JP2007310046A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact imaging lens suitable for a mobile camera. <P>SOLUTION: The imaging lens includes: an aperture diaphragm SD having a predetermined aperture; a first plane-convex lens 1 having positive refractive power; a second meniscus lens 2 having positive refractive power; and a third lens 3 having negative refractive power and formed so that its surface S6 on an image surface side may be an aspherical surface to have an inflection point where the direction of curvature changes within the range of an effective diameter in order from an object side. The focal length f of the lens system of the first lens 1 to the third lens 3, the focal length f1 of the first lens 1, the focal length f2 of the second lens 2, and a distance D2 on an optical axis between the first lens 1 and the second lens 2 satisfy conditional expressions (1) 0.9≤¾f1/f2¾≤1.0 and (2) 0.2≤D2/f≤0.3. Thus, the imaging lens whose entire length is shortened, which is compact and whose aberration is satisfactorily corrected is obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an imaging lens suitable for a small mobile camera using a solid-state imaging device such as a CCD mounted on a portable information terminal device such as a cellular phone, a portable personal computer, or a portable music player.

Conventionally, surveillance cameras, video cameras, and the like have been known as electronic imaging devices using a solid-state imaging device such as a CCD, and these have been mainly used for capturing moving images. Therefore, such a high performance is not required for the CCD and the imaging lens.
However, in recent years, with the widespread use of digital still cameras, it has become desirable to improve the performance of solid-state imaging devices and imaging lenses.
As a result, due to significant technological advances in solid-state imaging devices such as CCDs, it has become possible to obtain images close to those of film cameras, and at the same time, miniaturization and high pixel counts of CCDs and the like have been achieved. In addition to high performance, imaging lenses used in the field have been strongly demanded for miniaturization, thickness reduction, cost reduction, and the like.

By the way, as a conventional imaging lens, in order from the object side, a first lens having a meniscus positive refractive power having a convex surface directed toward the object side, an aperture stop having a predetermined aperture, and a positive refractive power having a biconvex shape. And a second lens having a negative refractive power in which an aspherical surface having an inflection point is formed on the image plane side and a concave surface facing the object side are known (for example, patents) Reference 1).
However, in this imaging lens, since the aperture stop is arranged between the first lens and the second lens, it is difficult to shorten the exit pupil position, shorten the exit angle, and shorten the overall length. is there.

As another imaging lens, in order from the object side, an aperture stop having a predetermined aperture, a first lens having a meniscus positive refractive power with a convex surface facing the object side, and a meniscus with a concave surface facing the object side A second lens having a positive refracting power and a third lens having a negative refracting power in which an aspherical surface having an inflection point on the image surface side and a concave surface facing the object side are formed. (For example, see Patent Document 2 and Patent Document 3).
However, in these imaging lenses, since the first lens has a meniscus shape, the manufacturing error sensitivity with respect to the optical axis of the lens becomes high, and it is difficult to maintain peripheral performance.

JP 2005-284153 A JP 2005-338234 A JP 2006-47858 A

  The present invention has been made in view of the above-described problems of the prior art, and its object is to correct various aberrations while reducing size, thickness, weight, and cost. To provide an imaging lens that is bright (small F value), has high optical performance, and is suitable for a mobile camera mounted on a portable information terminal device such as a mobile phone, a portable personal computer, and a portable music player. It is in.

An imaging lens of the present invention includes an aperture stop having a predetermined aperture, a first lens having a planoconvex shape having positive refractive power, a meniscus shape positive lens arrayed in order from the object side to the image plane side. A second lens having a refractive power, and a third lens having a negative refractive power formed on an aspherical surface having an inflection point where the direction of curvature changes within the effective diameter range of the image side surface. The focal length of the lens system of the first lens to the third lens is f, the focal length of the first lens is f1, the focal length of the second lens is f2, and on the optical axis of the first lens and the second lens. Conditional expressions (1) and (2) where D2 is the interval at
(1) 0.9 ≦ | f1 / f2 | ≦ 1.0
(2) 0.2 ≦ D2 / f ≦ 0.3
It is characterized by being composed of 3 pieces in 3 groups.
According to this configuration, by arranging the lenses so that the first lens and the second lens have a positive refractive power and the third lens has a negative refractive power, an appropriate lens back can be secured, and infrared light cut A filter or a cover glass can be easily arranged, and an aspherical surface having an inflection point in which the direction of curvature changes within the effective diameter range is formed on the image surface side surface of the third lens. Furthermore, by arranging the aperture stop in front and making the first lens a plano-convex shape, it is possible to suppress the emission angle while shortening the overall length.
Further, by satisfying the conditional expressions (1) and (2), it is possible to appropriately arrange the power, and it is possible to satisfactorily correct various aberrations, particularly astigmatism and distortion, while shortening the total length of the lens system. it can.

