JP2005091666A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized and compact imaging lens comprising three groups of four lenses, which can be adapted to a mega-order high pixel device. <P>SOLUTION: An imaging lens 10 has; a first lens group I (consisting of a first lens 1) which has a convex directed to the object side and has a positive power; a second lens group II (consisting of a concave lens 2 and a convex lens 3) which has a concave directed to the object side and has a positive power; and a third lens group III (consisting of a fourth lens 4) having a negative power. Lens surfaces 4a and 4b of the fourth lens 4 are aspherical surfaces together, and the imaging lens 10 satisfies 0.6<Σd<F<1.2, 0.8<Nd5/Nd4<1.2, and 1.4<νd5/νd4<2.5 wherein; F is the resultant focal length of an entire lens system; Σd is an interval on an optical axis between the object-side lens surface of the first lens and the image surface-side lens surface of the fourth lens; Nd4 is the refractive index of the second lens; Nd5 is the refractive index of the third lens; νd4 is the Abbe's number of the second lens; and νd5 is the Abbe's number of the third lens. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-quality, small and lightweight imaging lens used for an in-vehicle camera, a monitoring camera, a digital camera, a mobile phone camera, and the like using a light receiving element such as a CCD or a CMOS.

  An imaging lens incorporated in a monitoring camera or a digital camera using a light receiving element such as a CCD desirably has faithful subject reproducibility. Recently, CCDs and CCD cameras have been miniaturized as seen in cameras incorporated in mobile phones, and as a result, imaging lenses incorporated in these cameras have inevitably become smaller and more compact. The demand for is increasing. Furthermore, light receiving elements such as CCDs have become higher in the order of mega pixels, contrary to the miniaturization of CCDs. In addition, an imaging lens used in a camera using these must inevitably have a high optical performance. Conventionally, in order to exhibit such high optical performance, aberration correction has been performed using a large number of lenses.

  In addition, as a feature of a light receiving element such as a CCD or a CMOS, there is a restriction on a light ray angle taken into each pixel, and in an optical system ignoring this, an aperture efficiency is reduced and shading occurs. For this reason, the position of the exit pupil must be as far as possible from the image plane.

  Furthermore, since a space for inserting an infrared cut filter or the like is required between the imaging lens and the light receiving element, there is a restriction that the back focus must be long to some extent.

  An object of the present invention is to provide a compact and compact imaging that can prevent shading by making the maximum emission angle of the light receiving element with respect to the element surface smaller than the angle of view, and can correct aberrations so as to be compatible with mega pixels. To propose a lens.

  In order to solve the above-described problems, the imaging lens of the present invention is a lens having a four-element configuration in three groups, and a first lens group having a positive power from the object side toward the imaging surface, and a positive power. A second lens group having a negative power and a third lens group having a negative power are arranged. The first lens group is composed of a single first lens having a convex surface directed toward the object side, and the second lens group is a composite lens having a positive power with a concave surface directed toward the object side and having a different dispersion. It consists of a combination of a (second lens) and a convex lens (third lens). The third lens group consists of a single concave lens. Furthermore, at least one of the lens surfaces of the first, second, and third lenses is an aspheric surface. Further, the lens surfaces of the fourth lens are both aspherical.

  In the imaging lens of the present invention, since the first lens is a meniscus lens having a convex surface directed toward the object side, the overall length of the lens optical system can be shortened. Further, by making the second lens and the third lens of the second lens group have different dispersions, the on-axis and off-axis chromatic aberration can be effectively corrected. Further, in the case of a wide-angle lens, it is easy to correct aberrations such as distortion and coma aberration by making the object-side lens surface of the second lens group concave on the object side.

  Here, as a feature when the imaging plane is a CCD or CMOS, there is a restriction on the angle of light rays taken into each pixel, and in general, the angle gradually increases toward the periphery of the screen. In the imaging lens of the present invention, the lens surface on the object side of the second lens group has a concave surface directed toward the object side, and an aspheric surface is adopted for the lens surface of the third lens group to obtain the aspheric shape. By utilizing this, the maximum emission angle of the chief ray can be made 30 degrees or less. In addition, by adopting an aspherical surface for the lens surface of the third lens group, it is possible to satisfactorily correct fine adjustment of the field curvature and distortion, and in addition, it is possible to reduce the maximum emission angle of the principal ray. Is possible.

Next, it is desirable that the imaging lens of the present invention satisfies the following conditional expressions (1) to (3).
0.6 <Σd <F <1.2 (1)
0.8 <Nd5 / Nd4 <1.2 (2)
1.4 <νd5 / νd4 <2.5 (3)
Where F: combined focal length of all lens systems Σd: distance on the optical axis from the lens surface on the object side of the first lens to the lens surface on the image forming side of the fourth lens Nd4: refractive index of the second lens Nd5 : Refractive index of the third lens νd4: Abbe number of the second lens νd5: Abbe number of the third lens

  Conditional expression (1) is a conditional expression for miniaturization. If the lower limit is not reached, miniaturization is possible, but the lens becomes small and processing becomes difficult. If the upper limit is exceeded, it becomes difficult to compact the entire lens system.

