JP2004302057A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens having a high performance and a compact constitution by effectively using an aspherical surface while attaining cost reduction with a small number of lenses. <P>SOLUTION: The imaging lens is provided with: a 1st meniscus lens G1 having a convex facing the object side; a 2nd lens G2 where a convex face is formed on the object side, and which has positive power; a diaphragm St; a 3rd meniscus lens G3, at least one face of which is aspherical, and where a concave face is formed on the object side, and which has positive or negative power; and a 4th meniscus lens G4, two faces of which are aspherical, and also, where a convex faces the object side as for the shape near the optical axis, in this order from an object side. The imaging lens satisfies the following conditional inequality (1): 0.8<fA/f<2.0 (provided that f denotes the whole focal distance, fA denotes the synthetic focal distance of the 1st lens G1 and the 2nd lens G2). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an imaging lens particularly suitable for mounting on a small-sized imaging device.
[0002]
[Prior art]
2. Description of the Related Art In recent years, with the spread of personal computers to general households and the like, digital still cameras (hereinafter, simply referred to as digital cameras) capable of inputting image information such as captured scenery and portraits to personal computers have rapidly spread. It is getting. Further, with the advancement of functions of mobile phones, a module camera for image input (portable module camera) is often mounted on the mobile phone.
[0003]
In these image pickup devices, image pickup devices such as charge coupled devices (CCDs) and complementary metal oxide semiconductors (CMOSs) are used. In such an image pickup device, the size of the image pickup device has been reduced in recent years, so that the size of the entire device has been extremely reduced. In addition, the number of pixels of the image sensor has been increased, and higher resolution and higher performance have been achieved.
[0004]
As an imaging lens used in such an imaging device, for example, there is one described in the following patent document. Patent Documents 1 to 3 disclose a three-element imaging lens. Patent Document 4 describes a four-lens imaging lens. In the imaging lens described in Patent Document 4, there is a stop position between the first lens and the second lens from the object side.
[0005]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 10-48516 [Patent Document 2]
Japanese Patent Application Laid-Open No. 2002-221659 [Patent Document 3]
U.S. Pat. No. 6,441,971 [Patent Document 4]
JP 2002-517773 A
[Problems to be solved by the invention]
As described above, in recent years, image sensors have been reduced in size and pixels, and accordingly, imaging lenses for digital cameras have been required to have high resolution performance and compact configuration. On the other hand, the imaging lens of a portable module camera has conventionally been mainly required to be cost-effective and compact. Demands are also increasing.
[0007]
For this reason, there is a demand for the development of a wide variety of lenses that comprehensively consider cost, performance, and compactness. For example, it is desired to develop a low-cost, high-performance imaging lens that satisfies compactness that can be mounted on a portable module camera and that has a view to mounting on a digital camera in terms of performance.
[0008]
In response to such demands, for example, it is conceivable to use three or four lenses in order to achieve compactness and low cost, and to actively use an aspherical surface in order to achieve high performance. . In this case, the aspherical surface contributes to compactness and high performance, but is disadvantageous in terms of manufacturability and tends to increase the cost. Therefore, it is desirable to use the aspherical surface in consideration of manufacturability. The lenses described in the above patent documents have a three- or four-element configuration using an aspherical surface. However, the overall performance described above is insufficient. Is missing. In general, the performance of a three-lens lens is sufficient for a portable module camera in terms of performance, but is likely to be insufficient for a digital camera. Moreover, although the performance of the four-lens configuration can be improved as compared with the three-lens configuration, it is likely to be disadvantageous in terms of cost and compactness.
[0009]
The present invention has been made in view of such a problem, and an object of the present invention is to provide an imaging lens capable of realizing a high-performance and compact configuration by effectively using an aspheric surface while reducing cost with a small number of lenses. To provide.
