JP2002267928A - Image pickup lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image pickup lens which is small-sized and light and has excellent telecentricity, where astigmatism or the like is easily corrected and which is easily worked and assembled. SOLUTION: This image pickup lens is constituted of a 1st lens being a negative meniscus whose convex surface faces to an object side and a 2nd lens being a positive meniscus whose convex surface faces to an image surface through a diaphragm in order from the object side, and the two lenses (1st lens and 2nd lens) are constituted so that at least each one surface may be formed to be aspherical. Then, the pickup lens satisfies following conditions: -2.6<Pair /P0 <-1.3, 0.7<ΣD/f0 <1.2, 0.2<|R4 /f0 |<0.5, -1.0<f0 /f1 <-0.1 and 0.1<D2 /f0 <0.3, provided that Pair means the refractive power of an air lens, P0 means the refractive power of a lens entire system, ΣD means axial length from the surface of the 1st leans on the object side to the surface of the 2nd lens on an image side, f0 means the focal distance of the lens entire system, R4 means the radius of curvature (object side of L2 of an aspherical apex on a 4th surface, f1 means the focal distance of the 1st leans and D2 means the axial space between the 1st lens and the 2nd lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a taking lens, and more particularly, in order from the object side, a negative first lens (L _1), via a throttle, a positive second lens (L ₂₎ The present invention relates to a two-lens imaging lens configured, and particularly to an imaging lens suitable for a digital camera or a videophone using a light receiving element such as a small CCD or CMOS as a photosensitive member.

[0002]

2. Description of the Related Art In recent years, as an imaging lens for a digital camera or a videophone using a light-receiving element such as a small CCD or CMOS as a photosensitive member, particularly, it is small in size, low in cost, has a wide angle of view, and departs from telecentricity. There is an increasing demand for a high-quality imaging lens with little image quality. To meet such demands, a two-element lens that can achieve higher image quality than a single element lens is generally suitable. As a two-lens lens system, one having a configuration as shown in FIGS. 11 to 13, which are conceptual diagrams, is disclosed. Hereinafter, each conventional technique will be described.

The lens type (A) shown in FIG. 11 is an example composed of two lenses composed of a positive lens, an aperture, and a negative lens in order from the object side, and is disclosed in Japanese Patent No. 3055790, for example. . Such a lens type (A)
A so-called telephoto type (short f _B), it is excellent in compactness, because a large emission angle compared to the angle of incidence, since the telecentricity is poor, although there is no particular problem in the photoreceptor of the silver halide film, CCD An imaging lens using a light receiving element such as a CMOS has difficulty in widening the angle.

The lens type (B) shown in FIG. 12 is an example composed of two lenses consisting of a positive lens, an aperture, and a positive lens in order from the object side, and is disclosed in, for example, Japanese Patent Application Laid-Open No. 6-230278. Have been. The lens type (B) is a form in which the positive power is shared by using two positive lenses, the edge thickness of the convex lens can be easily secured, the workability is good, and both sides of the stop are substantially symmetric. Because of power, it is also advantageous for correcting various aberrations including distortion. However, since the first lens is a positive lens, the light flux after passing through the first lens is emitted at a steep angle when the angle of view is widened, and the emission angle of the light flux is relaxed by the positive refractive power of the second lens. Therefore, the fluctuation of the aperture interval error strongly affects the change of the aberration. Therefore, when widening the angle, measures such as darkening the FNo and increasing the depth are required.

A lens type (C) shown in FIG.
Is an example composed of two lenses composed of a negative lens, an aperture, and a positive lens in order from the object side. For example, Japanese Patent No. 28485
The technology is disclosed in Japanese Patent Publication No. 23 and the like. The lens type (C) is a so-called retrofocus type (reverse telephoto type), which is a well-known configuration suitable for widening the angle of view, but often has a long back focus and a large lens shape. This tendency is particularly strong when the distance between the negative first lens and the aperture is long. That is, even when the thickness of the first lens increases, the outer diameter of the first lens becomes large.