Here, in addition to the conditional expression (1), the air gap formed by the first lens group and the second lens group also contributes to the miniaturization and the peripheral light amount. If this air gap is da,
0.05 <da / F <0.5 (4)
Is desirable. If the lower limit is not reached under this condition, it becomes difficult to correct spherical aberration and curvature of field, and if the upper limit is exceeded, it becomes difficult to ensure the amount of peripheral light and at the same time miniaturization cannot be achieved.

  Conditional expression (2) is for keeping the spherical aberration well and keeping the curvature of the image surface stable. When the lower limit is exceeded, correction of spherical aberration and coma becomes difficult, and when the upper limit is exceeded. The field curvature cannot be kept stable.

  Conditional expression (3) is a condition for performing on-axis and off-axis achromaticity. If the upper limit is exceeded, correction of chromatic aberration becomes easy, but correction of spherical aberration and curvature of field becomes difficult. If the lower limit is not reached, correction of chromatic aberration becomes difficult.

Furthermore, in the present invention, the lens surface on the object side of the second lens has a concave surface on the object side. That is, if the curvature of the lens surface on the object side is R4,
R4 / F <0 (5)
It is. Thus, the correction of off-axis coma, field curvature, and distortion can be kept stable. If this condition is exceeded, negative distortion increases, and the aspherical shape of the third lens group becomes extremely complicated.

In addition, as described above, an aspherical surface is used for the lens surface of the third lens group, and at least one inflection point is provided, so that fine adjustment of the field curvature and distortion can be corrected well. In addition to this, it is possible to reduce the maximum emission angle of the principal ray, but in addition to this, when the curvature of the lens surface on the imaging surface side of the second lens group is R6,
0.1 <R6 / R4 <1.0 (6)
Is desirable. If the lower limit of this condition is not reached, the maximum emission angle of the chief ray can be reduced, but the power of the second lens group increases and negative field curvature occurs, and at the same time, it is difficult to ensure back focus. On the other hand, if the upper limit is exceeded, the maximum emission angle of the chief ray increases and the back focus becomes longer, making it difficult to reduce the size.

  The imaging lens according to the present invention is a lens having four elements in three groups, and the first lens, which is the first lens group, is a meniscus lens having a convex surface facing the object side, so that the total length of the lens optical system can be shortened. Further, by making the second lens and the third lens of the second lens group have different dispersions, the on-axis and off-axis chromatic aberration can be effectively corrected. Further, in the case of a wide-angle lens, the first lens surface of the second lens group is concave on the object side, so that aberrations such as distortion and coma can be easily corrected.

  Furthermore, by using the first lens surface of the second lens group to be concave toward the object side and the aspherical shape of the third lens group, the maximum emission angle of the chief ray is 30 degrees or less. It becomes possible to do.

  In addition, by adopting an aspherical surface for the third lens group, it is possible to finely adjust the field curvature and correct distortion, and at the same time, reduce the maximum emission angle of the principal ray. Is possible.

  Therefore, according to the present invention, it is possible to realize a compact, lightweight, and compact imaging lens that can cope with high-order pixels and can prevent shading.

  An imaging lens to which the present invention is applied will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram illustrating the imaging lens of the first embodiment. The imaging lens 10 of the present example includes a first lens 1 of the first lens group I, a second lens 2 and a third lens 3 of the second lens group II, and a third lens from the object side toward the imaging surface. The group III fourth lens 4 is arranged in this order. A diaphragm 5 is disposed between the first lens 1 and the second lens 2. Further, a cover glass 6 is disposed between the fourth lens 4 and the image plane 7.

  The first lens 1 of the first lens group is a positive meniscus lens having a convex surface facing the object side. The second lens group has a positive power with the concave surface facing the object side, and is composed of a cemented lens composed of a combination of the second lens 2 having a negative power and the third lens 3 having a positive power. The fourth lens 4 of the third lens group is configured by a lens having negative power.

  In this example, the lens surfaces 1a and 1b on both sides of the first lens 1 and the lens surfaces 4a and 4b on both sides of the fourth lens 4 are both aspheric. The lens surface 4a on the object side of the fourth lens 4 has an aspherical inflection point at about 32% of the effective diameter, and the lens surface 4b on the imaging surface side has about 40% of the effective diameter. Yes.

The lens data of the entire optical system of the imaging lens 10 is as follows.
F number: 2.8
Focal length: F = 3.55mm
Total lens length: Σd = 3.87mm

  Table 1A shows lens data of the imaging lens 10, and Table 1B shows aspheric coefficients for defining the aspherical shape of the aspherical lens surface. In Table 1A, i represents the order of the lens surfaces counted from the object side, R represents the radius of curvature of the lens surfaces, d represents the distance between the lens surfaces, Nd represents the refractive index of each lens surface, νd Represents the Abbe number of each lens. In addition, a lens surface with a star mark attached to the number i indicates an aspherical surface.

