JP2002296496A - Monofocal lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a monofocal lens for which the entire length can be shortened in simple constitution. SOLUTION: A first lens G1 and a second lens G2 are disposed successively from an object side. For the first lens G1, at least one surface is aspheric and it has positive refractive power. For the second lens G2, at least one surface is aspheric and it is constituted of a lens having the positive refractive power for which a convex surface is turned to the object side. It is desirable that the first lens G1 is constituted of optical resin. The first lens G1 can be constituted not only in a meniscus shape turning a concave surface to the object side but also in the meniscus shape turning the convex surface to the object side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographic lens used for, for example, an electronic camera (digital camera).
In particular, the present invention relates to a single focus lens suitable as a photographing lens of a small portable electronic camera with high portability.

[0002]

2. Description of the Related Art In recent years, CCDs have been replaced by silver halide films.
2. Description of the Related Art Electronic cameras that take an image of a subject using a solid-state imaging device such as a (charge-coupled device) are widely used. Electronic cameras generally include a still camera for taking a still image and a video camera for taking a moving image.

[0003]

By the way, recently,
With the downsizing of the CCD, the downsizing of the photographing lens is also required. In general, in order to reduce the size of the photographing lens, it is better that the number of lenses is small. As a photographing lens having a small number of lenses, for example,
There is a triplet type having a configuration of three groups. However, the triplet type is insufficient in terms of simplicity of configuration, compactness, and cost. In order to further solve these points, it is conceivable to make the lens configuration into two groups and two lenses.

Japanese Patent Publication No. 7-050246 discloses a technique relating to a photographic lens having two groups and two elements. The photographing lens described in this publication, in order from the object side,
A concave meniscus lens having a negative power and a biconvex lens having a positive power are provided, and one or more lens surfaces are formed as aspheric surfaces. However, the photographic lens described in this publication is still insufficient, in particular, in terms of compactness, particularly as a photographic lens for a recent small electronic camera. For example, in an electronic camera equipped with a CCD having a diameter of about 2 mm, a first lens is used as a photographing lens.
In some cases, it is required that the distance from the lens surface to the image forming surface is about 3 to 4 mm or less. The photographic lens described in the above-mentioned publication does not have performance enough to meet such a demand.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a single focus lens having a simple configuration and an extremely short overall length.

[0006]

A single focus lens according to the present invention comprises, in order from the object side, a first lens having at least one surface formed of an aspheric surface and having a positive refractive power, and at least one surface having a positive refractive power. A second lens having an aspherical surface and having a positive refractive power with a convex surface facing the object side is provided, and is configured to satisfy the following conditional expression (1). 50 <νd2 (1) where νd2 is the Abbe number of the second lens with respect to the d-line.

In the single focus lens according to the present invention, at least one surface of each lens is formed of an aspherical surface and has a positive refractive power. This makes it easy to correct various aberrations and to make the overall length extremely short. When the conditional expression (1) is satisfied, chromatic aberration can be easily corrected.

In the single focus lens according to the present invention, the first lens is formed of an optical resin and has a meniscus shape having a convex surface facing the object side, and satisfies the following conditional expression (2). It is desirable to be constituted. | F / f ₁ | <0.65 (2) where, f: overall focal length, f ₁ : focal length of the first lens

Since the first lens is made of an optical resin, it is easy to reduce the cost.
Aspherical surface processing is facilitated, which is advantageous in terms of manufacturability. Further, by satisfying conditional expression (2), the refractive power of the first lens is restricted, and deterioration of the optical performance due to an environmental change due to the use of the optical resin is prevented.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows a configuration of a single focus lens 1 according to an embodiment of the present invention. In FIG. 1,
The side indicated by the symbol Z _OBJ is the object side, that is, the side on which, for example, a subject for photography exists. In FIG. 1, the side indicated by reference numeral Z _IMG is the image forming side (image surface side), that is, the side on which the object image on the object side is formed. In FIG. 1, the reference symbol Ri indicates the radius of curvature of the i-th lens surface that sequentially increases toward the image surface, with the lens surface closest to the object being the first. The symbol Di indicates a surface interval on the optical axis between the i-th lens surface and the (i + 1) -th lens surface.