[0010]
[Means for Solving the Problems]
The imaging lens according to the present invention includes, in order from the object side, a meniscus-shaped first lens having a convex surface facing the object side, a second lens having a positive object-side surface having a convex surface, an aperture, and at least one lens. A meniscus-shaped third lens having a positive or negative power having a concave surface on the object side and a concave surface on the object side, and both surfaces being aspherical and having a shape near the paraxial surface convex toward the object side. And a fourth lens having a meniscus shape directed toward the lens, and is configured to satisfy the following conditional expression (1).
[0011]
0.8 <fA / f <2.0 (1)
Here, f indicates the entire focal length, and fA indicates the combined focal length of the first lens and the second lens.
[0012]
In the imaging lens according to the present invention, as described above, the first lens is a meniscus lens, the second lens is a positive lens, the third and fourth lenses are aspherical lenses, and the diaphragm is the second lens and the second lens. By arranging it between three lenses, it is possible to obtain the minimum necessary optical performance that can be applied not only to a portable module camera but also to a digital camera, while having a lens configuration as small as four. In particular, the third and fourth aspherical lenses greatly contribute to aberration correction. In addition, by satisfying Expression (1) with respect to the combined focal length of the front group (first and second lenses) closer to the object side than the aperture, the emission light angle (telecentricity) suitable for an imaging device such as a CCD is satisfied. While reducing the overall length. Thus, by effectively using an aspherical surface while achieving low cost with a small number of lenses, high performance and compactness can be achieved.
[0013]
In this imaging lens, the lens material of the second lens is preferably a glass material. In particular, by making the second lens a glass spherical lens that can be polished, it mainly contributes to cost reduction.
[0014]
Further, the fourth lens has an aspheric shape in which the surface on the object side has a weaker positive power toward the periphery within the range of the effective diameter, and the negative power as the surface on the image side moves toward the periphery. Desirably, the aspherical shape becomes weaker. By adopting such a special aspherical shape, correction of curvature of field and the like becomes easy, and it mainly contributes to high performance.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0016]
FIG. 1 shows a configuration example of an imaging lens according to an embodiment of the present invention. This configuration example corresponds to a lens configuration of a first numerical example (FIGS. 3 and 4) described later. FIG. 2 shows another configuration example of the imaging lens according to the present embodiment. The configuration example of FIG. 2 corresponds to a lens configuration of a second numerical example (FIGS. 5 and 6) described later. In FIG. 1 and FIG. 2, reference numeral Ri denotes an i-th (i) number, which is sequentially increased toward the image side (imaging side) with the surface of the component closest to the object being the first. = 1 to 10). Reference sign Di indicates a surface interval on the optical axis Z1 between the i-th surface and the (i + 1) -th surface. Since the basic configuration is the same in each configuration example, the following description will be made based on the configuration of the imaging lens shown in FIG.
[0017]
This imaging lens is used by being mounted on, for example, a portable module camera or a digital camera using an imaging element such as a CCD or a CMOS. This imaging lens has a configuration in which a first lens G1, a second lens G2, a stop St, a third lens G3, and a fourth lens G4 are arranged in order from the object side along an optical axis Z1. . An image pickup device (not shown) such as a CCD is arranged on an image forming surface (image pickup surface) of the image pickup lens. A cover glass CG for protecting the imaging surface is arranged near the imaging surface of the CCD. Between the fourth lens G4 and the imaging plane (imaging plane), other optical members such as an infrared cut filter and a low-pass filter may be arranged in addition to the cover glass CG.
[0018]
The first lens G1 has a meniscus shape with the convex surface facing the object side. The first lens G1 has, for example, a negative power. The first lens G1 is a spherical lens or an aspheric lens. When an aspherical lens is used, the object-side surface of the first lens G1 may be formed into an aspherical shape. The second lens G2 has a convex surface on the object side and has positive power. The image-side surface of the second lens G2 has, for example, a convex shape.
[0019]
The third lens G3 has at least one aspheric surface and a meniscus shape having a positive or negative power with a concave surface on the object side.
