JP2011242414A - Imaging lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging lens capable of favorably correcting aberration although the lens is small in size.SOLUTION: The imaging lens is constituted by sequentially arranging, from an object side to an image plane side, a positive first lens L1 having a meniscus shape with a convex face near the optical axis opposing to the object side, a negative second lens L2 having both-side concave faces near the optical axis, and a positive third lens L3 having a meniscus shape with a convex face near the optical axis opposing to the object side. In this configuration, a focal length f of the whole lens system, and focal lengths f1, f2 and f3 of the first to third lenses L1, L2 and L3, respectively, satisfy conditional expressions of f1<|f2|, f1<f3 and 0.5<f1/f<1.0.

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.

  The imaging lens mounted on the above-mentioned small camera is required to have a lens structure with high resolution that can be used not only for downsizing but also for recent imaging elements with high pixels. Conventionally, various lens configurations have been proposed. Among them, an imaging lens composed of three lenses has various aberrations corrected relatively well and is suitable for miniaturization. It is used in cameras.

  For example, an imaging lens described in Patent Document 1 is known as a three-lens imaging lens. The imaging lens includes, in order from the object side, a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power. When the focal length of the third lens is shortened relative to the entire focal length, that is, the refractive power of the third lens is relatively strong and the refractive power of the second lens is stronger than that of the first lens, We are trying to correct coma and the like.

JP 2008-76594 A

  In recent years, the downsizing and the increase in the number of pixels of a camera such as a mobile phone are rapidly progressing, and the performance required for the imaging lens is also stricter than before. According to the imaging lens described in Patent Document 1, although aberration can be corrected satisfactorily, the combined focal length of the lens system is relatively long, so that the optical axis from the object-side surface of the first lens to the image plane. It is difficult to shorten the distance above.

  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 first lens having a positive refractive power, a second lens having a negative refractive power, and a positive refractive power The first lens is formed into a shape in which the curvature radius of the object side surface and the curvature radius of the image side surface are both positive, and the second lens is formed on the object side surface. A shape in which the radius of curvature is negative and the curvature radius of the image-side surface is positive, and the third lens is shaped so that both the curvature radius of the object-side surface and the curvature radius of the image-side surface are positive. Where the focal length of the entire lens system is f, the focal length of the first lens is f1, the focal length of the second lens is f2, and the focal length of the third lens is f3. It was configured to satisfy (2a) and (3).
f1 <| f2 | (1)
f1 <f3 (2a)
0.5 <f1 / f <1.0 (3)

In the present invention, in order from the object side to the image plane side, a first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a negative refractive power are provided. The first lens is formed into a shape in which both the radius of curvature of the object side surface and the curvature radius of the image side surface are positive, and the second lens has a negative radius of curvature of the object side surface; A lens system is formed in a shape in which the curvature radius of the image side surface is positive, and the third lens is formed in a shape in which both the curvature radius of the object side surface and the curvature radius of the image side surface are positive. When the entire focal length is f, the focal length of the first lens is f1, the focal length of the second lens is f2, and the focal length of the third lens is f3, the following conditional expressions (1), (2b), (3 ).
f1 <| f2 | (1)
f1 <| f3 | (2b)
0.5 <f1 / f <1.0 (3)

  Conditional expressions (1), (2a), and (2b) are conditions for shortening the length (thickness) along the optical axis of the imaging lens and reducing the size of the imaging lens. As shown in these conditional expressions, by making the refractive power of the first lens stronger than that of each of the second lens and the third lens, the refractive power of the entire lens system is mainly concentrated on the first lens. It will be. In the present invention, the first lens is formed into a shape in which the curvature radius of the object side surface and the curvature radius of the image side surface are both positive, that is, a meniscus lens having a convex surface facing the object side in the vicinity of the optical axis. Is done. For this reason, the position of the principal point of the first lens having a strong refractive power moves to the object side, and as a result, the position of the principal point of the entire lens system moves to the object side. Preferably.

  In the present invention, since the radius of curvature of the object side surface of the third lens is positive, that is, a shape in which the convex surface faces the object side, the second lens depends on the surface shape of the second lens on the image surface side. The distance on the optical axis between the lens and the third lens may be long. Therefore, in the present invention, the shape of the second lens is a shape in which the radius of curvature of the object side surface is negative and the radius of curvature of the image side surface is positive, that is, a shape that forms a biconcave lens near the optical axis. Thus, the increase in the distance on the optical axis between the second lens and the third lens is suppressed, and the imaging lens can be more efficiently reduced in size.

