JP2006106176A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact imaging lens whose aberration is improved. <P>SOLUTION: The imaging lens is constituted by successively disposing a 1st lens 1, a diaphragm 3, and a 2nd lens 2 from an object side. The 1st lens is a meniscus lens turning its convex surface to the object side and having positive refractive power, the 2nd lens is a meniscus lens turning its convex surface to an image surface side and having positive refractive power. Assuming that the surface on the object side is the 1st surface and the surface on the image surface side is the 2nd surface in the 1st lens and the 2nd lens respectively, the imaging lens satisfies a following condition; [1] 0<TL/f<1.35 (provided that f: the focal distance of an entire lens system, and TL: air length converted distance from the 1st surface of the 1st lens to an image formation surface). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an imaging lens, and more particularly to a small imaging lens that uses a solid-state imaging device such as a CCD image sensor or a CMOS image sensor and is used for image input such as a mobile phone or a PDA.

In recent years, small imaging lenses using a solid-state imaging device such as a CCD image sensor or a CMOS image sensor have been mounted on communication devices such as mobile phones and PDAs, and they are rapidly spreading. For these applications, there is an increasing demand for further miniaturization.
Therefore, as lenses for such applications, various lens systems having two lenses or three lenses having excellent optical performance as compared with lens systems having one lens have been proposed.
In a three-lens lens system, it is possible to obtain extremely high optical performance. However, since the number of parts is large, it is difficult to reduce the size, and high accuracy is required for each component, resulting in an increase in manufacturing cost. There is a problem that it ends up.
In contrast, a two-lens lens system cannot achieve the optical performance of a three-lens lens system, but can obtain higher optical performance than a single-lens lens, and is small and has high resolution. It can be said that this is suitable for the solid-state imaging device.
As such a two-lens lens system, a retrofocus type imaging lens or a lens having a positive refractive power, which is a combination of a lens having a negative refractive power and a lens having a positive refractive power, which is conventionally referred to as a retrofocus type. There has been proposed a telephoto imaging lens in which a lens having a negative refractive power is combined. Although the retrofocus type can be reduced in cost, there is a problem that it is impossible to downsize as much as a single lens system because the back focus distance becomes long. On the other hand, the telephoto type has a back focus distance that is too short and has a problem of telecentricity, so that it is practically impossible to apply it as an imaging lens for a solid-state imaging device.
Therefore, an imaging lens in which two lenses having positive refractive power are combined has been proposed (see Patent Documents 1 to 3). According to these documents, it is described that a small image pickup lens that is small and has good aberration correction can be provided.
However, recently, along with the downsizing of mobile phones and the like, further downsizing of imaging lenses has been demanded, and the lenses described in the above documents have not yet met the requirements, and further improvements are necessary. It is said that.

JP 2003-215446 A JP 2003-232990 A JP 2003-232991 A

  Accordingly, an object of the present invention is to provide an imaging lens that can be further reduced in size and improved in various aberrations compared to the conventional one.

Thus, as means for solving the above-mentioned purpose,
(1) A first lens, an aperture, and a second lens are sequentially arranged from the object side,
The first lens is a meniscus lens having a positive refractive power with a convex surface facing the object side,
The second lens is a meniscus lens having a positive refractive power with a convex surface facing the image surface side,
In each of the first lens and the second lens, an imaging lens satisfying the following conditions when the object-side surface is the first surface and the image-side surface is the second surface;
[1] 0 <TL / f <1.35
However,
f: focal length TL of the entire lens system: air length conversion distance from the first surface of the first lens to the imaging surface,
(2) The imaging lens according to (1), wherein the imaging lens further satisfies the following conditions;
[2] 0.5 <f ₁ / f ₂ <1.0
[3] 0.2 <R ₁ /f<0.4
[4] −0.6 <R ₄ /f<−0.3
However,
f ₁ : focal length of the first lens f ₂ : focal length of the second lens R ₁ : radius of curvature of the first surface of the first lens R ₄ : radius of curvature of the second surface of the second lens,
(3) In the lens according to (1) or (2), at least one surface of the first lens and the second lens is an aspherical surface,
(4) In the lens according to any one of (1) to (3), an imaging lens using a resin as a material constituting the first lens and the second lens, and (5) the above The imaging lens according to any one of (1) to (4), further satisfying the following condition;
[5] 0.05 <D ₂ /f<0.13
However,
D ₂ : distance from the vertex of the second surface of the first lens to the vertex of the first surface of the second lens,
Are provided respectively.