[0020]
The fourth lens G4 has an aspheric surface on both surfaces, and a shape near the paraxial axis has a meniscus shape with the convex surface facing the object side. The fourth lens G4 has an aspheric shape in which the surface on the object side has a weaker positive power as it goes to the periphery within the range of the effective diameter, and the negative power becomes weaker as the surface on the image side goes to the periphery. It is desirable to have an aspherical shape. Thus, the fourth lens G4 has, for example, a surface on the object side having a convex shape near the paraxial axis and a concave shape on the peripheral portion, and a surface on the image side having a concave shape near the paraxial axis and a convex shape on the peripheral portion. It has a shape.
[0021]
In the present embodiment, the lens shape in the paraxial vicinity, for example in the aspheric expression below (A), is represented by the portion of the factor K (portion excluding the polynomial part of the coefficients A _i).
[0022]
This imaging lens is configured to satisfy the following conditional expression (1). However, in equation (1), f indicates the entire focal length, and fA indicates the combined focal length of the first lens G1 and the second lens G2.
0.8 <fA / f <2.0 (1)
[0023]
In this imaging lens, the first lens G1 and the second lens G2 in the group before the stop St are desirably made of, for example, a glass material in consideration of workability and manufacturing cost. In particular, when the second lens G2 is a spherical lens made of polished glass, cost can be reduced. When the first lens G1 has an aspherical shape, it is desirable to use a glass mold lens. On the other hand, the third lens G3 and the fourth lens G4 in the group behind the stop St are desirably made of an optical resin material (plastic lens) in order to perform special aspherical shape processing.
[0024]
Next, the operation and effects of the imaging lens configured as described above will be described.
[0025]
In this imaging lens, the first lens G1 has a meniscus lens, the second lens G2 has a positive lens, the third and fourth lenses G3 and G4 have aspherical lenses, and the diaphragm St has a second lens G2. By arranging the third lens G3 and the third lens G3, high performance and a compact lens system are realized with a small number of lenses such as four.
[0026]
In this imaging lens, since the third and fourth lenses G3 and G4 are aspherical lenses, a large aberration correction effect can be obtained. In particular, the fourth lens G4 has an aspheric shape in which the surface on the object side has a weaker positive power as it goes to the periphery and the negative power becomes weaker as it goes to the periphery in the range of the effective diameter. With such an aspherical shape, a greater effect can be obtained with respect to aberration correction, including correction of field curvature.
[0027]
Conditional expression (1) relates to the combined focal length of the front group (the first and second lenses G1 and G2) on the object side of the stop St. When the value exceeds the numerical range of the conditional expression (1), the power of the front group becomes too small, and it becomes difficult to shorten the total length. In general, in a digital camera or the like, it is desirable that light rays enter the imaging surface in a state nearly perpendicular to the imaging device such as a CCD. Therefore, it is desirable that the imaging lens mounted on a digital camera or the like has telecentricity. If the value falls below the numerical range of the conditional expression (1), the angle of the emitted light beam becomes large, and the telecentricity deteriorates.
[0028]
In addition, the cost can be reduced by optimizing the material of each lens and the processing method according to the lens shape as described above.
[0029]
As described above, according to the imaging lens of the present embodiment, by effectively using the aspheric surface with a small number of lenses of four, the compactness that can be mounted on a portable module camera is satisfied, and the performance is improved. Thus, a low-cost, high-performance imaging lens that can be mounted on a digital camera can be realized.
[0030]
【Example】
Next, specific numerical examples of the imaging lens according to the present embodiment will be described. Hereinafter, the first and second numerical examples (Examples 1 and 2) will be described together. 3 and 4 show specific lens data (Example 1) corresponding to the configuration of the imaging lens shown in FIG. FIGS. 5 and 6 show specific lens data (Example 2) corresponding to the configuration of the imaging lens shown in FIG. 3 and 5 show a basic data portion of the lens data of the embodiment, and FIGS. 4 and 6 show a data portion relating to the aspherical shape of the lens data of the embodiment.