  Conditional expression (3) is a condition for suppressing the field curvature within a favorable range while reducing the size of the imaging lens. When the upper limit “1.0” is exceeded, the refractive power of the first lens becomes relatively weak compared to the refractive power of the entire lens system, and therefore it is difficult to reduce the thickness of the imaging lens. In addition, since the refractive power of the second lens and the third lens is relatively stronger than that of the first lens, it is difficult to suppress the curvature of field within a favorable range. On the other hand, if the value is below the lower limit “0.5”, the refractive power of the first lens becomes relatively stronger than the refractive power of the entire lens system, which is advantageous for downsizing the imaging lens. The focus is shortened. Usually, an insert such as an infrared cut filter or a cover glass is often inserted between a lens system such as the imaging lens according to the present invention and the image plane of the imaging device. When the back focus is shortened, it is difficult to secure a space for inserting such an insert. Further, as the refractive power of the first lens becomes relatively strong, the image plane falls to the object side, and it becomes difficult to ensure good imaging performance.

  By satisfying the above conditional expressions (1) to (3), it is possible to achieve both the downsizing of the imaging lens and good aberration correction.

In the imaging lens having the above configuration, when the combined focal length of the second lens and the third lens is f23, and the refractive power of the third lens is positive, the following conditional expression (4a) It is desirable that the following conditional expression (4b) is satisfied when the refractive power of is negative.
−1.5 <f23 / f3 <−0.8 (4a)
0.5 <f23 / f3 <1.2 (4b)

  The above conditional expressions (4a) and (4b) suppress the on-axis and off-axis chromatic aberration within a good range while suppressing the incident angle of the light beam emitted from the imaging lens to the image sensor. It is a condition to do. As is well known, a so-called maximum incident angle is provided as a limit on the incident angle of the light beam that can be taken into the image sensor due to the structure of the image sensor. When light rays outside the range of the maximum incident angle are incident on the image sensor, a dark image in the peripheral portion is generated due to the shading phenomenon. Therefore, it is necessary to suppress the incident angle of the light beam emitted from the imaging lens to the imaging element within a certain range.

  When the upper limit value of the conditional expression (4a) is exceeded, or when the lower limit value of the conditional expression (4b) is exceeded, the refractive power of the third lens becomes relatively weak, and the on-axis and off-axis chromatic aberration is reduced. Although it is easy to suppress within a good range, it is difficult to suppress the incident angle of the light beam emitted from the imaging lens to the imaging element within a certain range. On the other hand, when the lower limit value of the conditional expression (4a) is not reached or when the upper limit value of the conditional expression (4b) is exceeded, the refractive power of the third lens becomes relatively strong and is emitted from the imaging lens. Although it is easy to suppress the incident angle of light rays to the image sensor within a certain range, the on-axis and off-axis chromatic aberration is undercorrected (short wavelength increases in the minus direction with respect to the reference wavelength), and good imaging performance It becomes difficult to get.

In the imaging lens having the above configuration, it is preferable that the following conditional expression (5) is further satisfied.
−1.0 <f1 / f2 <−0.5 (5)

  Conditional expression (5) is a condition for suppressing on-axis chromatic aberration, off-axis magnification chromatic aberration, and field curvature within a favorable range while reducing the thickness of the imaging lens. Exceeding the upper limit “−0.5” is effective for shortening the thickness of the imaging lens, but the axial chromatic aberration is undercorrected (short wavelength increases in the negative direction with respect to the reference wavelength). Off-axis lateral chromatic aberration is undercorrected. Further, the image plane falls to the object side. This makes it difficult to obtain good imaging performance. On the other hand, when the value is below the lower limit “−1.0”, the off-axis lateral chromatic aberration is overcorrected (the short wavelength increases in the positive direction with respect to the reference wavelength), and the image plane falls to the image plane side. However, it is difficult to obtain good imaging performance.

In the imaging lens having the above configuration, it is desirable that the following conditional expression (6) is satisfied, where Rf is the curvature radius of the object side surface of the second lens and Rr is the curvature radius of the image side surface.
−0.30 <Rf / Rr <0 (6)

  Conditional expression (6) is a condition for suppressing the aberration within a favorable range while reducing the thickness of the imaging lens. If the upper limit value “0” is exceeded, the position of the principal point of the lens system moves to the image plane side, making it difficult to reduce the size of the imaging lens. On the other hand, if the value falls below the lower limit “−0.30”, the position of the principal point of the lens system moves to the object side, which is advantageous for downsizing the imaging lens, but the image plane is overcorrected (in the positive direction). Increase). Further, since outward coma also increases, it becomes difficult to obtain good imaging performance with corrected aberration.

