JPH09159907A - Image pickup lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain excellent image-formation performance with two-lens constitution by constituting a lens of a homogeneous lens having negative refractive power and a specified radial type refractive index distribution lens which has positive refractive power and where a refractive index is distributed in a radial direction from an optical axis, and satisfying a specified condition. SOLUTION: This lens is constituted of the homogeneous lens having the negative refractive power and the radial type refractive index distribution lens which has the positive refractive power and where the refractive index is distributed in the radial direction from the optical axis, and is expressed by an expression N(r)=N00 +N10 r<2> + N20 r<4> , and satisfies the condition -0.05<1/V10 <0.01. Provided (r) expresses a distance from the optical axis in the radial direction, N(r) expresses the refractive index distribution at a point being the distance (r), N10 expresses the coefficient of 2i-order refractive index distribution, and V10 expresses the 2i-order distribution of the radial type refractive index distribution lens, and they are given by expressions Voo =(Nood -1)/(Noo F-Noo C) provided (i=0), and Vio =Niod /(Nio F-Nio C) provided (i=1, 2 and 3). Nood , Noo F and Fio C the refractive indexes on the optical axis of the lines (d), F and C, and Niod , Nio F and Nio C and the coefficients of 2i-order refractive index distribution of the lines (d), F and C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup lens for electronic image pickup such as a video camera.

[0002]

2. Description of the Related Art In recent years, with the widespread use of video cameras, videophones, door intercoms with cameras, etc., which use solid-state image pickup devices such as CCDs, the lens systems used for them are required to be further downsized and reduced in cost. Has been. Generally, in order to obtain good optical performance in a fixed focus lens system used for these, about 3 to 6 lenses are required, but it is desired to further reduce the number of lenses.

As a means for reducing the number of lenses while maintaining desired performance, it has been conventionally known to use a radial type gradient index lens element in an optical system. For example, as a conventional example of a lens system composed of one radial type gradient index lens, there is known one described in JP-A-6-175016. However, in this conventional example, in order to improve the imaging performance, the F number is made extremely dark to 9.8 to 13.5 to balance various aberrations. Further, as a conventional example of a lens system composed of two lenses, Japanese Patent Application Laid-Open No. 60-2186.
The one described in Japanese Patent Publication No. 14 is known. In this conventional example, various aberrations are well corrected, but the cost is high because two radial type gradient index lenses are used. Moreover, since the radial type gradient index lens has a spherical shape, it is difficult to easily match the optical axis of the surface and the optical axis of the medium with high accuracy in the actual processing scene, and a high-performance lens system is required. It is not preferable in terms of realizing a lens system which can be realized or whose manufacturing cost is low.

In order to solve the above-mentioned drawbacks, it is conceivable to make the radial type gradient index lens element into a biplanar shape, but the Petzval sum is undercorrected as compared with the spherical radial type gradient index lens element. Not preferable.

Further, as a conventional example composed of a radial type gradient index lens element having a biplanar shape and a homogeneous lens having a flat surface on one side and a spherical surface processed on the other side, Japanese Patent Laid-Open No. 58-
There are known lens systems described in JP-A-59420, JP-A-4-114112 and the like. However, these conventional examples relate to a pickup lens used under a monochromatic light source, and the publication does not describe correction of chromatic aberration. Further, in Japanese Patent Application Laid-Open No. 1-285514 and Japanese Patent Application Laid-Open No. 2-284107, similarly,
Nothing is said about the correction of chromatic aberration.

Further, as a conventional lens system considering chromatic aberration, there is known one disclosed in Japanese Patent Laid-Open No. 58-184113. In this conventional example, a radial type gradient index lens is used as a rigid mirror relay lens. Since the difference in refractive index between the optical axis and the peripheral portion of the radial type gradient index lens is small and the lens length is very long, a relay lens of a rigid mirror that transmits an image multiple times with one lens is used. It can only be used for lens systems for such special purposes.

Further, as a lens system considering chromatic aberration, there is a conventional example described in Japanese Patent Laid-Open No. 50-29238. In this conventional example, the difference in refractive index from the optical axis to the periphery of the radial type gradient index lens is known. However, it is difficult to actually manufacture a radial type gradient index lens material.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lens system having a good image forming performance with two lenses.

[0009]

According to the present invention, there is provided an imaging lens comprising:
It consists of a homogenous lens having negative refractive power and a radial type gradient index lens having a positive refractive power and having a refractive index distributed in the radial direction from the optical axis, represented by the following formula (a). A lens system characterized by satisfying (1).

N (r) = N ₀₀ + N ₁₀ r ² + N ₂₀ r ⁴ (a) (1) −0.05 <1 / V ₁₀ <0.01 However, r is in the radial direction from the optical axis. , N (r) is the refractive index distribution at the distance r, N _i0 is the 2i-th order refractive index distribution coefficient, V _i0 is the 2i-th order dispersion of the radial type gradient index lens, and the following equation (b) ), (C).

V ₀₀ = (N _00d −1) / (N _00F −N _00C ) (i = 0) (b) V _i0 = N _i0d / (N _i0F −N _i0C ) (i = 1,2,3.・) (C) where N _00d , N _00F and N _00C are d, F and C, respectively.
Refractive indices on the optical axis of the line, N _i0d , N _i0F , and N _i0C are 2i-th order refractive index distribution coefficients of the d, F, and C lines, respectively.

In order to solve the above problems, the present invention contemplates the following photographic lens having two lenses. That is, it is an imaging lens composed of one homogeneous lens and one radial type gradient index lens element. If the number of lenses is two in this way, the cost required for polishing and assembling can be reduced, and since the lens frame structure can be simplified, an extremely inexpensive imaging lens can be achieved. Moreover, by using a radial type gradient index lens element having a greater degree of freedom in aberration correction than a homogeneous lens, an imaging lens having high imaging performance can be achieved.

It is particularly important to correct chromatic aberration in order to achieve an image pickup lens having high image forming performance, and the radial type gradient index lens element has excellent characteristics for correcting chromatic aberration. Thought.

The axial chromatic aberration PAC of the radial type gradient index lens is approximated by the following equation (d).

PAC = K (φ _S / V ₀₀ + φ _m / V ₁₀ ) (d) where K is a constant depending on the ray height of the axial ray and the final paraxial ray angle, and φ _S and φ _m are radial The refractive powers of the thin-walled surface of the lens and the refractive power of the medium.

As is apparent from the equation (d), the amount of axial chromatic aberration can be controlled by setting V ₁₀ of the radial type gradient index lens element to an appropriate value.

The image pickup lens of the present invention satisfies the above condition (1) in order to satisfactorily correct the axial chromatic aberration.

If this condition (1) is satisfied, it becomes possible to satisfactorily correct the chromatic aberration generated in the radial type gradient index lens element. If the lower limit of condition (1) is -0.05
When it exceeds, the axial chromatic aberration is overcorrected. On the other hand, if the upper limit of 0.01 is exceeded, axial chromatic aberration will be insufficiently corrected, which is not preferable.

Further, the lens system of the present invention makes it possible to satisfactorily correct the Petzval sum of the entire lens system by giving a negative refractive power to the homogeneous lens. If the homogeneous lens has a positive refractive power, the Petzval sum of the entire lens system is undercorrected, and the image plane falls to the object side, which is not preferable.

The second structure of the present invention is represented by the above-mentioned formula (a) in which the refractive index is distributed in the radial direction from the homogeneous lens having the negative refractive power and the optical axis having the positive refractive power. The radial type gradient index lens element is characterized in that the radial type gradient index lens element has a planar shape on both sides and satisfies the following condition (2).

(2) 1 / V ₁₀ <1 / ν _{h where} ν _h is the Abbe number of a homogeneous lens.

By using two lenses, a relatively inexpensive image pickup lens system can be achieved. However, in order to obtain a more inexpensive lens system, it is desirable that the radial type gradient index lens element has a biplanar shape. If the shape of the surface is a flat surface, it can be processed at an extremely low cost as compared with the case of processing a spherical surface. Further, if the radial type gradient index lens element is formed in the shape of both planes, decentering of the surface of the radial type gradient index lens element and the medium, which is a problem in the actual processing, can be prevented, and the assembling adjustment becomes easy. .

However, if the radial type gradient index lens element is formed in the shape of both planes, the degree of freedom of aberration correction is reduced, and in particular, Petzval sum may be insufficiently corrected.

In the present invention, if the refractive power arrangement of the homogenous lens and the radial type gradient index lens element is set to an appropriate value, the Petzval sum is corrected well, the workability is excellent, the image pickup is inexpensive and has high image forming performance. You can get a lens.