The single focus lens 1 according to the present embodiment is used, for example, as a photographing lens of a small electronic camera, and an image pickup surface of an image pickup device such as a CCD is arranged on an image forming surface 3 thereof. Is done.

As shown in FIG. 1, the single focus lens 1 includes a first lens G in order from the object side along the optical axis Z _0.
1 and a second lens G2. The first lens G1 has at least one surface formed of an aspheric surface and has a positive refractive power. Second lens G
Reference numeral 2 denotes a lens having at least one surface formed of an aspheric surface and having a positive refractive power with a convex surface facing the object side.

The single focus lens 1 has the following conditional expression (1):
It is configured to satisfy. Where νd2
Represents the Abbe number of the second lens G2 with respect to the d-line. 50 <νd2 (1)

In the single focus lens 1, it is desirable that the first lens G1 is made of an optical resin (a so-called plastic lens) and is configured to satisfy the following conditional expression (2). However, where, f is, indicates the focal length of the entire lens system, f ₁ denotes a focal length of the first lens G1. | F / f ₁ | <0.65 (2)

In FIG. 1, the first lens G1 has a meniscus shape with the concave surface facing the object side.
May be configured in a meniscus shape with the convex surface facing the object side, like the single focus lens 1A shown in FIG.

Next, the single focus lens 1 having the above configuration
The optical action and effect brought about by this will be described.

In the single focus lens 1, the first lens G1
And at least one surface of each of the second lenses G2 is made up of an aspheric surface and has a positive refractive power, so that various aberrations are corrected while having a simple structure of two elements in two groups. And the overall length can be made extremely short. For this reason, for example, a CC having a diameter of about 2 mm
When used in an electronic camera equipped with D, the distance from the first lens surface to the imaging surface can be easily reduced to about 3 to 4 mm or less.

Further, in the single focus lens 1, the first lens G1 is made of an optical resin, so that the price can be reduced despite the use of an aspheric surface. Further, since aspherical surface processing is easily performed, it is advantageous in terms of manufacturability.

Conditional expression (1) is for limiting the optical characteristics of the lens material of the second lens G2, and mainly contributes to the correction of chromatic aberration. When a lens material exceeding the range of the conditional expression (1) is used, it becomes particularly difficult to correct chromatic aberration.

Conditional expression (2) limits the refractive power of the first lens G1. In general, in a lens using an optical resin, optical performance such as a focal length is easily changed by changes in environment such as temperature and humidity. This change in optical performance becomes more remarkable as the refractive power of the lens becomes stronger. Therefore,
When an optical resin is used for the first lens G1, by restricting the refractive power of the lens so as to satisfy the range of the conditional expression (2), it is possible to suppress the deterioration of the optical performance due to a change in the environment. it can. When the value exceeds the range of conditional expression (2),
The refractive power of the first lens G1 becomes strong, and is strongly affected by temperature, humidity, and the like, and there is a possibility that the optical performance is greatly deteriorated due to environmental changes.

As described above, according to the single focus lens 1 of the present embodiment, by satisfying the above-described configuration and each conditional expression as needed, it is possible to achieve a simple configuration and extremely short overall length. And an optical system with extremely high cost performance can be realized. Thus, by combining the single focus lens 1 with a small CCD, a small portable electronic camera with high portability can be realized.

[Examples] Next, specific numerical examples of the single focus lens 1 of the present embodiment will be described.

<Example 1> First, a first numerical example of the single focus lens 1 according to the present embodiment will be described. The cross-sectional structure of the single focus lens of the present embodiment (hereinafter, referred to as a single focus lens 1-1) is the same as the configuration shown in FIG.