In the imaging lens having the above configuration, when the distance between the first lens and the second lens is dA and the distance between the second lens and the third lens is dB, the following conditional expression (7) is satisfied. It is desirable.
0.25 <dA / dB <0.7 (7)

  Conditional expression (7) is a condition for suppressing spherical aberration and coma aberration within a favorable range. By disposing the second lens within the range defined by conditional expression (7), spherical aberration and coma aberration can be suppressed within a favorable range. If the upper limit “0.7” is exceeded, spherical aberration will be overcorrected, and axial chromatic aberration will be undercorrected. In addition, inward coma due to off-axis rays increases, making it difficult to suppress each aberration within a favorable range. On the other hand, below the lower limit “0.25”, the spherical aberration is insufficiently corrected, and the outward coma aberration due to off-axis rays increases. Therefore, also in this case, it is difficult to suppress each aberration within a favorable range.

Furthermore, in the imaging lens having the above-described configuration, it is desirable that the following conditional expression (7A) is satisfied in order to suppress spherical aberration and coma aberration in a better range.
0.3 <dA / dB <0.65 (7A)

  According to the imaging lens of the present invention, it is possible to provide both a compact imaging lens in which various types of aberrations are favorably corrected while achieving both a reduction in size of the imaging lens and good aberration correction.

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. 3 is an aberration diagram illustrating lateral aberration of the imaging lens illustrated in FIG. 1. FIG. 2 is an aberration diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens illustrated in FIG. 1. FIG. 6 is a lens cross-sectional view illustrating a schematic configuration of an imaging lens according to Numerical Example 2 according to an embodiment of the present invention. FIG. 5 is an aberration diagram showing lateral aberration of the imaging lens shown in FIG. 4. FIG. 5 is an aberration diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens illustrated in FIG. 4. FIG. 5 is a lens cross-sectional view illustrating a schematic configuration of an imaging lens according to Numerical Example 3 according to an embodiment of the present invention. FIG. 8 is an aberration diagram showing lateral aberration of the imaging lens shown in FIG. 7. FIG. 8 is an aberration diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens illustrated in FIG. 7. FIG. 10 is a lens cross-sectional view illustrating a schematic configuration of an imaging lens according to Numerical Example 4 according to an embodiment of the present invention. FIG. 11 is an aberration diagram illustrating lateral aberration of the imaging lens illustrated in FIG. 10. FIG. 11 is an aberration diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens illustrated in FIG. 10. FIG. 10 is a lens cross-sectional view illustrating a schematic configuration of an imaging lens according to Numerical Example 5 according to an embodiment of the present invention. FIG. 14 is an aberration diagram illustrating lateral aberration of the imaging lens illustrated in FIG. 13. FIG. 14 is an aberration diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens illustrated in FIG. 13.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings.

  1, 4, and 7 are lens cross-sectional views corresponding to Numerical Examples 1 to 3 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 has an aperture stop ST, a first lens L1 having a positive refractive power, and a negative refractive power in order from the object side to the image plane side. The second lens L2 and the third lens L3 having positive refractive power are arranged. A cover glass 10 is disposed between the third lens L3 and the image plane IM of the image sensor. The cover glass 10 can be omitted.

  The first lens L1 is formed into a shape in which both the radius of curvature of the object side surface and the curvature radius of the image side surface are positive, that is, a meniscus lens having a convex surface facing the object side in the vicinity of the optical axis X. Has been. The second lens L2 is formed in a shape in which the radius of curvature of the object side surface is negative and the radius of curvature of the image side surface is positive, that is, a shape that becomes a biconcave lens in the vicinity of the optical axis X. The third lens L3 is formed in a shape in which the curvature radius of the object side surface and the curvature radius of the image side surface are both positive, that is, a shape that becomes a meniscus lens having a convex surface facing the object side in the vicinity of the optical axis X. Has been. In the present embodiment, the third lens L3 has both an object-side surface and an image-side surface that are convex toward the object side in the vicinity of the optical axis X and concave toward the object side at the periphery. It is formed in an aspherical shape.

In the present embodiment, all the lens surfaces of the first lens L1 to the third lens L3 are aspherical. The aspherical shape employed for these lens surfaces is that the axis in the optical axis X direction is Z, the height in the direction orthogonal to the optical axis X is H, the conic coefficient is k, and the aspherical coefficients are A ₄ , A ₆ , A _8. , A ₁₀ , A ₁₂ , A ₁₄ , and A ₁₆ are expressed by the following formulas (the same applies to the second embodiment described later).