[0006]

Referring to FIGS. 11 to 13, two types of imaging lenses are used as a lens type (A) (positive and negative lens type) and a lens type (B) (positive and positive lens type). As described above, it can be seen that the lens type (C) (negative and positive lens types) is more suitable for widening the angle of view and has better telecentricity than the lens type). However, lens type (C)
As a further problem of (negative and positive lens types), in order to obtain an imaging lens having higher image quality, smaller size, and suitable for a digital camera using a light receiving element such as a CCD, a videophone, etc. In a two-lens lens system composed of a negative lens and a positive lens, it is desired that an astigmatism can be more easily corrected, a coma flare is small, and an image with good contrast can be obtained. Further, in a low-cost and lightweight two-lens lens system, there is a demand for an imaging lens which has a small outer diameter of the lens system and is suitable for downsizing the imaging apparatus body including the lens barrel.

The present invention has been made in view of the above problems, and is compact, has good telecentricity, is easy to correct lens aberrations, is a bright two-piece lens, has a large imaging angle of view, and further has a process for each lens. It is an object of the present invention to provide an imaging lens having good characteristics.

Further, the present invention has been made in view of the above-mentioned problems, and has a small outer diameter difference between the first lens and the second lens, is compact, has good telecentricity, easily corrects lens aberration, and has two bright lenses. Another object of the present invention is to provide a lens having a large imaging angle of view and a good workability of each lens.

[0009]

The imaging lens according to the first aspect of the present invention includes a first meniscus lens (L ₁ ) having a convex surface facing the object side, which is arranged in order from the object side, and a diaphragm. Positive meniscus second lens (L
₂ ) and satisfying the following conditions. −2.6 <P _air / P ₀ <−1.3 (1) where P _{0 is} the refractive power of the entire lens system P _air : the image side surface of R ₁ (R ₂ ) and the object side surface of L ₂ (R ₃ ) The refractive power is the reciprocal of the focal length, and P _air can be obtained by the following equation (2). _{P air = (1-N 1} ) / R 2 + (N 3 -1) / R 3 + {((N 1 -1) · (N 3 -1)) / (R 2 · R 3)} × D ₂ (2) The symbols used in equation (2) are as described below. N ₁ : refractive index of first lens (L ₁ ) for d-line N ₃ : refractive index of second lens (L ₂ ) for d-line R ₂ : aspherical vertex on the image side of first lens (L ₁ ) Curvature radius R ₃ : radius of curvature of the aspherical vertex on the object side of the second lens (L ₂ ) D ₂ : air gap on the axis between the first lens and the second lens

According to the first aspect of the present invention, a negative meniscus first lens (L ₁ ) having a convex surface facing the object side and a convex surface facing the image side via a stop are arranged in order from the object side. And a second lens (L ₂ ) having a positive meniscus. Since both the first lens (L ₁ ) and the second lens (L ₂ ) have a meniscus shape, the refracting action of each lens surface is such that the incident first face R ₁ has a positive action, and
The surface R ₂ and the third surface R ₃ have a negative effect, and the fourth surface R
₄ has a positive effect, that is, if we focus only on the function of the surface, it can be seen as a (positive) (negative, negative), (positive) configuration,
The power distribution is relatively symmetric, and the coma can be made symmetric. The use of an aspherical surface can also reduce the occurrence of coma flare. Further, since two surfaces having a negative action are provided in the vicinity of the diaphragm,
An air lens having a so-called diverging effect is provided by a positive incident surface R.
Because near ₁ and positive middle exit surface R _4, it is suitable for the correction of Pebbaru sum.

