JPS60220305A - Image-forming lens - Google Patents

Abstract

PURPOSE:To obtain an image-forming lens which has aberrations compensated well and has such wide field angle that it can be applied to a photographic lens or the like, by using a medium having a refractive index distribution and constituting a lens system with a small number of lenses. CONSTITUTION:This lens system is formed with two lenses consisting of a medium whose refractive index is changed continuously in accordance with the distance from the optical axis. The first lens arranged in the object side has such concave shape that the thickness is smallest on the optical axis and is increased according as going away from the optical axis, and further, the first lens has a meniscus shape whose concave is directed to the image side. The second lens arranged in the image side has a concave shape, and the image-side face of the second lens is plane or a concave. The first and the second lenses have the refractive index distribution where the refractive index is highest on the optical axis and is reduced gradually according as going away from the optical axis.

Description

Detailed Description of the Invention The cA of the present invention has a so-called refractive index distribution in which the refractive index gradually changes around the optical axis in a plane perpendicular to the optical axis. , for example, relates to an imaging lens suitable for photographic lenses and the like.

Conventionally, most imaging lenses have been constructed using a medium with a uniform refractive index, such as a half angle of view of 23° and an FA of 1.4.
1.8 Ordinary imaging lenses are generally composed of 6 to 7 homogeneous medium lenses, similar to photographic lenses.
If this could be constructed with fewer lenses, it would be extremely effective in terms of reducing the labor required for lens processing, simplifying the lens holding mechanism, and reducing the number of interfering factors such as eccentricity and other manufacturing errors. It's advantageous.

However, with the prior art, it is extremely difficult to reduce the number of constituent elements from the current level while maintaining optical performance.

On the other hand, in recent years, gradient index lenses have attracted attention as a new optical technology, such as array lenses used as erect equal-magnification imaging elements, skin collimator lenses that focus only on imaging near the axis -F, and optical discs. Many applications have been proposed for pickup lenses, etc.;
, 21, /466 [1982] reports its application to photographic lenses. This photographic lens is 2
Consists of a gradient index lens, with a meniscus shape on the object side that is thin on the optical axis, increases in thickness as it moves away from the optical axis, and has a convex surface facing the object side. The first lens has a refractive index distribution that has the highest refractive index on the optical axis and continuously decreases as it moves away from the optical axis, and the first lens has the highest refractive index on the optical axis on the image field side. It is thin, and the wall thickness increases as it moves away from the optical axis, and the surface convex to the image field side has a meniscus shape, and has the highest refractive index on the optical axis.
A second lens having a refractive index distribution in which the refractive index value continuously decreases with increasing distance rB from the optical axis is disposed. With this configuration, a standard lens with I'/I6 of 2 is obtained, and the number of lenses is significantly reduced compared to a conventional homogeneous lens.

Outside of this report, there are very few examples of applying a gradient index lens to an imaging lens with a relatively large angle of view.

An object of the present invention is to provide a condensing fS lens that uses a medium with a refractive index distribution and has an extremely small number of lenses, yet has aberrations well corrected and a wide angle of view that can be applied to photographic lenses. Our goal is to provide the following.

The image forming lens according to the present invention is formed by two lenses made of a medium whose refractive index continuously changes depending on the distance from the optical axis. The shape of the m-lens placed on the object side is such that it has the thinnest wall thickness on the optical axis and increases in thickness as it moves away from the optical axis (hereinafter, in this specification, such a shape will be referred to as a concave shape). The shape of the second lens arranged on the object world side is the above-mentioned concave shape, and the concave surface facing the image field side is a meniscus shape. The surface is a flat surface or has a concave shape facing the image field side. Furthermore, the refractive index distribution of both the first lens and the second lens is as follows.

A refractive index distribution in which the refractive index is highest on the optical axis and gradually decreases as the distance from the optical axis increases (hereinafter, in this specification, such a refractive index distribution will be referred to as positive refraction) rate distribution).

The power of the above-mentioned gradient index lens is determined by the gradient index type F, but as shown in the embodiments of the present invention, it is desirable that both the first lens and the second lens have positive power. 6. Furthermore, it is preferable that the object-world side surface of the second lens has a concave shape facing the object-world side.