In the imaging lens according to the present embodiment, the focal length of the entire lens system is f, the focal length of the first lens L1 is f1, the focal length of the second lens L2 is f2, the focal length of the third lens L3 is f3, The combined focal length of the second lens L2 and the third lens L3 is f23, the radius of curvature of the object side surface of the second lens L2 is Rf, the radius of curvature of the image side surface of the second lens L2 is Rr, and the first lens L1 When the distance between the second lens L2 and the second lens L2 is dA, and the distance between the second lens L2 and the third lens L3 is dB, the following conditional expressions are satisfied.
f1 <| f2 | (1)
f1 <f3 (2a)
0.5 <f1 / f <1.0 (3)
−1.5 <f23 / f3 <−0.8 (4a)
−1.0 <f1 / f2 <−0.5 (5)
−0.30 <Rf / Rr <0 (6)
0.25 <dA / dB <0.7 (7)
0.3 <dA / dB <0.65 (7A)

  Note that it is not necessary to satisfy all of the above conditional expressions. By satisfying each of the conditional expressions independently, it is possible to obtain the effects corresponding to each conditional expression, and aberrations are better than those of conventional imaging lenses. Thus, it is possible to construct a small imaging lens that has been corrected.

  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. In addition, 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 X, Nd indicates a refractive index with respect to the d line, νd indicates the Abbe number with respect to the d line. An aspheric surface is indicated by adding a symbol of * (asterisk) after the surface number i (the same applies to the second embodiment described later).

Numerical example 1
Basic lens data is shown below.
f = 2.778mm, Fno = 2.781, ω = 32.21 °
Unit mm
Surface data Surface number i R d Nd νd
(Surface) ∞ ∞
1 (Aperture) ∞ 0.0101
2 * 0.991 0.4377 1.52470 56.2
3 * 11.252 0.2000 (= dA)
4 * -1.888 (= Rf) 0.2995 1.61420 26.0
5 * 12.583 (= Rr) 0.4300 (= dB)
6 * 1.256 0.6616 1.52470 56.2
7 * 2.003 0.3500
8 ∞ 0.3000 1.51633 64.2
9 ∞ 0.5936
(Image plane IM) ∞

Aspheric data 2nd surface k = 0.000000, A ₄ = 7.328630E-02, A ₆ = -2.929952E-01, A ₈ = 9.756204E-01
Third surface k = 0.000000, A ₄ = -1.402649E-01, A ₆ = 8.559119E-01, A ₈ = -1.002302
4th surface k = 0.000000, A ₄ = -6.915185E-01, A ₆ = 5.621113, A ₈ = -1.574395E + 01, A ₁₀ = 1.659025E + 01
, A ₁₂ = -1.707965
5th surface k = 0.000000, A ₄ = -6.135715E-01, A ₆ = 4.249213, A ₈ = -9.380550, A ₁₀ = 1.078944E + 01,
A ₁₂ = -3.227468
6th surface k = -9.002869, A ₄ = -7.304172E-02, A ₆ = -3.563316E-01, A ₈ = 1.145037, A ₁₀ = -1.886568,
A ₁₂ = 1.748350, A ₁₄ = -8.718579E-01, A ₁₆ = 1.812686E-01
7th surface k = -1.563085, A ₄ = -2.460672E-01, A ₆ = 9.489244E-02, A ₈ = -1.609943E-02,
A ₁₀ = -3.619836E-02, A ₁₂ = 2.638370E-02, A ₁₄ = -5.759371E-03, A ₁₆ = -3.783857E-04

The focal lengths f1 to f3 of the lenses L1 to L3 and the combined focal length f23 of the second lens L2 and the third lens L3 are shown below.
f1 = 2.041
f2 = −2.652
f3 = 4.919
f23 = −5.834

The values of conditional expressions (3) to (7) are shown below.
(3) f1 / f = 0.735
(4a) f23 / f3 = −1.186
(5) f1 / f2 = −0.770
(6) Rf / Rr = −0.150
(7), (7A) dA / dB = 0.465
As described above, the imaging lens of Numerical Example 1 satisfies the conditional expressions.

  FIG. 2 shows the lateral aberration corresponding to the half angle of view ω divided into the tangential direction and the sagittal direction for the imaging lens of Numerical Example 1 (in FIGS. 5, 8, 11, and 14). the same). 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. The aberration diagrams show the aberration amount on the sagittal image surface S and the aberration amount on the tangential image surface T (the same applies to FIGS. 6, 9, 12, and 15).

  As shown in FIGS. 2 and 3, according to the imaging lens according to Numerical Example 1, various aberrations are favorably corrected. In addition, the air-converted distance from the object side surface of the first lens L1 to the image surface is as short as 3.170 mm, and the image pickup lens is suitably reduced in size.