Further, the imaging lens according to claim 1 is
The conditional expression (1) is satisfied. Conditional expression (1)
Is a condition for correcting the Pebbal sum by making the power of the air lens in the center of the two-lens lens system appropriate and for making the image plane flat. If the value is below the upper limit, the negative power of the air lens can be strongly maintained, so that the so-called Pebbal sum, which represents the curvature of the image plane near the paraxial axis, decreases
In particular, the sagittal image plane can be favorably corrected. On the other hand, if the lower limit is exceeded, the radius of the second and third surfaces sandwiching the aperture is weakened, so that the second and third surfaces are separated off the axis, and a sufficient air gap for inserting the aperture can be secured. . Further, the possibility that the second surface and the third surface intersect is reduced, and the FNo can be made bright. Further, it is not necessary to increase R of the exit surface (fourth surface) of the second lens, and the edge thickness of the second lens can be secured.
It is more preferable that the imaging lens according to claim 1 satisfies −2.4 <P _air / P ₀ <−1.5 (1 ′).

An imaging lens according to a second aspect is characterized by further satisfying the following condition. 0.7 <ΣD / f ₀ <1.2 (3) where ΣD: axial length from the object side surface of the first lens to the image side surface of the second lens f ₀ : focal length of the entire lens system .

Conditional expression (3) is a condition for achieving a reduction in size and weight of the lens system in combination with conditional expression (1). In the imaging lens according to the second aspect, by falling below the upper limit value in conditional expression (3), the length of the lens system can be reduced, and the outer diameter of the lens can be reduced synergistically. In particular, even if the negative power of the air lens described in the conditional expression (1) is increased, the shape of the second lens can be suppressed to a small size, so that the size can be reduced. On the other hand, when it exceeds the lower limit,
Any of the center thickness of the concave lens, the aperture interval, the edge thickness of the positive lens, and the like can be sufficiently secured. Here, if the aperture distance is insufficient, the R of the second surface and the third surface cannot be increased, and the image surface is insufficiently corrected, so that it is difficult to form a wide-angle lens. It is more preferable that the imaging lens according to claim 2 satisfies 0.8 <ΣD / f ₀ <1.1 (3 ′).

An imaging lens according to a third aspect is characterized by satisfying the following condition. 0.2 <│R ₄ / f ₀ │ <0.5 (4) where R ₄ : radius of curvature of the image-side surface of the second lens f ₀ : focal length of the whole lens system

Condition (4) is a condition for correcting the spherical aberration and improving the telecentricity by optimizing the image-side surface of the second lens. In the imaging lens according to the third aspect, when the value goes below the upper limit in conditional expression (4), the light condensing power on the surface close to the image surface becomes strong, and the telecentricity becomes good. On the other hand, when the value exceeds the lower limit, the radius of curvature of the fourth surface can be increased. Since the occurrence of negative spherical aberration is reduced and the rim of the lens is sufficiently secured, workability is improved.

The imaging lens according to claim 4, wherein the first lens is
The lens and the second lens have at least one aspheric surface. The aspherical surface of the lens is necessary to correct spherical aberration and coma flare that cannot be corrected by a spherical lens alone when pursuing brightness and a wide angle of view with a configuration of two lenses with a small number of lenses. Things. By making the lens surface aspherical, the degree of freedom in changing the lens surface shape increases, and advanced aberration correction becomes possible.
When this is constituted only by a spherical lens, spherical aberration is insufficiently corrected, an image with insufficient contrast is obtained, and a bright lens cannot be obtained.

The imaging lens according to claim 5, wherein the first lens is
The lens and the second lens are made of plastic. In addition, by making the first lens and the second lens made of plastic, it is possible to reduce cost and weight. Further, by using plastic as the lens material, it is easy to make the lens surface aspherical, and it is possible to provide an imaging lens with good various aberrations.

According to a sixth aspect of the present invention, there is provided an imaging lens having a negative meniscus first lens having a convex surface facing the object side and a positive meniscus having a positive meniscus facing the image surface via an aperture. And a second lens, which satisfies the following condition. −1.0 <f ₀ / f ₁ <−0.1 (5) 0.1 <D ₂ / f ₀ <0.3 (6) where f ₀ : focal length of the whole lens system f ₁ : first lens D ₂ : Air gap on the axis of the first lens and the second lens.