The present invention will be explained in detail below.

Like the lenses constituting the imaging lens of the present invention, the refractive index N
The refractive index of the so-called radial gradient lens, N (rl, is N(r)=No+N1r"+N, r'+H@r"+
It is expressed as -・. However, NO+ N1 +Nt e
Nm... is a constant, and r represents the distance from the optical axis. A lens with such a refractive index distribution has the following characteristics: (1) The refractive index distribution itself has refractive power. Therefore, there is a large degree of freedom in design in the paraxial region, r2) If the refractive index distribution primarily contributes to the Petzval sum, and if the refractive index distribution has positive refractive power, then the same refractive power can be applied to the refractive surface. Its contribution to the Petzval sum is small compared to the amount it can hold. Therefore, it is relatively easy to set the Petzval sum to an appropriate value and correct the field curvature.

(3) It has an aberration correction effect similar to that of an aspherical surface. That is,
Various aberrations can be controlled within a certain range without affecting the paraxial front by using the fourth-order or higher distribution coefficients (N7.N311.).

(4) Has an achromatic effect. That is, the wavelength dependence of the second-order coefficient N1 has a stronger influence on the chromatic aberration coefficient, thereby making it possible to control the chromatic aberration.

The present invention takes advantage of these features and has the following configuration. That is, 1q-y5Wc arranged on the object side
The shape of the runes, gj! It is a concave meniscus with a concave surface facing the side, and the shape of the second lens is a concave shape with a concave surface on the image side, and the refractive index of the first and second lenses is Assuming that the refractive powers of the distribution are both positive, t and n are given.

As I have already mentioned, the second son is advantageous in correcting field curvature by sharing positive refractive power in the refractive power distribution.

Therefore, in the present invention, the refractive index distribution of the positive refractive power of both the first and second lenses is made narrower, thereby reducing the positive contribution to the Petzval sum. Furthermore, by making both lenses concave in shape, it is possible to maintain a negative contribution to the Petzval sum and to correct field curvature, as well as to correct other aberrations such as spherical aberration.

In the imaging lens mentioned above and shown in the paper by Atkin8, if the surface of the second lens on the image field side is convex with respect to the image field side, the surface of the second lens on the object world side is strong against the object world side. Since the surface is concave, high-order aberrations are likely to occur on this surface. Therefore, in the present invention, the image field side surface of the second lens is made concave or flat, and @
The share of negative refractive power between the object-world side surfaces of the two lenses is reduced. This makes it possible to particularly prevent high-order spherical aberrations and comatic aberrations from occurring, and to perform good aberration correction.

Additionally, when performing spherical processing on a medium with a refractive index distribution,
It must be processed without eccentricity with respect to the central axis of the refractive index distribution, which is more difficult than conventional spherical processing of homogeneous media.
In the present invention, it can be said that it is extremely advantageous in terms of child-friendliness if the final surface, that is, the surface on the image field side of the second lens is made flat.

In order to perform even better aberration correction, it is desirable to satisfy the following conditions: 1.7≦r, /f≦8r1) -1≦r3/f≦-0,55 (2) 0.23≦d, / f≦0,42 (3)0.42≦(
13/f≦0.75 (4) Here, f is the focal length of the entire system, and rl+rj is the radius of curvature of the first and third surfaces counted from the object side, that is, r.

is the radius of curvature of the second lens on the object world side, r3 is the radius of curvature of the second lens on the object world side, d of the second lens is the axial thickness of the second lens, r, / f is When the lower limit of conditional expression (1) is exceeded, the spherical aberration generated on the object-world side surface of the lens becomes a dog, making it difficult to correct it in the entire system. If the upper limit is exceeded, the refraction of off-axis rays on the object-world side surface of the first lunion will increase, and off-axis aberrations will worsen.

rs /f f(2) f) When the value exceeds lower +5, the Petzval sum becomes large and it becomes difficult to correct the curvature of field, and when the upper limit is exceeded, the occurrence of higher-order aberrations at the third surface becomes large, and it becomes difficult to correct the field curvature. It becomes difficult to correct spherical aberration and coma.

