JP2002250862A - Image pickup lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive image pickup lens with which an excellent image quality is obtained suitable to a video camera and a still picture video camera for consumer use. SOLUTION: The image pickup lens 1 is constituted of a first lens L1 of a convex meniscus lens whose concave face is faced to the object side, a second lens L2 of the convex meniscus lens whose concave face is faced to the image side and a cemented lens consisting of a third lens L3 of a concave lens and a fourth lens L4 of a convex lens in this order from the object side, and at least one surface of the second lens and the face of the fourth lens on the image side are constituted of spherical surfaces, and when the Abbe number of an i-th lens is set as νi (i=1, 2, 3, and 4), the respective conditions of ν1<ν2 and ν3<ν4 are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to a technique for providing a small and high-performance imaging lens used for a consumer video camera and a still picture video camera. Related to technology.

[0002]

2. Description of the Related Art Conventionally, a retro-focus type is the mainstream of a wide-angle lens having an angle of view exceeding 55 degrees and an imaging lens for a video camera having a brightness of about F2.8.

That is, the retrofocus type is often used for the imaging lens because an infrared cut filter, an optical low-pass filter, and the like must be arranged between the lens closest to the image and the imaging device. The back focus is required, and microlenses called on-chip lenses are arranged for each pixel in the image sensor in order to enhance the light-gathering effect, and the exit pupil is required to properly exert the effect of this on-chip lens. Is required to be appropriately long.

[0004] As an example of the retrofocus lens used as the above-mentioned conventional imaging lens, Japanese Patent Application Laid-Open No.
-113799, JP-A-9-281487, JP-A-10-78545, JP-A-10-30
There are those described in JP-A-1025, JP-A-8-5908 and JP-A-11-142730.

A small and inexpensive retrofocus type imaging lens used in an image capturing camera or a film with a lens incorporated in a personal computer is disclosed in Japanese Patent Application Laid-Open Nos. 231132, JP-A-11-
There are those described in 337820 and JP-A-9-304695.

By the way, in the above-mentioned retrofocus type imaging lens, chromatic aberration can be satisfactorily corrected and high performance can be realized by incorporating a flint type concave lens in the rear lens unit. However, there is a drawback that the overall length becomes long.

Further, the two-lens configuration using a plastic lens has a drawback that chromatic aberration cannot be corrected in principle even though monochromatic aberration can be corrected well, and it is possible to cope with an image sensor having about 100,000 pixels. Also, there is a problem that it cannot be applied to a high-pixel type image sensor, for example, an image sensor having 1 million pixels or more.

[0008]

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention achieves a miniaturization comparable to a two-lens construction of a plastic lens, and at the same time, the exit pupil distance becomes about twice as long as the focal length. It is an object of the present invention to provide an inexpensive high-quality imaging lens which is suitable for a consumer video camera and a still image video camera, which has good adaptability to, and in which various aberrations including chromatic aberration are corrected extremely well.

[0009]

In order to solve the above problems, the imaging lens of the present invention has, in order from the object side, a first meniscus lens having a concave surface facing the object side and a concave surface facing the image side. The second lens of the convex meniscus lens, the cemented lens of the third lens of the concave lens and the fourth lens of the convex lens, and at least one surface of the second lens and the image side surface of the fourth lens are aspherical. Νi (i
= 1, 2, 3, 4) is the Abbe number of the i-th lens.
This is to satisfy the conditions of ν1 <ν2 and ν3 <ν4.

Therefore, it is possible to construct a small and inexpensive imaging lens suitable for a consumer video camera and a still picture video camera.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the accompanying drawings.

In the following description, "ri" is the i-th lens surface (i-th surface) and its radius of curvature counted from the object side, and "di" is the surface from the i-th lens surface to the (i + 1) -th lens surface. The interval, [ni] is the refractive index of the material constituting the i-th lens Li at the d-line, and “νi” is the Abbe number of the material constituting the i-th lens Li (i is FL and in the above di, ni and νi, Are the spacing, refractive index, and Abbe number for each filter).

The definition of the aspherical surface is as follows: "xi" is the depth of the aspherical surface, "H" is the height from the optical axis, and "Aj" is the j-th order aspherical coefficient. Xi = H ² / ri ｛1+ (1-H ² / ri ² ) ^1/2 ｝ + ΣA
jH ^j .

First, the outline of the imaging lens will be described.