Numerical example 2
Basic lens data is shown below.
f = 2.845mm, Fno = 2.904, ω = 31.60 °
Unit mm
Surface data Surface number i R d Nd νd
(Surface) ∞ ∞
1 (Aperture) ∞ 0.0000
2 * 0.900 0.4300 1.52470 56.2
3 * 4.740 0.1850 (= dA)
4 * -1.780 (= Rf) 0.3200 1.61420 26.0
5 * 15.960 (= Rr) 0.4300 (= dB)
6 * 1.300 0.6700 1.52470 56.2
7 * 2.130 0.3500
8 ∞ 0.3000 1.51633 64.2
9 ∞ 0.6208
(Image plane IM) ∞

Aspherical data second surface k = 1.013885, A ₄ = -1.331635E-01, A ₆ = 7.683254E-02, A ₈ = -1.386429
3rd surface k = 0.000000, A ₄ = -9.391646E-02, A ₆ = 1.435346, A ₈ = -3.848211
4th surface k = 1.896196, A ₄ = -5.575749E-01, A ₆ = 3.882751, A ₈ = -1.577240E + 01, A ₁₀ = 4.975935E + 01
, A ₁₂ = -1.268720E + 02
5th surface k = 0.000000, A ₄ = -5.792134E-01, A ₆ = 3.832462, A ₈ = -1.006235E + 01, A ₁₀ = 1.788974E + 01
, A ₁₂ = -1.464777E + 01
6th surface k = -2.812535E-01, A ₄ = -4.687720E-01, A ₆ = 2.145791E-01, A ₈ = 5.335429E-03,
A ₁₀ = -5.103890E-02, A ₁₂ = -1.924959E-02, A ₁₄ = 1.989603E-02, A ₁₆ = -2.036795E-03
7th surface k = 0.000000, A ₄ = -2.036252E-01, A ₆ = 3.404486E-03, A ₈ = 1.593126E-02,
A ₁₀ = 1.467323E-03, A ₁₂ = -4.172784E-03

The focal lengths f1 to f3 of the lenses L1 to L3 and the combined focal length f23 of the second lens L2 and the third lens L3 are shown below.
f1 = 2.039
f2 = −2.590
f3 = 4.976
f23 = −5.559

The values of conditional expressions (3) to (7) are shown below.
(3) f1 / f = 0.717
(4a) f23 / f3 = −1.117
(5) f1 / f2 = −0.787
(6) Rf / Rr = −0.112
(7), (7A) dA / dB = 0.430
Thus, the imaging lens of Numerical Example 2 satisfies the conditional expressions.

  FIG. 5 shows lateral aberration corresponding to the half angle of view ω for the imaging lens of Numerical Example 2. FIG. 6 shows spherical aberration SA (mm), astigmatism AS (mm), and distortion. Each aberration DIST (%) is shown. As shown in FIG. 5 and FIG. 6, various aberrations are satisfactorily corrected by the imaging lens according to Numerical Example 2 as well as Numerical Example 1. In this numerical example, the air-converted distance from the object-side surface of the first lens L1 to the image plane is 3.204 mm, and the image pickup lens is suitably reduced in size.

Numerical Example 3
Basic lens data is shown below.
f = 2.784mm, Fno = 2.786, ω = 32.15 °
Unit mm
Surface data Surface number i R d Nd νd
(Surface) ∞ ∞
1 (Aperture) ∞ 0.0000
2 * 1.000 0.4250 1.52470 56.2
3 * 10.500 0.2250 (= dA)
4 * -1.950 (= Rf) 0.3000 1.61420 26.0
5 * 13.000 (= Rr) 0.4200 (= dB)
6 * 1.267 0.6700 1.52470 56.2
7 * 1.975 0.3500
8 ∞ 0.3000 1.51633 64.2
9 ∞ 0.5778
(Image plane IM) ∞

Aspheric data second surface k = 0.000000, A ₄ = -7.576999E-02, A ₆ = 8.172900E-01, A ₈ = -1.400075
3rd surface k = 0.000000, A ₄ = -1.328168E-01, A ₆ = 1.622385, A ₈ = -3.233796
4th surface k = 0.000000, A ₄ = -5.711823E-01, A ₆ = 5.742309, A ₈ = -1.719770E + 01, A ₁₀ = 1.750999E + 01
, A ₁₂ = 4.502805E-01
5th surface k = 0.000000, A ₄ = -6.015480E-01, A ₆ = 4.288483, A ₈ = -9.464158, A ₁₀ = 9.509792,
A ₁₂ = -1.288798
6th surface k = -8.025975, A ₄ = -7.341897E-02, A ₆ = -3.577822E-01, A ₈ = 1.143415, A ₁₀ = -1.888013,
A ₁₂ = 1.747451, A ₁₄ = -8.718979E-01, A ₁₆ = 1.823664E-01
7th surface k = -1.243082, A ₄ = -2.450392E-01, A ₆ = 9.299420E-02, A ₈ = -1.694575E-02,
A ₁₀ = -3.649049E-02, A ₁₂ = 2.631459E-02, A ₁₄ = -5.742159E-03, A ₁₆ = -3.280138E-04