In the imaging lens according to the sixth aspect, in order from the object side, a negative meniscus first lens having a convex surface facing the object side, and a positive meniscus second lens having a convex surface facing the image surface via the diaphragm. It consists of: All lens surfaces have concave surfaces facing the stop.Especially for wide angle of view, the refraction effect of on-axis light beam and off-axis light beam is not extremely different, making the structure suitable for wide angle of view. ing. Further, the imaging lens according to claim 6 satisfies the conditional expressions (5) and (6). Conditional expression (5) shows conditions for suppressing an increase in lens size by making the focal length of the first lens appropriate, and for achieving flattening of an image plane at a wide angle. More specifically, when the value falls below the upper limit, the power of the negative first lens is weakened, and at the same time, the power of the positive second lens is also weakened, and the back focus is shortened. That is, although the telecentricity is slightly deteriorated, the outer diameter of the second lens becomes smaller, and the occurrence of aberrations is also weaker in the negative power at the front side of the aperture, and the positive power is weaker at the rear side of the aperture. The occurrence of coma flare is reduced. On the other hand, when the value exceeds the lower limit, the Pebbal sum decreases and the image plane can be corrected well. It is more preferable that the imaging lens according to claim 6 satisfies −0.8 <f ₀ / f ₁ <−0.2 (5 ′).

Further, the conditional expression (6), together with the conditional expression (5), shows conditions for suppressing an increase in the lens size and flattening the image plane. 7. The imaging lens according to claim 6, wherein when the value goes below the upper limit in conditional expression (5), one or both of the first lens and the second lens approach the stop, so that the outer diameter of the lens decreases, and the axial luminous flux is reduced. Since the difference between the position where the light flux and the off-axis light beam hit the lens surface is reduced, aberration correction can be satisfactorily performed. On the other hand, if the lower limit is exceeded, both the first lens and the second lens are separated from the stop, so the lens outer diameters are slightly increased for both lenses. It becomes possible. It is more preferable that the imaging lens according to claim 6 satisfies 0.15 <D ₂ / f ₀ <0.25 (6 ′).

An imaging lens according to a seventh aspect is characterized by satisfying the following condition. 0.2 <│R ₄ / f ₀ │ <0.5 (7) where R _{4 is} the radius of curvature of the image-side surface of the second lens f ₀ is the focal length of the entire lens system.

Conditional expression (7) is a condition for correcting the spherical aberration and improving the telecentricity by optimizing the image-side surface of the second lens. In the imaging lens according to the seventh aspect, when the value goes below the upper limit in conditional expression (7), the condensing power on the surface close to the image surface becomes strong, and the telecentricity becomes good. On the other hand, when the value exceeds the lower limit, the radius of curvature of the fourth surface can be increased. At the same time, the occurrence of negative spherical aberration is reduced, and at the same time, the rim of the lens can be sufficiently secured, so that workability is improved.

The imaging lens according to claim 8, wherein the first lens is
The lens and the second lens have at least one aspheric surface. The aspherical surface of the lens is necessary to correct spherical aberration and coma flare that cannot be corrected by a spherical lens alone when pursuing brightness and a wide angle of view with a configuration of two lenses with a small number of lenses. Things. By making the lens surface aspherical, the degree of freedom in changing the lens surface shape increases, and advanced aberration correction becomes possible.
When this is constituted only by a spherical lens, spherical aberration is insufficiently corrected, an image with insufficient contrast is obtained, and a bright lens cannot be obtained.

According to a ninth aspect of the present invention, there is provided the imaging lens according to the first aspect.
The lens and the second lens are made of plastic. In addition, by making the first lens and the second lens made of plastic, it is possible to reduce cost and weight. Further, by using plastic as the lens material, it is easy to make the lens surface aspherical, and it is possible to provide an imaging lens with good various aberrations.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the imaging lens of the present invention will be described below, but the present invention is not limited to these embodiments. Here, the symbols used in each embodiment are as follows. ω: imaging half angle of view (2ω: full imaging angle of view) Ri: radius of curvature of aspherical vertex on a surface (i = 1 to 4) Di: distance between surfaces on axis (i = 1 to 4) Nd: lens Refractive index at d-line of material νd: Abbe number of lens material F: F number (FNo) f ₀ : focal length of whole system f _B : back focus

In each embodiment, the shape of the aspherical surface is such that the X axis is taken in the optical axis direction and the height is made y in the direction perpendicular to the optical axis, and K, A
₄ , A ₆ , A ₈ , A ₁₀ , and A ₁₂ are represented by [Equation 1] as aspherical coefficients.