  According to the present invention, it is possible to provide a small, inexpensive and mass-produced imaging lens with improved various aberrations.

An embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows an embodiment of an imaging lens according to the present invention. This imaging lens is made of, for example, glass or a resin material, and has a meniscus shape having a positive refractive power with a convex surface facing the object side. The first lens 1 and the meniscus second lens 2 having positive refractive power with the convex surface facing the image surface side. Here, the lens surfaces of the first lens 1 and the second lens 2 are referred to as a first surface and a second surface in order from the object side.

  A diaphragm 3 is disposed between the second surface of the first lens 1 and the first surface of the second lens 2. A cover glass is disposed on the second surface side of the second lens 2. Various filters 5 such as an IR cut filter and a low-pass filter, and an imaging surface 4 which is a light receiving surface of an image sensor such as a CCD or a CMOS are provided. The various filters 5 can be omitted as necessary.

In the present invention, the following conditions are satisfied.
[1] 0 <TL / f <1.35
Here, f is the focal length of the entire lens system, and TL is the air length converted distance from the first surface of the first lens to the imaging surface.
The expression [1] defines the size of the lens system. When TL / f is equal to or greater than the upper limit, the entire lens becomes long and it is difficult to reduce the size.

In the present invention, it is preferable that the following conditions are satisfied.
[2] 0.5 <f ₁ / f ₂ <1.0
[3] 0.2 <R ₁ /f<0.4
[4] −0.6 <R ₄ /f<−0.3
Where f ₁ is the focal length of the first lens, f ₂ is the focal length of the second lens, R ₁ is the radius of curvature of the first surface of the first lens, and R ₄ is the radius of curvature of the second surface of the second lens. is there.

The expression [2] is a condition for reducing the size by making the power of the first lens strong. In the formula [2], when f ₁ / f ₂ is less than or equal to the lower limit value, it is difficult to correct various aberrations. Conversely, when f ₁ / f ₂ is greater than or equal to the upper limit value, the total optical length becomes longer, and the imaging lens is difficult to downsize.

The formula [3], the radius of curvature R ₁ of the first surface of the first lens having positive refractive power by an appropriate value, coma, condition for properly correcting curvature of field, chromatic aberration of magnification It is. In Formula [3], when R ₁ / f is less than or equal to the lower limit, it becomes difficult to correct coma aberration, field curvature, and lateral chromatic aberration with the second lens. It is difficult to reduce the size of the imaging lens.

The equation [4] is a condition for canceling the aberration generated by the first lens. In the equation [4], when R ₄ / f is equal to or lower than the lower limit value, the aberration generated in the first lens is canceled out, so that the absolute value of R ₄ increases and the optical total length increases and the imaging lens becomes smaller. On the other hand, when the value exceeds the above upper limit, it becomes difficult to cancel the various aberrations in a balanced manner.

  In the present invention, it is preferable that at least one surface of the first lens and the second lens is an aspherical surface. Since at least one surface of the first lens and the second lens is an aspherical surface, various aberrations such as spherical aberration and coma can be corrected satisfactorily even though the lens has two lenses.