The focal lengths f1 to f3 of the lenses L1 to L3 and the combined focal length f23 of the second lens L2 and the third lens L3 are shown below.
f1 = 2.075
f2 = −2.740
f3 = 5.081
f23 = −5.914

The values of conditional expressions (3) to (7) are shown below.
(3) f1 / f = 0.745
(4a) f23 / f3 = −1.164
(5) f1 / f2 = −0.757
(6) Rf / Rr = −0.150
(7), (7A) dA / dB = 0.536
Thus, the imaging lens of Numerical Example 3 satisfies the conditional expressions.

  FIG. 8 shows lateral aberration corresponding to the half angle of view ω for the imaging lens of Numerical Example 3, and FIG. 9 shows spherical aberration SA (mm), astigmatism AS (mm), and distortion. Each aberration DIST (%) is shown. As shown in FIG. 8 and FIG. 9, various aberrations are satisfactorily corrected by the imaging lens according to Numerical Example 3 as well as Numerical Example 1. In this numerical example, the air-converted distance from the object-side surface of the first lens L1 to the image plane is 3.166 mm, and the image pickup lens is also suitably reduced in size.

(Second Embodiment)
Next, a second embodiment of the present invention will be described in detail with reference to the drawings. In the imaging lens according to the present embodiment, unlike the imaging lens according to the first embodiment, the refractive power of the third lens L3 is negative.

  10 and 13 show lens cross-sectional views corresponding to Numerical Examples 4 and 5 of the present embodiment, respectively. Since all the numerical examples have the same basic lens configuration, the lens configuration of the imaging lens according to the present embodiment will be described here with reference to a lens cross-sectional view of Numerical Example 4.

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

  The first lens L1 is formed into a shape in which both the radius of curvature of the object side surface and the curvature radius of the image side surface are positive, that is, a meniscus lens having a convex surface facing the object side in the vicinity of the optical axis X. Has been. The second lens L2 is formed in a shape in which the radius of curvature of the object side surface is negative and the radius of curvature of the image side surface is positive, that is, a shape that becomes a biconcave lens in the vicinity of the optical axis X. The third lens L3 is formed in a shape in which the curvature radius of the object side surface and the curvature radius of the image side surface are both positive, that is, a shape that becomes a meniscus lens having a convex surface facing the object side in the vicinity of the optical axis X. Has been. In the third lens L3, as in the first embodiment, both the object-side surface and the image-side surface are convex on the object side in the vicinity of the optical axis X, and on the object side in the peripheral portion. It is formed in an aspherical shape that becomes a concave shape.

In the imaging lens according to the present embodiment, the focal length of the entire lens system is f, the focal length of the first lens L1 is f1, the focal length of the second lens L2 is f2, the focal length of the third lens L3 is f3, The combined focal length of the second lens L2 and the third lens L3 is f23, the radius of curvature of the object side surface of the second lens L2 is Rf, the radius of curvature of the image side surface of the second lens L2 is Rr, and the first lens L1 When the distance between the second lens L2 and the second lens L2 is dA, and the distance between the second lens L2 and the third lens L3 is dB, the following conditional expressions are satisfied.
f1 <| f2 | (1)
f1 <f3 (2b)
0.5 <f1 / f <1.0 (3)
0.5 <f23 / f3 <1.2 (4b)
−1.0 <f1 / f2 <−0.5 (5)
−0.30 <Rf / Rr <0 (6)
0.25 <dA / dB <0.7 (7)
0.3 <dA / dB <0.65 (7A)

  Note that it is not necessary to satisfy all of the above conditional expressions. By satisfying each of the conditional expressions independently, it is possible to obtain the effects corresponding to each conditional expression, and aberrations are better than those of conventional imaging lenses. Thus, it is possible to construct a small imaging lens that has been corrected.

Numerical examples of the imaging lens according to the present embodiment will be described.