As shown in FIGS. 1 and 3, the imaging lenses 1 and 2 each have a three-group, four-lens configuration.
A first lens L1 of a convex meniscus lens having a concave surface facing the object side and a second lens L1 of a convex meniscus lens having a concave surface facing the image side
A lens L2, a cemented lens of a third lens L3 of a concave lens and a fourth lens L4 of a convex lens,
At least one surface of the second lens L2 and the image-side surface of the fourth lens L4 are formed by aspheric surfaces. The first lens group GR1 is formed by the first lens, the second lens group GR2 is formed by the second lens L2, and the third lens group GR3 is formed by the third lens L3 and the fourth lens L4.

If νi (i = 1, 2, 3, 4) is the Abbe number of the i-th lens, the following conditions are satisfied: ν1 <ν2 (conditional expression 1) and ν3 <ν4 (conditional expression 2). It has been like that.

It is preferable that the second lens L2 is formed of a plastic lens having both aspheric surfaces.

Further, the imaging lenses 1 and 2 have a radius of curvature (i = 1, i) of the i-th surface (i-th surface) counting i from the object side.
2, <), where f is the focal length of the entire lens system, 0.2 <r4 / f <0.3 (conditional expression 3), 0.4 <| r7 / f | <0.5 (conditional expression 4) It is desirable to satisfy each of the conditions.

Incidentally, an effective aberration correcting means in a small imaging lens is to introduce an aspherical surface. The introduction of an aspherical surface has a great effect on the correction of spherical aberration, astigmatism and distortion. However, those not affected by the aspherical surface include the Petzval sum (ΣP) and chromatic aberration. Therefore, how to correct Petzval's sum and chromatic aberration without hindering the miniaturization of the lens system is a point for achieving both miniaturization and high performance in a small imaging lens.

The present invention relates to, for example, JP-A-10-1045.
As described in Japanese Patent Laid-Open Publication No. 11-111, a two-lens system including a negative lens and a positive lens is developed to
An object is to provide means for effectively correcting axial chromatic aberration and chromatic aberration of magnification that cannot be corrected by a single-plate configuration.

The negative lens, which corresponds to the front group of the prior art, exerts an effect of reducing the Petzval sum by the difference in the radius of curvature of the front and rear surfaces and the effect of an appropriate thickness. However, since the principal ray intersects the optical axis in the rear group convex lens, the ray height of the principal ray passing through the front group convex lens becomes high, which causes chromatic aberration of magnification.

Therefore, in order to correct the chromatic aberration of magnification without changing the correction effect of the Petzval sum, in the present invention, the negative lens of the front group is divided into the first lens L1 of the convex meniscus lens and the second lens L2 of the concave meniscus lens. The chromatic aberration of magnification is effectively corrected by satisfying the above conditional expression 1 by separating the lens into a single sheet.

Further, the first lens L1 and the second lens L2 having a high principal ray height are lenses that easily exert an effect on correction of astigmatism. It is effective. Here, there is a problem as to which surface of the first lens L1 and the second lens L2 is made aspherical. However, it is necessary to consider the manufacturing method and cost of the lens. There are three methods of manufacturing an aspherical lens: a glass mold, a composite aspherical surface, and plastic injection molding.

However, the glass mold has the drawback that it is difficult to form a concave lens, and that there are many restrictions on the type of glass suitable for molding. The compound aspherical surface forms a plastic aspherical film on the surface of the spherical polished lens, but has the drawback that the outer diameter of the polished lens becomes considerably larger than the effective diameter. Is not suitable. Plastic injection molding has the advantages of low cost and easy molding of concave lenses, but has the disadvantage of having little freedom in refractive index and Abbe number.

In order to reduce the Petzval sum, it is convenient to set the refractive index of the i-th lens Li to ni and n1> n2, and to correct the chromatic aberration of magnification, the above-mentioned conditional expression 1
Must be satisfied, the second lens L of the concave lens
2 is a plastic aspherical surface, the minimum curvature and the aspherical surface are manufactured at low cost, and the first lens L1 of the convex lens is a spherical polished lens made of glass having a higher refractive index and a higher dispersion than the second lens L2. At the same time, the cost should be increased and the cost increase due to glass molding should be avoided.
By configuring the first lens L1 and the second lens L2 by such a method, the Petzval sum, chromatic aberration of magnification, astigmatism, and distortion in the front group can be mainly corrected.