  The aspherical shape adopted for the lens surface is as follows, assuming that the axis in the optical axis direction is X, the height in the direction orthogonal to the optical axis is H, the conical coefficient is k, and the aspherical coefficients are A, B, C, and D. It can be expressed by a formula.

  The meaning of each symbol and the expression representing the aspherical shape are the same in the embodiments described later.

  The imaging lens 10 of this example satisfies the conditional expressions (1) to (3) as shown in Table 3 which will be described after the description of the second example.

  FIG. 3 is an aberration diagram showing various aberrations in the imaging lens 10. 3A shows spherical aberration SA, FIG. 3B shows astigmatism AS, and FIG. 3C shows distortion. In FIG. 3B, T represents a tangential image, and S represents a sagittal image surface. FIG. 3D shows lateral aberration, DY is the lateral Y aberration with respect to the Y pupil coordinates, and DX is the lateral X aberration with respect to the X pupil coordinates. The meaning of these symbols is the same as in Example 2 described later.

  FIG. 2 is a configuration diagram illustrating the imaging lens of the second embodiment. The imaging lens 20 of the present example includes a first lens 11 of the first lens group I, a second lens 12 and a third lens 13 of the second lens group II, and a third lens from the object side toward the imaging surface. The group III fourth lens 14 is arranged in this order. A diaphragm 15 is disposed between the first lens 11 and the second lens 12. A cover glass 16 is disposed between the fourth lens 14 and the imaging plane 17.

  The first lens 11 of the first lens group is a positive meniscus lens having a convex surface facing the object side. The second lens group has a positive power with the concave surface facing the object side, and is composed of a cemented lens composed of a combination of a second lens 12 having a negative power and a third lens 13 having a positive power. The fourth lens 14 of the third lens group is configured by a lens having negative power.

  In this example, the lens surfaces 11a and 11b on both sides of the first lens 11 and the lens surfaces 14a and 14b on both sides of the fourth lens 14 are both aspherical. Further, the lens surface 14a on the object side of the fourth lens 14 has an aspherical inflection point at about 32% of the effective diameter and the lens surface 14b on the resolution surface side of about 40% of the effective diameter. Yes.

The lens data of the entire optical system of the imaging lens 20 is as follows.
F number: 2.8
Focal length: F = 3.65mm
Total lens length: Σd = 3.8mm

  Table 2A shows lens data of each lens surface of the imaging lens 20, and Table 2B shows aspheric coefficients for defining the aspheric shape of the aspheric lens surface. FIG. 4 shows various aberrations in the imaging lens 20.

  Also in the imaging lens 20 of this example, as shown in Table 3, the conditional expressions (1) to (3) are satisfied.

(Other embodiments)
In the imaging lenses of the above-described embodiments, the second lens group is a cemented lens. However, it is needless to say that the imaging lens may be constituted by two separated lenses. The second lens group is formed of a glass lens, but it is needless to say that a plastic lens may be used.

It is a block diagram of the imaging lens of Example 1 of this invention. It is a block diagram of the imaging lens of Example 2 of this invention. It is an aberration diagram of the imaging lens of Example 1 of FIG. FIG. 6 is an aberration diagram of the imaging lens of Example 2 in FIG. 2.

Explanation of symbols

I First lens group
II Second lens group
III Third lens group 1, 11 First lens 2, 12 Second lens 3, 13 Third lens 4, 14 Fourth lens 5, 15 Aperture 6, 16 Cover glass 7, 17 Imaging surface 10, 20 Imaging lens d1 , D4, d5, d7 Lens thickness da (d2 + d3) Air spacing R1, R2, R4, R5, R6, R7, R8 between the first lens group and the second lens group The radius of curvature on the optical axis of the lens

Claims (6)

A first group lens having a positive power, arranged in order from the object side, a second group lens having a positive power, and a third group lens having a negative power;
The first lens group comprises a single first lens with a convex surface facing the object side,
The second group lens is a combination of a second lens made of a concave lens with a concave surface facing the object side and a third lens made of a convex lens,
The third group lens includes a single fourth lens,
Of the lens surfaces of the first, second, and third lenses, at least one lens surface is an aspheric surface,
The lens surface of the fourth lens is an imaging lens in which both lens surfaces are aspherical surfaces. In claim 1,
When the combined focal length of the imaging lens is F, and the distance on the optical axis from the object-side lens surface of the first lens to the imaging surface-side lens surface of the fourth lens is Σd,
0.6 <Σd <F <1.2
An imaging lens. In claim 1 or 2,
When the refractive index of the second lens is Nd4 and the refractive index of the third lens is Nd5,
0.8 <Nd5 / Nd4 <1.2
An imaging lens. In claim 1, 2 or 3,
When the Abbe number of the second lens is νd4 and the Abbe number of the third lens is νd5,
1.4 <νd5 / νd4 <2.5
An imaging lens. In any one of claims 1 to 4,
When the air gap between the first lens group and the second lens group is da,
0.05 <da / F <0.5
An imaging lens. In any one of claims 1 to 5,
When the curvature of the lens surface on the imaging surface side of the second lens group is R6,
0.1 <R6 / R4 <1.0
An imaging lens.

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