JP2009265528A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens capable of satisfactorily correcting aberration though it is compact. <P>SOLUTION: The imaging lens is constituted by arranging a positive first lens L1 that is a biconvex lens, a second lens L2 that is a negative meniscus lens turning its concave surface to an object side, and a third lens L3 that is a negative meniscus lens turning its convex surface to the object side in order from the object side toward an image surface side. When an Abbe number of the first lens L1 on a d-line is defined as νd1, an Abbe number of the second lens L2 on the d-line is defined as νd2, and an Abbe number of the third lens L3 on the d-line is defined as νd3, an expression (1): νd1>50, an expression (2): 20<νd2<35 and an expression (3): 20<νd3<35 are satisfied. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an imaging lens that forms a subject image on an imaging element such as a CCD sensor or a CMOS sensor, and is relatively small such as a mobile phone, a digital still camera, a portable information terminal, a security camera, an in-vehicle camera, and a network camera. The present invention relates to an imaging lens suitable for being mounted on a camera.

  An imaging lens mounted on the above-described small camera is required to have a high-resolution lens configuration that can cope with a recent imaging device with a high number of pixels as well as a small number of lenses. Conventionally, various lens configurations have been proposed. Among them, a lens configuration consisting of three lenses can correct various aberrations relatively well and is suitable for downsizing. It is used in cameras.

  For example, an imaging lens described in Patent Document 1 is known as a three-lens imaging lens. This imaging lens includes, in order from the object side, a first lens having positive refractive power (power), a second lens having negative refractive power, and a third lens having negative refractive power. An inflection point is provided on the lens surface of the third lens. As is well known, when an imaging element such as a CCD sensor or a CMOS sensor is employed, it is necessary to suppress the incident angle of light within a certain range. In this regard, according to the imaging lens described in Patent Document 1, the angle of light emitted from the imaging lens is suppressed by the surface shape of the third lens, and the entrance pupil is further separated from the image plane by the positive and negative lens arrangement. Since they are arranged far away, it is possible to obtain a bright captured image of the peripheral part in which shading is well suppressed.

  On the other hand, in recent years, a so-called front aperture type lens configuration in which an aperture stop is disposed in front of the imaging lens is often employed in order to improve the ease of assembly of the imaging lens. The imaging lens described in Patent Document 1 also employs such a front diaphragm type lens configuration. Incidentally, in the case of a front diaphragm type lens configuration, the aberration is minimized when the first lens is plano-convex with respect to spherical aberration and coma, and when the first lens is biconvex with respect to field curvature. Therefore, in order to perform balanced aberration correction, it is better to make the shape of the first lens a biconvex shape.

JP 2007-3768 A

  In the above-described positive and negative lens arrangement, since there is only one lens having positive refractive power, when further downsizing is attempted, the refractive power of the first lens is made relatively stronger than the refractive power of other lenses. There is a need. In the imaging lens employing the above-described positive / negative lens arrangement, the chromatic aberration is corrected by making the Abbe number of the second lens smaller than the Abbe numbers of the first and third lenses. For this reason, in an imaging lens employing such a configuration, if the refractive power of the first lens is excessively increased, the correction of chromatic aberration is insufficient and the image quality of the captured image is deteriorated. Although the imaging lens described in Patent Document 1 is excellent in terms of suppressing the incident angle of light to the imaging device, it has been difficult to achieve both downsizing and good aberration correction.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide an imaging lens that can correct aberrations satisfactorily while being small in size.

In order to solve the above problems, in the present invention, in order from the object side to the image plane side, a biconvex first lens having a positive refractive power, a negative refractive power, and a concave surface on the object side Is an imaging lens comprising a second lens of a meniscus lens having a negative refractive power and a third lens of a meniscus lens having negative refractive power and a concave surface facing the object side, the Abbe number at the d-line of the first lens is represented by νd1 When the Abbe number at the d-line of the second lens is νd2, and the Abbe number at the d-line of the third lens is νd3,
νd1> 50 (1)
20 <νd2 <35 (2)
20 <νd3 <35 (3)
Configured to satisfy.

  According to the imaging lens having the above configuration, the Abbe number of the second and third lenses is relatively smaller than the Abbe number of the first lens, that is, the dispersion of the second and third lenses is larger than the dispersion of the first lens. Since it is large, the axial chromatic aberration (particularly short wavelength) generated in the first lens is corrected by the two lenses of the second and third lenses. Therefore, it is possible to correct aberrations satisfactorily while being small.

In the present invention, the imaging lens is configured to satisfy the following conditional expression (4).
νd2 = νd3 (4)
By configuring the imaging lens so as to satisfy the conditional expression (4), the glass material of the second lens and the third lens can be made common, and the manufacturing cost can be reduced.