Next, aberration correction will be described in detail.

In order to obtain a good optical performance by constructing an image pickup lens with two lenses, that is, a homogeneous lens and a radial type gradient index lens element of both planes, first of all, Petzval sum and chromatic aberration are corrected. You have to think about it. This is because the amount of both aberrations generated is almost determined by the refractive power arrangement of the lens system, and once determined, it becomes difficult to correct it by bending of the lens or the like.

Here, the Petzval sum PTZ of the radial type gradient index lens element is approximated by the following equation (e).

PTZ = φ _S / N ₀₀ + φ _m / N ₀₀ ² (e) Further, the refractive power φ _m of the medium of the radial type gradient index lens element
Is approximated by the following equation (f).

Φ _m ≈−2N ₁₀ t _G (f) where t _G is the thickness of the radial type gradient index lens element.

In the image pickup lens of the present invention, since the radial type gradient index lens element has a planar shape on both sides in order to improve the manufacturability, φ _S ≈0, and the formula (d) and the formula (e) have only the second term. Becomes Further, since the homogeneous lens is closely attached, the axial chromatic aberration and Petzval sum of the imaging lens of the present invention are expressed by the following equations (g) and (h).

PAC = K (φ _h / ν _h + φ _m / V ₁₀ ) (g) PTZ = φ _h / n _h + φ _m / N ₀₀ ² (h) where φ _h is the refractive power of the thin homogeneous lens, and ν _h is the Abbe number of the homogeneous lens, and n _h is the refractive index of the homogeneous lens.

From the above equations (g) and (h), the refractive power of the homogeneous lens and the radial type gradient index lens and V ₁₀
It can be seen that the desired values of axial chromatic aberration and Petzval sum can be obtained by changing the parameters in the equations such as

In order to satisfactorily correct the axial chromatic aberration, considering that the axial chromatic aberration is reduced as compared with a homogeneous lens having an Abbe number ν _h and the same refracting power φ as the image pickup lens, the following formula is satisfied. There is a need to.

K (φ _h / ν _h + φ _m / V ₁₀ ) <K (φ / ν _h ) However, if φ = φ _m + φ _h is expanded, φ _m / V ₁₀ <φ _m / ν _h From this, the conditional expression (2) is derived.

That is, if the Abbe number ν _h of the homogeneous lens and the Abbe number V ₁₀ of the medium of the radial type gradient index lens element satisfy the condition (2), it is possible to satisfactorily correct the axial chromatic aberration. Become. The condition (2) is a necessary condition for satisfactorily correcting the axial chromatic aberration in the imaging lens of the present invention, and if the condition (2) is not satisfied, the axial chromatic aberration will be insufficiently corrected, which is not preferable.

Further, from the expression (g), in order to satisfactorily correct the axial chromatic aberration, in addition to the condition (2), it is necessary to set the refractive power arrangements of the homogeneous lens and the radial type gradient index lens element to appropriate values. is there. However, in setting the refractive power arrangement, it is necessary to sufficiently consider the Petzval sum correction in addition to the chromatic aberration. This is because the Petzval sum is substantially determined by the refractive power arrangement of the lens system as is the case with chromatic aberration, as described above and as is clear from the expression (h).

Here, the arrangement of the refractive power for simultaneously correcting the axial chromatic aberration and the Petzval sum value of the image pickup lens will be considered from the equations (g) and (h). First, when the refractive power ratio a of the medium given by the following expression (i) is introduced into the expressions (g) and (h), the expressions (g ′) and (h ′) are obtained.

A = φ _m / φ (φ = φ _m + φ _h ) (i) PAC = Kφ {(ν _h −V ₁₀ ) a + V ₁₀ } / (ν _h V ₁₀ ) (g ′) PTZ = φ {( n _h −N ₀₀ ² ) a + N ₀₀ ² } / (n _h N ₀₀ ² ) (h ′) Here, in order to make it easy to understand, FIG. 20 is a graph of the expression (g ′) and the expression (h ′). is there. In FIG. 20, the horizontal axis represents the refractive power ratio a of the medium, and the vertical axis represents the axial chromatic aberration PAC of the imaging lens and Petzval sum PTZ. In addition, a in the figure
_PAC is the value of the refractive power ratio a of the medium when PAC = 0 in the formula (g '), and a _PTZ is the value of the refractive power ratio a of the medium when PTZ = 0 in the formula (h'). , These are given by the following equations (j) and (k).

A _PAC = V ₁₀ / (V ₁₀ −ν _h ) (j) a _PTZ = N ₀₀ ² / (N ₀₀ ² −n _h ) (k) From FIG. 20, axial chromatic aberration and Petzval sum are simultaneously corrected. To realize this, first, it is necessary that a _PAC and a _PTZ have close values. For example, if the signs of a _PAC and a _PTZ are different, it becomes impossible for the imaging lens to simultaneously correct axial chromatic aberration and Petzval sum. In addition to this, if the value of the refractive power ratio a of the medium also has a value close to these values, it is possible to achieve an imaging lens in which axial chromatic aberration and Petzval sum are extremely well corrected.

Here, since the refractive index N ₀₀ on the optical axis does not greatly differ from the refractive index of the existing homogeneous glass in manufacturing the gradient index material, N ₀₀ is about 1.5 to 1.9. It becomes a value. Also, the refractive index n _h of the homogeneous glass is 1.5 to 1.9.
Since it is a value of the order, the formula (k) satisfies the following condition (k ′).

A _PTZ = N ₀₀ ² / (N ₀₀ ² −n _h )> 1 (k ′) If the imaging lens of the present invention satisfies the condition (1),
Expression (j) satisfies the following condition (j ′).

A _PTC = V ₁₀ / (V ₁₀ −ν _h )> 0 (j ′) Therefore, the values of a _PAC and a _PTZ when the axial chromatic aberration and Petzval sum are corrected well are 0 or more. Is. Therefore, the refractive power ratio of the medium also needs to be 0 or more.

Further, from the relational expression (k '), the refractive power ratio a of the medium is
It is more desirable that the value of is 1 or more.

That is, it is desirable that the refractive power of the medium of the radial type gradient index lens and the refractive power arrangement of the homogeneous lens have the relationship shown in the following expression (1).

Φ _m / (φ _m + φ _h )> 1 (l) If the refractive powers of the radial type gradient index lens and the homogeneous lens forming the imaging lens satisfy the expression (l), the axial chromatic aberration and Petzval sum are obtained. Can be satisfactorily corrected.
If the condition (l) is not satisfied, it becomes difficult to correct the axial chromatic aberration and Petzval sum at the same time.

Further, since the image pickup lens of the present invention has a positive refractive power, it is necessary to satisfy φ _m > 0 and φ _h <0 in order to satisfy the expression (l). That is, it is necessary that the refractive power of the medium of the radial type gradient index lens has a positive value and the refractive power of the homogeneous lens has a negative value. If the homogeneous lens has a positive value, it becomes difficult to satisfactorily correct Petzval sum and axial chromatic aberration.

In order to make the optical axes of the homogeneous lens and the radial type gradient index lens element coincide with each other with high accuracy, at least one surface of at least one side of the homogeneous lens has a planar shape, and at the plane portion thereof. Adhesion or close contact with the radial type gradient index lens is desirable. For example, if the outer diameters of the homogenous lens and the radial type gradient index lens are made equal, and the two lenses are adhered or adhered to each other at the flat portion with the outer diameters as a boundary, the optical axes of both lenses can be easily matched with high accuracy. It is possible. As another method, the optical axis position of one or both of the homogeneous lens and the radial type gradient index lens is measured, and the lens positions are adjusted so that the optical axes of both lenses coincide with each other, and the two lenses are bonded or closely attached. For example, it is possible to match the optical axes. At this time, both lenses have a flat surface portion, and if both the lenses are in contact with each other on the flat surface portion, adjustment is extremely easy. Further, if the two lenses are adhered or brought into close contact with each other, there is an advantage that the lens frame structure can be simplified. Further, even if a ring (spacer) for adjusting the lens interval is inserted between the homogeneous lens and the radial type gradient index lens element, the same effect as described above can be obtained.

In the third structure of the present invention, a homogeneous lens having a negative refractive power and a refractive index distributed in the radial direction from the optical axis having a positive refractive power are arranged in this order from the object side. ) Is a radial type gradient index lens, and is characterized in that focusing is performed on an object at a close range by changing the distance between the homogeneous lens and the radial gradient index lens.

When the image pickup lens of the present invention is applied to a video camera or the like which requires relatively high optical performance, it is necessary to sufficiently consider focusing. That is, it is necessary that various aberrations are properly corrected from the object at infinity to the object at a close range. Considering further miniaturization, it is desirable that the load on the driving mechanism for focusing be as small as possible.

