JP2008225332A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens which constitutes a lens unit used for a cellular phone or a personal computer (PC) or the like, and whose axial chromatic aberration is suppressed. <P>SOLUTION: The imaging lens includes a first lens L<SB>1</SB>having positive refractive power, which is a meniscus turning its convex surface to an object side, a stop S, a second lens L<SB>2</SB>having negative refractive power, which is a meniscus turning its convex surface to an image side, and a third lens L<SB>3</SB>having positive refractive power, whose image-side surface is plane or convex to the image side from the object side to the image side, and satisfies conditional expressions (1): 0.5≤f<SB>1</SB>/f≤0.9, (2):-5≤f<SB>2</SB>/f≤-0.8 and (3) 2≤f<SB>3</SB>/f≤1000. Provided that f: focal length of an entire optical system, f<SB>1</SB>: focal length of the first lens, f<SB>2</SB>: focal length of the second lens, and f<SB>3</SB>: focal length of the third lens. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an imaging lens constituting a lens unit used in a mobile phone, a personal computer (PC), and the like.

  In recent years, imaging devices mounted on mobile phones, personal computers, and the like are required to have high resolution, low cost, and small size. In addition, while the cell pitch of an image pickup device such as a charge-coupled device (CCD) or a complementary mental-oxide semiconductor device (CMOS) is reduced to increase the number of pixels, the area of the image pickup device itself is decreasing. For this reason, the optical system of the imaging apparatus is required to have a high imaging performance with reduced optical aberration, particularly axial chromatic aberration, and an incident angle of a light beam incident on the imaging element, as compared with the conventional art.

  For this reason, as a conventional imaging lens used in an imaging device mounted on a mobile phone, a personal computer, or the like, for example, a lens having a three-group three-lens configuration has been proposed (for example, see Patent Document 1).

JP 2005-352317 A

  The imaging lens described in Patent Document 1 has a configuration in which a lens having a positive refractive power, a diaphragm, a lens having a negative refractive power, and a lens having a positive refractive power are arranged in order from the object side. The image forming performance is improved by using it a lot.

  However, when the pixel size is small and an attempt is made to use it for a high-resolution image pickup device, there is a problem that axial chromatic aberration is large and sufficient resolution cannot be obtained. In addition, there is a problem that the total length is long and is not suitable for miniaturization, particularly thinning.

  The present invention has been made to solve such a problem, and an object thereof is to provide an imaging lens in which axial chromatic aberration is suppressed.

In order to solve the above-described problem, an imaging lens according to the present invention includes a first lens having a meniscus shape having a convex surface facing the object side from the object side to the image side, a stop, and a convex surface facing the image side. A second lens having negative meniscus shape with negative refractive power and a third lens having positive refractive power having an image-side surface that is flat or convex with respect to the image side. Conditional expressions (1) to (3) are satisfied.
0.5 ≦ f ₁ /f≦0.9 (1)
−5 ≦ f ₂ /f≦−0.8 (2)
2 ≦ f ₃ / f ≦ 1000 (3)
However,
f: focal length of the entire optical system,
f ₁ : focal length of the first lens,
f ₂ : focal length of the second lens,
f ₃ is the focal length of the third lens.

  The imaging lens of the present invention is realized by a three-group, three-lens configuration. By increasing the power of the first lens, optical aberration, particularly axial chromatic aberration, is reduced, and at the same time, the optical length is shortened. In addition, the third lens has a larger diameter than the first lens and the second lens, and cannot enter the lens barrel having the same diameter as the first lens and the second lens. There are many factors to do. Therefore, in order to make it insensitive to eccentricity, the power was made as weak as possible to obtain an optimum configuration for manufacturing.

  Note that the ghost is less likely to occur by making the curvature of the object side surface of the third lens negative, that is, by making it convex toward the image side. In addition, the incident angle with respect to the image sensor is relaxed so that optimum optical performance can be obtained. Also, enough back focus was taken. Further, the third lens has a thick central thickness so that axial chromatic aberration can be further corrected.

  Also, all three lenses are made of plastic so that they can be mass-produced at low cost.

  Here, an imaging apparatus can be realized by including the imaging lens of the present invention described above and an imaging element that converts an optical image formed by the imaging lens into an electrical signal. In addition, such an imaging apparatus may be equipped with an autofocus mechanism that controls the focal position. If an autofocus mechanism is installed, it is possible to correct the shift of the focal position due to environmental temperature changes, and even if all three lenses are made of plastic, it is possible to cope with temperature changes and the like.