(Equation 1)

(Embodiment 1) Embodiment 1 is an imaging lens according to the first aspect of the present invention. FIG. 1 is a cross-sectional view of the imaging lens of Example 1, and Tables 1 and 2 show lens data of the imaging lens of Example 1.

[Table 1]

[Table 2] FIG. 2 illustrates spherical aberration, astigmatism, and the like of the imaging lens of Example 1.
And each aberration diagram of distortion

(Embodiment 2) Embodiment 2 is an imaging lens according to the first and second aspects of the present invention. FIG. 3 is a cross-sectional view of the imaging lens of Example 2, and Tables 3 and 4 show lens data of the imaging lens of Example 2.

[Table 3]

[Table 4] FIG. 4 shows the spherical aberration and astigmatism of the imaging lens of Example 2.
FIG. 4 is a diagram showing aberrations of the lens and distortion.

(Embodiment 3) Embodiment 3 is an imaging lens according to the first and second aspects of the present invention. FIG. 5 is a sectional view of the imaging lens of the third embodiment. Tables 5 and 6 show lens data of the imaging lens of the third embodiment.

[Table 5]

[Table 6] FIG. 6 shows spherical aberration, astigmatism, and the like of the imaging lens of Example 3.
FIG. 4 is a diagram showing aberrations of the lens and distortion.

(Embodiment 4) Embodiment 4 is an imaging lens according to the first and second aspects of the present invention. FIG. 7 is a cross-sectional view of the imaging lens of Example 4, and Tables 7 and 8 show lens data of the imaging lens of Example 4.

[Table 7]

[Table 8] FIG. 8 is a diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens of the fourth embodiment.

(Embodiment 5) Embodiment 5 is an imaging lens according to the first and second aspects of the present invention. FIG. 9 is a sectional view of the imaging lens of the fifth embodiment. Tables 9 and 10 show lens data of the imaging lens of the fifth embodiment.

[Table 9]

[Table 10] FIG. 10 is a diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens of Example 5.

Table 11 shows specific values in each embodiment corresponding to the values defined in the claims.

[Table 11]

[0033]

According to the present invention, it is possible to provide an imaging lens having a small size, good telecentricity, easy correction of astigmatism, and good lens workability.

Further, according to the present invention, the outer diameter is small,
An imaging lens with good telecentricity and easy correction of astigmatism can be provided.

[Brief description of the drawings]

FIG. 1 is a lens cross-sectional view of an imaging lens of Example 1.

FIG. 2 illustrates spherical aberration, astigmatism, and the like of the imaging lens of Example 1.
FIG. 4 is an aberration diagram of distortion and distortion.

FIG. 3 is a lens cross-sectional view of an imaging lens of Example 2.

FIG. 4 shows the spherical aberration, astigmatism, and the like of the imaging lens of Example 2.
FIG. 4 is an aberration diagram of distortion and distortion.

FIG. 5 is a lens cross-sectional view of the imaging lens of Example 3;

FIG. 6 is an aberration diagram illustrating spherical aberration, astigmatism, and distortion of the imaging lens of Example 3;

FIG. 7 is a lens sectional view of the imaging lens of Example 4;

FIG. 8 illustrates spherical aberration, astigmatism, and the like of the imaging lens of Example 4;
FIG. 4 is an aberration diagram of distortion and distortion.

FIG. 9 is a lens cross-sectional view of the imaging lens of Example 5;

FIG. 10 is an aberration diagram of a spherical aberration, an astigmatism, and a distortion of the imaging lens of Example 5;

FIG. 11 is a schematic configuration diagram of a lens in a conventional example.