In the above configuration, the first lens, the second lens, and the third lens adopt a configuration in which at least one aspheric surface is formed in addition to the aspheric surface on the image plane side of the third lens. be able to.
According to this configuration, by providing at least one aspherical surface in addition to the aspherical surface on the image plane side of the third lens, various aberrations can be favorably corrected and bright (small F value). High optical performance can be ensured with a lens.

In the above configuration, a configuration in which at least one plastic lens is included in the first lens, the second lens, and the third lens can be employed.
According to this configuration, by including at least one plastic lens, it is possible to reduce the production cost, reduce the weight, etc., and easily form an aspheric surface, so that the degree of freedom in correcting aberrations is increased. A compact configuration is possible, and a complicated shape can be easily formed or processed as compared with the case of using a glass material.

In the above configuration, when the Abbe number of the first lens is ν1, the Abbe number of the second lens is ν2, and the Abbe number of the third lens is ν3, the conditional expressions (3) and (4)
(3) ν1-ν2 <15
(4) ν1-ν3 <15
A configuration that satisfies the above can be adopted.
According to this configuration, by satisfying the conditional expressions (3) and (4), it is possible to satisfactorily correct the lateral chromatic aberration and the longitudinal chromatic aberration in the entire region from the central portion to the peripheral portion, and the bright (F value) ) And high resolution can be obtained.

In the above configuration, the focal length of the lens system of the first lens to the third lens is f, the distance between the first lens and the second lens on the optical axis is D2, and the distance between the second lens and the third lens on the optical axis. When D4, conditional expression (5)
(5) (D2 + D4) /f≦0.25
A configuration that satisfies the above can be adopted.
According to this configuration, by satisfying conditional expression (5), the overall length of the lens system can be shortened, and the outer diameter of the lens system can be reduced, thereby achieving downsizing and thinning. In addition, various aberrations, particularly astigmatism and distortion can be corrected well.

  According to the imaging lens having the above-described configuration, various aberrations are corrected satisfactorily while achieving downsizing, thinning, weight reduction, cost reduction, and the like, and it is bright (F value is small) and has high optical performance. An imaging lens suitable for a mobile camera mounted on a portable information terminal device such as a mobile phone, a portable personal computer, or a portable music player can be obtained.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Embodiments of the invention will be described below with reference to the accompanying drawings.
1 and 2 show an embodiment of an imaging lens according to the present invention. FIG. 1 is a configuration diagram thereof, and FIG. 2 is an optical path diagram thereof.
As shown in FIG. 1, the imaging lens includes an aperture stop SD having a predetermined aperture and a first lens 1 having a plano-convex positive refractive power, which are sequentially arranged from the object side to the image plane side. Second meniscus-shaped second lens 2 having a positive refractive power, a first lens having a negative refractive power formed on an aspherical surface having an inflection point where the direction of curvature changes within the range of the effective diameter of the image side surface. Three lenses 3 are provided.
Then, glass plates 4 and 5 such as an infrared light cut filter or a cover glass are arranged behind the third lens 3, and an image plane P is arranged behind the glass plate 5.

  Here, in the aperture stop SD to the first lens 1 to the third lens 3 to the glass plates 4 and 5, as shown in FIG. 1, each surface is Si (i = 0 to 10), and each surface Si is made of Si. The radius of curvature is Ri (i = 0 to 10), the refractive indices of the first to third lenses 3 and the glass plates 4 and 5 with respect to the d-line are Ni (i = 1 to 5), and the Abbe number is νi (i = 1-5). The distance (thickness, air interval) on the optical axis L from the aperture stop SD to the image plane P is represented by Di (i = 0 to 10). Further, the focal length of the lens system of the first lens 1 to the third lens 3 is represented by f, the focal length of the first lens 1 is represented by f1, and the focal length of the second lens 2 is represented by f2.