In the present invention, it is preferable to use a resin as the material constituting the first lens and the second lens, and it is more preferable that the Abbe number is larger than 50. By using a resin, it is possible to provide an imaging lens that realizes an aspheric surface at low cost and has high performance. Furthermore, when the Abbe number is larger than 50, an imaging lens excellent in chromatic aberration, particularly off-axis magnification chromatic aberration, can be provided.
Examples of the resin to be used include resins such as a polymer having an alicyclic structure, polymethyl methacrylate, and polycarbonate. Among these, a polymer having an alicyclic structure is preferable from the viewpoint of dimensional stability due to low hygroscopicity and lightness. Examples of the polymer having an alicyclic structure include (1) norbornene polymer, (2) monocyclic olefin polymer, (3) cyclic conjugated diene polymer, and (4) vinyl alicyclic hydrocarbon heavy. A coalescence, and these hydrogenated substances are mentioned. Among these, a norbornene-based polymer is more preferable from the viewpoint of transparency and moldability.

In the present invention, it is preferable that the following conditions are further satisfied.
[5] 0.05 <D ₂ /f<0.13
D ₂ is the distance from the vertex of the second surface of the first lens to the vertex of the first surface of the second lens. Here, the vertex of the second surface of the first lens is the intersection of the optical axis and the second surface of the first lens, and the vertex of the first surface of the second lens is the optical axis and the first surface. Hit the intersection.

The above formula [5] has a concentric configuration and regulates the air space between the first lens and the second lens. By appropriately setting the space for disposing the aperture for limiting the light amount, the total lens length and the aperture space are set. Is secured. In the above equation [5], when D ₂ / f is less than or equal to the lower limit, it is difficult to correct aberrations and secure a diaphragm space.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.
In this embodiment, f is the focal length, FNo is the F number, R is the radius of curvature of the bent surfaces, D is the distance between the bent surfaces, Nd is the refractive index of the d-line, and νd is the Abbe number of the d-line. The surface number indicates the position of the surface in order from the object side. The aspherical shape of the lens is expressed by Equation [6], where the z-axis is taken in the optical axis direction, the x-axis is taken in the direction perpendicular to the optical axis, and the light traveling direction is positive. In equation [6], R is the radius of curvature of the apex of each lens, h is the height from the optical axis of the lens, k is the conic coefficient, and A ₄ , A ₅ , A ₆ , A ₈ , A ₁₀ are aspheric surfaces. Represents a coefficient.

FIG. 2 is a schematic configuration diagram showing an embodiment of an imaging lens according to the present invention. The imaging lens shown in FIG. 1 includes a first lens 1, an aperture 3, and a second lens 2 from the object side. The first lens is a positive meniscus lens and has an aspherical surface convex toward the object side. The second lens is also a positive meniscus lens and has an aspherical shape convex toward the image plane side. A cover glass 5 of the image sensor is disposed between the second lens 2 and the imaging plane 4.
In this example, a norbornene polymer (manufactured by Zeon Corporation, ZEONEX 480R, Abbe number is 56) was used as a material constituting the lens.

Example 1
The lens data of Example 1 are shown in Tables 1 and 2. In addition, the value of R in Table 1 being 0 means that it is a plane. The same applies to Table 3 of Example 2 and Table 5 of Example 3 described later.

  FIG. 3 is an aberration diagram showing various aberrations in the imaging lens of Example 1. In the astigmatism, M represents meridional aberration, and S represents sagittal aberration. In coma aberration, curve 1 represents the d line, curve 2 represents the F line, and curve 3 represents the C line. In spherical aberration, curve 1 represents the d line, curve 2 represents the F line, curve 3 represents the C line, and curve 4 represents the sine condition. The meanings of these symbols are the same in FIGS. 3 and 4 showing various aberrations of Example 2 and Example 3 described later.