  According to the imaging lens of the present invention, with a simple lens configuration of 3 elements in 3 groups, axial chromatic aberration can be suppressed and high imaging performance can be obtained, and the optical length can be shortened. Miniaturization and cost reduction can be achieved.

  Hereinafter, embodiments of the imaging lens of the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view showing a lens configuration of Example 1 in the present embodiment, and FIG. 2 is a cross-sectional view showing a lens configuration of Example 2 in the present embodiment.

  The imaging lens of the present embodiment is a single focus lens, and is represented by a small imaging device mounted on a small information terminal device such as a mobile phone or a notebook personal computer called a mobile PC. It is used by being mounted on various devices.

The imaging lens 1 includes a first lens L ₁ , an aperture S, a second lens L ₂ , and a third lens L ₃ in order from the object side, and each lens has at least one aspherical shape. For example, it is assumed that an imaging element such as a CCD or a CMOS is disposed on the imaging surface I of the single focus lens. Further, between the third lens L ₃ and the image plane I, an optical member P may be disposed in addition to a cover glass made of resin or glass, an infrared cut filter, a low-pass filter, or the like.

  The imaging element is a photoelectric conversion element having a rectangular light receiving surface with an imager size of about 1/4 inch, for example, and pixels are arranged in a substantially lattice shape on the light receiving surface, and the amount of light received for each pixel is Output electrical signal based on. The number of pixels is assumed to be about 2 million to 5 million pixels.

  The imaging element is adjusted so that the light receiving surface is aligned with an appropriate position in the optical axis direction of the opposing imaging lens 1.

  The imaging apparatus configured as described above images an object by forming an object image on the light receiving surface of the imaging element by the imaging lens 1. The image sensor outputs an electrical signal based on the amount of light received by each pixel to the control circuit. The electric signal output to the control circuit is converted into a digital signal in the control circuit, and is recorded on various storage media as image data.

The imaging lens 1 of the present embodiment is configured to satisfy the following conditional expressions (1) to (9).
0.5 ≦ f ₁ /f≦0.9 (1)
−5.0 ≦ f ₂ /f≦−0.8 (2)
2.0 ≦ f ₃ / f ≦ 1000 (3)
40 ≦ νL ₁ ≦ 70 (4)
20 ≦ νL ₂ ≦ 35 (5)
νL ₃ −νL ₂ ≧ 0 (6)
3.3 ≦ R ₃₁ ≦ 7.0 (7)
−∞ ≦ R ₃₂ ≦ −7 (8)
1.0 ≦ T ₃ ≦ 3.0 (9)

Here, definitions of symbols used in the conditional expressions (1) to (9) are as follows.
f: focal length of the entire optical system (as well as units mm.f ₁ ~f ₃₎
f _1: focal length of the first lens L ₁ f _2: the second lens L ₂ of the focal length f _3: the focal length NyuL ₁ of the third lens L _3: Abbe number of the first lens L _{1 νL 2:} the second lens Abbe number of L ₂ νL ₃ : Abbe number of the third lens L ₃ R ₃₁ : radius of curvature of the object side surface of the third lens L ₃ (a positive sign when the vertex is on the object side axis. R ₃₂ also The same shall apply (unit: mm)
R ₃₂ : curvature radius of the image side surface of the third lens L ₃ T ₃ : center thickness of the third lens L ₃

  The above conditional expressions (1) to (9) are common to the first embodiment and the second embodiment described below, and are suitable for individual image pickup devices or image pickup apparatuses by appropriately adopting them as necessary. More preferable imaging performance and a small optical system are realized.

  The operation of the conditional expression will be described below.

The imaging lens 1 according to the present embodiment is a three-group, three-element lens. From the object side, the first lens L ₁ has a positive power and a large Abbe number, the second lens L ₂ has a negative power and an Abbe number. Small, the third lens L ₃ has a positive power and a larger Abbe number than the second lens L ₂ , and has the same basic configuration as a lens widely known as “Cook triplet”, and corrects aberrations well. .

  In the present invention, the axial chromatic aberration is suppressed to 8.5 μm by appropriately adopting conditional expressions (1) to (9) from the conventional example in which the axial chromatic aberration was 16 μm.

Conditional expressions (1), (2), and (3) relate to the power (refractive power) of the three lenses, and conditional expressions (4), (5), and (6) It relates to dispersion of glass materials. Conditional expressions (7) and (8) relate to the paraxial radius of curvature of the third lens L ₃ , and conditional expression (9) relates to the center thickness of the third lens L ₃ .