For the above reasons, in the imaging lens of the present invention, focusing is performed by changing the distance between the homogeneous lens and the radial type gradient index lens element as described above. In other words, if focusing is performed using only one of the lenses, the movable lens can be light and the load on the drive mechanism is extremely small, and the lens frame and drive mechanism can be made into a compact structure, thus achieving a compact imaging lens. obtain. It is also possible to move the lens system as a whole to perform focusing in order to reduce fluctuations in aberrations during focusing, but it is also possible to perform so-called total extension, but since the weight of the movable lens increases and the load on the drive mechanism increases. However, it is not preferable because an image pickup lens having a compact structure cannot be achieved.

Further, in order to reduce the aberration variation during focusing, it is desirable to perform focusing by moving only the homogeneous lens having negative refractive power. Further, it is desirable that, in order from the object side, a negative homogeneous lens and a positive radial type gradient index lens element be configured, and the negative first lens be extended to the object side for focusing on an object at a close range. When the first lens is extended to the object side to perform focusing on an object at a close range, the fluctuation of the ray height of the axially incident light ray is extremely small, so that the fluctuation of spherical aberration particularly during focusing can be reduced. Further, since the amounts of on-axis chromatic aberration and Petzval sum generated do not change much, it is possible to obtain good imaging performance.

Further, as described above, in order to easily match the optical axis of the homogeneous lens and the optical axis of the radial type gradient index lens element with high accuracy, at least one surface of at least one surface of the homogeneous lens is made flat. It is desirable that the flat portion be adhered or adhered to the radial type gradient index lens element directly or via a space ring. However, when focusing is performed by changing the distance between the homogenous lens and the radial type gradient index lens, both lenses should be brought into close contact with each other directly or through a gap ring at the time of assembly, and the optical axis positions at that time should match. It is desirable to adjust the positions of both lenses as described above.

Further, the lens system of the present invention can be applied to an image pickup system using a solid-state image pickup device such as a CCD or an image guide consisting of a fiber bundle. In that case, in order to secure sufficient peripheral light quantity, It is desirable that the first lens having a negative refracting power and the second lens having a positive refracting power be formed in order from the side. If the negative-positive configuration is adopted in order from the object side, the incident angle of the off-axis ray on the image plane can be made close to vertical. In other words, it is possible to bring the optical axis close to the optical axis and to secure a sufficient amount of peripheral light. If from the object side,
A positive-negative configuration is not preferable because it becomes difficult to make the incident angle of the off-axis light rays to the image surface close to vertical, and it becomes difficult to secure a sufficient peripheral light amount.

Further, in the image pickup lens of the present invention, in order to satisfactorily correct the axial chromatic aberration and Petzval sum, it is desirable that the homogeneous lens has a large negative refractive power. As described above, the refractive power of the medium of the radial type gradient index lens has a positive value, and the refractive power of the homogeneous lens has a negative value in order to satisfactorily correct axial chromatic aberration and Petzval's sum. It is a condition. In this case, if the refractive power of the homogeneous lens is as small as possible, it will be difficult to satisfactorily correct the Petzval sum.

For the above reasons, it is desirable that the lens system of the present invention satisfies the following condition (3).

(3) −1.5 <f / f _h <−0.05 Here, f is the focal length of the entire photographing lens system, and f _h is the focal length of the homogeneous lens.

If the imaging lens of the present invention satisfies the above condition (3), the Petzval sum can be corrected well. If the upper limit of -0.05 of condition (3) is exceeded, the Petzval sum will be undercorrected and the image plane will tilt toward the object side, which is not preferable. If the lower limit of -1.5 is exceeded, the Petzval sum will be overcorrected and the image plane will tilt away from the object, which is not preferable.

Further, in the case where the first lens is a negative lens and the second lens is a positive lens in order from the object side and the first lens is used for focusing, when the first lens is extended to the object side, Outer aberrations, especially distortion, tend to be large, but if the negative refracting power of the first lens is set to an appropriate value, then distortion can be corrected well. Therefore, in the imaging lens of the present invention, when the first lens is used for focusing, it is desirable to satisfy the above condition (3).

If the focal length of the first lens satisfies the condition (3), it is possible to reduce the fluctuation of off-axis aberrations due to focusing. If the lower limit of condition (3) is -1.5
When the value exceeds, the negative refractive power of the first lens becomes too large and the off-axis aberration is deteriorated. If the upper limit of -0.05 is exceeded, the negative refracting power becomes weak and the moving distance for focusing becomes long, which makes it difficult to downsize the lens system.

Since the image pickup lens of the present invention has a positive refractive power, it tends to generate negative spherical aberration in the entire system. However, a homogeneous lens having a negative refractive power and a radial type gradient index lens having both planes are used. With this configuration, it is possible to satisfactorily correct. For example, when a radial type gradient index lens element having a positive refractive power is provided with a concave surface instead of forming both flat surfaces, the refractive index of this concave surface becomes smaller as it goes from the optical axis to the periphery. Has the effect of reducing. Therefore, in such an imaging lens, negative spherical aberration is undercorrected.

However, since the radial type gradient index lens used in the imaging lens of the present invention has a planar shape, spherical aberration does not occur in the surface, and positive spherical aberration occurs in a homogeneous lens having a negative refractive power. Therefore, it becomes possible to satisfactorily correct spherical aberration in the entire system.

As described above, the imaging lens of the present invention is excellent in manufacturability, and axial chromatic aberration and Petzval sum are well corrected, and in addition, spherical aberration can be well corrected. is there.

When the image pickup lens of the present invention is applied to a video camera or the like, it is desired that the field angle be wide to some extent. In the imaging lens of the present invention, by adding a negative lens to the object side of the radial type gradient index lens, it becomes possible to widen the angle of view. In addition to this, it is desirable to increase the refractive power of the radial type gradient index lens element to some extent. If the refractive power of the radial type gradient index lens is large, a focal length of the entire system is short and a wide field angle lens system can be achieved.

Here, it is considered that the refractive power of the medium of the radial type gradient index lens element is increased. From the above formula (f), it is necessary to increase N ₁₀ or t _{g in order} to increase the refractive power of the medium. However, it is not preferable because it is difficult to produce a material having an extremely large N ₁₀ in terms of producing a material having a refractive index distribution. Therefore, in the present invention, it was considered that the radial type gradient index lens element has an appropriate lens thickness so as to obtain a sufficient refractive power without making N ₁₀ extremely large. That is, in the lens system of the present invention, it is desirable that the radial type gradient index lens element satisfies the following condition (4).

(4) 0.5 <t _G / f <9 where f is the focal length of the entire image pickup lens system.

If the condition (4) is satisfied, it is possible to effectively use the effect of correcting axial chromatic aberration and Petzval sum. If the lower limit of 0.5 of the condition (4) is exceeded, the refractive power of the radial type gradient index lens element will be weakened, and it will be difficult to effectively utilize the effect of correcting axial chromatic aberration and Petzval sum. If the upper limit of 9 is exceeded,
This is not preferable because the lens becomes thick and an image may be formed inside the lens.

Further, when the image pickup lens of the present invention is applied to an image pickup system in which the pixel pitch of the solid-state image pickup element is fine and higher performance is required, it is desirable to further satisfy the following condition (5).

(5) 1.5 <t _G / f <7 If the lower limit of 1.5 of the condition (5) is exceeded, the refracting power of the imaging lens becomes weak, and axial chromatic aberration and Petzval sum are corrected. It is difficult to effectively use the effect in a lens system that requires high-performance imaging performance. On the other hand, when the upper limit of 7 is exceeded, the lens becomes thick and the transmittance and flare deteriorate, so that it becomes difficult to achieve high-performance imaging performance.

Further, when the image pickup lens of the present invention is applied to a lens system which requires higher performance of image formation, it is desirable to satisfy the following condition (6).

(6) −0.01 <1 / V ₁₀ <0.008 If the radial type gradient index lens component satisfies the condition (6), it may be possible to correct axial chromatic aberration more favorably. If the upper limit of 0.008 of the condition (6) is exceeded, axial chromatic aberration will be undercorrected, which is not preferable.
If the lower limit value of -0.01 is exceeded, axial chromatic aberration will be overcorrected, which is not preferable.

The radial type gradient index lens element can control the amount of spherical aberration generated by changing the value of the fourth-order term N ₂₀ of the refractive index distribution. In order to satisfactorily correct spherical aberration in the imaging lens of the present invention, it is desirable to satisfy the following condition (7).

