JP2008281873A - Imaging lens - Google Patents

Abstract

An imaging lens with reduced axial chromatic aberration is provided.
An imaging lens, the image side from the object side, a first lens L ₁ having a positive refractive power, a second lens L ₂ having a negative refractive power, a stop S, the convex surface on the image side The four groups of four imaging lenses including the directed meniscus third lens L ₃ and the fourth lens L ₄ satisfy the following conditional expressions (i) and (ii).
0.5 ≦ f ₁ /f≦1.5 (i)
−9 ≦ f ₂ /f≦−0.6 (ii)
However,
f: focal length of the entire optical system,
f ₁ : focal length of the first lens,
f ₂ is the focal length of the second lens.
[Selection] Figure 1

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 device is required to have a high imaging performance that suppresses optical aberrations, 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 for an imaging device mounted on a mobile phone, a personal computer, or the like, for example, a lens having a configuration of four elements in three groups has been proposed (see, for example, Patent Document 1). Further, a lens having a four-group four-lens configuration has been proposed (see, for example, Patent Document 2).

JP 2006-309043 A Japanese Patent No. 3411565

  In the imaging lens described in Patent Document 1, the imaging performance is improved with the configuration of four elements in three groups. However, since the first lens and the second lens are cemented lenses, the productivity is low and the production cost is low. There is a problem that is high.

  On the other hand, in the imaging lens described in Patent Document 2, a four-group four-lens configuration has a negative refractive power, an aperture, a lens having a positive refractive power, a negative refractive power lens, and a positive lens in order from the object side. In order to improve the imaging performance, a lens having a refractive power of 1 is arranged.

  However, since the first lens is a lens having a negative refractive power, there is a problem that the optical path length becomes long and it is not suitable for miniaturization, particularly thinning, and is not suitable for use in a mobile phone or the like.

  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 problems, an imaging lens according to the present invention includes a first lens having a positive refractive power, a second lens having a negative refractive power, a diaphragm, and an image side from the object side to the image side. A four-group four-lens imaging lens including a third meniscus lens having a convex surface and a fourth lens, characterized in that the following conditional expressions (i) and (ii) are satisfied.
0.5 ≦ f ₁ /f≦1.5 (i)
−9 ≦ f ₂ /f≦−0.6 (ii)
However,
f: focal length of the entire optical system,
f ₁ : focal length of the first lens,
f ₂ is the focal length of the second lens.

  The imaging lens of the present invention is realized by a lens configuration of four elements in four groups, and by making the first lens positive power, optical aberration, particularly axial chromatic aberration, is reduced and at the same time, the optical length is shortened.

  In addition, the axial chromatic aberration can be corrected by the first lens and the second lens in the front group and the third lens and the fourth lens in the rear group with the stop interposed therebetween, so that the chromatic aberration can be extremely reduced as a whole. did.

  Furthermore, the second lens and the third lens before and after the stop are composed of negative power lenses, so that distortion can be reduced.

  In addition, astigmatism and coma can be reduced by setting the center of the radius of curvature of both surfaces of the third lens behind the stop approximately to the center of the stop.

  Further, by making the fourth lens a positive power lens, the incident angle with respect to the image sensor is relaxed so that optimum optical performance can be obtained. This also ensures sufficient back focus.

  Furthermore, all four 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 a focus position shift due to environmental temperature changes, and even if all four 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 4 elements in 4 groups, it is possible to obtain high imaging performance by suppressing axial chromatic aberration, and to shorten the optical length, thereby improving the resolution, and Miniaturization and cost reduction can be achieved, and an optimum lens unit for a high-resolution image sensor can be realized.

  Further, since it is not necessary to use a cemented lens, productivity can be improved.

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

  1 is a cross-sectional view showing the lens configuration of Example 1 in the present embodiment, FIG. 2 is an optical path diagram of the lens of Example 1 shown in FIG. 1, and FIG. 3 is Example 2 of the present embodiment. FIG. 4 is a sectional view showing the lens configuration, and FIG. 4 is an optical path diagram of the lens of Example 2 shown in FIG.