Numerical Example 4
Basic lens data is shown below.
f = 3.621mm, Fno = 3.625, ω = 25.79 °
Unit mm
Surface data Surface number i R d Nd νd
(Surface) ∞ ∞
1 (Aperture) ∞ 0.0000
2 * 0.817 0.5000 1.52470 56.2
3 * 1.651 0.3694 (= dA)
4 * -3.006 (= Rf) 0.3000 1.61420 26.0
5 * 15.697 (= Rr) 0.7083 (= dB)
6 * 1.620 0.6200 1.61420 26.0
7 * 1.380 0.3500
8 ∞ 0.3000 1.51633 64.2
9 ∞ 0.4400
(Image plane IM) ∞

Aspheric data second surface k = 9.156504E-01, A ₄ = -2.001875E-01, A ₆ = 1.385200E-01, A ₈ = -4.868637E-01,
A ₁₀ = -7.798491
Third surface k = -3.106284, A ₄ = 4.166545E-01, A ₆ = -6.228740E-02, A ₈ = 6.554742,
A ₁₀ = -1.423451E + 01, A ₁₂ = 5.774142E + 01
4th surface k = 2.224686E + 01, A ₄ = -6.057120E-01, A ₆ = 2.680953, A ₈ = -1.045722E + 01,
A ₁₀ = 2.191227E + 01, A ₁₂ = -1.356551E + 01
Fifth surface k = 0.000000, A ₄ = -7.601373E-01, A ₆ = 3.334531, A ₈ = -9.723123, A ₁₀ = 1.683559E + 01,
A ₁₂ = -1.501090E + 01, A ₁₄ = 5.837649
6th surface k = -1.035029E + 01, A ₄ = -1.045628E-01, A ₆ = -2.940384E-01, A ₈ = 2.507408E-01,
A ₁₀ = 1.091151E-01, A ₁₂ = -1.173489E-01, A ₁₄ = -1.263306E-01, A ₁₆ = 9.209900E-02
7th surface k = -1.189318E-01, A ₄ = -4.099832E-01, A ₆ = 7.581204E-02, A ₈ = -4.964783E-03,
A ₁₀ = 5.659021E-03, A ₁₂ = -1.126572E-02, A ₁₄ = 3.011777E-03, A ₁₆ = -4.222558E-04

The focal lengths f1 to f3 of the lenses L1 to L3 and the combined focal length f23 of the second lens L2 and the third lens L3 are shown below.
f1 = 2.555
f2 = −4.083
f3 = −889.682
f23 = −3.470

The values of conditional expressions (3) to (7) are shown below.
(3) f1 / f = 0.706
(4b) f23 / f3 = 0.850
(5) f1 / f2 = −0.626
(6) Rf / Rr = −0.192
(7), (7A) dA / dB = 0.522
Thus, the image pickup lens of Numerical Example 4 satisfies the conditional expressions.

  FIG. 11 shows lateral aberration corresponding to the half angle of view ω for the imaging lens of Numerical Example 4, and FIG. 12 shows spherical aberration SA (mm), astigmatism AS (mm), and distortion. Each aberration DIST (%) is shown. As shown in FIGS. 11 and 12, according to the imaging lens according to Numerical Example 4, various aberrations are favorably corrected. In this numerical example, the air conversion distance from the object-side surface of the first lens L1 to the image plane is 3.486 mm, and the image pickup lens is suitably reduced in size.

Numerical Example 5
Basic lens data is shown below.
f = 3.628mm, Fno = 3.630, ω = 25.79 °
Unit mm
Surface data Surface number i R d Nd νd
(Surface) ∞ ∞
1 (Aperture) ∞ 0.0000
2 * 0.822 0.5000 1.52470 56.2
3 * 1.689 0.3710 (= dA)
4 * -3.009 (= Rf) 0.3000 1.61420 26.0
5 * 15.420 (= Rr) 0.7326 (= dB)
6 * 1.620 0.6000 1.61420 26.0
7 * 1.380 0.3500
8 ∞ 0.3000 1.51633 64.2
9 ∞ 0.4390
(Image plane IM) ∞

Aspherical data second surface k = 9.364151E-01, A ₄ = -1.986518E-01, A ₆ = 1.380496E-01, A ₈ = -4.787480E-01,
A ₁₀ = -7.872170
3rd surface k = -3.087456, A ₄ = 4.160485E-01, A ₆ = -6.131399E-02, A ₈ = 6.229271,
A ₁₀ = -1.425891E + 01, A ₁₂ = 5.609518E + 01
4th surface k = 2.222221E + 01, A ₄ = -6.004975E-01, A ₆ = 2.685851, A ₈ = -1.048226E + 01,
A ₁₀ = 2.188325E + 01, A ₁₂ = -1.370214E + 01
5th surface k = 0.000000, A ₄ = -7.592701E-01, A ₆ = 3.334627, A ₈ = -9.723934, A ₁₀ = 1.684345E + 01,
A ₁₂ = -1.501855E + 01, A ₁₄ = 5.845048
6th surface k = -1.035886E + 01, A ₄ = -1.046718E-01, A ₆ = -3.007844E-01, A ₈ = 2.495829E-01,
A ₁₀ = 1.103298E-01, A ₁₂ = -1.185574E-01, A ₁₄ = -1.270811E-01, A ₁₆ = 9.251858E-02
7th surface k = -1.182990E-01, A ₄ = -4.108258E-01, A ₆ = 7.541289E-02, A ₈ = -4.932627E-03,
A ₁₀ = 5.622894E-03, A ₁₂ = -1.129859E-02, A ₁₄ = 3.010675E-03, A ₁₆ = -4.223644E-04