FIGS. 2A and 2B show this single focus lens 1-.
2 shows specific numerical data relating to the configuration of FIG. More specifically, FIG. 2A shows basic lens data, and FIG. 2B shows data on an aspheric surface. 2 (A) and 2 (B), in the column of surface number Si, the lens surface of the present single focal length lens 1-1, which has the lens surface closest to the object side as the first lens surface and increases gradually toward the image surface side. Indicates a number. In the column of the radius of curvature Ri, the value of the radius of curvature of the i-th lens surface from the object side is shown in association with the symbol Ri shown in FIG. Also in the field of the surface interval Di, corresponding to the code Di shown in FIG.
The distance on the optical axis between the first lens surface Si and the (i + 1) th lens surface Si + 1 is shown. Radius of curvature Ri and surface spacing Di
The unit of the value of is millimeter (mm). Ndj and ν
The column of dj shows the values of the refractive index and Abbe number of the j-th lens (J = 1, 2) from the object side with respect to the _d -line (wavelength λ _d = 587.6 nm), respectively. FIG. 2A shows the focal length f (= 1.00 m) of the entire system in the single focus lens 1-1.
m), F number (F _NO . = 2.8) and angle of view 2ω
(= 68.1 °) is also shown.

In FIG. 2A, the symbol "*" attached to the left of the surface number indicates that the lens surface is aspheric. In this embodiment, all the lens surfaces are aspherical. FIG. 2A shows the values of the radii of curvature near the optical axis as the radii of curvature of these aspheric surfaces.

FIG. 2B shows five aspherical coefficients K, A ₄ , A ₆ , A ₈ , and A ₄ representing an aspherical shape as aspherical surface data.
It indicates the value of the A _10. These aspherical coefficients are coefficients in an aspherical polynomial represented by the following equation (A). Aspheric polynomial of equation (A) are those in the direction perpendicular to the optical axis Z ₀ taking the h-axis representing the aspherical shape. Aspherical surface, a curved surface obtained by rotating a curve represented by the formula (A) around the optical axis Z _0. In aspheric polynomial of equation (A), h is the distance from the optical axis Z ₀ to the lens surface (height) (unit: mm) represents the. Z (h) represents the sag amount of the lens surface at the height h. More specifically, Z (h) is, from a point on the aspherical surface at a position of height h from the optical axis Z _0, the length of the perpendicular drawn to the tangent plane of the vertex of the aspherical surface (perpendicular to the optical axis plane) Indicates the size (unit: mm). C is
The reciprocal of the paraxial radius of curvature R of the lens surface near the optical axis (1
/ R). K is the eccentricity (or conical constant)
A ₄ , A ₆ , A ₈ , and A ₁₀ are the fourth order and the sixth order, respectively.
Represents the next, eighth, and tenth order aspheric coefficients. Note that FIG.
In the numerical value representing the aspheric coefficient shown in (B), the symbol "E" indicates that the following numerical value is a "power exponent" with a base of 10, and an exponential function with the base of 10 Indicates that the number represented is multiplied by the number before "E". For example, “1.0E-02” becomes “1.0 × 1
0 ^-2 ".

Z (h) = Ch ² / {1+ (1−K · C ² · h ² ) ^1/2 } + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ + A ₁₀ h ¹⁰ (A)

FIG. 7 collectively shows values corresponding to the conditions of the above-mentioned conditional expressions (1) and (2) in each of the present embodiment and the second embodiment described later. As can be seen from this figure, in the present embodiment, the values corresponding to each condition are all within the range of each conditional expression.

FIGS. 3A to 3C show the single focus lens 1-.
Various aberrations for 1 are shown. More specifically, FIG.
3A shows spherical aberration, FIG. 3B shows astigmatism, and FIG. 3C shows distortion (distortion). In these figures, each aberration is based on the d-line. In each aberration diagram, symbols g, d,
The curves with C show aberrations for g-line, d-line, and C-line, respectively. The wavelengths of the g-line, d-line, and C-line are 435.8 nm, 587.6 nm, and 656.3 nm, respectively. FIG. 3 (B)
In, the solid line indicates the aberration with respect to the sagittal image plane S,
A broken line indicates an aberration with respect to a tangential (meridional) image plane T. In each aberration diagram, F _N0 .
Indicates an F number, and ω indicates a half angle of view.

<Example 2> Next, a second numerical example of the single focus lens 1 according to the present embodiment will be described.