  According to the imaging lens of the present invention, since the chromatic aberration is corrected by the second lens and the third lens, both the downsizing of the imaging lens and the favorable aberration correction are achieved, and various aberrations are corrected well. It is possible to provide a small imaging lens.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  1 and 4 show lens cross-sectional views corresponding to Numerical Examples 1 and 2 of the present embodiment, respectively. Since any of the numerical examples has the same basic lens configuration, the lens configuration of the present embodiment will be described with reference to the lens cross-sectional view of Numerical Example 1.

  As shown in FIG. 1, the imaging lens according to the present embodiment includes a first lens L1 having a positive refractive power and a second lens L2 having a negative refractive power in order from the object side to the image plane side. And a third lens L3 having the same negative refractive power is arranged. A cover glass 10 is disposed between the third lens L3 and the image plane of the image sensor. The cover glass 10 can be omitted.

  The first lens has a biconvex shape, and the surface on the image plane side is an aspherical shape in which the vicinity of the optical axis is convex on the image plane side and the peripheral portion is concave on the image plane side, that is, an inflection point. It is formed in the aspherical shape which has. The second lens is a meniscus lens having a concave surface facing the object side, and the third lens is a meniscus lens having a convex surface facing the object side. The aperture stop is disposed between the apex tangent plane of the object side surface of the first lens L1 and the image surface side surface of the first lens L1 (not shown).

In the present embodiment, all the lens surfaces of the first lens L1 to the third lens L3 are aspherical. The aspherical shape adopted for these lens surfaces is that the axis in the optical axis direction is Z, the height in the direction perpendicular to the optical axis is H, the conic coefficient is k, and the aspherical coefficients are A ₄ , A ₆ , A ₈ , A _{When 10} , it is represented by the following formula.

In the imaging lens according to the present embodiment, the Abbe number of the first lens L1 at the d-line is νd1, the Abbe number of the second lens L2 at the d-line is νd2, and the Abbe number of the third lens L3 at the d-line is νd3. When
νd1> 50 (1)
20 <νd2 <35 (2)
20 <νd3 <35 (3)
Satisfied. In the present embodiment, the following conditional expression (4) is further satisfied.
νd2 = νd3 (4)

  Next, numerical examples of the present embodiment will be shown. In each numerical example, f represents the focal length of the entire lens system, Fno represents the F number, and ω represents the half angle of view. Further, i indicates a surface number counted from the object side, R indicates a radius of curvature, d indicates a distance (surface interval) between lens surfaces along the optical axis, Nd indicates a refractive index with respect to d-line, and νd Indicates the Abbe number for the d line. An aspheric surface is indicated by adding a symbol of * (asterisk) after the surface number i.

Numerical example 1
Basic lens data is shown below.
f = 2.820mm, Fno = 2.850, ω = 31.82 °
Unit mm
Surface data Surface number i R d Nd νd
(Surface) ∞ ∞
1 * 0.994 0.4000 1.52470 56.2 (= νd1)
2 * -250.000 0.3338
3 * -0.800 0.2700 1.58500 29.0 (= νd2)
4 * -1.266 0.2906
5 * 2.445 0.5500 1.58500 29.0 (= νd3)
6 * 1.897 0.1200
7 ∞ 0.5000 1.51633 64.0
8 ∞ 0.7141
(Image plane) ∞

Aspherical data first surface k = -7.896940E-01, A ₄ = 8.737008E-02, A ₆ = 2.491520E-01, A ₈ = 1.067526E-01, A ₁₀ = -1.631444
2nd surface k = -2.000000E-01, A ₄ = -1.340621E-02, A ₆ = 1.021666, A ₈ = -1.187767, A ₁₀ = -3.031475
Third surface k = -3.004759, A ₄ = 9.566372E-01, A ₆ = 4.355547E-01, A ₈ = -3.929834, A ₁₀ = 1.490549
Fourth surface _{k = 1.187264, A 4 = 1.379204} , A 6 = 3.020918E-01, A 8 = -2.724590E-01, A 10 = -9.065297E-01
5th surface k = -3.506622E + 01, A ₄ = 3.768308E-03, A ₆ = -3.526439E-01, A ₈ = 3.760278E-01, A ₁₀ = -1.126834E-01
6th surface k = -2.597424E + 01, A ₄ = -4.827145E-02, A ₆ = -1.269599E-01, A ₈ = 8.016258E-02, A ₁₀ = -2.443238E-02

  As described above, the imaging lens according to Numerical Example 1 has νd1 = 56.2 and νd2 = νd3 = 29.0, which satisfies the conditional expressions (1) to (4).

  FIG. 2 shows the lateral aberration corresponding to the half angle of view ω in the tangential direction and the sagittal direction for the imaging lens of Numerical Example 1 (the same applies in FIG. 5). FIG. 3 shows spherical aberration SA (mm), astigmatism AS (mm), and distortion aberration DIST (%) for the imaging lens of Numerical Example 1. In these aberration diagrams, the spherical aberration diagram shows the amount of aberration for each wavelength of 587.56 nm, 435.84 nm, 656.27 nm, 486.13 nm, and 546.07 nm, as well as the sine condition violation amount OSC. In the aberration diagram, the amount of aberration on the sagittal image surface S and the amount of aberration on the tangential image surface T are shown (same in FIG. 6). As shown in FIGS. 2 and 3, according to the imaging lens according to Numerical Example 1, chromatic aberration is corrected well and various aberrations are corrected well.

