JP2004246168A - Single focus lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a single focus lens having short back focus and made compact in entire length. <P>SOLUTION: The single focus lens is equipped with a diaphragm St, a 1st lens G1 whose both surfaces are formed to be aspherical and which has positive power, and a 2nd lens G2 whose both surfaces are formed to be aspherical and which has positive power in order from an object side. The lens satisfies a conditional expression: f1/f2<1.0 concerning the ratio of the focal distances f1 and f2 of the 1st and the 2nd lenses G1 and G2. It satisfies a conditional expression: 3.0<R2/R1 concerning the paraxial radius of curvature of both surfaces of the 1st lens G1. <P>COPYRIGHT: (C)2004,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 Conventionally, an image pickup apparatus using an image pickup device such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) has been known. In such an imaging apparatus, an image of a subject is formed on an image sensor, and the image is electronically read to perform photographing. 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. Particularly, in recent years, miniaturization of a module camera for image input and a digital still camera (hereinafter simply referred to as a digital camera) in a mobile phone has been remarkable.
[0003]
2. Description of the Related Art Conventionally, as an imaging lens that can be used in a small-sized imaging device, for example, there is one described in the following publication. The lens described in Patent Document 1 is a three-element lens. The lens described in Patent Document 2 has a two-lens configuration.
[0004]
[Patent Document 1]
Japanese Patent Application Laid-Open No. H10-301022 [Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-258684
[Problems to be solved by the invention]
By the way, in recent years, the performance of an image pickup device has been improved, and a device having a small size and a high pixel count without increasing the size of the entire device has been developed. With the increase in the number of pixels, higher optical performance is required for an imaging lens used for the imaging lens.
[0006]
In order to obtain optical performance that can withstand the increase in the number of pixels of the imaging element, it is conceivable to increase the number of lenses. However, as in the case of the three-lens configuration described in Patent Document 1, the imaging performance can be improved by increasing the number of lenses.・ Portability may be lost. Therefore, development of an imaging lens that satisfies the required optical performance, has a short back focus, and has a compact overall length for compactness and portability has been demanded. Although the lens described in Patent Literature 2 is simplified with a two-lens configuration, development of a lens with higher performance than this is desired.
[0007]
The present invention has been made in view of such a problem, and an object of the present invention is to provide a single focal length lens having a short back focus and capable of reducing the overall length.
[0008]
[Means for Solving the Problems]
The single focus lens according to the present invention includes, in order from the object side, a stop, a first lens having positive power with both surfaces being aspherical, and a second lens having positive power with both surfaces being aspherical, The first lens has an aspherical surface in which the surface on the object side has a stronger positive power as it goes to the periphery, and the surface on the image side has a concave shape near the paraxial and a convex shape as it goes to the peripheral portion. The second lens has an aspherical shape, and the second lens has an aspherical surface in which the negative power becomes stronger as the object side surface goes to the periphery, and the image side surface has a concave shape near the paraxial and has a concave shape near the periphery. It is configured to have an aspherical shape that becomes convex as it goes, and is configured to satisfy the following conditional expressions (1) and (2).
[0009]
f1 / f2 <1.0 (1)
3.0 <R2 / R1 (2)
Here, f1 indicates the paraxial focal length of the first lens, f2 indicates the paraxial focal length of the second lens, and R1 indicates the paraxial radius of curvature of the front (object side) surface of the first lens. R2 indicates the paraxial radius of curvature of the rear (image side) surface of the first lens.
[0010]
In the single focus lens according to the present invention, all the surfaces are made aspherical, so that it is easy to obtain good optical performance while reducing the total number of lenses while reducing the number of lenses to two. In particular, by satisfying the expression (1), the positive power of the first lens becomes larger than that of the second lens, and the back focus is easily shortened. In particular, by satisfying the expression (2), it is easy to correct the field curvature.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0012]
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 (FIG. 3) 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 (FIG. 4) described later. In FIGS. 1 and 2, the reference sign Ri is assigned such that the surface of the component closest to the object including the stop St is set to the 0th surface, and sequentially increases toward the image side (imaging side). The radius of curvature of the i-th (i = 0 to 6) surface is shown. 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.
[0013]
This single focus lens is used, for example, as an imaging lens for a small imaging device using an imaging device such as a CCD or a CMOS. This single focus lens has, in order from the object side, along the optical axis Z1, a stop St, a first lens G1 having aspherical surfaces on both surfaces and having positive power, and a positive power having an aspherical surface on both surfaces. A second lens G2. 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, an optical member such as an infrared cut filter or a low-pass filter may be disposed between the second lens G2 and the imaging surface (imaging surface) in addition to the cover glass CG.
[0014]
The first lens G1 has an aspherical surface in which the front (object side) surface has a positive power that becomes stronger toward the periphery, and a rear (image side) surface has a concave shape in the vicinity of the paraxial and has a peripheral surface. It is configured with an aspherical shape that becomes convex as it goes to the portion.
[0015]
The second lens G2 has an aspherical surface in which the negative power increases as the object side surface goes to the periphery, and the image side surface has a concave shape near the paraxial axis and a convex shape as it goes to the peripheral portion. It has an aspherical shape. The object-side surface of the second lens G2 has a convex shape near the paraxial.
[0016]
This single focus lens is configured to satisfy the following conditional expressions (1) and (2). Here, in the formulas (1) and (2), f1 indicates the focal length (paraxial focal length) of the first lens G1, and f2 indicates the focal length (paraxial focal length) of the second lens G2. ), R1 indicates the paraxial radius of curvature of the front (object side) surface of the first lens G1, and R2 indicates the paraxial radius of curvature of the rear (image side) surface of the first lens G1. ing.
f1 / f2 <1.0 (1)
3.0 <R2 / R1 (2)
[0017]
In the present embodiment, the "near the optical axis", represented in the aspherical surface equation below (A), by only a portion of the coefficient K (portion excluding the polynomial part of the coefficients A _i) shape Refers to the part.
[0018]
Next, the operation and effect of the single focus lens configured as described above will be described.
[0019]
In this single focus lens, both the first lens G1 and the second lens G2 have a unique aspherical shape on both sides, so that the number of lenses is as small as two compared to a case where only a spherical lens is used. While miniaturizing the overall length, field curvature, distortion, and coma are corrected in a well-balanced manner, and good optical performance is obtained. For example, since the image-side surface of the second lens G2 has an aspheric surface shape that is concave near paraxial and convex toward the periphery, field curvature can be easily corrected.
[0020]
Conditional expression (1) relates to the ratio of the focal lengths f1 and f2 between the first lens G1 and the second lens G2. By satisfying the numerical range of the conditional expression (1), the positive power of the first lens G1 becomes larger than that of the second lens G2, and it becomes easy to shorten the back focus. On the other hand, if the value is out of the numerical range of the conditional expression (1), the back focus becomes long, and it is difficult to make the overall length compact.
[0021]
Conditional expression (2) relates to the shape of the first lens G1. By satisfying the numerical range of the conditional expression (2), it becomes easy to correct the field curvature. On the other hand, when the value is out of the numerical range of the conditional expression (2), it becomes difficult to correct the curvature of field.
[0022]
As described above, according to the single focus lens according to the present embodiment, it is possible to achieve a short back focus and a compact overall length while maintaining good optical performance while maintaining a small number of lenses of two. . This makes it possible to provide an imaging lens particularly suitable for mounting on a small-sized imaging device.
[0023]
【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. FIGS. 3A and 3B show specific lens data (Example 1) corresponding to the configuration of the single focus lens shown in FIG. FIGS. 4A and 4B show specific lens data (Example 2) corresponding to the configuration of the single focus lens shown in FIG. 3 (A) and 4 (A) show basic data portions of the lens data of the embodiment, and FIGS. 3 (B) and 4 (B) show the lens data of the embodiment. Shows the data portion related to the aspherical shape.
[0024]
In the column of the surface number Si in the lens data shown in each drawing, the surface of the component closest to the object side including the stop St is set to 0th for the single focus lens of each embodiment, and sequentially increases toward the image side. The number of the i-th surface (i = 0 to 6), which has been given a reference numeral, 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 3) lens element from the object side with respect to the d-line (587.6 nm), including the cover glass CG, respectively. . The values of the radii of curvature R5 and R6 on both surfaces of the cover glass CG are 0 (zero), which indicates that the cover glass CG is flat.
[0025]
FIGS. 3A and 4A simultaneously show the values of the focal length f (mm), F number (FNO.), And angle of view 2ω (ω: half angle of view) of the entire system as various data. Show.
[0026]
In each lens data of FIGS. 3A and 4A, a symbol “*” attached to the left of the surface number indicates that the lens surface has an aspherical shape. In each of the embodiments, both surfaces (first and second surfaces) of the first lens G1 and both surfaces (third and fourth surfaces) of the second lens G2 are aspherical. 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.
[0027]
In the numerical values of each aspherical surface data in FIG. 3B and FIG. 4B, the symbol “E” indicates that the next numerical value is an “exponent” with a base of 10. Indicates that the value represented by the base exponential function is multiplied by the value before "E". For example, “1.0E-02” indicates “1.0 × 10 ⁻² ”.
[0028]
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.
[0029]
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 : an i-th order (i = 3 to 10) aspherical surface coefficient
FIG. 5 shows the values corresponding to the above-mentioned conditional expressions (1) and (2) collectively for each embodiment. As shown in FIG. 5, the values of the examples fall within the numerical ranges of the conditional expressions (1) and (2).
[0031]
FIGS. 6A to 6C 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, FIGS. 7A to 7C show various aberrations of the second embodiment.
[0032]
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 back focus is short, and the overall length is made compact.
[0033]
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.
[0034]
【The invention's effect】
As described above, according to the single focus lens of the present invention, in order from the object side, the diaphragm, the first lens having both surfaces having an aspherical shape and having positive power, and the positive power having both surfaces having an aspherical shape. A first lens having an aspheric surface in which the positive power becomes stronger toward the periphery, and an image-side surface having a concave shape near the paraxial and having a concave surface near the periphery. The surface on the object side of the second lens has an aspheric shape in which the negative power becomes stronger toward the periphery, and the surface on the image side has a concave shape near the paraxial. The first lens has a predetermined conditional expression (1) with respect to the focal length of each lens, and has a predetermined conditional expression (2) with respect to the paraxial radius of curvature of both surfaces of the first lens. So that the spherical lens In compared with the case where the structure, yet small number of lenses as two, while achieving compactness of the entire length, it is possible to obtain a good optical performance. In particular, by satisfying conditional expression (1), the positive power of the first lens becomes larger than that of the second lens, and the back focus can be shortened.
[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 lens data of a single focus lens according to Example 1.
FIG. 4 is a diagram showing lens data of a single focus lens according to Example 2.
FIG. 5 is a diagram illustrating values of conditional expressions satisfied by a single focus lens according to each example.
FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the single focus lens according to Example 1.
FIG. 7 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 (1)

In order from the object side, an aperture, a first lens having both surfaces aspherical and having positive power, and a second lens having both surfaces aspherical and having positive power,
The first lens has an aspherical surface in which the object-side surface has a positive power that becomes stronger toward the periphery, and an image-side surface has a concave shape near the paraxial axis and a convex shape toward the periphery. It is composed of
The second lens has an aspherical surface on the object side, where negative power becomes stronger toward the periphery, and an image-side surface becomes concave near the paraxial and convex toward the periphery. It is composed of an aspherical shape,
A single focus lens characterized by satisfying the following conditional expressions (1) and (2).
f1 / f2 <1.0 (1)
3.0 <R2 / R1 (2)
However,
f1: paraxial focal length of the first lens f2: paraxial focal length of the second lens R1: paraxial radius of curvature R2 of the front (object side) surface of the first lens R2: rear side (image side) of the first lens Radius of paraxial curvature of surface

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