration and spherical aberration with one lens, but on the other hand, it produces a large amount of coma. To suppress the occurrence of this coma,
It is advantageous to bring the entire lens system closer to symmetry.

In the lens system of the present invention, an aperture is arranged between the second lens and the third lens, which is a radial inhomogeneous lens, so that the coma aberration generated in the radial inhomogeneous lens is caused by the first lens and the second lens. It is desirable to have a structure that can be canceled by a lens.

[0029]

EXAMPLES Next, examples of the lens system of the present invention will be shown. Example 1 f = 50.0, F / 2.0, 2ω = 46.7 °, maximum image height = 21.6 r ₁ = 44.9342 d ₁ = 8.7694 n ₁ = 1.77250 ν ₁ = 49.66 r ₂ = -409.5004 d ₂ = 7.2432 r ₃ =- _{42.8274 d 3 = 1.2000 n 2 =} 1.72151 ν 2 = 29.24 r 4 = 94.5537 d 4 = 1.2000 r 5 = ∞ ( _{stop) d 5 = 7.3475 r 6 =} -57.1982 d 6 = 11.7438 n 3 ( heterogeneous lens) r ₇ = -174.6559 Inhomogeneous lens N ₀ = 1.72000, N ₁ = -1.4647 × 10 ^-3 , N ₂ = 6.42
The terms of 16 × 10 ⁻⁷ N _{3 and} above are 0, v ₀ = 50.24, v ₁ = 74.72, v
₂ = 74.72 φ _s / φ _M = -0.245, N ₂ f ⁴ = 4.01 Example 2 f = 50.0, F / 1.4, 2ω = 46.7 °, maximum image height = 21.6 r ₁ = 51.3222 d ₁ = 4.8000 n ₁ = 1.78650 ν ₁ = 50.00 r ₂ = -914.1988 d ₂ = 10.9963 r ₃ = -43.6092 d ₃ = 1.2000 n ₂ = 1.70055 ν ₂ = 30.11 r ₄ = 120.5201 d ₄ = 1.8000 r ₅ = ∞ (diaphragm) d ₅ = 9.4723 r ₆ = -88.4646 d ₆ = 10.1310 n ₃ (non-uniform lens) r ₇ = -189.0898 non-uniform lens N ₀ = 1.77250, N ₁ = -1.5475 × 10 ^-3 , N ₂ = 4.64
40 × 10 ^-7 N ₃ = -7.9995 × 10 ^-11 , N _{4 and} above terms are 0, v ₀ = 4
9.61 v ₁ = 82.81, v ₂ = 82.81, v ₃ = 82.81 φ _s / φ _M = -0.146, N ₂ f ⁴ = 2.90 where r ₁ , r ₂ , ... Are the radius of curvature of each lens surface, d
₁ , d ₂ , ... Are the wall thickness and air gap of each lens, n
₁ , n ₂ , ... Is the refractive index of each lens, ν ₁ , ν ₂ ,.
Is the Abbe number of each lens.

Example 1 has a lens configuration shown in FIG. 1, in which a first lens having a positive refractive power, a second lens having a negative refractive power, and a radial type heterogeneous lens having a positive refractive power are sequentially arranged from the object side. The lens is the third lens, and the diaphragm is between the second lens and the third lens. In this embodiment, the focal length of the lens system is 50 mm and the F number is 2.0, which is bright.

The second embodiment is shown in FIG. 2. The first lens having a positive refractive power, the second lens having a negative refractive power, and the radial type inhomogeneous lens having a positive refractive power are arranged in this order from the object side. And the diaphragm is between the second lens and the third lens. In this embodiment, the focal length is 50 mm
So, the F number is very bright at 1.4.

[0032]

The lens system of the present invention is composed of a very small number of lenses of three and is bright and has high performance. Moreover, only one of the three radial type inhomogeneous lenses is used, and even if the refractive index distribution is slightly deviated from the design value when the lens system is assembled at low cost, the remaining 6
The deviation of the distribution can be easily corrected only by changing the curvature of the surface, and the tolerance for the refractive index distribution shape can be loosened.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is an aberration curve diagram of Example 1 of the present invention.

FIG. 4 is an aberration curve diagram of Example 2 of the present invention.

FIG. 5 is a cross section of a conventional lens system.

Claims (2)

(57) [Claims] 1. A positive refractive power is provided in order from the object side.homogeneous
Of the lensHas a negative refractive power with the first lensHomogeneous lens
Radial heterogeneous lens having positive refractive power with the second lens
The above-mentioned radial type inhomogeneous lens consisting of the third lens
Satisfy the following conditions (1) to (4):
Lens system. (1) N₁ <0 (2) -0.5 <φ_S/ φ_M <0 (3) v₁ > 40 (4) N₂ ・ F^Four > 1 However, N₁ , N₂ IsRadial type inhomogeneous lensRefractive index
Coefficient representing the distribution, φ_S , Φ_M Are radial type
The power of the homogeneous lens surface and the medium, v₁ IsRadial type
Heterogeneous lensIs the coefficient of Abbe's number, f is the focal point of the whole system
It is a distance.Here, the refractive index component of the radial type inhomogeneous lens
The cloth has a distance y in the direction orthogonal to the optical axis and a radius y
N (y) is the refractive index at, and N is the refractive index on the optical axis. ₀ , Distribution coefficient
To N ₁ , N ₂ , ... is expressed by the following equation.
It n (y) = N ₀ + N ₁ y ² + N ₂ y ^Four + ... Also, the Abbe number v of the radial type inhomogeneous lens ₁ Is the next
Given by the formula. v ₁ = N _1d / (N _1F -N _1C ) Where N _1d , N _1F , N _1C For d line, F line and C line respectively
Coefficient N ₁ Is. 2. Between the second lens and the third lens
The lens system according to claim 1, further comprising a diaphragm.