The correction of the Petzval sum will be described in more detail. In the imaging lenses 1 and 2, the first surface r1
Since the fourth surface r4 has the smallest radius of curvature among the seventh surface r7, it is necessary to define the radius of curvature of the fourth surface r4. In conditional expression 3 defining the radius of curvature of the fourth surface r4, if the value of r4 / f exceeds the upper limit, the Petzval sum cannot be sufficiently reduced, and if the value exceeds the lower limit, the back focus becomes longer. It becomes impossible to achieve miniaturization, and it becomes difficult to correct distortion, astigmatism, and coma generated from the fourth surface r4.

Regarding the structure of the rear lens group, in the structure of the rear lens group, satisfying conditional expression 1 is in a direction to worsen axial chromatic aberration, and the deteriorated axial chromatic aberration is sufficiently corrected by the rear lens group. There is a need to. Further, it is necessary to satisfy conditional expression 2 for achromatism, and the third lens L3
The fourth lens L4 needs to be made of a material whose Abbe number is as far apart as possible, and the fourth lens L4 must be made of a material that can be easily processed as a glass mold.

As described above, when the materials of the third lens L3 and the fourth lens L4 are determined, the radius of curvature of the joint surface for correcting chromatic aberration is determined.

The positive refractive power of the rear group is concentrated on the seventh surface r7. Therefore, when the value of | r7 / f | exceeds the lower limit of conditional expression 4, the fourth lens L4 must be made thinner in order to maintain the focal length of the entire lens system, and the radius of curvature of the cemented surface is inevitably reduced. Since it cannot be reduced, the correction of chromatic aberration becomes insufficient. Conversely, if the value of | r7 / f | exceeds the upper limit, the distance from the fourth surface r4 to the seventh surface r7 increases, so that the back focus becomes longer, which is disadvantageous for miniaturization. Would.

Next, Numerical Examples 1 and 2 specifically showing the imaging lens of the present invention will be shown.

As shown in FIG. 1, the imaging lens 1 according to Numerical Example 1 is of a three-group, four-element configuration, and has a first convex meniscus lens having a concave surface facing the object side in order from the object side. A lens L1, a second lens L2 of a convex meniscus lens having a concave surface facing the image side, and a third lens L3 of a concave lens
And a cemented lens of a convex lens and a fourth lens L4. The object-side surface r3 and the image-side surface r4 of the second lens L2 and the image-side surface r7 of the fourth lens L4 are formed by aspheric surfaces. The fourth lens L4 and the image plane IM
A suitable filter FL such as an infrared cut filter or an optical low-pass filter is disposed between the filter G and the filter G.

Table 1 below shows numerical values of the imaging lens 1.

[0033]

[Table 1]

Table 2 shows each optical characteristic of the imaging lens 1.

[0035]

[Table 2]

Table 3 shows the surface r constituted by an aspherical surface.
The fourth, sixth, eighth and tenth order aspherical coefficients of 3, r4 and r7 are shown. Note that “e” in the table represents an exponential expression with a base of 10. For example, “e−3” represents “× 1”.
0 ^-3 "(the same applies to Table 6 described later).

[0037]

[Table 3]

FIG. 2 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1. In the spherical aberration curve diagram, the solid line shows the numerical value at the d-line, the broken line shows the numerical value at the g-line, and the dashed line shows the numerical value at the C-line. In the astigmatism curve diagram, the solid line shows the sagittal field curvature, and the broken line shows the meridional. This shows the numerical value of the field curvature (the same applies to FIG. 4 described later).

As shown in FIG. 3, the imaging lens 2 according to Numerical Example 2 is of a three-group, four-lens configuration, and has a first convex meniscus lens having a concave surface facing the object side in order from the object side. A lens L1, a second lens L2 of a convex meniscus lens having a concave surface facing the image side, and a third lens L3 of a concave lens
And a cemented lens of a convex lens and a fourth lens L4. The object-side surface r3 of the second lens L2,
The image-side surface r7 of the lens L4 is formed of an aspheric surface. Note that an appropriate filter FL such as an infrared cut filter or an optical low-pass filter is disposed between the fourth lens L4 and the image plane IMG.

Table 4 below shows numerical values of the imaging lens 2.

[0041]

[Table 4]

Table 5 shows the optical characteristics of the imaging lens 2.

[0043]

[Table 5]

Table 6 shows a surface r constituted by an aspherical surface.
The fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients of 3 and r7 are shown.