(7) −0.2 <N ₂₀ × f ⁴ <0.2 If the radial type gradient index lens element satisfies the condition (7), it becomes possible to excellently correct spherical aberration. If the upper limit of 0.2 of the condition (7) is exceeded, spherical aberration will be overcorrected, which is not preferable. If the lower limit of -0.2 is exceeded, spherical aberration will be undercorrected, which is not preferable.

Further, when the image pickup lens of the present invention is applied to a lens system which requires higher performance of image formation, it is desirable to satisfy the following condition (8).

(8) −0.05 <N ₂₀ × f ⁴ <0.05 If the radial type gradient index lens element satisfies the condition (8), spherical aberration can be corrected more favorably.
If the upper limit of 0.05 of the condition (8) is exceeded, spherical aberration will be overcorrected and high-performance imaging performance cannot be achieved, which is not preferable. Also, the lower limit of -0.05
If it exceeds, spherical aberration is insufficiently corrected and high-performance imaging performance cannot be achieved, which is not preferable.

Further, in the image pickup lens of the present invention, it is desirable to satisfy the following condition (9) in order to satisfactorily correct lateral chromatic aberration in addition to axial chromatic aberration.

(9) 1 / ν _h <0.03 In the image pickup lens of the present invention, since the ray heights of the off-axis chief ray passing through the homogeneous lens and the radial type gradient index lens component are different, axial chromatic aberration is reduced. In addition, in order to correct the chromatic aberration of magnification, it is desirable that all lenses have a small amount of chromatic aberration. With respect to the radial type gradient index lens element, it is possible to reduce the amount of chromatic aberration generated by satisfying the condition (2). Therefore, it is desirable that the condition (9) be satisfied for the homogeneous lens. If the condition (9) is satisfied, lateral chromatic aberration as well as lateral chromatic aberration can be reduced. If the condition (9) is not satisfied, it becomes difficult to satisfactorily correct lateral chromatic aberration.

Further, when the image pickup lens of the present invention is applied to a lens system requiring higher performance of image formation, it is desirable to satisfy the following condition (10).

(10) 1 / ν _h <0.025 If the condition (10) is satisfied, it becomes possible to satisfactorily correct lateral chromatic aberration. If the condition (10) is not satisfied, chromatic aberration of magnification will be insufficiently corrected and high-performance imaging performance cannot be achieved, which is not preferable.

Further, in order to satisfactorily correct the Petzval sum in the image pickup lens system of the present invention, it is desirable to satisfy the following condition (11).

(11) N _00d > 1.55 If the condition (11) is satisfied, the value of the second term of the formula (h) becomes smaller, and the Petzval sum generated in the medium of the radial type gradient index lens element is sufficiently small. It becomes possible to make it small. If the condition (11) is not satisfied, the Petzval sum generated in the medium will be undercorrected, which is not preferable.

When the lens system of the present invention is applied to a lens system having a relatively wide angle of view, it is necessary to increase the refractive power φ of the entire lens system. In addition to this, in order to satisfactorily correct the axial chromatic aberration, it is desirable to satisfy the following condition (12).

(12) 0.5 <a _PAC <1.7 When the refractive power φ (= φ _m + φ _h ) of the entire lens system is increased, the refractive power ratio a of the medium given by the equation (i) is a Will have a value close to 1. In addition, as described above, in order to satisfactorily correct the axial chromatic aberration, it is desirable that the value of a _{PAC and} the value of the refractive power a of the medium be close to each other. Therefore, it is desirable for the lens system of the present invention to satisfy the above (12). That is, if the condition (12) is satisfied, it is possible to excellently correct the axial chromatic aberration. If the upper limit of 1.7 of condition (12) is exceeded, axial chromatic aberration will be undercorrected. If the lower limit of 0.5 is exceeded, overcorrection will occur, which is not preferable.

In the image pickup lens system of the present invention, a homogeneous lens or a radial type gradient index lens is used as an optical element having an effect such as a low pass filter, an infrared cut filter or a band cut filter for cutting a specific wavelength component. Therefore, it is possible to realize a high-performance imaging lens.

Further, by making the homogeneous spherical lens an aspherical surface, it is possible to satisfactorily correct various aberrations.

When the load on the driving mechanism for focusing does not pose a problem, it is also possible to integrally move the homogeneous lens and the radial type gradient index lens, ie, so-called total extension.

[0086]

BEST MODE FOR CARRYING OUT THE INVENTION Next, an embodiment of the present invention will be described below.
A description will be given based on each example. Example 1 f = 4.2 mm, F number 2.0, 2ω = 42 ° r₁ = -10.4898 d₁ = 1.0000 n₁ = 1.48749 ν₁ = 70.21 r_Two = ∞ (aperture) d _Two = 0 r_Three = ∞d_Three = 8.0000 n_Two (Refractive index distribution lens) r_Four = ∞d_Four = 1.0000 n_Three = 1.51633 ν_Three = 64.15 r_Five = ∞ Gradient index lens N₀₀ N_Ten N₂₀ d-line 1.75000, -0.19934 × 10^-1， -0.28601 × 10^-Four C line 1.74500, -0.19904 × 10^-1、 -0.28557 × 10^-Four F line 1.76167, -0.20005 × 10^-1， -0.28703 × 10^-Four 1 / V_Ten= 0.0051, 1 / ν_h = 0.0142, f / f_h = -0.1952 t_G /F=1.9048, N₂₀× f^Four = -0.0089, N_00d = 1.7500, a_PAC = 1.5572

Example 2 f = 4.88 mm, F number 3.5, 2ω = 28.6 ° r ₁ = 10.11312 d ₁ = 1.8000 n ₁ = 1.84666 ν ₁ = 23.78 r ₂ = 4.3922 (aspherical surface) d ₂ = 0.5000 r ₃ = ∞ d ₃ = 8.0000 n ₂ (refractive index distribution lens) r ₄ = ∞ d ₄ = 1.0000 n ₃ = 1.51633 ν ₃ = 64.15 r ₅ = ∞ aspherical coefficient P = 1, A ₄ = -0.63278 × 10 ^-2 , A ₆ = 0.38524 × 10 ^-2 A ₈ = -0.12709 × 10 ^-2 Refractive index distribution lens N ₀₀ N ₁₀ N ₂₀ d line 1.75000, -0.19934 × 10 ^-1 , -0.28601 × 10 ^-4 C line 1.74500, -0.19904 × 10 ^-1 , 、 0.2.55 557 × 10 ^-4 F line 1.76167 、 -0.20005 × 10 ^-1 、 -0.28703 × 10 ^-4 1 / V ₁₀ ＝ 0.0051、1 / ν _h ＝ 0.0421 、 f / f _h ＝ -0.4559 t _G / f = 1.6407, N ₂₀ × f ⁴ = -0.0162, N _00d = 1.7500, a _PAC = 1.1379

Example 3 Focal length = 0.844 mm, object point distance = 11 mm, NA = 0.073, 2ω = 129.2 ° r ₁ = ∞ d ₁ = 0.3600 n ₁ = 1.88300 ν ₁ = 40.78 r ₂ = 0.6800 d ₂ = 0.8000 r ₃ = ∞ d ₃ = 3.8000 n ₂ (refractive index distribution lens) r ₄ = ∞ d ₄ = 2.8000 n ₃ = 1.51633 ν ₃ = 64.15 r ₅ = ∞ refractive index distribution lens N ₀₀ N ₁₀ N ₂₀ N ₃₀ d line 1.70000, -0.12580 x 10, 0.78000 x 10 ^-2 , -0.47000 x 10 ^-3 C line 1.69475, -0.12567 x 10, 0.77922 x 10 ^-2 , -0.46953 x 10 ^-3 F line 1.71225, -0.12609 x 10, 0.78182 × 10 ^-2 , -0.47110 × 10 ^-3 1 / V ₁₀ = 0.0033, 1 / ν _h = 0.0245, f / f _h = -1.0961 t _G /f=4.5024, N ₂₀ × f ⁴ = 0.0040, N _00d = 1.7000, a _PAC = 1.1573

Example 4 f = 5 mm, F number 1.67, 2ω = 35 ° r ₁ = -12.8994 d ₁ = 4.5000 n ₁ = 1.43875 ν ₁ = 94.97 r ₂ = ∞ (aperture) d ₂ = 0r ₃ = ∞ d ₃ = 15.4727 n ₂ (refractive index distribution lens) r ₄ = ∞ refractive index distribution lens N ₀₀ N ₁₀ N ₂₀ d line 1.60000, -0.99200 × 10 ^-2 , 0.16991 × 10 ^-4 C line 1.59600, -0.99348 × 10 ^-2 , 0.16991 x 10 ^-4 F line 1.60933, -0.998852 x 10 ^-2 , 0.16991 x 10 ^-4 g line 1.61696, -0.998651 x 10 ^-2 , 0.16991 x 10 ^-4 1 / V ₁₀ = -0.0050, 1 / ν _h = 0.0105, f / f _h = -0.1701 t _G /f=3.0945, N ₂₀ × f ⁴ = 0.0106, N _00d = 1.6000, a _PAC = 0.6780

