JP4283554B2 - Single focus lens - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a single focus lens for photographing, and more particularly to a single focus lens suitable for mounting on a digital still camera (hereinafter simply referred to as a digital camera).
[0002]
[Prior art]
In recent years, with the spread of personal computers to ordinary homes and the like, digital cameras capable of inputting image information such as photographed landscapes and human images to personal computers are rapidly spreading.
[0003]
A digital camera converts an optical image into an electrical signal using an image pickup device such as a CCD (charge coupled device) and records it as digital image data. Conventionally, as such a photographic lens for a digital camera, there are those described in, for example, Japanese Patent Application Laid-Open Nos. H5-157962 and H11-125767.
[0004]
[Problems to be solved by the invention]
Incidentally, digital cameras are generally required to have high image quality, high resolution, and a compact configuration. Therefore, the photographic lens mounted thereon is also required to be compact and capable of withstanding high image quality. In a digital camera, in order to obtain a high-resolution image, the overall size of the CCD may be increased to increase the number of pixels. However, when the overall size of the CCD is increased, the lens system is enlarged accordingly, and if the conventional lens configuration is adopted, the compactness is lost. In addition, in a digital camera, it is desirable that a light beam is incident in a state of being nearly perpendicular to the imaging surface because of the characteristics of the CCD. Therefore, it is desirable that telecentricity is ensured in the taking lens mounted on the digital camera.
[0005]
The lenses described in Japanese Patent Laid-Open Nos. H5-157962 and H11-125767 are particularly easy to mount, are compact and cost-effective when mounted on a digital camera that requires high resolution. It does not have sufficient performance in terms of points.
[0006]
The present invention has been made in view of such problems, and an object of the present invention is to provide a single focus lens capable of obtaining good performance particularly suitable for a compact and high-resolution digital camera while having a simple configuration. There is.
[0010]
The single focus lens of the present invention has, in order from the object side, at least one aspherical surface, a negative first lens having a concave surface facing the image side, a stop, and a positive first lens having a convex surface facing the object side. Two lenses, a negative third lens having a concave surface facing the object side, and a lens surface on the image side are convex toward the image side in the paraxial region, and the positive refractive power becomes weaker toward the periphery. A fourth lens having an aspherical shape, and satisfying the following conditional expressions (1) to (3). It is desirable that the first lens has at least an aspheric lens surface on the image side.
[0011]
│R1 / R2│> 2 …… (1)
D5 / Ymax> 0.05 (2)
0.1> D2 / f (3)
Here, R1 represents the paraxial radius of curvature of the lens surface on the object side of the first lens, R2 represents the paraxial radius of curvature of the lens surface on the image side of the first lens, and D2 represents the image of the first lens. D5 indicates the distance from the lens surface on the side to the stop, D5 indicates the distance between the lens surface on the image side of the second lens and the lens surface on the object side of the third lens, Ymax indicates the maximum image height, and f Indicates the total focal length.
[0012]
In the single focus lens according to the present invention , by adopting the above-described configuration, various aberrations are corrected well while having a simple configuration, and good performance suitable for a compact, high-resolution digital camera can be obtained. In particular, since the lens surface on the image side of the fourth lens is aspheric, curvature of field and distortion are corrected well. Moreover, various aberrations are satisfactorily corrected by satisfying conditional expressions (1) and (2).
[0013]
In particular, the curvature of field is favorably corrected by making the image side lens surface of the first lens an aspherical surface. In particular, when the conditional expression (3) is satisfied, the aperture position can be easily set at a position away from the image plane, the exit pupil distance can be kept long, and telecentricity can be easily ensured.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0015]
<First Embodiment>
FIG. 1 shows a configuration example of a single focus lens according to an embodiment of the present invention. FIG. 2 shows another configuration example of the single focus lens according to the present embodiment. The configuration examples shown in FIGS. 1 and 2 correspond to the lens configurations of first and second reference examples 1-1 and 1-2 (FIGS. 3 and 4) described later, respectively. In FIG. 1 and FIG. 2, the symbol Ri is the i-th (i = 1) that sequentially increases toward the image side (imaging side) with the surface of the most object-side component including the stop St as the first. The curvature radius of the surface of the component of -11) is shown. A symbol Di indicates a surface interval on the optical axis between the i-th surface and the i + 1-th surface. The first embodiment is an embodiment that does not belong to the invention described in the claims of the present application.
[0016]
The single focus lenses 1A and 1B (hereinafter, 1A and 1B are collectively referred to as 1) according to the present embodiment shown in FIGS. 1 and 2 are particularly suitable for mounting on a compact digital camera. is there.
[0017]
The single focus lens 1 includes a first lens G1 to a fourth lens G4 in order from the object side along the optical axis Z1. An aperture stop St is provided between the first lens G1 and the second lens group G2. An imaging element such as a CCD (not shown) is disposed on the imaging plane (imaging plane) of the single focus lens 1. A cover glass CG for protecting the imaging surface is disposed near the imaging surface of the CCD.
[0018]
The first lens G1 is a negative lens with a concave surface facing the image side. For example, the first lens G1 has a negative meniscus shape. On the other hand, the second lens G2 is a positive lens having a convex surface facing the object side. The second lens G2 has a biconvex shape, for example.
[0019]
The third lens G3 is a negative lens with a concave surface facing the object side. For example, the third lens G3 has a biconcave shape. On the other hand, the fourth lens G4 has an aspherical shape in which the lens surface on the image side is convex toward the image side in the paraxial region and the positive refractive power becomes weaker toward the periphery. For example, the fourth lens G4 has a biconvex shape in the paraxial region. The fourth lens G4 is preferably aspheric on both sides for aberration correction.
[0020]
The single focus lens 1 is configured to satisfy the following conditional expressions (1) and (2). In conditional expressions (1) and (2), R1 represents the paraxial curvature radius of the object-side lens surface of the first lens G1, and R2 represents the paraxial curvature radius of the image-side lens surface of the first lens G1. D5 represents the distance between the image-side lens surface of the second lens G2 and the object-side lens surface of the third lens G3, and Ymax represents the maximum image height.
[0021]
│R1 / R2│> 2 …… (1)
D5 / Ymax> 0.05 (2)
[0022]
The single focus lens 1 has only one lens (fourth lens G4) using an aspheric surface as compared with the single focus lens 2 according to the second embodiment described later. The position of the diaphragm St is set to a position relatively close to the second lens G2. Since there is only one lens using an aspheric surface, it is suitable for implementation at low cost. In addition, the single focus lens 1 has a configuration suitable for reducing the F number.
[0023]
Next, optical actions and effects brought about by the single focus lens 1 configured as described above will be described.
[0024]
In the single focus lens 1, the first lens G1 to the fourth lens G4 are configured as described above, so that the configuration length (collapse length), that is, the most object-side refractive surface ( The distance from the first lens G1 on the object side) to the most image side refractive surface (the image side surface of the fourth lens G4) can be shortened. Thereby, the optical system at the time of retracting can be made compact. For example, when mounted on a digital camera, the optical device including the single focus lens 1 can be made compact.
[0025]
In the single focus lens 1, in particular, since the lens surface on the image side of the fourth lens G <b> 4 has an aspheric shape, curvature of field and distortion are favorably corrected.
[0026]
Moreover, various aberrations are satisfactorily corrected by satisfying conditional expressions (1) and (2). In particular, when the conditional expression (1) is satisfied, it is possible to satisfactorily correct field curvature and distortion. In particular, astigmatism and distortion can be favorably corrected by satisfying conditional expression (2).
[0027]
As described above, according to the single focus lens 1 according to the present embodiment, various aberrations are favorably corrected while having a simple configuration of four lenses, and is particularly suitable for a compact and high-resolution digital camera. Performance can be obtained.