  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 , a second lens L ₂ , a diaphragm S, a third lens L ₃ , and a fourth lens L ₄ in order from the object side. For example, the third lens L ₃ and the fourth lens L _At least two of ₄ are aspherical. 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 fourth lens L ₄ 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 (8).
0.4 ≦ f ₁ /f≦1.5 (1)
−30 ≦ f ₂ /f≦−0.6 (2)
−1000 ≦ f ₃ /f≦−0.5 (3)
0.7 ≦ f ₄ / f ≦ 1000 (4)
40 ≦ νL ₁ ≦ 70 (5)
20 ≦ νL ₂ ≦ 35 (6)
20 ≦ νL ₃ ≦ 35 (7)
3 ≦ (R ₃₂ + R ₃₁ ) / (R ₃₂ −R ₃₁ ) ≦ 10 (8)

Here, definitions of symbols used in the conditional expressions (1) to (8) are as follows.
f: focal length of the entire optical system (units mm.f ₁ ~f ₄ as well)
f ₁ : focal length of the first lens L ₁ f ₂ : focal length of the second lens L ₂ f ₃ : focal length of the third lens L ₃ f ₄ : focal length νL ₁ of the fourth lens L ₄ : first lens L ₁ Abbe number NyuL _2: Abbe number of the second lens L ₂ NyuL _3: third lens L ₃ of the Abbe number R _31: having a vertex in the third lens L curvature of the object side surface of the ₃ radius (on the object side shaft The case is a positive sign, the same applies to R _{32 (} unit: mm)
R ₃₂ : radius of curvature of the image side surface of the third lens L ₃

  The above conditional expressions (1) to (8) are common to the first embodiment and the second embodiment described below, and are suitable for individual image pickup devices or image pickup devices 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.
Conditional expressions (1), (2), (3), (4) relate to the power (refractive power) of the four lenses, and conditional expressions (5), (6), (7) This relates to the dispersion (Abbe number) of the glass material. Conditional expression (8) relates to bending of the third lens L ₃ .

In order to shorten the total lens length, it is desirable to set the first lens L ₁ to a positive power. If the lower limit of the conditional expression (1) is exceeded, the power becomes too strong, the eccentricity sensitivity becomes high, and problems such as frequent occurrence of defects occur. If the upper limit is exceeded, problems such as the fact that the overall length cannot be shortened and axial chromatic aberration cannot be suppressed occur. The optimum condition is the expression (1) including the conditional expression (i).

Further, in order to suppress the longitudinal chromatic aberration and magnification are those first lens L ₁ has a low dispersion, i.e., those greater Abbe number is required, the first lens L ₁ is an Abbe number of the conditional expression ( It is desirable to satisfy 5) and to have a function of a so-called crown type lens.

The second lens L ₂ preferably has a negative power and a small Abbe number in order to correct axial chromatic aberration. When the upper limit of conditional expression (2) is exceeded, the power becomes too strong, the eccentricity sensitivity becomes high, and problems such as frequent occurrence of defects occur. If the lower limit is exceeded, the correction of longitudinal chromatic aberration becomes weak. For this reason, the optimum condition is the expression (2) including the conditional expression (ii).

Further, it is desirable that the second lens L ₂ has an Abbe number satisfying the conditional expression (6) and has a function of a so-called flint-type lens. As described above, the first lens L ₁ and the second lens L ₂ are axially arranged on the condition that the first lens L ₁ has a positive power and a large Abbe number, and the second lens L ₂ has a negative power and a small Abbe number. Chromatic aberration can be corrected well.

In order to reduce distortion, it is desirable that the third lens L ₃ has a negative power as well as the second lens L ₂ . As a result, the incident angle of the light incident on the stop S is kept small. At the same time, the third lens L ₃ suppresses spherical aberration to a small extent by bending represented by the conditional expression (8).

In addition, since the center of curvature of the object side surface and the image side surface of the third lens L ₃ is approximately in the vicinity of the center of the stop S, astigmatism and coma are suppressed to a low level. In order for the center of curvature radius of both surfaces of the third lens L ₃ to be approximately near the stop S and for the third lens L ₃ to function as a negative power lens and bend optimally, the conditional expression (iii ) Including the conditional expression (3) and conditional expression (8) including the conditional expression (iv) are required. Further, if the upper limit of conditional expression (3) is exceeded, the eccentricity sensitivity becomes high, and problems such as frequent occurrence of defects occur.

Here, it is desirable that the Abbe number of the third lens L ₃ satisfies the conditional expression (7) for spherical aberration correction and axial chromatic aberration correction, and has a function of a so-called flint type lens.

In order to reduce the angle of light incident on the imaging surface such as a CCD or CMOS sensor, the fourth lens L ₄ is preferably a positive power lens. The conditional expression is the expression (4). If the lower limit of the conditional expression (4) is exceeded, the eccentricity sensitivity becomes high, and problems such as frequent occurrence of defects occur. If the upper limit is exceeded, the angle incident on the imaging surface cannot be reduced.

  Then, the power configuration of the entire lens is positive, negative, (aperture), negative, and positive, and the configuration is symmetrical with respect to the aperture S to reduce spherical aberration, coma aberration, and lateral chromatic aberration.