Example 5 f = 4.38 mm, F number 1.95, 2ω = 40.6 ° r ₁ = -17.2145 d ₁ = 7.1453 n ₁ = 1.48749 ν ₁ = 70.21 r ₂ = 6.6013 d ₂ = 0.4000 r ₃ = ∞ d ₃ = 1.7061 n ₂ (refractive index distribution lens) r ₄ = ∞ refractive index distribution lens N ₀₀ N ₁₀ N ₂₀ d line 1.60000, -0.88660 × 10 ^-2 , 0.90122 × 10 ^-5 C line 1.59600, -0.88542 × 10 ^-2 , 0.90122 × 10 ^-5 F line 1.60933, -0.88834 × 10 ^-2 , 0.90122 × 10 ^-5 1 / V ₁₀ = 0.0044, 1 / ν _h = 0.0142, f / f _h = -0.4914 t _G /f=4.5001, N ₂₀ × f ⁴ = 0.0033, N _00d = 1.6000, a _PAC = 1.4502

Example 6 f = 3.84 mm, F number 1.21, 2ω = 64.9 ° r ₁ = -8.1476 d ₁ = 2.8898 n ₁ = 1.72342 ν ₁ = 37.95 r ₂ = ∞ d ₂ = 0 r ₃ = ∞ d ₃ = 14.7528 n ₂ (refractive index distribution lens) r ₄ = ∞ refractive index distribution lens N ₀₀ N ₁₀ N ₂₀ N ₃₀ d line 1.65000, -0.13746 × 10 ^-1 , -0.13709 × 10 ^-4 , 0.41168 × 10 ^-5 C Wire 1.64675, -0.13747 x 10 ^-1 , -0.13709 x 10 ^-4 , 0.41168 x 10 ^-5 F wire 1.65758, -0.13743 x 10 ^-1 , -0.13709 x 10 ^-4 , 0.41168 x 10 ^-5 1 / V ₁₀ = -0.0003, 1 / ν _h = 0.0264, f / f _h = -0.3411 t _G /f=3.8399, N ₂₀ × f ⁴ = -0.0030, N _00d = 1.6500, a _PAC = 0.9897

Example 7 f = 2.05 mm, object point distance = 2.85 mm, NA = 0.085, magnification 1 ×, 2ω = 31.4 ° r ₁ = ∞ d ₁ = 8.6295 n ₁ (refractive index distribution lens) r ₂ = ∞ d ₂ = 0.5000 r ₃ = -1.7937 d ₃ = 1.4446 n ₂ = 1.51633 ν ₂ = 64.15 r ₄ = ∞ Refractive index distribution lens N ₀₀ N ₁₀ N ₂₀ d-line 1.66520, -0.40000 × 10 ^-1 , -0.22554 × 10 ^-3 C line 1.65981, -0.39936 x 10 ^-1 , -0.22526 x 10 ^-3 F line 1.67823, -0.40147 x 10 ^-1 , -0.22621 x 10 ^-3 1 / V ₁₀ = 0.0053, 1 / ν _h = 0.0156, f / f _h = -0.5912 t _G /f=4.2013, N ₂₀ × f ⁴ = -0.0040, N _00d = 1.6652, a _PAC = 1.5095

Example 8 f = 4 mm, F number 2.0, 2ω = 44.8 ° r ₁ = 10.3948 d ₁ = 1.0000 n ₁ = 1.74100 ν ₁ = 52.65 r ₂ = 7.5833 d ₂ = 0.3000 r ₃ = ∞ d ₃ = 23.7362 n ₂ (refractive index distribution lens) r ₄ = ∞ refractive index distribution lens N ₀₀ N ₁₀ N ₂₀ d line 1.70000, -8.0000 × 10 ^-3 , 6.3430 × 10 ^-6 C line 1.69533, -7.9912 × 10 ^-3 , 6.3430 × 10 ^-6 F wire 1.71089, -8.0204 × 10 ^-3 , 6.3430 × 10 ^-6 1 / V ₁₀ = 0.0037, 1 / ν _h = 0.0190, f / f _h = -0.6920 t _G /f=5.9341, N ₂₀ × f ⁴ = 0.0016, N _00d = 1.7000, a _PAC = 1.2383, where r ₁ , r ₂ , ... Are the radii of curvature of each lens surface, d
₁ , d ₂ , ... Is the thickness of each lens and the lens interval, n
₁ , n ₂ , ... Are refractive indices of d-line of each lens, ν ₁ , ν
₂ , ... Is the Abbe number of each lens.

The first embodiment of the present invention is shown in FIG. That is, it is an imaging lens in which two lenses, a first lens of a negative lens and a second lens of a positive lens, are sequentially arranged from the object side, and the second lens is a radial type gradient index lens element of both planes. The negative first lens has a concave surface on the object side and a flat surface on the image side, and is in close contact with the radial type gradient index lens element at this flat surface portion. Further, by providing the diaphragm S between the homogeneous lens and the radial type gradient index lens element, the height of the off-axis ray passing through the lens system is lowered, which is useful for reducing the diameter of the lens.

The image pickup lens of this embodiment has a CCD at the image plane position.
By arranging such solid-state image pickup elements, it can be used as an image pickup system used in, for example, a video camera, a videophone or an intercom. A filter F for cutting a specific wavelength component is adhered or adhered to the image side surface of the radial type gradient index lens element. Further, if the radial type gradient index lens or the homogeneous lens has a filter function for cutting a specific wavelength component, the filter F in FIG. 1 can be omitted. Further, the filter can be arranged apart from the radial type gradient index lens.

In Example 1, the first lens is extended to the object side to perform focusing on an object at a close range. Therefore, the homogeneous lens and the radial type gradient index lens are configured to be in close contact with each other without being adhered.

The near object (object point position 100
The distance between the first lens and the second lens is 3.810 mm when focusing to (mm), and the diaphragm is in close contact with the second lens and does not move.

Further, in Example 1, since the outer diameters of the homogeneous lens and the radial type gradient index lens element are the same, it is possible to easily match the optical axes of both lenses with relatively high accuracy.

When focusing is not performed in the lens system of the present invention, the homogeneous lens and the radial type inhomogeneous lens may be bonded to each other in order to simplify the lens frame and the like.

Further, in Example 1, since the homogeneous lens has a negative refractive power, 1 / V ₁₀ of the radial type gradient index lens element should have a small positive value in order to satisfactorily correct the axial chromatic aberration. did.

Further, considering that the image pickup lens of the present invention is manufactured at low cost, it is desirable that the homogeneous lens has a concave surface and a plane shape as in the first embodiment. If the surface on one side is flat, polishing is extremely easy and processing can be performed at low cost.

Further, the image pickup lens of the present invention has a merit of aberration correction which cannot be obtained by a radial type gradient index lens element having a directly spherical surface. FIG. 21 is a glass map in which the vertical axis is the refractive index n _d and the horizontal axis is the Abbe number ν _d, and the region surrounded by the thick line is the approximate existing range of existing glass,
The arrows indicate changes in the refractive index and Abbe number of the radial type gradient index lens, and the black circles indicate the refractive index and Abbe number on the optical axis. For example, the values of the refractive index and Abbe number distributed from the optical axis to the periphery of the radial type gradient index lens material manufactured by the ion exchange method or the sol-gel method are indicated by the arrow A in FIG. As shown, it is almost within the range of existing glass. Therefore, for example, as shown by the arrow B in FIG. 21, the refractive index on the optical axis and the Abbe number are within the range of the existing glass, and the refractive index and the Abbe number deviate from the range of the existing glass as it goes to the periphery. It is extremely difficult to manufacture such a radial type gradient index lens material. However, there are cases where such a material is desired to correct various aberrations. For example, the material represented by the arrow B has a small dispersion and is therefore advantageous for the correction of chromatic aberration. However, by using the imaging lens of the present invention, even if the radial type gradient index lens is a material within the range of the existing glass, it is almost the same as the above-mentioned material having a refractive index distribution outside the range of the existing glass. It is possible to obtain the effect of. Specifically, Example 1
This will be described with reference to FIG. An arrow A represents the radial type gradient index lens of the first embodiment, N ₀₀ = 1.75, V ₀₀ = 45, V _10.
The axial chromatic aberration PAC _A having a value of ≈200 is expressed by the following equation.