[0028]
<Second Embodiment>
Next, a second embodiment of the present invention will be described.
[0029]
FIG. 8 shows a configuration example of a single focus lens according to an embodiment of the present invention. FIG. 9 shows another configuration example of the single focus lens according to the present embodiment. The configuration examples shown in FIGS. 8 and 9 correspond to the lens configurations of first and second examples 2-1 and 2-2 (FIGS. 10 and 11) described later, respectively. In FIG. 8 and FIG. 9, parts corresponding to the components of the single focus lens 1 (FIGS. 1 and 2) of the first embodiment are given the same reference numerals.
[0030]
The single focus lenses 2A and 2B (hereinafter, 2A and 2B are collectively referred to as 2) according to the present embodiment shown in FIGS. 8 and 9 are the same as the single focus lens 1 of the first embodiment. Similarly, it is particularly suitable for mounting on a compact digital camera.
[0031]
The single focus lens 1 includes a first lens G1 to a fourth lens G4 in order from the object side along the optical axis Z1 as in the single focus lens 1 of the first embodiment. An aperture stop St is provided between the first lens G1 and the second lens group G2. An imaging element such as a CCD (not shown) is disposed on the imaging plane (imaging plane) of the single focus lens 1. A cover glass CG for protecting the imaging surface is disposed near the imaging surface of the CCD.
[0032]
In the single focus lens 2 of the present embodiment, the shape of the single focus lens 2 is different from that of the single focus lens 1 of the first embodiment, particularly in the portion of the first lens G1. That is, in the single focus lens 2, the first lens G1 is configured to have at least one aspheric surface. The first lens G1 preferably has at least an aspheric lens surface on the image side. Further, the first lens G1 has a relatively thin lens thickness as compared with the single focus lens 1 of the first embodiment.
[0033]
Further, in the single focus lens 2, the position of the stop St is set relatively close to the first lens G 1 as compared with the single focus lens 1 according to the first embodiment. The single focus lens 2 is configured to satisfy the following conditional expressions (1) to (3). Conditional expressions (1) and (2) are the same as the conditions satisfied by the single focus lens 1 according to the first embodiment. In conditional expression (3), D2 indicates the distance from the image-side lens surface of the first lens G1 to the stop St, and f indicates the overall focal length.
[0034]
│R1 / R2│> 2 …… (1)
D5 / Ymax> 0.05 (2)
0.1> D2 / f (3)
[0035]
In the single focus lens 2, in particular, the curvature of field is favorably corrected by making the image side lens surface of the first lens G 1 an aspherical surface. In particular, when the conditional expression (3) is satisfied, the aperture position St can be easily set at a position away from the image plane, and the exit pupil distance can be kept long. This makes it easy to maintain telecentricity in accordance with the characteristics of the image sensor, particularly when mounted on a digital camera. That is. The light beam can be easily incident in a state of being nearly perpendicular to the imaging surface, and performance suitable particularly for a digital camera can be obtained.
[0036]
Further, in this single focus lens 2, more aspheric surfaces are employed, and the lens thickness of the first lens G <b> 1 is relatively thin, so that it is compared with the single focus lens 1 according to the first embodiment. Thus, the configuration length can be further shortened. Thereby, the optical system at the time of retracting can be further compacted. For example, when the optical system is mounted on a digital camera, the optical device including the single focus lens 2 can be extremely compact.
[0037]
As described above, according to the single focus lens 2 according to the present embodiment, various aberrations are favorably corrected while having a simple configuration, and optimal performance can be obtained particularly for a compact and high-resolution digital camera. .
[0038]
【Example】
Next, specific numerical examples of the single focus lens according to the present embodiment will be described.
[0039]
[ Reference Examples 1-1 and 1-2]
First, as a reference example, a numerical example of the first embodiment will be described. Hereinafter, the first and second numerical examples ( Reference Examples 1-1 and 1-2) will be described together. 3A and 3B show specific lens data corresponding to the configuration of the single focus lens 1A shown in FIG. 1, and FIGS. 4A and 4B show the single lens data shown in FIG. Specific lens data corresponding to the configuration of the focus lens 1B is shown. 3A and 4A show basic data portions of the lens data of Reference Examples 1-1 and 1-2, respectively, and FIGS. 3B and 4B respectively show the basic data portions. The data regarding an aspherical shape are shown.
[0040]
In the column of the surface number Si in the lens data shown in FIGS. 3A and 4A, the surface of the most object side component including the stop St is set to 1 for the single focus lens of each reference example. The number of the surface of the component which increases sequentially toward the image side is shown as the 2nd. The column of the radius of curvature Ri indicates the value of the radius of curvature of the surface of the i-th component from the object side, corresponding to the symbol Ri shown in FIGS. Also in the column of the surface interval Di, the interval on the optical axis between the i-th surface Si and the i + 1-th surface Si + 1 from the object side is shown in correspondence with the reference numerals shown in FIGS. The unit of the value of the curvature radius Ri and the surface interval Di is millimeter (mm). In the columns Ndj and νdj, the refractive index and Abbe number for the d-line (wavelength 587.6 nm) of the j-th lens element (j = 1 to 5) from the object side, including the cover glass CG, are shown. Show. FIGS. 3A and 4A also show the focal length f (mm), the angle of view 2ω (°), and the F number (FNO.) Of the entire system.
[0041]
In FIGS. 3A and 4A, the symbol “*” attached to the left side of the surface number indicates that the lens surface is an aspherical surface. In Reference Examples 1-1 and 1-2, both surfaces (eighth surface and ninth surface) of the fourth lens G4 are aspherical. 3A and 4A show the numerical values of the curvature radii near the optical axis (paraxial region) as the curvature radii of these aspheric surfaces.
[0042]
The aspheric data shown in FIGS. 3B and 4B are constants or coefficients in an aspheric polynomial expressed by the following equation (A). The aspherical polynomial in equation (A) represents the shape of the aspherical surface with the Y axis taken in the direction orthogonal to the optical axis Z1. The aspherical surface is a curved surface obtained by rotating the curve represented by the formula (A) around the optical axis Z1. Y corresponds to the distance (height) from the optical axis Z1 to the lens surface. The value of Z is the length of a perpendicular drawn from a point on the aspheric surface at a height Y from the optical axis Z1 to the tangential plane (plane perpendicular to the optical axis) of the aspheric surface, that is, the aspherical surface. Indicates depth. C is the reciprocal (1 / R) of the paraxial radius of curvature R of the lens surface in the vicinity of the optical axis. KA represents an eccentricity (or conic constant), and A ₄ , A ₆ , A ₈ , and A ₁₀ represent 4th, 6th, 8th, and 10th order aspherical coefficients, respectively. In the numerical value representing the aspheric coefficient, the symbol “E” indicates that the subsequent numerical value is a “power exponent” with 10 as the base, and a numerical value represented by an exponential function with 10 as the base. Indicates that the numerical value before “E” is multiplied. For example, “1.0E-02” indicates “1.0 × 10 ⁻² ”.
[0043]
Z = [CY ² / {1+ (1−KA · C ² · Y ² ) ^1/2 }]
+ A ₄ Y ⁴ + A ₆ Y ⁶ + A ₈ Y ⁸ + A ₁₀ Y ¹⁰ ...... (A)
However,
Z: Depth of aspheric surface (mm)
Y: Height (mm)
KA: eccentricity C = 1 / R: paraxial curvature (R: paraxial radius of curvature)
[0044]
FIG. 5 shows values corresponding to the above-described conditional expressions (1) and (2), that is, values of the expressions “| R1 / R2 |” and “D5 / Ymax” for each reference example. As shown in FIG. 5, the value of each reference example is within the range of conditional expressions (1) and (2).