FIGS. 5A and 5B show specific numerical data relating to the configuration of the single focus lens of the present embodiment (hereinafter, referred to as a single focus lens 1-2). More specifically,
FIG. 5A shows basic lens data.
(B) shows data on the aspherical surface. FIG.
The meanings of the numerical values shown in FIGS.
(FIGS. 2A and 2B). Also in this embodiment, as in the first embodiment, all lens surfaces are aspherical.

The sectional structure of the single focus lens 1-2 is the same as that of the single focus lens 1A shown in FIG. That is, in the present embodiment, the first lens G1 has a meniscus shape with the convex surface facing the object side.

The single focus lens 1-2 also satisfies all the conditions of the conditional expressions (1) and (2) as shown in FIG.

FIGS. 6A to 6D show the single focus lens 1-.
2 shows various aberrations. More specifically, FIG. 6A shows spherical aberration, FIG. 6B shows astigmatism, and FIG.
(C) shows distortion. In FIG. 6B, the solid line indicates the aberration with respect to the sagittal image plane S, and the broken line indicates the aberration with respect to the tangential (meridional) image plane T. The meanings of the respective symbols attached to these aberration diagrams are the same as in the case of the first embodiment (FIGS. 3A to 3C).

As described above, in the single focus lens of each embodiment, various aberrations are satisfactorily corrected while satisfying the above-mentioned conditional expressions, and the overall length is extremely short.

The present invention is not limited to the above-described embodiment, and various modifications can be made. For example, the radius of curvature R, surface interval D, refractive index N, and Abbe number ν of each lens component
Is not limited to the value shown in each of the numerical examples, and may take other values.

The present invention can be applied not only to an electronic camera but also to a photographing lens of a so-called lens shutter camera using a silver halide film.

[0039]

As described above, claim 1 or claim 2
According to the described single focus lens, a first lens having a positive refractive power, at least one surface of which is formed by an aspheric surface;
A second lens having at least one surface formed of an aspherical surface and having a positive refractive power with a convex surface facing the object side, and a second lens having a positive refractive power disposed in order from the object side; Since the configuration is such that the predetermined conditional expression (1) for limiting the characteristics is satisfied, it is possible to correct various aberrations and extremely shorten the overall length while having a simple configuration of two elements in two groups. it can.

In particular, according to the single focus lens of the second aspect, in the single focus lens of the first aspect, the first lens is formed of an optical resin and has a meniscus shape having a convex surface facing the object side. Since the lens system is configured and satisfies the predetermined conditional expression (2) for limiting the refractive power of the first lens, the refractive power of the first lens is limited and the optical resin is used. It is possible to prevent optical performance from deteriorating due to environmental changes.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration example of a single focus lens according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a first numerical example (Example 1) of the single focus lens according to one embodiment of the present invention;
(A) shows basic lens data, and (B) shows data on an aspheric surface.

FIG. 3 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion in the single focus lens of Example 1;

FIG. 4 is a cross-sectional view showing another configuration example of the single focus lens according to one embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a second numerical example (Example 2) of the single focus lens according to one embodiment of the present invention;
(A) shows basic lens data, and (B) shows data on an aspheric surface.

FIG. 6 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion in the single focus lens of Example 1;

FIG. 7 is an explanatory diagram showing condition values satisfied by a single focus lens of each embodiment.

[Explanation of symbols]

G1 ... first lens, G2 ... second lens, Z ₀ ... optical axis, 1, 1A ... monofocal lens, 3 ... imaging plane.

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Claims (2)

[Claims] 1. A first lens having a positive refractive power, at least one surface of which is formed of an aspheric surface, and at least one surface of which is formed of an aspheric surface, with a convex surface facing the object side, in order from the object side. A second lens having a positive refracting power, and further configured to satisfy the following conditional expression (1): 50 <νd2 (1) where νd2 is the Abbe number of the second lens with respect to the d-line. 2. The first lens is made of an optical resin and has a meniscus shape with a convex surface facing the object side, and is further configured to satisfy the following conditional expression (2). The single focus lens according to claim 1, wherein: | F / f ₁ | <0.65 (2) where: f: overall focal length f ₁ : focal length of the first lens