Here, in order to reduce the axial chromatic aberration, it is necessary to increase the Abbe number of the fourth lens L _4, may reduce the Abbe number of the fourth lens L ₄ in order to reduce the overall optical length The Abbe number of the fourth lens L ₄ has a wide selection range.

At the same time, it is desirable that the curvature on the image plane side of the fourth lens L ₄ is negative (a convex shape protruding in the direction of the image plane I), but this condition is not an absolute condition.

The reason why it is desirable that the image plane side of the fourth lens L ₄ has a convex shape protruding in the direction of the imaging plane I is as follows. If the curvature on the image plane side of the fourth lens L ₄ is made positive, the peripheral surface protrudes to the image side rather than the vicinity of the optical axis, so that the light reflected by the imaging element is also reflected in the fourth lens L. Reflects on the side of the image of ₄ and tends to generate ghosts. At the same time, in the case of such a shape in which ghosts are likely to occur, the ambient light will be further jumped to the periphery, so that the incident angle with respect to the image sensor becomes tight. Further, there is a problem that the back focus is shortened by the protruding portion toward the image side.

  However, since the axial chromatic aberration is suppressed in the imaging lens to which the present invention is applied, these problems impair the merchantability of the lens unit to which the imaging lens of the present invention is applied and the imaging device in which the lens unit is mounted. Rather, there is a wide selection range.

  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. Also, surface No. 7, the object side surface of the fourth lens L _4, surface No. 8 shows the image side surface of the fourth lens L _4. The ninth surface is the object side surface of the cover glass, and the tenth surface is the image side surface of the bar glass.

  Moreover, the 1st surface-8th surface of the 1st-4th lens is comprised by the aspherical surface. The shape of this aspherical surface is expressed by the following equation (9). 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. .

  5 and 6 are graphs showing various aberrations (spherical aberration, chromatic aberration, astigmatism, distortion aberration in order from the left) of Example 1 and Example 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 four elements in four groups. 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. Yes.

In the imaging lens, the first lens L ₁ is a plastic-molded aspheric lens from the object side and has a positive power, the Abbe number is 55.8, and the second lens L ₂ is a plastic-molded aspheric lens and has a negative power. The Abbe number is 29.0, the stop is S, the third lens L ₃ is a plastic aspherical lens and has negative power, the Abbe number is 29.0, and the fourth lens L ₄ is It is a plastic molded aspheric lens with positive power and an Abbe number of 55.8.

  When the parameters of the conditional expressions (1) to (8) described above are set to the values shown in Table 1 below, the aberration is well corrected as shown in FIG. 5 by the synergistic effect of the four lenses. In Example 1, the axial chromatic aberration is suppressed to 6.0 μm, and the optical total length is suppressed to 4.7 mm, so that a lens unit optimal for a small high-resolution image sensor having 3 million pixels or more can be realized.

Example 1 Lens configuration data surface number R d n νd
1 1.400 0.748 1.530 55.8
2 -3.785 0.087
3 -2.495 0.300 1.580 29.0
4 -2112.9 0.734
5 -0.477 0.302 1.580 29.0
6 -0.724 0.080
7 2.478 1.068 1.530 55.8
8 -1078.1 0.300
9 ∞ 0.300 1.517 64.2
10 ∞

Example 1 Aspherical data first surface K = 0.106, A = -0.267E-01, B = -0.143E-01, C = -0.356E-01, D = -0.986E-02,
E = -0.395E-02, F = 0.507E-02
Second surface K = -4.998, A = 0.131E + 00, B = -0.166E + 00, C = 0.109E + 00, D = -0.272E-01,
E = -0.136E-01, F = 0.117E-01
Third surface K = -9.303, A = 0.326E + 00, B = -0.361E + 00, C = 0.272E + 00, D = -0.779E-01,
E = -0.864E-01, F = 0.124E + 00
4th surface K = -6.900, A = 0.199E + 00, B = -0.165E-01, C = -0.167E + 01, D = 0.257E + 01,
E = 0.107E + 00, F = 0.460E-02
Fifth surface K = -0.840, A = 0.326E + 00, B = -0.109E + 01, C = -0.295E + 01, D = 0.101E + 02,
E = 0.123E + 00, F = 0.126E-03
6th surface K = −0.340, A = 0.257E + 00, B = −0.723E-01, C = −0.542E + 00, D = 0.172E + 01,
E = 0.636E-01, F = -0.932E-01
7th surface K = -9.996, A = -0.975E-01, B = 0.450E-01, C = -0.836E-02, D = 0.619E-03,
E = -0.117E-04, F = 0.142E-05
8th surface K = -40.00, A = -0.394E-01, B = -0.550E-02, C = 0.345E-03, D = 0.132E-03,
E = 0.370E-05, F = 0.422E-05

Example 1 Various configuration data f (focal length) = 3.76 mm
F number (numerical aperture) = 2.8
ω (half angle of view) = 30.5 deg
H (lens total length) = 4.7 mm

<Example 2>
The imaging lens of Example 2 is a lens having four elements in four groups. 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. Yes.