[0031]
In the field of the surface number Si in the lens data shown in each drawing, a code is set in the imaging lens of each embodiment so that the surface of the component on the most object side is the first and the number increases sequentially toward the image side. The number of the attached i-th (i = 1 to 10) surface is shown. In the field of the radius of curvature Ri, the value of the radius of curvature of the i-th surface from the object side is shown in association with the symbol Ri attached in FIGS. The column of the surface distance Di also shows the distance on the optical axis between the i-th surface Si and the (i + 1) -th surface Si + 1 from the object side, corresponding to the reference numerals given in FIGS. The units of the values of the radius of curvature Ri and the surface interval Di are millimeters (mm). The columns of Ndj and νdj show the refractive index and Abbe number of the j-th (j = 1 to 5) lens element from the object side with respect to the d-line (587.6 nm), including the cover glass CG, respectively. . Note that the values of the curvature radii R9 and R10 on both surfaces of the cover glass CG are 0 (zero), which indicates that the surfaces are flat. 3 and 5 also show the values of the focal length f (mm), the F-number (FNO.), And the angle of view 2ω (ω: half angle of view) of the entire system as various data.
[0032]
In each lens data of FIGS. 3 and 5, a symbol “*” attached to the left side of the surface number indicates that the lens surface has an aspherical shape. In each of the embodiments, the object-side surface S1 of the first lens G1, the both surfaces S5 and S6 of the third lens G3, and the both surfaces S7 and S8 of the fourth lens G4 have an aspheric shape. In the basic lens data, numerical values of the radius of curvature near the optical axis (near the paraxial axis) are shown as the radius of curvature of these aspheric surfaces.
[0033]
In the numerical values of the respective aspherical surface data in FIGS. 4 and 6, the symbol “E” indicates that the next numerical value is a “power exponent” with a base of 10, and the exponential function with the base of 10 Indicates that the number represented is multiplied by the number before "E". For example, “1.0E-02” indicates “1.0 × 10 ⁻² ”.
[0034]
In each aspherical surface data, the value of each coefficient A _i , K in the aspherical surface expression represented by the following expression (A) is described. More specifically, Z is a length (mm) of a perpendicular line drawn from a point on the aspherical surface at a height h from the optical axis to a tangent plane of the apex of the aspherical surface (a plane perpendicular to the optical axis). Show.
[0035]
Z = C · h ² / {1+ (1-K · C ² · h ² ) ^1/2 } + A ₃ · h ³ + A ₄ · h ⁴ + A ₅ · h ⁵ + A ₆ · h ⁶ + A ₇ · h ⁷ + A ^{_{8 · h 8 + A 9 ·}} h 9 + A 10 · h 10 ...... (A)
However,
Z: Depth of aspherical surface (mm)
h: distance (height) from optical axis to lens surface (mm)
K: eccentricity C: paraxial curvature = 1 / R
(R: paraxial radius of curvature)
A _i : the i-th order (i = 3 to 10) aspherical surface coefficient
In each of the embodiments, the aspheric surfaces of the object-side surface S1 of the first lens G1 and both surfaces S5 and S6 of the third lens G3 are even-numbered coefficients A ₄ , A ₆ , A ₈ , as aspheric coefficients. It is represented by using effectively the only a _10. The aspherical shape of both surfaces S7 and S8 of the fourth lens G4 effectively uses odd-ordered aspherical coefficients A ₃ , A ₇ and A ₉ .
[0037]
FIG. 7 shows values corresponding to the above-described conditional expression (1) for each embodiment. As shown in FIG. 7, the value of each embodiment is within the numerical range of the conditional expression (1).