Moreover, the value of each conditional expression in Example 1 is shown.
In the conditional expression of 0 <TL / f <1.35, TL / f = 1.26
In the conditional expression of 0.5 <f ₁ / f ₂ <1.0, f ₁ / f ₂ = 0.81
In the conditional expression 0.2 <R ₁ /f<0.4, R ₁ /f=0.3
In the conditional expression −0.6 <R ₄ /f<−0.3, R ₄ /f=−0.44
In the conditional expression of 0.05 <D ₂ /f<0.13, D ₂ /f=0.08
Met.

(Example 2)
The lens data of Example 2 are shown in Tables 3 and 4.

  FIG. 4 is an aberration diagram showing various aberrations in the imaging lens of Example 2.

Moreover, the value of each conditional expression in Example 2 is shown.
In the conditional expression of 0 <TL / f <1.35, TL / f = 1.34
In the conditional expression of 0.5 <f ₁ / f ₂ <1.0, f ₁ / f ₂ = 0.9
In the conditional expression 0.2 <R ₁ /f<0.4, R ₁ /f=0.31
In the conditional expression −0.6 <R ₄ /f<−0.3, R ₄ /f=−0.43
In the conditional expression of 0.05 <D ₂ /f<0.13, D ₂ /f=0.12
Met.

(Example 3)
Tables 5 and 6 show lens data of Example 3.

  FIG. 5 is an aberration diagram showing various aberrations in the imaging lens of Example 3.

Moreover, the value of each conditional expression in Example 3 is shown.
In the conditional expression of 0 <TL / f <1.35, TL / f = 1.25
In the condition of 0.5 <f ₁ / f ₂ <1.0, f ₁ / f ₂ = 0.82
In the conditional expression 0.2 <R ₁ /f<0.4, R ₁ /f=0.3
In the conditional expression −0.6 <R ₄ /f<−0.3, R ₄ /f=−0.45
In the conditional expression of 0.05 <D ₂ /f<0.13, D ₂ /f=0.11
Met.

  As can be seen from FIGS. 3 to 5, all of astigmatism, distortion, spherical aberration, and coma are substantially satisfactory values, and sufficient optical characteristics can be obtained.

FIG. 1 shows an embodiment of an imaging lens according to the present invention. FIG. 2 is a schematic configuration diagram showing an embodiment of an imaging lens according to the present invention. FIG. 3 is an aberration diagram showing various aberrations in the imaging lens of Example 1. FIG. 4 is an aberration diagram showing various aberrations in the imaging lens of Example 2. FIG. 5 is an aberration diagram showing various aberrations in the imaging lens of Example 3.

Explanation of symbols

1: First lens 2: Second lens 3: Aperture 4: Imaging surface 5: Cover glass M: Meridional S: Sagittal

Claims (5)

A first lens, an aperture, and a second lens are sequentially arranged from the object side,
The first lens is a meniscus lens having a positive refractive power with a convex surface facing the object side,
The second lens is a meniscus lens having a positive refractive power with a convex surface facing the image surface side,
In each of the first lens and the second lens, when the object side surface is the first surface and the image side surface is the second surface, the imaging lens satisfies the following conditions.
[1] 0 <TL / f <1.35
However,
f: focal length of the entire lens system TL: air length conversion distance from the first surface of the first lens to the imaging surface The imaging lens according to claim 1, further satisfying the following condition.
[2] 0.5 <f ₁ / f ₂ <1.0
[3] 0.2 <R ₁ /f<0.4
[4] −0.6 <R ₄ /f<−0.3
However,
f ₁ : focal length of the first lens f ₂ : focal length of the second lens R ₁ : radius of curvature of the first surface of the first lens R ₄ : radius of curvature of the second surface of the second lens 3. The imaging lens according to claim 1, wherein at least one surface of the first lens and the second lens is an aspherical surface. The imaging lens according to claim 1, wherein a resin is used as a material constituting the first lens and the second lens. The imaging lens according to any one of claims 1 to 4, further satisfying the following condition:
[5] 0.05 <D ₂ /f<0.13
However,
D ₂ : Distance from the vertex of the second surface of the first lens to the vertex of the first surface of the second lens

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