Single focus lens

【0001】

[Technical field to which the invention belongs]

The present invention relates to a single focus lens particularly suitable for mounting on a small imaging device.

【0002】

[Conventional technology]

Conventionally, an image pickup device using an image pickup device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) has been known. In such an image pickup device, a subject image is formed on an image pickup device, and the image is electronically read to take a picture. Since such an image pickup device has been miniaturized in recent years, the device as a whole has been made extremely miniaturized. In particular, module cameras and digital still cameras (hereinafter, simply referred to as digital cameras) for image input in mobile phones have been remarkably miniaturized in recent years.

【0003】

Conventionally, as an imaging lens that can be used in a small imaging apparatus, for example, those described in the following publications are available. The lens described in Patent Document 1 is a three-element lens. The lens described in Patent Document 2 is a two-element lens.

【0004】

[Patent Document 1]

Japanese Unexamined Patent Publication No. 10-301022 [Patent Document 2]

Japanese Unexamined Patent Publication No. 2000-258684 [0005]

[Problems to be Solved by the Invention]

By the way, in recent years, the performance of an image sensor has been improved, and a compact and high-pixel image sensor has been developed without increasing the size of the entire element. With such an increase in the number of pixels, the image pickup lens used therein is also required to have higher optical performance than before.

【0006】

In order to obtain optical performance that can withstand an increase in the number of pixels of the image sensor, it is conceivable to increase the number of lenses. However, like the three-lens configuration described in Patent Document 1, the imaging performance can be improved by increasing the number of lenses, but it is disadvantageous in terms of overall length, and when mounted on an imaging device, it is small. -There is a risk of loss of portability. Therefore, there is a demand for the development of an imaging lens having a compact overall length in order to be compact and portable while satisfying the required optical performance. On the other hand, when mounted on an image pickup device, in general, an optical member such as an infrared cut filter or a cover glass is often arranged between the final surface of the lens and the image pickup element. Therefore, a certain amount of back focus is required in order to arrange these optical members while trying to reduce the overall length. Although the lens described in Patent Document 2 has been simplified by a two-lens configuration, it is desired to develop a lens having higher performance than this.

【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 focus lens capable of ensuring a necessary back focus while reducing the overall length.

【0008】

[Means for solving problems]

The single focus lens according to the present invention includes an aperture, a first lens having an aspherical shape on both sides and having positive power, and a second lens having an aspherical shape on both sides and having positive power in order from the object side. The first lens has an aspherical shape in which the positive power becomes stronger as the surface on the object side goes to the periphery, and the surface on the image side has a concave shape in the vicinity of the near axis and a convex shape as it goes to the peripheral portion. The second lens has an aspherical shape, and the surface on the object side has an aspherical shape in which the negative power becomes stronger as it goes to the periphery, and the surface on the image side has a concave shape in the vicinity of the near axis and is in the peripheral portion. It is configured to have an aspherical shape that becomes convex as it goes, and is configured to satisfy the following conditional equations (1) and (2).

【0009】

f1 / f2> 1.0 ...... (1) 1.0 <R2 / R1 <10.0 ...... (2) However, f1 indicates the paraxial focal length of the first lens, and f2 is the first. 2 Indicates the paraxial focal length of the lens, R1 indicates the paraxial radius of curvature of the surface on the front side (object side) of the first lens, and R2 indicates the paraxial radius of the surface on the rear side (image side) of the first lens. Shows the radius of curvature.

【0010】

In the single focus lens according to the present invention, by making all the surfaces aspherical, it is easy to obtain good optical performance while reducing the overall length while reducing the number of lenses as small as two. Further, by satisfying the equation (1), the positive power of the second lens becomes larger than that of the first lens, and it becomes easier to secure the back focus. In particular, satisfying Eq. (2) makes it easier to correct curvature of field.

【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 an embodiment of the present invention. This configuration example corresponds to the lens configuration of the first numerical embodiment (FIG. 3) described later. Further, FIG. 2 shows another configuration example of the single focus lens according to the present embodiment. The configuration example of FIG. 2 corresponds to the lens configuration of the second numerical embodiment (FIG. 4) described later. In addition, in FIGS. 1 and 2, the reference numeral Ri is assigned so that the surface of the component on the most object side including the aperture St is the 0th position and the reference numeral is gradually increased toward the image side (imaging side). The radius of curvature of the i-th (i = 0 to 6) plane is shown. The symbol Di indicates the distance between the i-th plane and the i + 1-th plane on the optical axis Z1. Since the basic configuration is the same for each configuration example, the configuration of the single focus lens shown in FIG. 1 will be used as a basis for the following description.