In the imaging lens, the first lens L ₁ is a plastic-molded aspheric lens from the object side and has a positive power, the Abbe number is 55.8, and the second lens L ₂ is a plastic-molded aspheric lens and has a negative power. The Abbe number is 29.0, the stop is S, the third lens L ₃ is a plastic aspherical lens and has negative power, the Abbe number is 29.0, and the fourth lens L ₄ is It is a plastic-molded aspherical lens with positive power and an Abbe number of 29.0.

  When the parameters of the conditional expressions (1) to (8) described above are set to the values shown in Table 1 above, the aberration is well corrected as shown in FIG. 6 by the synergistic effect of the four lenses. In Example 2, the axial chromatic aberration is suppressed to 10.4 μm, and the optical total length is suppressed to 4.47 mm, so that it is possible to realize a lens unit optimal for a small high-resolution imaging device having 3 million pixels or more.

Example 2 Lens Configuration Data Surface Number R d n νd
1 1.140 0.801 1.530 55.8
2 15.58 0.0552
3 -9.021 0.300 1.580 29.0
4 -7.101 0.608
5 -0.606 0.300 1.580 29.0
6 -0.887 0.050
7 5.350 1.352 1.580 29.0
8 -1100.0 0.300
9 ∞ 0.300 1.517 64.2
10 ∞

Example 2 Aspheric Data First Surface K = −0.638, A = 0.526E-01, B = 0.695E-02, C = 0.432E-01, D = −0.339E-01
Second surface K = 9.436, A = 0.273E-01, B = -0.304E + 00, C = 0.642E + 00, D = -0.387E + 00
Third surface K = -8.978, A = 0.108E + 00, B = -0.317E + 00, C = 0.896E + 00, D = -0.703E + 00
Fourth surface K = 3.495, A = 0.153E + 00, B = −0.233E + 00, C = 0.162E + 01, D = -0.354E + 01
5th surface K = -0.806, A = -0.231E-02, B = -0.141E + 00, C = -0.163E + 01, D = 0.242E + 01
6th surface K = -0.176, A = 0.177E + 00, B = 0.465E-01, C = -0.946E-01, D = 0.434E + 00
7th surface K = -10.00, A = -0.621E-01, B = 0.277E-01, C = -0.708E-02, D = -0.742E-03,
E = 0.113E-02, F = -0.208E-03
8th surface K = 40.00, A = -0.347E-01, B = -0.587E-03, C = -0.661E-03, D = 0.629E-04,
E = -0.486E-04, F = 0.112E-04

Example 2 Various configuration data f (focal length) = 3.80 mm
F number (numerical aperture) = 2.8
ω (half angle of view) = 30.5 deg
H (lens total length) = 4.47 mm

  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. FIG. 2 is an optical path diagram of the lens configuration of Example 1. It is sectional drawing which shows the lens structure of Example 2 in this Embodiment. FIG. 6 is an optical path diagram of the lens configuration of Example 2. 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, L ₄ ... 4th lens

Claims (4)

From the object side to the image side, a first lens having a positive refractive power, a second lens having a negative refractive power, a stop, a meniscus third lens having a convex surface facing the image side, and a fourth lens An imaging lens characterized by satisfying the following conditional expressions (i) and (ii):
0.5 ≦ f ₁ /f≦1.5 (i)
−9 ≦ f ₂ /f≦−0.6 (ii)
However,
f: focal length of the entire optical system,
f ₁ : focal length of the first lens,
f ₂ is the focal length of the second lens. The imaging lens according to claim 1, further satisfying the following conditional expressions (iii) and (iv).
−1000 ≦ f ₃ /f≦−0.5 (iii)
3 ≦ (R ₃₂ + R ₃₁ ) / (R ₃₂ −R ₃₁ ) ≦ 10 (iv)
However,
f ₃ : focal length of the third lens,
R ₃₁ : radius of curvature of the object side surface of the third lens,
R ₃₂ is the radius of curvature of the image side surface of the third lens. The imaging lens according to claim 1, wherein at least two of the first lens, the second lens, the third lens, and the fourth lens are aspherical lenses. 2. The imaging lens according to claim 1, wherein all of the first lens, the second lens, the third lens, and the fourth lens are formed of an aspheric lens made of a plastic material.

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