In order to reduce the axial chromatic aberration, it is necessary to increase the power of either the first lens L ₁ or the third lens L ₃ which is a positive power lens, or both. Here, when strong first lens L ₁ of the power, at the same time since the optical length is shortened, and strongly first lens L ₁ of the power. In addition, when the positive power of the first lens L ₁ is too strong, the eccentricity sensitivity is increased, and problems such as frequent occurrence of defects during the production occur. The optimum condition is the equation (1).

Since the second lens L ₂ is sandwiched between the first lens L ₁ having a strong positive power and the third lens L ₃ having a positive power, the second lens L ₂ has a negative power and corrects axial chromatic aberration. Further, the chromatic aberration of magnification and the curvature of field are corrected by arranging the first lens L ₁ and the second lens L ₂ approximately symmetrically with respect to the stop S.

Here, when the upper limit of conditional expression (2) is exceeded, the negative power of the second lens L ₂ becomes too strong, resulting in problems such as high eccentricity sensitivity and frequent defects during manufacturing. If the lower limit is exceeded, the correction of longitudinal chromatic aberration becomes weak. For this reason, the optimum condition is the expression (2).

Since the first lens L ₁ and the second lens L ₂ have substantially the same diameter and are approximately symmetrical with respect to the stop S, the first lens L ₁ and the second lens L ₂ can be placed in a lens barrel chamber that is coaxial and has the same radius. Can be reduced.

On the other hand, the third lens L ₃ has a larger exit light beam angle from the second lens L ₂ and therefore has a larger diameter than the other two lenses. For this reason, since it cannot be put in the lens barrel part which is coaxial and has the same radius as the first lens L ₁ and the second lens L ₂ , there are many factors that cause eccentricity. Therefore, it is desirable not to increase the power as much as possible in order to make it insensitive to eccentricity.

  When the lower limit of the conditional expression (3) is exceeded, it is easy to be affected by the eccentricity as described above, and problems such as frequent occurrence of defects during production occur. On the other hand, when the upper limit is exceeded, the correction of axial chromatic aberration is weakened. For this reason, the optimum condition is the expression (3).

Curvature of the third object side surface of the lens L ₃ is, the conditional expression (7) becomes too small curvature exceeds the lower limit of the eccentric sensitivity is increased, manufacturing defect problems such that frequently. If the upper limit is exceeded, the curvature becomes large and the correction of chromatic aberration becomes weak, which is not suitable.

When the image side surface of the third lens L ₃ exceeds the lower limit of the conditional expression (8) and the curvature becomes positive, the peripheral surface has a shape projecting toward the image side rather than the vicinity of the optical axis, and the light reflected by the image sensor. However, it is not preferable because it is reflected on the image side surface of the third lens L ₃ and is likely to generate a ghost.

  At the same time, in the case of such a shape in which ghosts are likely to occur, the surrounding light is more likely to jump to the periphery, which is not preferable because the incident angle with respect to the image sensor becomes tight. In addition, there is a problem that the back focus is shortened by the protruding portion.

  If the upper limit of conditional expression (8) is exceeded and the curvature becomes small, the curvature of field becomes negative and correction becomes difficult. Therefore, the optimum condition for securing sufficient back focus and ensuring desirable camera performance is expressed by equation (8).

At the same time, the third lens L ₃ corrects axial chromatic aberration better as the center thickness increases. For this reason, if the lower limit of conditional expression (9) is exceeded, the correction of axial chromatic aberration is weak. However, if the thickness exceeds the upper limit of conditional expression (9) and is too thick, the optical length cannot be shortened. Therefore, the optimum condition is Equation (9).

  Hereinafter, the configuration of the imaging lens to which the present invention is applied will be described more specifically with reference to numerical data and aberration diagrams.

  In the lens configuration data of each example, R indicates the radius of curvature (mm) of the surface specified by the surface number i (i = 1, 2,...) Counted from the object side, and d indicates the surface number. The axial upper surface distance (mm) of the surface specified by i is shown. Further, n represents the refractive index of the optical element identified by the surface number i with respect to the d-line (588 nm), and νd represents the Abbe number of the optical element identified by the surface number i with respect to the d-line.

Here, the first surface (surface number 1), the surface of the first lens L ₁ on the object side, second side, showing a surface of the first image side of the lens L _1. Further, the third surface, the surface of the second lens L ₂ on the object side, the fourth surface denotes the image side surface of the second lens L _2. Furthermore, the fifth surface, the object side surface of the third lens L _3, surface No. 6 shows the image-side surface of the third lens L _3. The seventh surface is the object side surface of the cover glass, and the eighth surface is the image side surface of the bar glass.