【0013】

This single focus lens is used, for example, as an image pickup lens for a small image pickup device using an image pickup element such as a CCD or CMOS. This single focus lens has an aperture St, a first lens G1 having an aspherical shape on both sides and having positive power, and a positive power having an aspherical shape on both sides in order from the object side along the optical axis Z1. It is equipped with a second lens G2. An image sensor such as a CCD (not shown) is arranged on the image pickup surface (imaging 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, an optical member such as an infrared cut filter or a low-pass filter may be arranged between the second lens G2 and the imaging surface (imaging surface).

【0014】

The first lens G1 has an aspherical shape in which the front side (object side) surface becomes stronger as the positive power goes to the periphery, and the rear side (image side) surface has a concave shape in the vicinity of the paraxial axis. It is composed of an aspherical shape that becomes convex as it goes to the part.

【0015】

The second lens G2 has an aspherical shape in which the negative power becomes stronger as the surface on the object side goes to the periphery, and the surface on the image side becomes concave in the vicinity of the paraxial axis and convex as it goes to the periphery. It is composed of an aspherical shape. The surface of the second lens G2 on the object side has a convex shape in the vicinity of the paraxial axis.

【0016】

This single focus lens is configured to satisfy the following conditional equations (1) and (2). However, in equations (1) and (2), f1 indicates the focal length of the first lens G1 (focal length on the paraxial axis), and f2 is the focal length of the second lens G2 (focal length on the paraxial axis). ), R1 indicates the paraxial radius of curvature of the surface on the front side (object side) of the first lens G1, and R2 indicates the paraxial curvature radius of the surface on the rear side (image side) of the first lens G1. ing. f1 / f2> 1.0 ...... (1) 1.0 <R2 / R1 <10.0 ...... (2) [0017]

In the present embodiment, "near coaxial axis" means only the portion related to the coefficient K in the aspherical surface equation (A) described later (coefficient A)._iIt refers to the shape part represented by (the part excluding the polynomial part related to).

【0018】

Next, the action 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 the number of lenses is as small as two compared to the case where only a spherical lens is used. Good optical performance can be obtained by correcting image plane curvature, distortion, and coma in a well-balanced manner while reducing the overall length. For example, by forming the surface of the second lens G2 on the image side into an aspherical shape that is concave in the vicinity of the paraxial and becomes convex toward the peripheral portion, curvature of field can be easily corrected.

【0020】

The 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 second lens G2 becomes larger than that of the first lens G1, and it becomes easier to secure the back focus. On the other hand, if it is out of the numerical range of the conditional expression (1), it becomes difficult to secure a predetermined back focus required for arranging an optical member such as a cover glass CG.

【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 curvature of field. On the other hand, if it 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, although the number of lenses is as small as two, the overall length is made compact while maintaining good optical performance, and the necessary back focus is achieved. Can be secured. This makes it possible to provide an image pickup lens particularly suitable for mounting on a small image pickup device.

【0023】

[Example]

Next, a specific numerical example 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 collectively described. FIGS. 3 (A) and 3 (B) show specific lens data (Example 1) corresponding to the configuration of the single focus lens shown in FIG. Further, 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 the basic data portion of the lens data of the embodiment, and FIGS. 3 (B) and 4 (B) show the lens data of the embodiment. The data part related to the aspherical shape is shown.

【0024】

In the column of the surface number Si in the lens data shown in each figure, for the single focus lens of each embodiment, the surface of the component on the most object side including the aperture St is set to 0th, and the number gradually increases toward the image side. The number of the i-th (i = 0 to 6) planes coded in this way is shown. In the column of radius of curvature Ri, the value of the radius of curvature of the i-th surface from the object side is shown corresponding to the reference numeral Ri attached in FIGS. 1 and 2. The column of the surface spacing 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. 1 and 2. The unit of the value of radius of curvature Ri and surface spacing Di is millimeter (mm). In the columns of Ndj and νdj, the values of the refractive index and Abbe number for the d-line (587.6 nm) of the j-th (j = 1 to 3) lens element from the object side are shown, including the cover glass CG, respectively. The values of the radii of curvature R5 and R6 on both sides of the cover glass CG are 0 (zero), which indicates that the cover glass is flat.

【0025】

In FIGS. 3 (A) and 4 (A), 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 are simultaneously shown as various data. Shown.

【0026】

In each lens data of FIGS. 3 (A) and 4 (A), the symbol * attached to the left side of the surface number indicates that the lens surface has an aspherical shape. In each embodiment, both sides of the first lens G1 (first and second surfaces) and both sides of the second lens G2 (third and fourth surfaces) have an aspherical shape. The basic lens data shows the numerical values of the radius of curvature near the optical axis (near the paraxial axis) as the radius of curvature of these aspherical surfaces.

【0027】

In the numerical values of each aspherical data in Fig. 3 (B) and Fig. 4 (B), the symbol "E" indicates that the numerical value following it is a "power exponential" with 10 as the base, and the 10 is used as the value. Indicates that the number represented by the base exponential function is multiplied by the number before "E". For example, in the case of "1.0E-02", "1.0 x 10"^-2".

【0028】

For each aspherical data, each coefficient A in the aspherical shape equation represented by the following equation (A)_iWrite the value of, K. More specifically, Z is the length (mm) of a perpendicular line drawn from a point on the aspherical lens at a height h from the optical axis to the tangent plane (plane perpendicular to the optical axis) of the aspherical apex. Shown.

【0029】

Z = C h²/ {1+ (1-K C² H²)^1/2} + A₃ H³+ A₄ H⁴+ A₅ H⁵+ A₆ H⁶+ A₇ H⁷+ A₈ H⁸+ A₉ H⁹+ A₁₀ H¹⁰ (A) However, Z: Depth of aspherical surface (mm) h: Distance from optical axis to lens surface (height) (mm) K: Decentering rate C: Paraxial curvature = 1 / R (R: Paraxial radius of curvature) A_i: Aspherical coefficient of order i (i = 3 ~ 10) [0030]

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 value of each embodiment is within the numerical range of the conditional expressions (1) and (2).

【0031】

6 (A) to 6 (C) show spherical aberration, astigmatism, and distortion (distortion) in the single focus lens of Example 1. Each aberration diagram shows aberrations with the d-line as the reference wavelength, but the spherical aberration diagram also shows the aberrations for the g-line (wavelength 435.8 nm) and 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 Example 2 are shown in FIGS. 7 (A) to 7 (C).

【0032】

As can be seen from the above lens data and each aberration diagram, aberration correction is satisfactorily performed for each embodiment. In addition, the overall length has been made compact and the necessary back focus has been secured.

【0033】

The present invention is not limited to the above-described embodiment and each embodiment, and various modifications can be performed. For example, the values of the radius of curvature, the interplanar spacing, and the refractive index of each lens component are not limited to the values shown in the above numerical examples, and may take other values.

【0034】

[Effect of the invention]

As described above, according to the single focus lens of the present invention, the aperture, the first lens having aspherical shape on both sides and having positive power, and the positive power having aspherical shape on both sides are applied in order from the object side. It is equipped with a second lens that has a second lens, and the surface of the first lens on the object side has an aspherical shape in which the positive power becomes stronger toward the periphery, and the surface on the image side has a concave shape in the vicinity of the near axis and is in the peripheral portion. The aspherical shape becomes convex as it goes, the surface of the second lens on the object side is made into an aspherical shape in which the negative power becomes stronger as it goes to the periphery, and the surface on the image side is concave in the vicinity of the near axis. It has an aspherical shape that becomes convex as it goes to the part, satisfies the predetermined conditional expression (1) with respect to the focal distance of each lens, and satisfies the predetermined conditional expression (2) with respect to the near-axis curvature radii on both sides of the first lens. Therefore, it is possible to obtain good optical performance while reducing the overall length while reducing the number of lenses, which is as small as two, as compared with the case where only a spherical lens is used. In particular, by satisfying the conditional expression (1), the positive power of the second lens becomes larger than that of the first lens, and the necessary back focus can be secured.

[Simple explanation of drawings]

FIG. 1 shows a configuration example of a single focus lens according to an embodiment of the present invention, and is a cross-sectional view of a lens corresponding to the first embodiment.

FIG. 2 shows another configuration example of a single focus lens according to an embodiment of the present invention, and is a cross-sectional view of a lens corresponding to the second embodiment.

FIG. 3 is a diagram showing lens data of a single focus lens according to the first embodiment.

FIG. 4 is a diagram showing lens data of a single focus lens according to a second embodiment.

FIG. 5 is a diagram showing values of conditional expressions satisfied by the single focus lens according to each embodiment.

FIG. 6 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the single focus lens according to the first embodiment.

FIG. 7 is an aberration diagram showing spherical aberration, astigmatism, and distortion of the single focus lens according to the second embodiment.

[Explanation of symbols]

CG ... cover glass, Gj ... jth lens from the object side, Ri ... radius of curvature of the ith lens surface from the object side, Di ... ith and ith from the object side +1 Surface spacing from the first lens surface, Z1 ... optical axis.