Numerical example 2
Basic lens data is shown below.
f = 2.820mm, Fno = 2.850, ω = 31.82 °
Unit mm
Surface data Surface number i R d Nd νd
(Surface) ∞ ∞
1 * 0.999 0.4000 1.52470 56.2 (= νd1)
2 * -250.000 0.3338
3 * -0.800 0.2700 1.58500 29.0 (= νd2)
4 * -1.252 0.2906
5 * 2.466 0.5500 1.58500 29.0 (= νd3)
6 * 1.910 0.1200
7 ∞ 0.5000 1.51633 64.0
8 ∞ 0.7206
(Image plane) ∞

Aspherical data first surface k = -7.957504E-01, A ₄ = 8.589716E-02, A ₆ = 2.760649E-01, A ₈ = 1.459224E-01, A ₁₀ = -1.758079
Second surface k = -2.000000E-01, A ₄ = -1.798138E-03, A ₆ = 1.037928, A ₈ = -1.172457, A ₁₀ = -2.728364
Third surface k = -2.934304, A ₄ = 9.619215E-01, A ₆ = 4.763801E-01, A ₈ = -3.825461, A ₁₀ = 1.648597
Fourth surface _{k = 1.134386, A 4 = 1.359959} , A 6 = 3.303322E-01, A 8 = -2.547396E-01, A 10 = -7.864106E-01
5th surface k = -2.656608E + 01, A ₄ = -3.360250E-02, A ₆ = -3.308488E-01, A ₈ = 3.865180E-01, A ₁₀ = -1.209835E-01
6th surface k = -2.412610E + 01, A ₄ = -4.935333E-02, A ₆ = -1.446683E-01, A ₈ = 9.793812E-02, A ₁₀ = -2.914553E-02

  Thus, the imaging lens according to Numerical Example 2 also satisfies νd1 = 56.2 and νd2 = νd3 = 29.0, which satisfies the conditional expressions (1) to (4).

  FIG. 5 shows lateral aberration corresponding to each half angle of view ω for the imaging lens of Numerical Example 2, and FIG. 6 shows spherical aberration SA (mm), astigmatism AS (mm), and The distortion aberration DIST (%) is shown respectively. As shown in FIG. 5 and FIG. 6, the imaging lens according to Numerical Example 2 also corrects chromatic aberration as well as various aberrations as well as Numerical Example 1.

  The imaging lens according to the present invention is not limited to the above embodiment. In the above embodiment, all the surfaces of the first lens L1 to the third lens L3 are aspherical surfaces, but it is not always necessary to make all the surfaces aspherical. For example, any one surface or both surfaces of the second lens L2 may be configured as a spherical surface.

  Moreover, in the said embodiment, in order to suppress the light incident angle to an image pick-up element, it was set as the structure which provided the inflexion point in the 3rd lens L3. However, when there is a margin for the light incident angle on the image sensor and it is not necessary to provide an inflection point on the third lens L3, the lens surface of the third lens is formed in an aspherical shape having no inflection point. Alternatively, either one or both surfaces of the third lens L3 may be configured as a spherical surface.

1 is a lens cross-sectional view illustrating a schematic configuration of an imaging lens according to Numerical Example 1 according to an embodiment of the present invention. FIG. 6 is an aberration diagram showing lateral aberration of the imaging lens according to Example 1 with the same numerical values. FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the imaging lens according to Example 1 of the same numerical value. FIG. 5 is a lens cross-sectional view illustrating a schematic configuration of an imaging lens according to Numerical Example 2 according to the embodiment of the present invention. FIG. 6 is an aberration diagram showing lateral aberration of the imaging lens according to Numerical Example 2; FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the imaging lens according to Example 2 of the same numerical value.

Explanation of symbols

L1 1st lens L2 2nd lens L3 3rd lens 10 Cover glass

Claims (2)

In order from the object side to the image surface side, a biconvex first lens having positive refractive power, a second lens of a meniscus lens having negative refractive power and having a concave surface facing the object side, and negative An imaging lens including a third lens of a meniscus lens having a refractive power of
When the Abbe number in the d-line of the first lens is νd1, the Abbe number in the d-line of the second lens is νd2, and the Abbe number in the d-line of the third lens is νd3,
νd1> 50 (1)
20 <νd2 <35 (2)
20 <νd3 <35 (3)
An imaging lens characterized by satisfying Furthermore, the following conditional expression (4),
νd2 = νd3 (4)
The imaging lens according to claim 1, wherein: