JP2008158122A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact imaging lens having superior optical characteristics which includes a first lens having positive power and a second lens having negative power. <P>SOLUTION: The imaging lens includes a diaphragm, the first lens which has one or more aspherical surfaces, is biconvex and has positive power, and the second lens which has one or more aspherical surfaces, turns its convex surface to an image surface side and is a meniscus lens having negative power, in the order starting from the object side to the image surface side. The center thickness d1 of the first lens, the center thickness d3 of the second lens and the focal length (f) of the overall imaging lens satisfy conditional Expressions (1) and (2), where the Expression (1) is 0.50<(d1+d3)/f<0.75 and the Expression (2) is 0.10<d3/f<0.29. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an imaging lens. In particular, it is composed of two lenses with compact and good optical characteristics suitable for small-sized imaging devices, photosensors, portable module cameras, WEB cameras, etc. using solid-state imaging devices such as CCDs and CMOSs for high pixels. The present invention relates to an imaging lens.

  In recent years, various imaging devices using a solid-state imaging device such as a CCD or a CMOS have been widely used. As these image sensors become higher in performance and smaller in size, an image pickup lens used in an image pickup apparatus is required to be more compact and have better optical characteristics than before.

  Conventionally, much research and development has been conducted on an imaging lens that satisfies both compactness and good optical characteristics. The demand for compactness and optical characteristics is increasing due to the high performance of solid-state imaging devices such as CCDs. In order to make the imaging lens compact, it is advantageous that the number of lenses constituting the imaging lens is small. On the other hand, as the number of lenses increases, the optical characteristics can be easily achieved. In consideration of these, an imaging lens composed of two lenses that balance compactness with good optical characteristics has been proposed.

  Patent Document 1 proposes an imaging lens that includes, in order from the object side, a first lens having a positive aperture and a biconvex positive power and a second lens having a concave meniscus positive power on the object side. Has been. However, even with this configuration, it cannot be said that the imaging lens is sufficiently compact, and there is a demand for further compactness.

  Patent Document 2 proposes an imaging lens that includes, in order from the object side, a first lens having a positive aperture and a biconvex positive power, and a second meniscus lens having a concave surface on the object side. As this imaging lens, a lens system in which “positive / positive” or “positive / negative” is combined as the power configuration of the first lens and the second lens is disclosed. These configurations are said to be advantageous for downsizing compared to the “negative / positive” combination. However, even with this proposal, it is still not sufficient to make the imaging lens compact, and there is a demand for further compactness.

  There are many proposals for taking into account the power configuration of the first lens and the second lens for making the imaging lens composed of two lenses compact, but the focus is on the lens thickness, which is one of the factors for making the imaging lens compact. I don't see any proposals that address this.

JP 2003-75719 A JP 2003-329921 A

  The present invention has been made in order to solve the above-described problems of the conventional example. In an imaging lens including a first lens having a positive power and a second lens having a negative power, the optical lens is further compact and has good optical characteristics. An object of the present invention is to provide an imaging lens having

In order to achieve the above object, according to the first aspect of the present invention, there is provided, in order from the object side to the image surface side, a first aperture having one or more aspheric surfaces and a biconvex positive power, and one surface. The above is an imaging lens that satisfies the following conditional expressions (1) and (2) in an imaging lens including an aspherical surface and a meniscus second lens having a negative power with the convex surface facing the image surface side.
0.50 <(d1 + d3) / f <0.75 (1)
0.10 <d3 / f <0.29 (2)
However,
f: focal length of the entire imaging lens,
d1: Core thickness of the first lens,
d3: the core thickness of the second lens. The lens core thickness is the thickness of the lens on the optical axis.

According to a second aspect of the present invention, in the imaging lens of the first aspect, the following conditional expression (3) is satisfied.
0.2 <d2 / d1 <1.0 (3)
However,
d1: Core thickness of the first lens,
d2: A surface interval on the optical axis between the image side surface of the first lens and the object side surface of the second lens.

  According to a third aspect of the present invention, in the imaging lens of the first or second aspect, the first lens is formed of a resin material having a refractive index of 1.50 to 1.55.

  As a result of intensive studies on the relationship between the thickness of the constituent lens and the optical characteristics in the imaging lens including the first lens having the positive power and the second lens having the negative power, the present inventors have determined that the thickness of the constituent lens and the imaging lens The present inventors have found that an imaging lens that is compact and has good optical characteristics can be obtained when the focal lengths are in a specific relationship, and the present invention has been achieved. That is, in order from the object side to the image plane side, the aperture, one or more aspherical surfaces, a biconvex first lens having positive power, and one or more aspherical surfaces with a convex surface on the image plane side. In an imaging lens including a meniscus second lens having negative power directed toward the center, the ratio of the sum of the core thicknesses of the first and second lenses to the focal length of the entire imaging lens, the core thickness of the second lens and the imaging When the ratio of the focal length of the entire lens to the condition shown in claim 1 is satisfied, an imaging lens having a two-lens configuration that is compact and has good optical characteristics can be obtained. Thereby, it is possible to contribute to high functionality, high performance, and compactness of various digital input devices such as a mobile phone in which the imaging lens according to the present invention is mounted.

  Furthermore, by satisfying the conditions described in claim 2, it is possible to regulate the enlargement of the imaging lens and to achieve a good balance between compactness and optical characteristics.

  Furthermore, by satisfying the conditions shown in claim 3, the core thickness of the first lens can be reduced, the sum of the core thicknesses of the first lens and the second lens can be kept small, and the imaging lens can be easily made compact. To.

  An embodiment of an imaging lens according to the present invention will be described with reference to the drawings. FIG. 1 shows a configuration of an imaging lens according to an embodiment of the present invention. The imaging lens LA is a two-lens lens system in which an aperture S1, a first lens L1, and a second lens L2 are arranged in order from the object side (not shown) toward the image plane. A glass flat plate GF is placed between the second lens L2 and the image plane. As this glass flat plate GF, what has functions, such as a cover glass, IR cut filter, or a low pass filter, can be used.

  By disposing the stop S1 closer to the object side than the first lens L1, the entrance pupil position can be set at a position far from the image plane. As a result, high telecentricity can be ensured, and the incident angle with respect to the image plane can be made suitable.

  The first lens L1 is a biconvex positive lens having one or more aspheric surfaces, preferably both aspheric surfaces. The second lens L2 is a meniscus lens having a negative power with one or more surfaces being aspheric, preferably both surfaces being aspheric and having a convex surface facing the image surface side. By combining a positive power lens having an aspheric surface as the first lens and a negative power lens having an aspheric surface as the second lens, the imaging lens LA can be made compact and good optical characteristics can be obtained. However, simply combining the first lens with the positive power and the second lens with the negative power is not sufficient for compactness. The power is an amount defined by the reciprocal of the focal length.

Therefore, in an imaging lens including a positive power first lens and a negative power second lens, as a result of earnestly examining the relationship between the thickness of the constituent lenses and the optical characteristics, the optical characteristics of the constituent lenses are within a range that is not impaired as much as possible. In order to reduce the thickness, as a first condition, a ratio between the sum of the core thickness d1 of the first lens L1 and the core thickness d3 of the second lens L2 and the focal length f of the entire imaging lens is expressed by the following conditional expression ( It was found that 1), more preferably, conditional expression (1-A) must be satisfied.
0.50 <(d1 + d3) / f <0.80 (1)
0.55 <(d1 + d3) / f <0.74 (1-A)

  If the value of (d1 + d3) / f becomes larger than the upper limit value of conditional expression (1), it may be difficult to achieve compactness. If the value of (d1 + d3) / f is smaller than the lower limit value of conditional expression (1), it may be difficult to correct aberrations. Therefore, outside the range of the conditional expression, it becomes difficult to obtain an imaging lens that achieves both compactness and good optical characteristics. Therefore, by satisfying the condition shown in the conditional expression (1), more preferably the conditional expression (1-A), an imaging lens having a two-lens configuration having a compact and good optical characteristic can be obtained. Thereby, it is possible to contribute to high functionality, high performance, and compactness of various digital input devices such as a mobile phone on which the imaging lens LA is mounted.

Further, as a second condition, the ratio between the core thickness d3 of the second lens and the focal length f of the entire imaging lens should satisfy the following conditional expression (2), more preferably the conditional expression (2-A). Thus, it is possible to regulate the increase in the size of the imaging lens LA and to appropriately balance compactness and optical characteristics.
0.10 <d3 / f <0.30 (2)
0.15 <d3 / f <0.27 (2-A)

  If the value of d3 / f is larger than the upper limit value of conditional expression (2), it may be difficult to achieve compactness. On the other hand, if the value of d3 / f is smaller than the lower limit value of the conditional expression (2), the core thickness of the second lens becomes too thin, making it difficult to manufacture the lens, and correcting aberrations. Sometimes. Therefore, when the imaging lens LA satisfies the conditional expressions (1) and (2) at the same time, it is possible to achieve compactness and good optical characteristics.

Further, a ratio d2 / d1 between the core thickness d1 of the first lens L1 and the surface distance d2 on the optical axis between the image-side surface of the first lens L1 and the object-side surface of the second lens L2 is as follows. By satisfying conditional expression (3), preferably conditional expression (3-A), more preferably conditional expression (3-B), the imaging lens LA can be made more compact while maintaining good optical characteristics. be able to.
0.2 <d2 / d1 <1.0 (3)
0.3 <d2 / d1 <0.8 (3-A)
0.5 ≦ d2 / d1 <0.7 (3-B)

  If the value of d2 / d1 is larger than the upper limit value of the conditional expression (3), compactification becomes insufficient, and if it is smaller than the lower limit value, good optical characteristics may not be obtained.

The power balance between the first lens L1 and the second lens L2 of the imaging lens LA is such that the relationship between the focal lengths f1 and f2 of the respective lenses and the focal length f of the entire imaging lens LA is the following conditional expressions (4) and (5). Is preferably satisfied. Conditional expression (4) shows the relationship between the ratio f1 / f of the focal length f1 of the first lens L1 and the focal length f of the entire imaging lens. Conditional expression (5) shows the relationship between the focal length f2 of the second lens L2 and the ratio f2 / f of the focal length f of the entire imaging lens.
0.4 <f1 / f <0.8 (4)
0.8 <| f2 | / f <2.0 (5)

  If the value of f1 / f is larger than the upper limit value of the conditional expression (4), the optical length becomes long and it may be difficult to make it compact. On the other hand, if the value is smaller than the lower limit, the correction of distortion may be insufficient. If the value of | f2 | / f is outside the range of the conditional expression (5), correction of spherical aberration or the like may be insufficient. Therefore, outside the range of conditional expressions (4) and (5), it is difficult to obtain a compact imaging lens with good optical characteristics.

  The first lens L1 and the second lens L2 are each formed of a resin material. The resin material used for the first lens L1 has a refractive index of d-line measured in accordance with ASTM D542 method of 1.50 to 1.55, preferably 1.50 to 1.53. The resin material may be a resin material having a light transmittance in the wavelength range of 450 to 600 nm of 80% or more, preferably 85% or more, and may be a thermoplastic resin or a thermosetting resin. Specific examples of a preferable resin material for the first lens L1 include an amorphous polyolefin-based resin having a cyclo ring or other cyclic structure.

  The resin material used for the second lens L2 has a d-line refractive index measured according to the ASTM D542 method of 1.50 to 1.65, preferably 1.55 to 1.64. The resin material may be a resin material having a light transmittance in the wavelength range of 450 to 600 nm of 80% or more, preferably 85% or more, and may be a thermoplastic resin or a thermosetting resin.

  Specific examples of the resin material used for the second lens L2 include a cyclo ring, an amorphous polyolefin resin having a cyclic structure, a polystyrene resin, an acrylic resin, a polycarbonate resin, and 9,9- Examples thereof include highly transparent polyester-based resins, epoxy-based resins, and silicon-based resins that include bis (4-hydroxyphenyl) fluorene as a constituent component. Among these, a highly transparent polyester-based resin having a polycarbonate-based resin, 9,9-bis (4-hydroxyphenyl) fluorene, or the like as a constituent component is preferable. The first lens L1 and the second lens L2 can be manufactured by using a known molding method such as injection molding, compression molding, cast molding, transfer molding, using the resin material.

  When manufacturing a lens with a resin material, an edge can be provided in the lens perimeter part of the 1st lens L1 and the 2nd lens L2. The shape of the edge is not particularly limited as long as the performance of the lens is not impaired. From the viewpoint of lens moldability, the edge thickness is preferably in the range of 70 to 130% of the thickness of the outer periphery of the lens. When the edge is provided on the outer peripheral portion of the lens, if light enters the edge portion, it may cause ghost or flare. In that case, a light shielding mask for restricting incident light may be provided between the lenses as necessary.

  Before the imaging lens LA is used in an imaging module or the like, the first lens L1 and the second lens L2 are subjected to a known surface treatment such as antireflection or surface curing on the lens surfaces on the object side and the image plane side. Also good. The imaging module using the imaging lens LA is used for a portable module camera, a WEB camera, a personal computer, a digital camera, an optical sensor of an automobile or various industrial devices, a monitor, and the like.

Hereinafter, specific examples of the imaging lens LA of the present invention will be described. The symbols described in each example indicate the following.
f: Focal length of entire imaging lens (unit: mm)
f1: focal length of first lens L1 (unit: mm)
f2: Focal length of second lens L2 (unit: mm)
Fno: F number r: radius of curvature of the optical surface, in the case of a lens, the center radius of curvature (unit: mm)
d: Lens core thickness or distance between lenses (unit: mm)
d1: Core thickness (unit: mm) of the first lens L1
d2: Surface distance (unit: mm) on the optical axis between the image side surface of the first lens L1 and the object side surface of the second lens L2.
d3: Core thickness of the second lens L2 (unit: mm)
d4: A surface interval (unit: mm) on the optical axis between the image plane side of the second lens L2 and the object side of the glass flat plate GF.
d5: thickness of the glass flat plate GF (unit: mm)
nd: Refractive index N1 at the d-line: Refractive index N2 of the first lens L1: Refractive index N3 of the second lens L2: Refractive index νd of the glass plate GF: Abbe number ν1 at the d-line: Abbe of the first lens L1 Number ν2: Abbe number of second lens L2 ν3: Abbe number of glass flat plate GF

  The aspherical shape of each lens surface of the first lens L1 and the second lens L2 of the imaging lens LA is such that y is an optical axis with the traveling direction of light as positive and x is an axis perpendicular to the optical axis. Is expressed by the following aspheric polynomial (6).

^{y = (x 2 / r)} / [1+ {1- (k + 1) (x / r) 2} 1/2] + A4x 4
+ A6x ⁶ + A8x ⁸ + A10x ¹⁰ (6)

  However, r is a radius of curvature on the optical axis, k is a conical coefficient, and A4, A6, A8, and A10 are aspherical coefficients.

  As an aspheric surface of each lens surface, an aspheric surface represented by Expression (6) is used for convenience. However, it is not particularly limited to this aspheric polynomial (6).

(Example 1)
FIG. 2 is a configuration diagram illustrating the arrangement of the imaging lenses according to the first embodiment. Curvature radius r, refractive index nd, Abbe number νd, lens core thickness, or surface between lenses of the first lens L1 and the second lens L2 constituting the imaging lens LA of Example 1 Table 1 shows values such as the distance d, the conical coefficient k, and the aspheric coefficient.

  Under these conditions, f = 2.09 mm, f1 = 1.16 mm, f2 = −2.58 mm, d1 = 1.17 mm, d2 = 0.37 mm, d3 = 0.37 mm, and Fno = 2.8. From these values, (d1 + d2) /f=0.74, d3 / f = 0.18, d2 / d1 = 0.32.

  The spherical aberration of the imaging lens of Example 1 is shown in FIG. 3, and astigmatism and distortion are shown in FIG. From the above results, it can be seen that the imaging lens of Example 1 is compact and has good optical characteristics. Note that the spherical aberration and astigmatism are the results of aberration with respect to three wavelengths of 486 nm, 588 nm, and 656 nm, and are aberrations at a wavelength of 486 nm, a wavelength of 588 nm, and a wavelength of 656 nm in this order from the left. Distortion is an aberration at a wavelength of 588 nm. Further, astigmatism S is an aberration with respect to the sagittal image surface, and T is an aberration with respect to the tangential image surface.

(Example 2)
FIG. 5 is a configuration diagram illustrating the arrangement of the imaging lenses of the second embodiment. The object side and image plane side curvature radii r, refractive index nd, Abbe number νd, lens core thickness, or between the lenses of the first lens L1 and the second lens L2 constituting the imaging lens LA of Example 2 Table 2 shows values such as the surface interval d, the conical coefficient k, and the aspheric coefficient.

  Under these conditions, f = 2.42 mm, f1 = 1.50 mm, f2 = −4.07 mm, d1 = 1.24 mm, d2 = 0.69 mm, d3 = 0.41 mm, and Fno = 2.8. From these values, they were (d1 + d2) /f=0.68, d3 / f = 0.17, d2 / d1 = 0.56.

  The spherical aberration of the imaging lens of Example 2 is shown in FIG. 6, and astigmatism and distortion are shown in FIG. From the above results, it can be seen that the imaging lens of Example 2 is compact and has good optical characteristics.

The figure which shows the structure of the imaging lens which concerns on one Embodiment of this invention. The figure which shows the structure of the specific Example 1 of the said imaging lens. FIG. 3 is a spherical aberration diagram of the imaging lens of Example 1. FIG. 3 is an astigmatism diagram and a distortion diagram of the imaging lens of Example 1. FIG. 6 is a diagram illustrating a configuration of Example 2 of the imaging lens. FIG. 6 is a spherical aberration diagram of the imaging lens of Example 2. FIG. 6 is an astigmatism diagram and a distortion diagram of the imaging lens of Example 2.

Explanation of symbols

LA: Imaging lens S1: Diaphragm L1: First lens L2: Second lens GF: Glass flat plate R1: Object side surface R2 of the first lens L1: Image surface side surface R3 of the first lens L1: Object side surface R4 of the second lens L2 : Image surface side surface R5 of the second lens L2: Object side surface R6 of the glass flat plate GL: Image surface side surface d1 of the glass flat plate GL: Core thickness d2 of the first lens L1: Image surface side of the first lens L1 and the second lens L2 Surface distance d3 on the optical axis to the object side of the second lens L2: core thickness d4 of the second lens L2: surface distance d5 on the optical axis between the image surface side of the second lens L2 and the object side of the glass flat plate GF: glass flat plate GF Thickness of

Claims (3)

In order from the object side to the image surface side, the aperture, one or more surfaces are aspherical, a biconvex first lens having positive power, and one or more surfaces are aspherical, with the convex surface facing the image surface side. An imaging lens including a meniscus second lens having negative power, wherein the following conditional expressions (1) and (2) are satisfied:
0.50 <(d1 + d3) / f <0.75 (1)
0.10 <d3 / f <0.29 (2)
However,
f: focal length of the entire imaging lens,
d1: Core thickness of the first lens,
d3: the core thickness of the second lens. The lens core thickness refers to the thickness of the lens on the optical axis. The imaging lens according to claim 1, wherein the following conditional expression (3) is satisfied.
0.2 <d2 / d1 <1.0 (3)
However,
d1: Core thickness of the first lens,
d2: A surface interval on the optical axis between the image side surface of the first lens and the object side surface of the second lens.   The imaging lens according to claim 1 or 2, wherein the first lens is formed of a resin material having a refractive index of 1.50 to 1.55.

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