FIG. 12 is a schematic configuration diagram of a lens in a conventional example.

FIG. 13 is a schematic configuration diagram of a lens in a conventional example.

[Explanation of symbols]

R _1: the curvature of the aspheric surface vertex on the first surface radius (L ₁ on the object side) R _2: a second curvature of the aspherical surface vertex in the surface radius (L ₁ on the object side) R _3: aspheric vertex in the third plane radius of curvature (L ₂ on the object side) R _4: fourth curvature of the aspherical vertices in surface radius (L ₂ on the object side) L _1: the first lens (negative) L _2: the second lens (positive) D ₁ : On-axis thickness of first lens D ₂ : on-axis distance between first and second lenses D ₃ : on-axis thickness of second lens Nd: refractive index at d-line νd: Abbe number ω: half image Angle (2ω: full imaging angle of view) f ₀ : focal length of whole lens system f _B : back focus of whole lens system F: F number P _air : refractive power of air lens P ₀ : refractive power of whole lens system ΣD: axial length from the object-side surface of the first lens to the image-side surface of the second lens f ₁ : focal length of the first lens f ₂ : distance of the second lens Focal length

Claims (9)

[Claims] 1. A negative meniscus first lens (L ₁ ) having a convex surface facing the object side, and a positive meniscus second lens (L) having a convex surface facing the side surface via a diaphragm arranged in order from the object side. ₂ ) An imaging lens, characterized by satisfying the following conditions. −2.6 <P _air / P ₀ <−1.3 (1) where P _{0 is} the refractive power of the entire lens system P _air : the image side surface of R ₁ (R ₂ ) and the object side surface of L ₂ (R ₃ ) The refractive power is the reciprocal of the focal length, and P _air can be obtained by the following equation (2). P _air = (1−N ₁ ) / R ₂ + (N ₃ −1) / R ₃ + {((N ₁ −1) · (N ₃ −1)) / (R ₂ · R ₃ )} × D ₂ (2) The symbols used in equation (2) are as described below. N ₁ : refractive index of first lens (L ₁ ) for d-line N ₃ : refractive index of second lens (L ₂ ) for d-line R ₂ : aspherical vertex on the image side of first lens (L ₁ ) Curvature radius R ₃ : radius of curvature of the aspherical vertex on the object side of the second lens (L ₂ ) D ₂ : air gap on the axis between the first lens and the second lens 2. The imaging lens according to claim 1, wherein the following condition is satisfied. 0.7 <ΣD / f ₀ <1.2 (3) where ΣD: axial length from the object side surface of the first lens to the image side surface of the second lens f ₀ : focal length of the entire lens system . 3. The imaging lens according to claim 1, wherein the following condition is satisfied. 0.2 <│R ₄ / f ₀ │ <0.5 (4) where R ₄ : radius of curvature of the image-side surface of the second lens f ₀ : focal length of the whole lens system 4. The imaging lens according to claim 1, wherein the first lens and the second lens have at least one aspheric surface. 5. The imaging lens according to claim 1, wherein the first lens and the second lens are made of plastic. 6. A negative meniscus first lens having a convex surface facing the object side, arranged in order from the object side, and a diaphragm,
An imaging lens comprising: a positive meniscus second lens having a convex surface facing the image surface, and satisfying the following conditions. −1.0 <f ₀ / f ₁ <−0.1 (5) 0.1 <D ₂ / f ₀ <0.3 (6) where f ₀ : focal length of the whole lens system f ₁ : first lens focal length D _2: air space on the axis of the first lens and the second lens 7. The imaging lens according to claim 6, wherein the following condition is satisfied. 0.2 <│R ₄ / f ₀ │ <0.5 (7) where R ₄ : radius of curvature of the image-side surface of the second lens f ₀ : focal length of the whole lens system 8. The imaging lens according to claim 6, wherein the first lens and the second lens have at least one aspheric surface. 9. The imaging lens according to claim 6, wherein said first lens and said second lens are made of plastic.