PAC _A = K (φ _s / 45 + φ _m / 200) Further, the axial chromatic aberration PAC _B of the arrow _B is expressed by the following equation.

PAC _B = K (φ _s 70.21 + φ _m / 200) However, in the present Example 1, n _h = 1.48749, ν _h
= 70.21 and a radial type gradient index lens, and the axial chromatic aberration PA of this combined lens is
C _C becomes the following formula.

[0105] _{PAC C = K (φ h /70.21+φ} m / 200) Therefore, if you choose the refractive power of the homogeneous lens such that φ _h ≒ φ _s, table by an arrow B is practically difficult to produce the It is possible to achieve an imaging lens having a chromatic aberration correction effect that is almost the same as that of the material used. As described above, the image pickup lens of the present invention has an advantage that the degree of freedom in aberration correction is increased by selecting a desired homogeneous lens.

The aberrations of the image pickup lens of the first embodiment are as shown in FIGS. 9 and 10. FIG. 9 shows an object point at infinity and FIG. 10 shows an object point at a short distance. As shown in these aberration diagrams,
It can be seen that various aberrations are well corrected despite the fact that the number of lenses is two.

The second embodiment of the present invention is shown in FIG. That is, it is an imaging lens in which two lenses, a first lens of a negative lens and a second lens of a positive lens, are sequentially arranged from the object side, and the second lens is a radial type gradient index lens element of both planes. Further, the first lens has a meniscus shape with a concave surface facing the image side, and has a flat surface portion 1 on the outer peripheral portion with respect to the effective diameter of the image surface side surface, and this flat surface portion 1 forms a radial type gradient index lens element. Adhered or adhered. A filter F for cutting a specific wavelength component is used on the image side of the radial type gradient index lens.

In Example 2, the homogeneous lens of the image pickup lens used in Example 1 is replaced with a homogeneous meniscus lens, and although the radial type gradient index lens is common, the focal length is different. This is an example of realizing an imaging lens. That is, the image pickup lens of the present invention has a common radial type gradient index lens component L _g as shown in FIG.
homogeneous lenses of various shapes with different d and n, such as L _h1 ,
By combining L _h2 and L _h3 , it is possible to achieve imaging lenses L _c1 , L _c2 , and L _c3 that have different focal lengths and the amounts of various aberrations, although there is only one type of radial type gradient index lens. it can. As described above, since the radial type gradient index lens can be used as a common component, it is possible to realize an inexpensive lens system in spite of being compatible with various lens systems used under a white light source.

Further, by providing the diaphragm s on the object side of the homogeneous lens, it becomes possible to satisfactorily correct particularly off-axis aberrations.

Further, in this embodiment, the focal length is relatively long,
In particular, since it is difficult to correct the axial chromatic aberration, a high-dispersion glass that has an Abbe number of 1 / ν _h > 0.03 is used for the homogeneous lens. Further, since the diaphragm is provided closest to the object side, it is desirable to use high-dispersion glass for the homogeneous lens in order to satisfactorily correct lateral chromatic aberration.

Further, by using an aspherical surface for the image-side surface of the homogeneous lens, various aberrations are well corrected. The aspherical shape used in the examples is represented by the following formula.

However, the above equation takes the x-axis in the optical axis direction,
The y axis is taken in the direction perpendicular to the optical axis, r is the radius of curvature on the optical axis, P is the conical constant, and A _2i is the aspherical coefficient.

In the second embodiment as well, focusing is performed by moving the first lens to the object side. The distance between the first lens and the second lens when focusing on a short-distance object (object distance: 100 mm) according to the second embodiment is 1.5151 mm, and the aperture is the first.
Move with the lens.

The aberrations of the second embodiment are as shown in FIGS. 11 and 12, where FIG. 11 is for an object point at infinity and FIG. 12 is for a near object point. It can be seen that the imaging lens of the second example has various aberrations well corrected despite the fact that the number of lenses is two.

The third embodiment of the present invention is shown in FIG. That is, it is an imaging lens in which two lenses, a first lens of a negative lens and a second lens of a positive lens, are sequentially arranged from the object side, and the second lens is a radial type gradient index lens element of both planes. Further, the first lens is a plano-concave lens, and has a flat surface portion 1 on the outer peripheral portion with respect to the effective diameter of the image-side surface.
And is adhered or adhered to the radial type gradient index lens. Further, the diaphragm S is provided at a position of 1 mm from the object side surface of the radial type gradient index lens element toward the image side. Further, a filter F for cutting a specific wavelength component is used on the image side of the radial type gradient index lens.

The third embodiment is an example in which a wide angle of the lens system is achieved by increasing the negative refracting power of the homogeneous lens, and it can be used for an objective lens of an endoscope, for example. When the imaging lens of the present invention is applied to a lens having a wide angle of view as in Example 3, it is desirable to satisfy the following condition (13).

(13) −1.5 <f / f _h <−0.4 If the upper limit of −0.4 to condition (13) is exceeded, it will be difficult to achieve a wide angle of view. In addition, the lower limit −
If it exceeds 1.5, the Petzval sum is overcorrected, and the image plane tilts away from the object, which is not preferable.

Further, it is difficult to correct Petzval's sum in particular because the present embodiment is a lens system having a wide angle of view, but this is satisfactorily _achieved by the radial type gradient index lens _component satisfying N _00d > 1.6. Correcting. However, N _00d > 1.65
It is more desirable to satisfy the above condition because the Petzval sum can be corrected even better.

Further, since the diaphragm is provided inside the radial type gradient index lens element, it is possible to correct the off-axis aberration particularly in addition to reducing the diameter of the lens.

When the homogenous lens of the imaging lens of the present invention and the radial type gradient index lens are adhered, an adhesive is applied to the plane portion indicated by 1 in FIG. 3 and the adhesive is applied to assemble the assembly. It is good, but the portion indicated by reference numeral B in FIG.
It is also possible to apply an adhesive to the outer peripheral portion of the lens for adhesion as indicated by 4 in (B).

The aberrations of Example 3 are as shown in FIG. The image pickup lens of Example 3 has two lenses.
It can be seen that the aberrations are well corrected despite the number of sheets.

The fourth embodiment of the present invention is shown in FIG. That is, it is an imaging lens in which two lenses, a first lens of a negative lens and a second lens of a positive lens, are sequentially arranged from the object side, and the second lens is a radial type gradient index lens element of both planes. The first lens is a concave flat lens, and is adhered or adhered to the radial type gradient index lens on the image side. In addition, the solid-state image sensor or the solid-state image sensor unit C is adhered or adhered to the image side portion of the radial type gradient index lens element, which is effective for downsizing or simplification of the lens frame structure.

When the image pickup lens of the present invention is applied to a lens system having a relatively narrow angle of view, it is not preferable that a homogeneous lens having a negative refractive power has an extremely large value, and the following condition (14) It is desirable to satisfy.

(14) −0.8 <f / f _h <−0.1 If the lower limit of −0.8 of condition (14) is exceeded, the Petzval sum will be overcorrected and the image plane will move away from the object. Falls in the direction. If the upper limit of -0.1 is exceeded, the Petzval sum will be undercorrected and the image plane will tilt toward the object side, which is not preferable.

When the imaging lens of the present invention is applied to an optical system in which transmittance and flare are problems, it is desirable that the radial type gradient index lens element has a total lens length of about 40 mm or less. Further, if the radial type gradient index lens element is about 25 mm or less as in this embodiment, a lens system having a better imaging performance can be achieved.

In order to prevent flare on the outer peripheral side surface of the lens, it is effective to make the outer peripheral side surface of the lens satin-finished or to apply a relatively dark color paint such as black to the outer peripheral side surface of the lens.

The aberrations of Example 4 are as shown in FIG. As shown in this figure, the imaging lens of Example 4 is
It can be seen that various aberrations are well corrected despite the fact that the number of lenses is two.

The fifth embodiment has the structure shown in FIG.
That is, it is an imaging lens in which two lenses, a first lens of a negative lens and a second lens of a positive lens, are sequentially arranged from the object side, and the second lens is a radial type gradient index lens element of both planes. The first lens is a biconcave lens, and is adhered or adhered to the radial type gradient index lens at the plane portion 1 on the image plane side. An aperture stop S is provided 5 mm from the object side surface of the radial type gradient index lens element to the image side. Further, a field stop FS for cutting flare components and the like is provided in the radial type gradient index lens. For these diaphragms, make a cut from the outer periphery of the radial type gradient index lens and apply paint that blocks the light flux to the cut surface, or cut the radial type gradient index lens and cut it on the cut surface. Can be produced by printing, vapor deposition or sandwiching a thin plate and adhering or adhering again.

