JP2004302060A - Single focal lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a single focal 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 single focal lens is provided with: a 1st lens G1, at least one face of which is aspherical, also, where both faces are formed to a convex shape near an optical axis, and which has positive power; a diaphragm St; a 2nd meniscus lens G2, at least one face of which is aspherical, also, where a concave faces the object side near the optical axis, and which has positive power; and a 3rd lens G3, both faces of which are aspherical, which has positive or negative power, where the surface on the object side is convex near the optical axis and which is constituted of plastic material, in this order from an object side. The single focal lens satisfies prescribed conditional expressions (1) to (3) as for the focal distance and Abbe number, etc., of the 1st lens G1. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a single focus 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 1, there is a stop position between the second lens and the third lens from the object side. In the imaging lens described in Patent Literature 2, there is a stop position between the first lens and the second lens from the object side. In the imaging lens described in Patent Literature 3, there is a stop position on the most object side of the lens system. In the imaging lens described in each patent document, the lens closest to the object has a meniscus shape.
[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 the above problems, and has as its object to achieve a high-performance and compact configuration by effectively using an aspheric surface while achieving low cost with a small number of lenses. Is to provide.
[0010]
[Means for Solving the Problems]
The single focus lens according to the present invention includes, in order from the object side, at least one surface having an aspherical shape, and a shape near the paraxial side having a biconvex shape having a positive power, a stop, and at least one surface. Has an aspherical shape, and a second lens having a meniscus shape having a positive power with a concave surface facing the object side in the vicinity of the paraxial, and both surfaces having an aspherical shape and having a positive or negative power, a paraxial In the vicinity, a third lens whose surface on the object side is made of a plastic material having a convex shape is provided, and is configured to satisfy the following conditional expressions (1) to (3).
[0011]
0.8 <f1 / f <2.0 (1)
60 <νd1 (2)
0.2 <D2 / f <0.5 (3)
Here, f indicates the overall focal length, f1 indicates the focal length of the first lens, vd1 indicates the Abbe number of the first lens with respect to the d line, and D2 indicates the air between the first lens and the second lens. The interval is shown.
[0012]
In the single focus lens according to the present invention, the first lens and the second lens having positive power in the paraxial direction, and the third lens having a double-sided aspherical surface made of a plastic material are arranged in order from the object side, and An aperture is provided between the first lens and the second lens, and the lens satisfies predetermined conditional expressions (1) to (3) relating to the focal length, Abbe number, and the like of the first lens, and the shape, power distribution, By using appropriate materials such as glass materials, the cost can be reduced with as few as three lenses, while the aspherical surface is effectively used to achieve high optical performance that can be used not only for portable module cameras but also for digital cameras. can get.
[0013]
In this single focus lens, it is preferable that the first lens has an aspherical surface in which the positive power becomes weaker toward the periphery within the effective diameter range. The second lens has an aspheric shape in which the negative power becomes weaker as the surface on the object side moves toward the periphery within the range of the effective diameter, and the positive power becomes higher as the surface on the image side moves toward the periphery. It is preferable to have an aspherical shape that becomes weaker. The third lens has an aspheric shape in which the positive power decreases as the surface on the object side moves toward the periphery within the effective diameter range, and the negative power decreases as the surface on the image side moves toward the periphery. It is preferable to have an aspherical shape that becomes weaker.
[0014]
In this single focus lens, it is preferable that the lens material of the first lens is a glass material, particularly a glass mold lens made of a low dispersion glass material in consideration of optical performance. Similar to the third lens, the lens material of the second lens is preferably a plastic (optical resin) material in consideration of performing special aspherical shape processing.
[0015]
By appropriately adopting these preferable configurations as needed, a more sophisticated and compact lens system is realized.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0017]
FIG. 1 shows a configuration example of a single focus lens according to one 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 single focus 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 8). 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 is based on the configuration of the single focus lens shown in FIG.
[0018]
This single focus lens is used, for example, mounted on a portable module camera or digital camera using an image pickup device such as a CCD or a CMOS. The single focus lens has a configuration in which a first lens G1, a stop St, a second lens G2, and a third lens G3 are arranged in this 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 single focus lens. A cover glass CG for protecting the imaging surface is arranged near the imaging surface of the CCD. In addition to the cover glass CG, other optical members such as an infrared cut filter and a low-pass filter may be arranged between the third lens G3 and the imaging plane (imaging plane).
[0019]
The first lens G1 has at least one aspheric surface. The first lens G1 has a biconvex shape near paraxial and has positive power. When the object-side surface of the first lens G1 has an aspherical shape, for example, it is preferable that the object-side surface has a shape such that the positive power becomes weaker toward the periphery within the range of the effective diameter. .
[0020]
The second lens G2 has at least one aspheric surface and a meniscus shape having a positive power with a concave surface facing the object near the paraxial axis. When both surfaces of the second lens G2 are aspherical, for example, within the range of the effective diameter, the surface on the object side has an aspherical shape in which the negative power becomes weaker toward the periphery, and the surface on the image side. It is preferable that the surface has an aspherical shape in which the positive power becomes weaker toward the periphery. Thus, the second lens G2 has, for example, a surface on the object side having a concave shape near the paraxial axis and a convex shape on the peripheral portion, and a surface on the image side having a convex shape near the paraxial axis and a concave shape on the peripheral portion. It has a shape.
[0021]
The third lens G3 has an aspheric surface on both surfaces, has positive or negative power, and has a convex surface on the object side near paraxial. The lens material of the third lens G3 is made of a plastic material. The aspherical shape of the third lens G3 is such that, within the range of the effective diameter, the positive power decreases as the object-side surface approaches the periphery, and the negative power decreases as the image-side surface approaches the periphery. It is preferable that the shape is such that the power of the first electrode becomes weak. Thus, the third lens G3 has, for example, a surface on the object side having a convex shape near the paraxial axis and a concave shape at the peripheral portion, and a surface on the image side having a concave shape near the paraxial axis and a convex surface at the peripheral portion. It has a shape.
[0022]
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).
[0023]
This single focus lens is configured to satisfy the following conditional expressions (1) to (3). In the formulas (1) to (3), f indicates the entire focal length, f1 indicates the focal length of the first lens G1, νd1 indicates the Abbe number of the first lens G1 with respect to the d line, and D2 indicates The air gap between the first lens G1 and the second lens G2 is shown.
0.8 <f1 / f <2.0 (1)
60 <νd1 (2)
0.2 <D2 / f <0.5 (3)
[0024]
In this single focus lens, the lens material of the first lens G1 is preferably a glass material, particularly a glass mold lens made of a low dispersion glass material in consideration of optical performance. On the other hand, the lens material of the second lens G2 and the third lens G3 is preferably made of an optical resin material (plastic lens) in order to perform special aspherical shape processing.
[0025]
Next, the operation and effect of the single focus lens configured as described above will be described.
[0026]
In this single focus lens, a first lens G1 and a second lens G2 having a positive power in the paraxial direction, and a third lens G3 made of a plastic material and having a double-sided aspherical shape are arranged in order from the object side, and , The stop St is disposed between the first lens G1 and the second lens G2, and a predetermined conditional expression (1) to (3) relating to the focal length and Abbe number of the first lens G1 is satisfied. By making the shape, power distribution, glass material, and the like appropriate, high performance and a compact lens system are realized while reducing cost with as few as three lenses.
[0027]
In order to improve on-axis performance, it is effective to reduce chromatic aberration. In this single focus lens, a chromatic aberration correction effect is provided by using, for example, a glass mold lens made of a low dispersion glass material as the lens material of the first lens G1. In order to improve the on-axis performance, the stop St is disposed between the first lens G1 and the second lens G2, and the shape of the first lens G1 near the paraxial is a biconvex shape.
[0028]
In addition, in this single focus lens, a large aberration correction effect can be obtained by using an aspheric surface for each lens. In particular, when the aspherical shape of each lens is a special shape in which the shape and power are different between the paraxial vicinity and the peripheral portion, a greater effect on aberration correction including correction of field curvature is obtained. can get.
[0029]
Conditional expression (1) relates to the focal length of the first lens G1. When the value exceeds the numerical range of the conditional expression (1), the power of the first lens G1 becomes too small, and it becomes difficult to correct the curvature of field. 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 a single focus 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.
[0030]
Conditional expression (2) relates to the Abbe number of the first lens G1. If the value is out of this range, it becomes difficult to correct chromatic aberration. Conditional expression (3) relates to the air gap between the first lens G1 and the second lens G2. Exceeding the numerical range of conditional expression (3) is not preferable because the overall length becomes too long and lacks compactness. If it is smaller, the angle of the emitted light beam becomes large, and the telecentricity deteriorates, which is not preferable.
[0031]
As described above, according to the single focus lens according to the present embodiment, by effectively using the aspheric surface with a small number of lenses of three, the compactness that can be mounted on a portable module camera is satisfied, and the performance is improved. On the surface side, a low-cost, high-performance imaging lens that can be mounted on a digital camera can be realized.
[0032]
【Example】
Next, specific numerical examples of the single focus 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 single focus lens shown in FIG. FIGS. 5 and 6 show specific lens data (Example 2) corresponding to the configuration of the single focus 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.