  The first lens 1 is made of a glass material. As shown in FIG. 1, the object side surface S1 is formed as a convex surface, and the image surface side surface S2 is formed as a flat surface. Shape lens. Here, the surface S1 on the object side and the surface S2 on the image plane side are formed into spherical surfaces, but may preferably be formed into an aspherical surface. By using an aspherical surface, it is possible to ensure high optical performance with a small F value and brightness, in which various aberrations are well corrected.

The second lens 2 is preferably made of a plastic material (is a plastic lens), and as shown in FIG. 1, the object-side surface S3 is formed as a concave surface, and the image-side surface S4 is formed as a convex surface. It is a meniscus lens having positive refractive power.
By making the second lens 2 a plastic lens, the lens system can be made lighter, lower in cost, lower in production cost, etc., compared to the case where it is molded from a glass material, and an aspherical surface can be easily formed. As a result, a compact configuration can be achieved as much as the degree of freedom of aberration correction is increased, and a complicated shape can be easily formed or processed as compared with the case of using a glass material. Here, the object-side surface S3 and the image-side surface S4 are formed into spherical surfaces, preferably aspherical surfaces. By using an aspherical surface, it is possible to ensure high optical performance with a small F value and brightness, in which various aberrations are well corrected.

The third lens 3 is preferably formed of a plastic material (a plastic lens), and as shown in FIG. 1, the object-side surface S5 is formed in a convex surface near the optical axis L, and the image-side surface S6 is formed. In the vicinity of the optical axis L, the lens has a negative refractive power formed on a convex surface having an inflection point where the direction of curvature changes within the range of the concave surface and the effective diameter.
By making the third lens 3 a plastic lens, the lens system can be made lighter, lower in cost, lower in production cost, etc., compared to the case where it is molded from a glass material, and an aspherical surface can be easily formed. As a result, a compact configuration can be achieved as much as the degree of freedom of aberration correction increases, and complex shapes with inflection points can be easily formed or processed as compared with the case of using a glass material. .
Here, the surface S6 on the image plane side is formed as an aspherical surface. Preferably, the object side surface S5 and the image surface side surface S6 are both aspherical. As a result, it is possible to ensure high optical performance in which the F value is small and bright and various aberrations are well corrected.

That is, by arranging the lenses so that the first lens 1 and the second lens 2 have a positive refractive power and the third lens 3 has a negative refractive power, an appropriate lens back can be secured, and an infrared light cut filter Glass filters 4 and 5 such as can be easily arranged.
Further, since the aspherical surface having the inflection point where the direction of curvature changes within the effective diameter range is formed on the image surface side surface S6 of the third lens 3, various aberrations can be corrected well.
Furthermore, by arranging the aperture stop SD at the forefront and making the first lens 1 a plano-convex shape, it is possible to suppress the emission angle while shortening the overall length.

In addition to the above configuration, the focal length f of the lens system of the first lens 1 to the third lens 3, the focal length f1 of the first lens 1, the focal length f2 of the second lens 2, the first lens 1 and the second lens 2. The distance D2 on the optical axis L of the lens 2 is defined by conditional expressions (1) and (2).
(1) 0.9 ≦ | f1 / f2 | ≦ 1.0
(2) 0.2 ≦ D2 / f ≦ 0.3
It is formed to satisfy.
Conditional expressions (1) and (2) define the power arrangement and arrangement relationship of the first lens 1 and the second lens 2. If the value of | f1 / f2 | exceeds the upper limit value or the lower limit value of conditional expression (1), the power of the focal length is shifted, making it difficult to correct astigmatism, and the manufacturing error sensitivity with respect to the optical axis L of the lens is low. It becomes high and it becomes difficult to maintain peripheral performance. If the value of D2 / f exceeds the lower limit value of conditional expression (2), it becomes difficult to correct astigmatism. If the value of D2 / f exceeds the upper limit value of conditional expression (2), the total length of the lens is increased. It becomes longer, and further, it becomes difficult to correct distortion.
That is, by satisfying the conditional expressions (1) and (2), it is possible to appropriately arrange power, and it is possible to satisfactorily correct various aberrations, particularly astigmatism and distortion, while shortening the total length of the lens system. it can.