The focal lengths f1 to f3 of the lenses L1 to L3 and the combined focal length f23 of the second lens L2 and the third lens L3 are shown below.
f1 = 2.547
f2 = −4.074
f3 = −311.068
f23 = −3.4448

The values of conditional expressions (3) to (7) are shown below.
(3) f1 / f = 0.702
(4b) f23 / f3 = 0.848
(5) f1 / f2 = −0.625
(6) Rf / Rr = −0.195
(7), (7A) dA / dB = 0.506
Thus, the imaging lens of the present numerical value example 5 satisfies each conditional expression.

  FIG. 14 shows lateral aberration corresponding to the half angle of view ω for the imaging lens of Numerical Example 5, and FIG. 15 shows spherical aberration SA (mm), astigmatism AS (mm), and distortion. Each aberration DIST (%) is shown. As shown in FIGS. 14 and 15, according to the imaging lens according to Numerical Example 5, various aberrations are favorably corrected. In this numerical example, the air-converted distance from the object-side surface of the first lens L1 to the image plane is 3.490 mm, and the image pickup lens is suitably reduced in size.

  Therefore, when the imaging lens according to the present embodiment is applied to an imaging optical system 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 camera and the like have high functionality and small size. Can be achieved simultaneously.

  The imaging lens according to the present invention is not limited to the above embodiments. In each of the above embodiments, 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.

  The present invention can be applied to an imaging lens mounted on a device that is required to have a small aberration as well as a good aberration correction capability, such as a mobile phone or a digital still camera.

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

Claims (7)

A first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a positive refractive power are arranged in order from the object side to the image plane side. An imaging lens,
The first lens is formed in a shape in which both the radius of curvature of the object side surface and the radius of curvature of the image side surface are positive,
The second lens is formed in a shape in which the radius of curvature of the object side surface is negative and the radius of curvature of the image side surface is positive.
The third lens is formed in a shape in which both the radius of curvature of the object side surface and the radius of curvature of the image side surface are positive,
When the focal length of the entire lens system is f, the focal length of the first lens is f1, the focal length of the second lens is f2, and the focal length of the third lens is f3, the following conditional expression is satisfied. A characteristic imaging lens.
f1 <| f2 |
f1 <f3
0.5 <f1 / f <1.0 The imaging lens according to claim 1, wherein the following conditional expression is satisfied when a combined focal length of the second lens and the third lens is f23.
−1.5 <f23 / f3 <−0.8 A first lens having a positive refractive power, a second lens having a negative refractive power, and a third lens having a negative refractive power are arranged in order from the object side to the image plane side. An imaging lens,
The first lens is formed in a shape in which both the radius of curvature of the object side surface and the radius of curvature of the image side surface are positive,
The second lens is formed in a shape in which the radius of curvature of the object side surface is negative and the radius of curvature of the image side surface is positive.
The third lens is formed in a shape in which both the radius of curvature of the object side surface and the radius of curvature of the image side surface are positive,
When the focal length of the entire lens system is f, the focal length of the first lens is f1, the focal length of the second lens is f2, and the focal length of the third lens is f3, the following conditional expression is satisfied. A characteristic imaging lens.
f1 <| f2 |
f1 <| f3 |
0.5 <f1 / f <1.0 The imaging lens according to claim 3, wherein the following conditional expression is satisfied when a combined focal length of the second lens and the third lens is f23.
0.5 <f23 / f3 <1.2 The imaging lens according to claim 1, wherein the following conditional expression is satisfied.
−1.0 <f1 / f2 <−0.5 6. The following conditional expression is satisfied, where Rf is a radius of curvature of the object side surface of the second lens and Rr is a radius of curvature of the image side surface. The imaging lens described in 1.
−0.30 <Rf / Rr <0 The following conditional expression is satisfied, where a distance between the first lens and the second lens is dA, and a distance between the second lens and the third lens is dB. The imaging lens according to any one of 1 to 6.
0.25 <dA / dB <0.7

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