[0045]

[Table 6]

FIG. 4 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1.

As described above, the imaging lens having three groups and four elements according to the present invention can be miniaturized so that the total length is about 2.6 times the focal length, and can be an infrared cut filter or an optical low-pass. It is possible to secure a sufficient back focus for disposing a filter or the like.

Further, since the Petzval sum of the imaging lens of the present invention can be set to 0.25 or less, it is possible to satisfactorily correct the curvature of field, and further, the first lens and the second lens which are the front units have a magnification. It is possible to correct chromatic aberration,
The axial chromatic aberration can be corrected by the third lens and the fourth lens which are the rear group, and spherical aberration and coma can be satisfactorily corrected by the effect of the aspheric surface.

Furthermore, while the above-mentioned conventional two-lens plastic lens structure can be applied only to an image sensor having about 100,000 pixels, the image pickup lens of the present invention has
Necessary and sufficient image quality can be obtained even for an image sensor having 1,000,000 pixels or more.

It should be noted that the specific shapes and structures of the respective parts shown in the above-described embodiments are merely examples for embodying the present invention, and the technical features of the present invention will be described below. The scope should not be construed as limiting.

[0051]

As described above, the imaging lens according to the present invention includes, in order from the object side, the first lens of the convex meniscus lens having the concave surface facing the object side and the second lens of the convex meniscus lens having the concave surface facing the image side. The second lens is constituted by a cemented lens of a third lens of a concave lens and the fourth lens of a convex lens, and at least one surface of the second lens and an image-side surface of the fourth lens are constituted by aspheric surfaces. i = 1,2,
If (3, 4) is the Abbe number of the i-th lens, ν1 <ν
2. Since each condition of ν3 <ν4 is satisfied, a sufficient back focus can be ensured and a compact imaging lens having good optical characteristics can be provided.

In the invention described in claim 2, the second
Since the lens is a plastic lens having two aspheric surfaces, the imaging lens can be manufactured at low cost.

According to the third and fourth aspects of the present invention, ri is counted from the object side, i-th surface (i-th surface)
.., Where f is the focal length of the entire lens system, 0.2 <r4 / f <0.3, 0.4 <|
Since each condition of r7 / f | <0.5 was satisfied,
It is possible to provide a high-quality image pickup lens that can further improve optical characteristics and can cope with an image pickup element having 1 million pixels or more.

[Brief description of the drawings]

FIG. 1 shows Numerical Example 1 in an embodiment of the imaging lens of the present invention together with FIG. 2, and this figure is a schematic diagram showing a lens configuration.

FIG. 2 is a diagram illustrating spherical aberration, astigmatism, and distortion.

FIG. 3 shows Numerical Example 2 in the embodiment of the imaging lens of the present invention together with FIG. 4, and FIG. 3 is a schematic diagram showing a lens configuration.

FIG. 4 is a diagram illustrating spherical aberration, astigmatism, and distortion.

[Explanation of symbols]

1: imaging lens, 2: imaging lens, L1: first lens,
L2: second lens, L3: third lens, L4: fourth lens

Claims (4)

[Claims] 1. A first lens of a convex meniscus lens having a concave surface facing the object side, a second lens of a convex meniscus lens having a concave surface facing the image side, and a third lens having a concave surface facing the object side.
It is constituted by a cemented lens of a lens and a fourth lens of a convex lens, and at least one surface of the second lens and an image-side surface of the fourth lens are constituted by aspheric surfaces, so as to satisfy the following conditions. An imaging lens, characterized in that: ν1 <ν2 ν3 <ν4 where νi (i = 1, 2, 3, 4): Abbe number of the ith lens. 2. The imaging lens according to claim 1, wherein the second lens is a plastic lens having two aspheric surfaces. 3. The imaging lens according to claim 1, wherein the following conditions are satisfied. 0.2 <r4 / f <0.3 0.4 <| r7 / f | <0.5, where ri is the radius of curvature of the i-th surface (i-th surface) counted from the object side (i = 1, 2, ...), f: focal length of the whole lens system. 4. The imaging lens according to claim 2, wherein the following conditions are satisfied. 0.2 <r4 / f <0.3 0.4 <| r7 / f | <0.5, where ri is the radius of curvature of the i-th surface (i-th surface) counted from the object side (i = 1, 2, ...), f: focal length of the whole lens system.