[0045]
FIGS. 6A to 6D and FIGS. 7A to 7D show various aberrations of the single focus lens of each reference example. Specifically, FIGS. 6 (A) and 7 (A) show the spherical aberration for each reference example, and FIGS. 6 (B) and 7 (B) show the astigmatism for each reference example. Show. 6 (C) and 7 (C) show distortion (distortion aberration) for each reference example, and FIGS. 6 (D) and 7 (D) show chromatic aberration of magnification for each reference example. ing. In FIGS. 6B and 7B showing astigmatism, the solid line shows the aberration with respect to the sagittal image surface, and the broken line shows the aberration with respect to the tangential (meridional) image surface. The spherical aberration indicates values for d-line (wavelength 587.6 nm), F-line (wavelength 486.1 nm), and C-line (wavelength 656.3 nm). In each aberration diagram, those not particularly specifying the wavelength indicate aberrations with respect to the d-line. In each aberration diagram, FNO. Indicates the F number, and ω indicates the half angle of view.
[0046]
As can be seen from the above lens data and aberration diagrams, in each reference example of the first embodiment, various aberrations are corrected satisfactorily, and optimal performance is obtained particularly for a compact and high-resolution digital camera. .
[0047]
[Examples 2-1 and 2-2]
Next, an example of the second embodiment will be described. Hereinafter, the first and second numerical examples (Examples 2-1 and 2-2) will be described together. 10A and 10B show specific lens data corresponding to the configuration of the single focus lens 2A shown in FIG. 8, and FIGS. 11A and 11B show the single lens data shown in FIG. Specific lens data corresponding to the configuration of the focus lens 2B is shown. FIGS. 10A and 11A respectively show basic data portions of the lens data of Examples 2-1 and 2-2, and FIGS. 10B and 11B respectively show the basic data portions. The data regarding an aspherical shape are shown. The meanings of these lens data are the same as those in Reference Examples 1-1 and 1-2 (FIGS. 3A and 3B, FIGS. 4A and 4B).
[0048]
In Examples 2-1 and 2-2, both surfaces (first surface and second surface) of the first lens G1 and the image side surface (9th surface) of the fourth lens G4 are aspherical. It has become.
[0049]
FIG. 12 shows values corresponding to the conditional expressions (1) to (3) described above, that is, the values of the expressions “| R1 / R2 |”, “D5 / Ymax”, and “D2 / f” for each example. It is shown. As shown in FIG. 12, the value of each Example is in the range of conditional expressions (1) to (3).
[0050]
FIGS. 13A to 13D and FIGS. 14A to 14D show various aberrations of the single focus lens of each example. Specifically, FIGS. 13A and 14A show the spherical aberration for each example, and FIGS. 13B and 14B show the astigmatism for each example. Show. FIGS. 13C and 14C show distortion (distortion aberration) for each example, and FIGS. 13D and 14D show chromatic aberration of magnification for each example. ing. The meanings of the symbols attached to the respective aberration diagrams are the same as those in the respective aberration diagrams for Reference Examples 1-1 and 1-2.
[0051]
As can be seen from the above lens data and aberration diagrams, various aberrations are corrected satisfactorily for each example of the second embodiment, and optimal performance is obtained particularly for a compact, high-resolution digital camera. .
[0052]
The present invention is not limited to the above-described embodiments and examples, and various modifications can be made. For example, the values of the radius of curvature R, the surface interval D, the refractive index N, and the Abbe number ν of each lens component are not limited to the values shown in the above numerical examples, and may take other values.
[0054]
As described above , according to the single focus lens according to claim 1 or 2, in order from the object side, the negative first lens having at least one aspherical surface and having a concave surface facing the image side; An aperture, a positive second lens with a convex surface facing the object side, a negative third lens with a concave surface facing the object side, and a lens surface on the image side convex toward the image side in the paraxial region, and Since it is configured to include an aspherical fourth lens whose positive refractive power decreases toward the periphery, and is configured to satisfy the following conditional expressions (1) to (3), a simple configuration However, good performance suitable for a compact, high-resolution digital camera can be obtained. In particular, since the lens surface on the image side of the fourth lens is aspherical, curvature of field and distortion can be corrected well. Moreover, various aberrations can be favorably corrected by satisfying conditional expressions (1) and (2).
[0055]
In particular, when the lens surface on the image side of the first lens is aspheric, the curvature of field can be corrected well. In particular, by satisfying conditional expression (3), it becomes easy to set the aperture position away from the image plane, and the exit pupil distance can be kept long. This makes it easy to maintain telecentricity in accordance with the characteristics of the image sensor, particularly when mounted on a digital camera.
[Brief description of the drawings]
[1] show a configuration example of a single focus lens according to the first embodiment is a lens sectional view corresponding to Example 1-1.
FIG. 2 is a lens cross-sectional view illustrating another configuration example of the single focus lens according to the first embodiment and corresponding to Reference Example 1-2.
3 is an explanatory diagram showing a basic lens data and (A) and aspherical data (B) for the first reference example 1-1 of the single focus lens according to the first embodiment.
FIG. 4 is an explanatory diagram showing basic lens data (A) and aspherical data (B) for a second reference example 1-2 of the single focus lens according to the first embodiment.
FIG. 5 is an explanatory diagram showing data of condition values satisfied by single focus lenses according to Reference Examples 1-1 and 1-2.
6 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration for the single focus lens of Reference Example 1-1. FIG.
7 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration for the single focus lens of Reference Example 1-2. FIG.
FIG. 8 illustrates a configuration example of a single focus lens according to a second embodiment of the present invention, and is a lens cross-sectional view corresponding to Example 2-1.
FIG. 9 is a lens cross-sectional view illustrating another configuration example of a single focus lens according to the second embodiment of the present invention and corresponding to Example 2-2.
FIG. 10 is an explanatory diagram showing basic lens data (A) and aspherical data (B) for a first example 2-1 of a single focus lens according to a second embodiment of the present invention.
11 is an explanatory diagram showing basic lens data (A) and aspherical data (B) for a second example 2-2 of a single focus lens according to a second embodiment of the present invention. FIG.
FIG. 12 is an explanatory diagram illustrating data of condition values satisfied by the single focus lenses according to Examples 2-1 and 2-2.
13 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration for the single focus lens of Example 2-1. FIG.
FIG. 14 is an aberration diagram showing spherical aberration, astigmatism, distortion, and lateral chromatic aberration for the single focus lens of Example 2-2.
[Explanation of symbols]
CG: cover glass, G1 to G4: first to fourth lenses, St: stop, Z1: optical axis, 1 (1A, 1B), 2 (2A, 2B): single focus lens.

Claims (2)

From the object side,
A negative first lens having at least one aspheric surface and having a concave surface facing the image side;
Aperture,
A positive second lens with a convex surface facing the object side;
A negative third lens with a concave surface facing the object side;
A fourth lens having an aspherical shape in which the lens surface on the image side is convex toward the image side in the paraxial region and the positive refractive power decreases toward the periphery;
And it is comprised so that the following conditional expressions (1)-(3) may be satisfied. The single focus lens characterized by the above-mentioned.
│R1 / R2│> 2 …… (1)
D5 / Ymax> 0.05 (2)
0.1> D2 / f (3)
However,
R1: Paraxial radius of curvature of the lens surface on the object side of the first lens R2: Paraxial radius of curvature of the lens surface on the image side of the first lens D2: Distance from the lens surface on the image side of the first lens to the stop D5: Distance between the image-side lens surface of the second lens and the object-side lens surface of the third lens Ymax: Maximum image height f: Overall focal length Wherein the first lens, at least, a single focus lens according to claim 1, wherein the surface on the image side is an aspherical surface.

Images