The aberrations of the fifth embodiment are as shown in FIG. As shown in this figure, the imaging lens of Example 5 is
It can be seen that various aberrations are well corrected despite the fact that the number of lenses is two.

The sixth embodiment of the present invention is shown in FIG. That is, it is an imaging lens in which two lenses, a first lens of a negative lens and a second lens of a positive lens, are sequentially arranged from the object side, and the second lens is a radial type gradient index lens element of both planes. The first lens is a concave flat lens, and is adhered or adhered to the radial type gradient index lens element on the image side surface.

Further, the diameter of the radial type gradient index lens element is made different from the diameter of the homogeneous lens element, and the lens frame or the part 6 of the lens frame can be efficiently arranged, thereby achieving downsizing of the lens system. There is.

The aberrations of Example 6 are as shown in FIG. As can be seen from the figure, the various aberrations are well corrected in the imaging lens of Example 6 even though the number of lenses is two.

The seventh embodiment of the present invention is shown in FIG. That is, the first lens is a radial type inhomogeneous lens having a positive refracting power and the second lens is a homogenous lens having a negative refracting power in order from the object side. The homogeneous lens of the second lens has a concave flat shape and has a flat surface portion 1 on the outer peripheral portion with respect to the effective diameter of the object side surface, and the flat surface portion is bonded or adhered to the radial type inhomogeneous lens. Further, the stop S is 2.7 closer to the image side than the object side surface of the radial type inhomogeneous lens.
It is provided at a position of 369 mm.

The seventh embodiment is an optical system of equal-magnification imaging, and can be used as an image pickup lens system for a video microscope or the like by disposing a solid-state image pickup element at the image plane position.

The aberrations in Example 7 are as shown in FIG. As shown in this figure, it is understood that the imaging lens of Example 7 has various aberrations well corrected despite the fact that the number of lenses is two.

The eighth embodiment has the structure shown in FIG.
That is, it is an imaging lens in which two lenses, a first lens of a negative lens and a second lens of a positive lens, are sequentially arranged from the object side, and the second lens is a radial type gradient index lens element of both planes. The first lens is a biconcave lens, and includes a flat surface portion 1 on the image side and a flat surface portion 8 of the radial type gradient index lens element.
The interval ring 7 is sandwiched between. An aperture stop S is provided 3.887 mm from the surface on the object side of the radial type gradient index lens element. When focusing is performed by changing the distance between the first lens and the second lens, it is desirable to dispose the first lens, the distance ring 7, and the radial type gradient index lens element without adhering to each other in FIG. 8B. It is also possible to bond the first lens and the spacing ring 7, or the spacing ring 7 and the radial type gradient index lens. Example 8
In the above, the spacing ring 7 is a thin plate having a shape of both planes. However, for example, the spacing ring 7 may be shaped so as to have a surface oblique to the optical axis as shown by the hatched portion in FIG. Is.
In the case of FIG. 23, the flat part of the interval ring 7 and the flat part 6 of the radial type gradient index lens L _g are brought into close contact with each other and the contact surface is slid so that the optical axis positions of both lenses are adjusted to coincide with each other. be able to. Further, as shown in FIG. 24, it is also possible to bring the relatively sharp portion 8 of the homogeneous concave lens into close contact with the radial type gradient index lens element, and make an adjustment so that the optical axes of both lenses coincide with each other without using a spacing ring. Is.

Further, in order to achieve higher image forming performance, it is desirable that the homogeneous lens has a biconcave shape as in this embodiment. If it has a biconcave shape, the negative refracting power can be distributed to both surfaces, so that it is possible to reduce various aberrations generated in this lens.

In Example 8, the distance between the first lens and the second lens when focusing on a short-distance object (object distance 100 mm) was 0.6148 mm, and the aperture was the second lens (refractive index distribution lens). It is fixed to.

The aberrations of Example 8 are as shown in FIG. As can be seen from this figure, the imaging lens of Example 8 has various aberrations satisfactorily corrected despite the fact that the number of lenses is two.

Next, a case of manufacturing the image pickup lens of the present invention will be described in detail with reference to FIGS.

FIG. 25 shows a conventional example, which is a model in which a radial type gradient index lens element is directly subjected to spherical surface processing. FIG. 25A is a diagram showing a cylindrical radial type gradient index lens and a gradient index distribution before spherical processing, where L _g is a radial type gradient index lens and n (r) is a radius r.
The optical axis 10 of the medium coincides with the central axis of the outer peripheral portion 13. FIG. 25B is an ideal example in which one surface 14 is processed into a spherical surface, and the optical axes of the surface and the medium coincide with each other. However, in the actual processing scene,
It is difficult to easily process the surface R ₁ with high accuracy as shown in FIG. 5B, and as shown in FIG.
0 and the optical axis 15 of the plane R ₁ are the directions 16 perpendicular to the optical axis of the medium.
In some cases, eccentricity of δ and tilt angle ε may occur. In the case of a homogeneous lens, even if such eccentricity occurs, decentering is not a problem because centering can be performed by cutting the lens outer periphery with the optical axis of the polished surface as a collision, but in the case of a radial type gradient index lens the medium is used. Since the optical axis of is eccentric with respect to the outer peripheral portion, centering cannot be performed.

Therefore, in the lens system of the present invention, as in the model shown in FIG. 26, a radial type gradient index lens having a shape of both planes and a homogeneous spherical lens having a plane portion 19 are used by adhering or adhering. FIG. 26A shows a radial type gradient index lens L _g whose both surfaces are processed into a flat surface and a homogeneous lens L _h having a flat surface portion 19 on one side.
The optical axis 10 of the medium of the radial type gradient index lens L _g coincides with the central axis of the outer peripheral portion 13, and the homogeneous lens L
_{Since h} can be centered, the optical axis 16 of the surface R ₁ can be easily aligned with the central axis of the outer peripheral portion 21 with high accuracy. FIG. 26B shows a diagram in which the radial type gradient index lens L _g and the homogeneous lens L _h are adhered or brought into close contact with the respective plane portions 20 and 19. At this time,
For example, if as the outer peripheral portion 21 of the outer peripheral portion 13 and the homogeneous lens L _h of the radial type refractive index distribution lens coincides, the optical axis of the homogeneous lens of the medium of the radial type refractive index distribution lens easily Can be matched.

Further, in the conventional method shown in FIG. 25, when decentering occurs during spherical surface processing, the radial type gradient index lens L _g cannot be used, resulting in poor yield and high cost. In the method shown in FIG. 26, since the eccentricity at the time of spherical surface processing does not pose any problem as described above, it is possible to increase the yield and perform processing at an extremely low cost. Further, since the radial type gradient index lens has flat surfaces on both sides, it can be processed with high accuracy and at low cost.

FIG. 27 shows an example in which the image pickup lens of the present invention is incorporated in a lens frame having an extremely simple structure. FIG.
Medium L _h is a concave lens, L _g is a radial type gradient index lens, and 25 is a lens frame, which is a cylindrical shape in this case. Also, 2
Reference numerals 6 and 27 denote a solid-state image sensor and its processing circuit section, and 28
Is a cover glass for protecting the lens. With the configuration shown in FIG. 27, a homogeneous lens and a radial type gradient index lens can be dropped into the lens frame 25, so that the assembly is extremely easy. Further, when the image pickup lens is held by the lens frame as in the present embodiment, the homogeneous lens and the radial type gradient index lens do not necessarily have to be bonded. In addition, if the homogeneous lens and the radial type gradient index lens are bonded together in advance and then mounted in a lens frame, they can be assembled with high accuracy. At this time, it is also possible to bond the image pickup element to the radial type gradient index lens element in advance.

Further, the cover glass 28 can be constructed with an optical filter for cutting a specific wavelength component.

FIG. 28 shows the structure of the lens frame for focusing with the imaging lens of the present invention. In FIG. 28, L _h is a concave lens, L _g is a radial type gradient index lens, and 25 is a lens frame, which holds the radial type gradient index lens L _g in this case. Further, reference numerals 26 and 27 represent a solid-state image sensor and its processing circuit section, and 29 is a lens frame which holds a homogeneous lens L _h . As shown in FIG. 28, the homogeneous lens L _h and the radial type gradient index lens component L _g are held by different lens frame parts 25 and 29, thereby enabling focusing. In FIG. 28, the lens frame 25
The lens frame 29 and the lens frame 29 are normally connected to each other via a cam groove used in a lens frame of a silver halide camera or the like, but they are connected to each other via a screw portion or the contact surface has a smooth shape. The mirror frame of may be slid for focusing.