[0033]
In the column of the surface number Si in the lens data shown in each drawing, the surface of the component on the object side is set to be the first for the single focus lens of each embodiment, and the surface number is sequentially increased toward the image side. Indicates the number of the i-th (i = 1 to 8) surface to which. 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 4) 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 radii of curvature R7 and R8 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.
[0034]
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 embodiment, all the surfaces S1 to S6 of the first to third lenses G1 to G3 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.
[0035]
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 ”.
[0036]
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.
[0037]
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 example both aspherical shape of the first lens surfaces of G1 S1, S2 and the second lens G2 of the double-sided S3, S4, as the aspherical coefficients, coefficients of even-order _{A 4, A 6, A 8} , A 10 Only are used effectively. The aspherical shape of both surfaces S5 and S6 of the third lens G3 effectively uses odd-ordered aspherical coefficients A ₃ , A ₇ and A ₉ .
[0039]
FIG. 7 shows the values corresponding to the above-mentioned conditional expressions (1) to (3) collectively for each embodiment. As shown in FIG. 7, the value of each embodiment is within the numerical range of the conditional expressions (1) to (3).
[0040]
FIGS. 8A to 8C show spherical aberration, astigmatism, and distortion (distortion) in the single focus 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.
[0041]
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.
[0042]
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.
[0043]
【The invention's effect】
As described above, according to the single focus lens of the present invention, the first lens and the second lens having a positive power in the paraxial direction and the third lens having a double-sided aspherical surface made of a plastic material are provided on the object side. , And a stop is provided between the first lens and the second lens to satisfy predetermined conditional expressions (1) to (3) relating to the focal length and Abbe number of the first lens. Since the shape, power distribution, glass material and the like of each lens are optimized, an aspherical surface can be effectively used, and a high-performance and compact configuration can be realized while reducing cost with a small number of lenses.
[0044]
In particular, in the single focus lens of the present invention, the lens material of the first lens is made of a glass material, for example, a glass mold lens made of a low dispersion glass material, so that high performance can be easily achieved. Further, when the lens material of the second lens and the third lens is made of a plastic material, it becomes easy to perform special aspherical processing on the second lens and the third lens.
[Brief description of the drawings]
FIG. 1 illustrates a configuration example of a single focus lens according to an embodiment of the present invention, and is a lens cross-sectional view corresponding to Example 1.
FIG. 2 illustrates another configuration example of the single focus lens according to one embodiment of the present invention, and is a lens cross-sectional view corresponding to Example 2.
FIG. 3 is a diagram showing basic lens data of a single focus lens according to Example 1.
FIG. 4 is a diagram illustrating data on an aspheric surface of the single focus lens according to the first example.
FIG. 5 is a diagram showing basic lens data of a single focus lens according to Example 2.
FIG. 6 is a diagram showing data on the aspherical surface of the single focus lens according to Example 2.
FIG. 7 is a diagram illustrating values of conditional expressions satisfied by a single focus lens according to each example.
FIG. 8 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion of the single focus lens according to the first example.
FIG. 9 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the single focus lens according to Example 2.
[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 (3)

In order from the object side,
A first lens having a positive power and at least one surface having an aspherical shape, and having a biconvex shape near paraxial;
Aperture and
A second lens having a meniscus shape having at least one aspheric surface and having a positive power with a concave surface facing the object side near paraxial;
A third lens made of a plastic material whose surface on the object side in the vicinity of paraxial has a positive or negative power, and has a positive or negative power,
In addition, a single focus lens is configured to satisfy the following conditional expressions (1) to (3).
0.8 <f1 / f <2.0 (1)
60 <νd1 (2)
0.2 <D2 / f <0.5 (3)
However,
f: Overall focal length f1: Focal length of the first lens νd1: Abbe number of the first lens with respect to the d line D2: Air gap between the first and second lenses The object-side surface of the first lens has an aspherical shape, and the shape is such that the positive power becomes weaker toward the periphery within an effective diameter range,
Both surfaces of the second lens are aspherical shapes, and the shape is such that the object-side surface is such that the negative power becomes weaker toward the periphery within the range of the effective diameter, and the image-side surface However, it is a shape where the positive power becomes weaker as it goes to the periphery,
The aspherical shape of the third lens is such that, within the range of the effective diameter, the object-side surface becomes weaker in positive power as it goes to the periphery, and the image-side surface becomes more negative as it goes to the periphery. The single focus lens according to claim 1, wherein the single focus lens has a shape in which the power of the lens is weakened. The first lens is made of a glass material,
The single focus lens according to claim 1, wherein the second lens is made of a plastic material.

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