Conditional expressions (5) and (4) are conditions related to lens thickness, and dl + dS is the lower 1 of conditional expressions (3) and (4).
If the value exceeds 100 mm, the refractive power distribution of each lens becomes strong, various aberrations worsen, and it becomes difficult to manufacture the medium. Also (3
) If the upper limit of (4) is exceeded, the entire system will become larger and the back focus will not be sufficient and force will be exerted.

Examples of the present invention will be described below.

Table 1 shows lens data of the first to fifth embodiments of the present invention. Each example has a half angle of view of 24° and a focal length of Vi1, the first and second rows have an F/F of 1.8, and the third to fifth examples have an F/F of 1.8.
It is F61.4. 1st. In the fourth embodiment, the surface on the image field side of the lens 2 is concave with respect to the image field, and in the second, fifth, and fifth embodiments, the surface on the image field side of the brush 2 lens is a flat surface. be.

Dear friend, the third embodiment is an example in which the first and @2 lenses have exactly the same refractive index distribution. If the same medium is used for both lenses in this way, it is extremely advantageous in terms of production costs.

Table 2 is shown in Table 1, and the third-order spherical aberration coefficient ■, coma aberration coefficient ■, and astigmatism coefficient ≡ when the object is short to infinity for each example are shown.
Table 1 shows the values of the Petzval sum P and the distortion aberration coefficient V. Table 1 First Example Second Example Third Example Fourth Example Fifth Example Table 2 As seen in this table, the joint aberration of each example The coefficients are well corrected.

Table 3 shows the lens data of the sixth embodiment of the present invention, and Table 4 shows its third-order aberration coefficients and axial chromatic aberration coefficients fiL.
It shows rust with 1x chromatic aberration T.

This example is an example in which chromatic aberration is corrected, and as shown in Table 4, the chromatic aberration coefficient is well corrected as well as the monochromatic third-order aberration coefficient. FIG. 1 shows a sectional view of the lens of the sixth embodiment, and FIG. 2(A) shows the dI9i of the lens of the sixth embodiment.
, and the refractive index distribution corresponding to the g-line, and FIG. 2(B) similarly shows the refractive index distribution corresponding to d@ and gllj of the second lens. FIG. 3 is a diagram of various aberrations of the sixth embodiment, and it can be seen that aberrations are well corrected up to high orders.

Note that when correcting chromatic aberration, it is desirable to satisfy the following conditions because the chromatic aberration caused by surface refraction is corrected by refractive power distribution.

N Iol (a) > N, G, (g) N, G2 (d
)> N,, 2(g) However, N+o+(d), '132 (g) are respectively d line,
Represents the quadratic distribution coefficient of the refractive index distribution of the lunth with respect to the g-line, ',02(d)*NIO2(8) are each 68
It represents the quadratic distribution coefficient of the refractive index distribution of the second lens with respect to the 1g line.

As explained above, in the present invention, by using a medium having a radial gradient, various aberrations can be well corrected and a secondary imaging lens can be realized with an extremely small number of lenses (two lenses). 1! Ta.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a cross-sectional view of an embodiment of the imaging lens according to the present invention, and FIGS. 2(A) and 2(B) are views of the imaging lens shown in FIG. FIG. 3 is a diagram showing the refractive index distribution of each lens to be formed, and FIG. 3 is a diagram showing various aberrations of the imaging lens shown in FIG. 1. rl...Radius of curvature of the first lens surface counting from the object side Air interval so...Sine condition dissatisfaction theorem Mlo, meridional surface S...Sagittal my surface Applicant: Canon Co., Ltd. 7 MouthLI EA/2 0 EA/2

Claims (1)

[Claims] (1) The lens is formed by two lenses made of a medium whose refractive index continuously changes depending on the distance from the optical axis, and the refractive index changes as the distance from the optical axis increases. It is not small and has a refractive index distribution, and its shape is the thinnest on the optical axis and has a constant meniscus shape with a concave surface facing the image field side. , has a refractive index distribution in which the refractive index decreases as it moves away from the optical axis, and its shape is such that it is the thinnest on the optical axis and the surface on the image field side is flat or concave toward the image field side. An imaging lens characterized by