  Moreover, the 1st surface-6th surface of the 1st-3rd lens is comprised by the aspherical surface. The shape of this aspherical surface is expressed by the following equation (10). Here, the distance from the tangent plane of the aspheric vertex of the coordinate point on the aspheric surface where the height from the optical axis is y is x, and the curvature (1 / r) of the aspheric vertex is c. K represents a conical coefficient, and A, B, C, D, E, and F represent a fourth-order, sixth-order, eighth-order, tenth-order, twelfth-order, and fourteenth-order aspheric coefficients, respectively. .

  3 and 4 are graphs showing various aberrations (spherical aberration, chromatic aberration, astigmatism, distortion aberration in order from the left) of Examples 1 and 2. FIG. In spherical aberration and chromatic aberration, the broken line (d) indicates spherical aberration and chromatic aberration at 587.56 nm (d line), the solid line (e) indicates spherical aberration and chromatic aberration at 546.07 nm (e line), and the alternate long and short dash line (F). This shows spherical aberration and chromatic aberration at 486.13 nm (F line). In astigmatism, the solid line (S) represents the sagittal image plane, and the broken line (M) represents the meridional image plane. In distortion aberration, the horizontal axis represents the distortion percentage.

<Example 1>
The imaging lens of Example 1 is a lens having three elements in three groups. For example, it is assumed that an object image is formed on an imaging element having an imager size of 1 / 4.5 inches and 2 million pixels. In the imaging lens, the first lens L ₁ from the object side is an aspheric surface of a plastic mold and has a positive power, has an Abbe number of 55.8, then has an aperture S, and the second lens L ₂ is a non-plastic mold. The spherical surface has a negative power and an Abbe number of 24.0, and the third lens L ₃ is an aspheric surface of a plastic mold and has a positive power and an Abbe number of 55.8.

The first lens L ₁ has a meniscus shape with a convex surface facing the object side, the second lens L ₂ has a meniscus shape with a convex surface facing the image side, and the third lens L ₃ is a biconvex lens. The thickness is as thick as 1.414 mm, and the aberration is well corrected as shown in FIG. In the present invention, the longitudinal chromatic aberration is suppressed to 8.5 μm. The parameters are shown in Table 1 below.

Example 1 Lens configuration data surface number R d n νd
1 0.864 0.518 1.530 55.8
2 2.399 0.427
3 -0.543 0.330 1.627 24.0
4 -0.838 0.080
5 4.262 1.414 1.530 55.8
6 -1100.0 0.300
7 ∞ 0.300 1.517 64.2
8 ∞

Example 1 Aspherical data first surface K = 0.049, A = 0.355E-01, B = 0.165E + 00, C = -0.704E + 00, D = 0.157E + 01,
E = 0.173E + 01, F = -0.952E + 01
Second surface K = -3.757, A = 0.546E-01, B = 0.160E-01, C = -0.3370E + 01, D = 0.278E + 01,
E = 149E-06, F = 0.224E-07
Third surface K = 0.122, A = 0.709E + 00, B = 0.163E + 01, C = 0.327E + 01, D = 0.968E-01,
E = 0.194E-07, F = 0.347E-07
4th surface K = -0.976, A = 0.217E + 00, B = 0.425E + 00, C = 0.302E + 00, D = -0.821E + 00,
E = -0.150E + 01, F = 0.236E + 01
5th surface K = 2.901, A = 0.296E-02, B = −0.195E-02, C = −0.346E-02, D = 0.104E-02,
E = 0.266E-03, F = -0.192E-03
6th surface K = 40.00, A = -0.757E-01, B = 0.267E-01, C = -0.457E-02, D = -0.549E-03,
E = 0.443E-03, F = -0.825E-04

Example 1 Various configuration data f (focal length) = 3.19 mm
F number (numerical aperture) = 3.2
ω (half angle of view) = 32.5 deg
H (lens total length) = 4.00 mm

<Example 2>
The imaging lens of the second embodiment is a three-group, three-lens configuration lens. For example, it is assumed that an object image is formed on an imaging element of 3 to 5 million pixels with an imager size of 1/4 inch. . In the imaging lens, the first lens L ₁ from the object side is an aspheric surface of a plastic mold and has a positive power, has an Abbe number of 55.8, then has an aperture S, and the second lens L ₂ is a non-plastic mold. The spherical surface has a negative power and an Abbe number of 24.0, and the third lens L ₃ is an aspheric surface of a plastic mold and has a positive power and an Abbe number of 55.8.