[0038]
FIGS. 8A to 8C show spherical aberration, astigmatism, and distortion (distortion) in the imaging lens of Example 1. FIG. Each aberration diagram shows aberrations with the d-line as the reference wavelength, while the spherical aberration diagrams also show aberrations for the g-line (wavelength 435.8 nm) and the C-line (wavelength 656.3 nm). In the astigmatism diagram, the solid line shows the aberration in the sagittal direction, and the broken line shows the aberration in the tangential direction. Similarly, various aberrations of the second embodiment are shown in FIGS.
[0039]
As can be seen from the lens data and the aberration diagrams described above, the aberrations are satisfactorily corrected in each example. In addition, the overall length is reduced.
[0040]
The present invention is not limited to the above embodiment and each example, and various modifications can be made. For example, the values of the radius of curvature, the surface spacing, the refractive index, and the like of each lens component are not limited to the values shown in each of the numerical examples, and may take other values.
[0041]
【The invention's effect】
As described above, according to the imaging lens of the present invention, the first lens is a meniscus lens, the second lens is a positive lens, and the third and fourth lenses are aspherical lenses. Since it is arranged between the second lens and the third lens and satisfies the predetermined conditional expression (1) with respect to the combined focal length of the front group on the object side of the stop, low cost can be achieved with a small number of lenses. By effectively using the aspherical surface, a high-performance and compact configuration can be realized.
[0042]
In particular, in the imaging lens of the present invention, when the lens material of the second lens is a glass material, the cost can be further reduced by making the second lens a spherical lens made of, for example, polished glass. it can. Further, the fourth lens has an aspheric shape in which the surface on the object side has a weaker positive power toward the periphery within the range of the effective diameter, and the negative power is weaker as the surface on the image side moves toward the periphery. In the case of an aspherical shape, correction of curvature of field and the like becomes easy, and higher performance can be achieved.
[Brief description of the drawings]
FIG. 1 illustrates a configuration example of an imaging lens according to an embodiment of the present invention, and is a lens cross-sectional view corresponding to Example 1.
FIG. 2 shows another example of the configuration of the imaging lens according to the embodiment of the present invention, and is a lens cross-sectional view corresponding to Example 2.
FIG. 3 is a diagram illustrating basic lens data of the imaging lens according to the first embodiment.
FIG. 4 is a diagram illustrating data on an aspheric surface of the imaging lens according to the first embodiment.
FIG. 5 is a diagram showing basic lens data of the imaging lens according to Example 2.
FIG. 6 is a diagram illustrating data on an aspheric surface of the imaging lens according to the second embodiment.
FIG. 7 is a diagram illustrating values of conditional expressions satisfied by the imaging lens according to each example.
FIG. 8 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the imaging lens according to Example 1.
FIG. 9 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion of the imaging lens according to the second embodiment.
[Explanation of symbols]
CG: cover glass; Gj: j-th lens from the object side; Ri: radius of curvature of the i-th lens surface from the object side; Di: surface distance between the i-th and (i + 1) th lens surfaces from the object side , Z1 ... optical axis.

Claims (2)

In order from the object side,
A first meniscus lens having a convex surface facing the object side,
A second lens having a positive power with the object side surface having a convex shape;
Aperture and
A meniscus-shaped third lens having a positive or negative power, wherein at least one surface has an aspherical shape and the object-side surface has a concave shape;
A fourth lens having a meniscus shape in which both surfaces are aspherical and the shape near the paraxial surface is convex toward the object side;
An imaging lens characterized by satisfying the following conditional expression (1).
0.8 <fA / f <2.0 (1)
However,
f: Overall focal length fA: Composite focal length of the first lens and the second lens The second lens is made of a glass material,
The fourth lens has an aspheric shape in which the surface on the object side has a weaker positive power toward the periphery within the range of the effective diameter, and the power on the image side has a lower negative power as it goes to the periphery. The imaging lens according to claim 1, wherein the imaging lens has an aspherical shape.

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