The imaging lens shown in each of the following items can also achieve the object of the present invention.

(1) An imaging lens which satisfies the following condition (3) in the lens system described in claims 1, 2 or 3 of the claims.

(3) −1.5 <f / f _f <−0.05 (2) The lens system according to claim 1, 2 or 3 of the claims or (1) above. Then, an imaging lens that satisfies the following condition (4).

(4) 0.5 <t _G / f <9 (3) The lens system according to any one of claims 1, 2 and 3 or (1) or (2) above. Then, an imaging lens that satisfies the following condition (5).

(5) 1.5 <t _G / f <7 (4) Claims 2 or 3 of the claims or (1), (2) or (3) above. An imaging lens that satisfies the following condition (6) in the lens system.

(6) −0.01 <1 / V ₁₀ <0.008 (5) Claims 1, 2 or 3 of the claims or the above (1), (2), (3) or ( An imaging lens satisfying the following condition (7) with the lens system described in the item 4).

(7) −0.2 <N ₂₀ × f ⁴ <0.2 (6) Claims 1, 2 or 3 in the claims or (1), (2), (3) or An imaging lens satisfying the following condition (8) with the lens system described in the item (4).

(8) −0.05 <N ₂₀ × f ⁴ <0.05 (7) Claims 1, 2 or 3 of the claims or (1), (2), (3), An imaging lens satisfying the following condition (9) in the lens system described in the item (4), (5) or (6).

(9) 1 / ν _h <0.03 (8) Claims 1, 2 or 3 in the claims or (1), (2), (3), (4), (5) ) Or (6), the following conditions (10)
Imaging lens that satisfies the requirements.

(10) 1 / ν _h <0.025 (9) Claims 1, 2 or 3 in the claims or (1), (2), (3), (4), (5) ),
An imaging lens according to the lens system described in (6), (7), or (8), which satisfies the following condition (11).

(11) N _00d > 1.55 (10) Claims 1, 2 or 3 in the claims or (1), (2), (3), (4), (5),
An imaging lens satisfying the following condition (12), which is the lens system according to (6), (7), (8), or (9).

(12) 0.5 <a _PAC <1.7 (11) Claims 1, 2 or 3 of the claims or (1), (2), (3), (4), (5),
(6), (7), (8), (9) or the lens system described in (10), which satisfies the following condition (13).

(13) −1.5 <f / f _h <−0.4 (12) Claims 1, 2 or 3 in the claims or (1), (2), (3), (4), (5),
(6), (7), (8), (9) or the lens system described in (10), the imaging lens satisfying the following condition (14).

(14) −0.8 <f / f _h <−0.1

[0161]

The image pickup lens of the present invention has excellent workability and various aberrations, particularly axial chromatic aberration and Petzval sum, are well corrected.

[Brief description of the 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 a sectional view of a third embodiment of the present invention.

FIG. 4 is a sectional view of a fourth embodiment of the present invention.

FIG. 5 is a sectional view of a fifth embodiment of the present invention.

FIG. 6 is a sectional view of a sixth embodiment of the present invention.

FIG. 7 is a sectional view of a seventh embodiment of the present invention.

FIG. 8 is a sectional view of Embodiment 8 of the present invention.

FIG. 9 is an aberration curve diagram for an object point at infinity according to the first embodiment of the present invention.

FIG. 10 is a short-distance object point (object point distance 1 according to the first embodiment of the present invention.
Aberration curve diagram for (00 mm)

FIG. 11 is an aberration curve diagram for an object point at infinity according to Example 2 of the present invention.

FIG. 12 is a short-distance object point (object point distance 1 according to the second embodiment of the present invention.
Aberration curve diagram for (00 mm)

FIG. 13 is a short-distance object point (object point distance 1 according to the third embodiment of the present invention.
Aberration curve diagram for 1mm)

FIG. 14 is an aberration curve diagram for an object point at infinity according to Example 4 of the present invention.

FIG. 15 is an aberration curve diagram for an object point at infinity according to Example 5 of the present invention.

FIG. 16 is an aberration curve diagram for an object point at infinity according to Example 6 of the present invention.

FIG. 17 is an aberration curve diagram for a near object point (object point distance 2.85 mm) according to Example 7 of the present invention.

FIG. 18 is an aberration curve diagram for an object point at infinity according to Example 8 of the present invention.

FIG. 19 is a short-distance object point (object point distance 1 according to Example 8 of the present invention.
Aberration curve diagram for (00 mm)

FIG. 20 is a graph showing the relationship between the refractive power ratio a of the medium, the axial chromatic aberration, and the Petzval sum.

FIG. 21 is a diagram showing a distribution of refractive index-Abbe number of a glass material.

FIG. 22 is a diagram showing an example of a combination of a radial type gradient index lens and a homogeneous lens.

FIG. 23 is a view showing a contact means between the flat surface of the radial type gradient index lens element and the concave surface side of the homogeneous lens.

FIG. 24 is a view showing another contact means between the flat surface of the radial type gradient index lens element and the concave surface side of the homogeneous lens element.

FIG. 25 is a view showing an example in which a radial type gradient index lens, which is a conventional manufacturing example, is directly subjected to spherical surface processing.

FIG. 26 is a diagram showing an example of a combination of imaging lenses according to the present invention.

FIG. 27 is a view showing an example of assembling the imaging lens of the present invention into a lens frame.

FIG. 28 is a view showing another example of assembling the imaging lens of the present invention into the lens frame.

Claims (3)

[Claims] 1. A homogenous lens having a negative refractive power and a radial type gradient index lens represented by the following formula (a) in which the refractive index is distributed in the radial direction from the optical axis having a positive refractive power. An imaging lens characterized by satisfying the following condition (1). _{N (r) = N 00 +} N 10 r 2 + N 20 r 4 ··· (a) (1) -0.05 <1 / V 10 <0.01 where, r is the distance in the radial direction from the optical axis, N (r) is the refractive index distribution at the point of distance r, N _i0 is the 2i-th order refractive index distribution coefficient, and V _i0 is the i-th order dispersion of the radial type gradient index lens, and the following formula (b), ( given in c). _{V 00 = (N 00d -1)} / (N 00F -N 00C) (i = 0) (b) V i0 = N i0d / (N i0F -N i0C) (i = 1,2,3 ··) ( c) where N _00d , N _00F and N _00C are d, F and C, respectively.
Refractive indices on the optical axis of the line, N _i0d , N _i0F , and N _i0C are 2i-th order refractive index distribution coefficients of the d, F, and C lines, respectively. 2. A homogenous lens having a negative refractive power and a radial type gradient index lens represented by the following formula (a) in which the refractive index is distributed in the radial direction from the optical axis having a positive refractive power. That is, the radial type gradient index lens element has a biplanar shape, and a part or all of the surface of the homogeneous lens on the radial type gradient index lens side has a planar shape, and the following condition (2) is satisfied. Characteristic imaging lens. N (r) = N ₀₀ + N ₁₀ r ² + N ₂₀ r ⁴ (a) (2) 1 / V ₁₀ <1 / ν _h where r is the distance from the optical axis in the radial direction, N (r) Is the refractive index distribution at the distance r, N _i0 is the 2i th order refractive index distribution coefficient, V _i0 is the i th order dispersion of the radial type gradient index lens, and is given by the following equations (b) and (c). Ν _h is the Abbe number of a homogeneous lens. _{V 00 = (N 00d -1)} / (N 00F -N 00C) (i = 0) (b) V i0 = N i0d / (N i0F -N i0C) (i = 1,2,3 ··) ( c) where N _00d , N _00F and N _00C are d, F and C, respectively.
Refractive indices on the optical axis of the line, N _i0d , N _i0F , and N _i0C are 2i-th order refractive index distribution coefficients of the d, F, and C lines, respectively. 3. A homogenous lens having a negative refractive power and a radial type gradient index lens having a positive refractive power and having a refractive index distributed in a radial direction from an optical axis represented by the following formula (a): In other words, the image pickup lens is characterized in that the distance between the homogeneous lens and the radial type gradient index lens element is changed to perform focusing on an object at a close range. ^{_{N (r) = N 00 +}} N 10 r 2 + N 20 r 4 ··· (a) (1) -0.05 <1 / V 10 <0.01 where, r is the distance in the radial direction from the optical axis, N (r) is the refractive index distribution at the distance r, and N _i0 is the 2i-th order refractive index distribution coefficient.