JP4382423B2 - Three-lens single focus lens - Google Patents

Description

  The present invention relates to a single focal length lens having a short total length consisting of three lenses, and in particular, a three-lens single focal point having a simple configuration suitable for an imaging lens of a small digital camera or an imaging lens of a PC or a portable terminal. It relates to lenses.

  Conventionally, as a technical field of imaging lenses, the overall length is short and inexpensive as imaging lens modules mounted on relatively thin devices such as simple digital cameras, PC web cameras, and portable terminals. However, there are some optical performances that are a little loose.

  In order to satisfy such a requirement, conventionally, an imaging lens has been configured with a single lens. Since the lens module used for such applications has a small electric image sensor such as a CCD and a low resolution, the image size of the image sensor is small, and the lens system itself can be miniaturized with a small number of lenses. It was done.

  However, in recent years, since the resolution (megapixel) and the size of the image sensor have increased, the aberration generated by a single lens configuration becomes too large and sufficient optical performance cannot be obtained.

  Therefore, it has become necessary to develop a lens system having a short overall length suitable for an image sensor that has been improved in resolution and size. In order to cope with such a problem, for example, an aspheric surface is provided on at least one surface. A three-lens CCD lens made of a plastic material is known (see Patent Document 1 below).

  Further, Patent Document 1 also describes an example in which three lenses are formed of the same plastic material, which is advantageous in terms of production cost and manufacturability. In particular, today's imaging lenses for mobile terminals and the like are required to be mass-produced at low cost, so forming all the lenses with the same plastic material can meet that demand, desirable.

Japanese Patent Laid-Open No. 10-48516

  By the way, since display of a PC, a portable terminal, and the like is standard in color image display today, it is strongly demanded to improve chromatic aberration. However, in the technique described in Patent Document 1, In particular, in terms of chromatic aberration, it was not always good.

  In a small-sized lens such as an imaging lens of a portable terminal or the like, the longitudinal chromatic aberration is not so conspicuous, but the lateral chromatic aberration is required to be greatly improved.

  Further, depending on the technique described in Patent Document 1, there is room for improvement in terms of downsizing the system including the image sensor.

  In particular, in the case of a small image pickup device for image pickup such as a PC or a portable terminal, conventionally, it has been necessary to make the chief ray incident at an angle within about 15 ° with respect to the image pickup surface. The above incident angles, for example, about 25 ° or more have been allowed. Therefore, it is desired to develop an imaging lens that can correct various aberrations well even when the incident angle of the chief ray is set to be relatively large.

  The present invention has been made in view of the above-described circumstances, has a configuration corresponding to recent image sensors, can satisfactorily correct optical performance, particularly lateral chromatic aberration, and reduce the size of the system including the image sensor. An object of the present invention is to provide an inexpensive three-element single focus lens that can be satisfactorily satisfied.

The three-element single focal length lens according to the present invention includes, in order from the object side, a first lens having a positive refractive power, a diaphragm, and a second lens having a positive refractive power and having at least one aspheric surface. A third lens for correcting aberration having at least one aspherical surface and having a positive refractive power in the vicinity of the optical axis, and the first, second and third lenses are made of the same plastic material. It is formed and further configured to satisfy the following conditional expressions (1) and (2).

1.0 <f ₁ '/f'<5.0 (1)
50 <ν _d <60 (2)
However,
f 'focal length of the whole system,
f ₁ 'first lens focal length,
v _d Abbe number of the first, second and third lenses at the d-line

  The first lens is a meniscus lens having a convex surface on the object side, the second lens is a meniscus lens having a concave surface on the object side, and the third lens is a convex surface on the object side. It is desirable to use a meniscus lens.

  In addition, it is desirable that at least one of the aspheric surfaces formed on the third lens has a shape represented by an aspheric expression having an odd-order aspheric coefficient including a third order.

  According to the three-element single focus lens of the present invention, all the lenses are formed of the same plastic material, and the second lens and the third lens are formed with aspheric surfaces, so that the formation of the aspheric surface is easy. Although it is a simple and inexpensive lens system having a three-lens configuration, it is possible to improve optical performance, particularly lateral chromatic aberration.

  Furthermore, by satisfying two predetermined conditional expressions, the distance to the pupil can be ensured while improving the curvature of field and the lateral chromatic aberration.

Hereinafter, a three-lens configuration single focus lens according to an embodiment of the present invention will be described with reference to the drawings. The three-lens configuration single focus lens of the embodiment shown in FIG. 1 (representing the lens of Example 1 as a representative) is a first lens L composed of a positive meniscus lens having a convex surface facing the object side in order from the object side. ₁ , a second lens L ₂ composed of a positive meniscus lens having a concave surface facing the object side, and a third lens L ₃ for aberration correction composed of a meniscus lens having a convex surface facing the object side (positive in the vicinity of the optical axis) from now, these two sides of the second lens L ₂ and third lens L ₃ is a respective aspheric so as to efficiently focus the light beam incident along the optical axis X to the position P on the imaging surface of the imaging device This is a three-element single focus lens.