The first lens L ₁ has a meniscus shape with a convex surface facing the object side, the second lens L ₂ has a meniscus shape with a convex surface facing the image side, and the third lens L ₃ is a biconvex lens. The thickness is as thick as 2.373 mm, and the aberration is well corrected as shown in FIG. In the present invention, the longitudinal chromatic aberration is suppressed to 8.5 μm. Various parameters are shown in Table 1.

Example 2 Lens Configuration Data Surface Number R d n νd
1 0.993 0.641 1.530 55.8
2 2.779 0.535
3 -0.671 0.471 1.627 24.0
4 -1.181 0.080
5 -4.193 2.373 1.530 55.8
6 -1100.0 0.300
7 ∞ 0.300 1.517 64.2
8 ∞

Example 2 Aspherical data first surface K = −0.073, A = 0.416E-01, B = 0.422E-01, C = 0.315E-01, D = −0.153E + 00,
E = 0.713E + 00, F = -0.840E + 00
Second surface K = -0.209, A = 0.590E-01, B = -0.242E + 00, C = 0.121E + 01, D = -0.649E + 01,
E = 0.947E + 01, F = -0.158E-07
Third surface K = 0.213, A = 0.328E + 00, B = 0.409E-01, C = 0.259E + 01, D = −0.302E + 01
E = 0.224E-07, F = 0.668E-08
Fourth surface K = -1.294, A = 0.332E-01, B = 0.338E-01, C = 0.112E + 00, D = 0.225E + 00
E = -0.598E + 00, F = 0.291E + 00
Fifth surface K = -10.00, A = -0.181E-01, B = 0.265E-01, C = -0.922E-02, D = -0.358E-02
E = 0.338E-02, F = -0.735E-03
6th surface K = -40.00, A = -0.390E-01, B = 0.240E-02, C = 0.727E-04, D = 0.209E-04
E = 0.299E-04, F = -0.756E-05

Example 2 Various configuration data f (focal length) = 3.91 mm
F number (numerical aperture) = 3.2
ω (half angle of view) = 30.00 deg
H (lens total length) = 5.0mm

  An imaging lens to which the present invention is applied can be used not only for a camera built in or externally attached to a mobile phone or a mobile PC, but also for a camera incorporated or externally attached to a digital still camera, a video camera, or the like. .

It is sectional drawing which shows the lens structure of Example 1 in this Embodiment. It is sectional drawing which shows the lens structure of Example 2 in this Embodiment. 3 is a graph showing various aberrations of Example 1. 6 is a graph showing various aberrations of Example 2.

Explanation of symbols

L ₁ ... 1st lens, L ₂ ... 2nd lens, L ₃ ... 3rd lens

Claims (4)

A first lens having a positive refractive power with a meniscus shape having a convex surface facing the object side from the object side to the image side, and a second lens having a negative refractive power with a meniscus shape having a convex surface facing the image side And a third lens having a positive refractive power in which the image side surface has a flat surface or a convex shape with respect to the image side,
An imaging lens satisfying the following conditional expressions (1) to (3):
0.5 ≦ f ₁ /f≦0.9 (1)
−5 ≦ f ₂ /f≦−0.8 (2)
2 ≦ f ₃ / f ≦ 1000 (3)
However,
f: focal length of the entire optical system,
f ₁ : focal length of the first lens,
f ₂ : focal length of the second lens,
f ₃ is the focal length of the third lens. The imaging lens according to claim 1, further satisfying the following conditional expressions (4) to (6).
40 ≦ νL ₁ ≦ 70 (4)
20 ≦ νL ₂ ≦ 35 (5)
νL ₃ −νL ₂ ≧ 0 (6)
However,
νL ₁ : Abbe number of the first lens,
νL ₂ : Abbe number of the second lens,
νL ₃ : Abbe number of the third lens. The imaging lens according to claim 1, further satisfying the following conditional expressions (7) to (9).
3.3 ≦ R ₃₁ ≦ 7.0 (7)
−∞ ≦ R ₃₂ ≦ −7 (8)
1.0 ≦ T ₃ ≦ 3.0 (9)
However,
R ₃₁ : radius of curvature of the object side surface of the third lens,
R ₃₂ : radius of curvature of the image side surface of the third lens,
T ₃ is the center thickness of the third lens. The imaging lens according to claim 1, wherein at least one of the first lens, the second lens, and the third lens is formed of an aspheric plastic mold lens.

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