Of the aspheric surfaces, the image surface side surface of the second lens L ₂ and the object side surface of the third lens L ₃ are aspheric surfaces having a positive refractive power that increases toward the periphery. the image-side surface of the third lens L ₃ of the aspherical negative refractive power increases toward the periphery is a non-spherical surface is weakened.

The first lens L ₁ and between the second lens L ₂ diaphragm 2 is disposed, the cover glass 1 of the image pickup device is disposed between the third lens L ₃ and the image pickup device.

The aspheric shapes of both surfaces of the second lens L ₂ and the third lens L ₃ are represented by the following aspheric expression.

Furthermore, this three-lens configuration single focus lens satisfies the following conditional expressions (1) and (2).
1.0 <f ₁ '/f'<5.0 (1)
50 <ν _d <60 (2)
However,
f 'focal length of the whole system,
f ₁ 'first lens focal length,
v _d Abbe number of the first, second and third lenses at the d-line

Next, the function and effect of this embodiment will be described.
The number of lenses of three is the minimum number necessary to obtain a desired level of good optical performance. By configuring the power of each of the lenses L _{1 to} L ₃ as described above, the following The following effects can be obtained.

That is, the first lens L ₁ and second lens L ₂ facing each other across the diaphragm 2 by a positive lens, to ensure most of the refractive power of the lens system, the two lenses L _1, L ₂ each aberration generated by, has little power has two aspheric corrected by the third lens L _3. That is, the aspherical surface formed on both surfaces of the third lens L ₃ are odd order because it is a shape represented by aspheric expression having an aspherical surface coefficients, the vicinity of the optical axis lens shape including a tertiary The shape can be controlled with high precision for each lens surface position corresponding to each image height position, and various aberrations, particularly lateral chromatic aberration, can be corrected well. it can. Also in case of providing the aspherical surface as described above only one surface of the third lens L _3, it is possible to obtain the advantageous effects to some extent above.

Further, by the first lens L ₁ and second lens L ₂ positive lens, it is possible to shorten the overall length of the lens system, in particular, a lens for imaging such as a PC or a portable terminal or small digital, It can be suitable as an imaging lens of a camera. Further, by configuring the third lens L ₃ to have a positive refractive power in the vicinity of the optical axis, the aspherical surface formed on each surface of the third lens L ₃ can be more effectively operated. It becomes possible.

In addition, the maximum incident angle of the incident light beam on the image plane can be as large as about 25 °, for example, corresponding to a recent image sensor, and thereby the distance from the third lens L ₃ to the image sensor. The system including the image sensor can be made compact.

Further, as the lens material, all the lenses L ₁ , L ₂ , and L ₃ are made of the same plastic material, so that the cost can be reduced and the productivity when forming an aspherical surface is improved. be able to.

  In the three-lens configuration single focus lens of the present embodiment, since the conditional expressions (1) and (2) described above are satisfied, the following operational effects can be achieved.

That is, the conditional expression (1) is intended to define the focal length of the first lens L _1, above this upper limit becomes difficult to correct field curvature, whereas the distance to the pupil falls below the lower limit too short It is not preferable. Therefore, by satisfying the conditional expression (1), it is possible to secure the distance to the pupil while improving the field curvature.

Conditional expression (2) defines the Abbe numbers of all the lenses L ₁ , L ₂ , and L ₃ , and it is difficult to correct chromatic aberration if the numerical value is out of this numerical range. Therefore, chromatic aberration can be improved by satisfying conditional expression (2).

  Note that various modifications can be made to the three-lens configuration single focus lens of the present invention. For example, the radius of curvature of each lens, the lens interval (or lens thickness), and the shape of the aspheric surface can be changed as appropriate.

<Example 1>
FIG. 1 shows a schematic configuration of a three-lens single-focus lens according to the first embodiment. The configuration of this single focus lens is as described in the embodiment. In this single focus lens, an aspheric surface is formed on both the second lens L ₂ and the third lens L ₃ . Moreover, all the forming materials of the lenses L ₁ , L ₂ , and L ₃ are COP (cycloolefin polymer).

The radius of curvature R (mm) of each lens surface of this single focus lens, the center thickness of each lens, and the air spacing between the lenses (hereinafter collectively referred to as the axial top surface spacing) D (mm), d of each lens The values of the refractive index N and the Abbe number ν in the line are shown in Table 1. The numbers in the table indicate the order from the object side. Table 2 shows the aspheric constants K, A ₃ , A ₄ , A ₅ , A ₆ , A ₇ , A ₈ , A ₉ , A ₁₀ ( the second lens _{L 2} shows the values of _{K, a 4, a 6,} a 8, a 10). As shown in the middle part of Table 1, the focal length f of the entire lens system in Example 1 is set to 3.93 mm, the F number Fno. Is set to 4.0, and the field angle 2ω is set to 64.6 degrees.

  Further, as shown in the lower part of Table 1, in this example, the conditional expressions (1) and (2) are satisfied.

  FIG. 3 is an aberration diagram showing various aberrations (spherical aberration, astigmatism, distortion, and lateral chromatic aberration) of the single focus lens of Example 1. In the astigmatism diagram, aberrations with respect to the sagittal image surface and the tangential image surface are shown. In this aberration diagram, ω represents a half angle of view. As is clear from these aberration diagrams, according to the single focus lens of Example 1, various aberrations including lateral chromatic aberration can be corrected well.

<Example 2>
FIG. 2 shows a schematic configuration of a three-lens configuration single focus lens according to the second embodiment. The configuration of the single focus lens is substantially the same as that of the first embodiment, and the same reference numerals are given to the same elements in the explanation of the corresponding drawings, and the duplicate explanation is omitted. Also in the single-focus lens, aspherical in any of the lens surfaces of the second lens L ₂ and third lens L ₃ are formed. In addition, the materials for forming the lenses L ₁ , L ₂ , and L ₃ are all PMMA.

Table 3 shows values of the radius of curvature R (mm) of each lens surface of this single focus lens, the axial top surface distance D (mm) of each lens, and the refractive index N and Abbe number ν of each lens at the d-line. The numbers in the table indicate the order from the object side. Table 4 shows the aspheric constants K, A ₃ , A ₄ , A ₅ , A ₆ , A ₇ , A ₈ , A ₉ , A ₁₀ ( the second lens _{L 2} shows the values of _{K, a 4, a 6,} a 8, a 10). Further, as shown in the middle of Table 3, in Example 2, the focal length f of the entire lens system is set to 3.93 mm, the F number Fno. Is set to 4.0, and the field angle 2ω is set to 64.6 degrees.

  Further, as shown in the lower part of Table 3, in this example, the conditional expressions (1) and (2) are satisfied.

  FIG. 4 is an aberration diagram showing various aberrations (spherical aberration, astigmatism, distortion, and lateral chromatic aberration) of the single focus lens of Example 2. In the astigmatism diagram, aberrations with respect to the sagittal image surface and the tangential image surface are shown. In this aberration diagram, ω represents a half angle of view. As is clear from these aberration diagrams, according to the single focus lens of Example 2, various aberrations including lateral chromatic aberration can be corrected well.

1 is a schematic diagram showing a lens configuration of a three-element single focus lens according to Example 1 of the present invention. FIG. Schematic showing the lens configuration of a three-lens configuration single focus lens according to Example 2 of the present invention Aberration diagrams showing various aberrations (spherical aberration, astigmatism, distortion, and lateral chromatic aberration) of the three-lens single-focus lens of Example 1. Aberration diagram showing various aberrations (spherical aberration, astigmatism, distortion, and lateral chromatic aberration) of the three-lens configuration single focus lens of Example 2

Explanation of symbols

L ₁ ~L ₃ lens R ₁ to R ₈ lens surface (optical member surface) radius of curvature D ₁ to D ₇ axial distance X optical axis P imaging position 1 cover glass 2 aperture

Claims (3)

In order from the object side, a first lens having a positive refractive power, a stop, a second lens having a positive refractive power, at least one surface being an aspheric surface, and at least one surface being an aspheric surface , An aberration correcting third lens having a positive refractive power in the vicinity of the axis, and the first, second and third lenses are made of the same plastic material;
Furthermore, the three-element single focal point lens is configured to satisfy the following conditional expressions (1) and (2).
1.0 <f ₁ '/f'<5.0 (1)
50 <ν _d <60 (2)
However,
f 'focal length of the whole system,
f ₁ 'first lens focal length,
v _d Abbe number of the first, second and third lenses at the d-line The first lens is a meniscus lens having a convex surface on the object side,
The second lens is a meniscus lens having a concave surface on the object side,
The third lens is a meniscus lens having a convex surface on the object side;
2. The three-element single focal lens according to claim 1, wherein:   2. At least one of the aspherical surfaces formed on the third lens has a shape represented by an aspherical expression having an odd-order aspherical coefficient including a third order. Alternatively, the three-piece single-focus lens according to 2.

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