In the first lens 1 to the third lens 3, the aspherical surface is defined by the following equation.
Z = Cy ² / [1+ (1-εC ² y ² ) ^1/2 ] + Dy ⁴ + Ey ⁶ + Fy ⁸ + Gy ¹⁰
Where Z: distance from the tangent plane at the apex of the aspheric surface to a point on the aspheric surface whose height from the optical axis L is y, y: height from the optical axis, C: curvature at the apex of the aspheric surface ( = 1 / R), ε: conical constant, D, E, F, G: aspherical coefficients.

In the imaging lens having the above configuration, the Abbe number ν1 of the first lens 1, the Abbe number ν2 of the second lens 2, and the Abbe number ν3 of the third lens 3 are preferably conditional expressions (3) and (4).
(3) ν1-ν2 <15
(4) ν1-ν3 <15
It is formed so as to satisfy.
Conditional expressions (3) and (4) define the Abbe number range of the first lens 1 to the third lens 3. If the values of (ν1−ν2) and (ν1−ν3) deviate from the conditional expressions (3) and (4), the lateral chromatic aberration and axial chromatic aberration near the optical axis L can be corrected. Even so, since the image side surface S6 of the third lens 3 has a convex shape with the curvature direction changing within the effective diameter range, it is difficult to correct lateral chromatic aberration and axial chromatic aberration in the peripheral portion. .
That is, when the values of (ν1−ν2) and (ν1−ν3) satisfy the conditional expressions (3) and (4), the lateral chromatic aberration and the axial chromatic aberration are excellent in the entire region from the central portion to the peripheral portion. It can be corrected to be bright (because the F value is small) and high resolution can be obtained.

In the imaging lens having the above configuration, the focal length f of the lens system of the first lens 1 to the third lens 3, the distance D2 on the optical axis L between the first lens 1 and the second lens 2, the second lens 2, The distance D4 on the optical axis L of the third lens 3 is preferably conditional expression (5).
(5) (D2 + D4) /f≦0.25
It is formed so as to satisfy.
Conditional expression (5) defines the lens interval with respect to the focal length of the lens system. When the value of (D2 + D4) / f satisfies the conditional expression (5), the overall length of the lens system can be shortened, and the outer diameter of the lens system can be reduced, thereby achieving downsizing and thinning. In addition, various aberrations, particularly astigmatism and distortion can be corrected well.

  Next, specific numerical examples of the imaging lens will be described below as Example 1 and Example 2.

The main specification specifications, various numerical data (setting values), and values of conditional expressions (1) to (5) in the first embodiment are as follows. Further, the spherical aberration, astigmatism, distortion, and lateral chromatic aberration in Example 1 are the results shown in FIG. In the astigmatism in FIG. 3, S indicates an aberration on the sagittal plane, and M indicates an aberration on the meridional plane.
In Example 1, the first lens 1 is made of a glass material, and the second lens 2 and the third lens 3 are made of a resin material (plastic material). The aspheric surfaces are formed on both surfaces S3 and S4 of the second lens 2 and both surfaces S5 and S6 of the third lens 3.

<Value of conditional expression>
(1) | f1 / f2 | = | 3.6 / 3.9 | = 0.92 → 0.9 ≦ 0.92 ≦ 1.0
(2) D2 / f = 0.8 / 3.7 = 0.22 → 0.2 ≦ 0.22 ≦ 0.3
(3) ν1-ν2 = 70.2-56.3 = 13.9 → 13.9 <15
(4) ν1-ν3 = 70.2-56.3 = 13.9 → 13.9 <15
(5) (D2 + D4) / f = (0.8 + 0.09) /3.6=0.25 → 0.25 ≦ 0.25

<Specification specifications>
Object distance = 60 cm, focal length f of the lens system of the first lens 1 to the third lens 3 = 3.7 mm, focal length f1 of the first lens 1 = 3.6 mm, focal length f2 of the second lens 2 = 3. 9 mm, F value (F number) = 2.9, exit pupil position = -3.6 mm, exit angle of outermost ray = -22.1 °, total length of lens system (first lens 1 to image plane P) = 5 0.0 mm, total lens length (first lens 1 to third lens 3) = 3.7 mm, back focus (air conversion) = 1.3 mm, angle of view 2ω = 61.4 °, distortion (at 600 mm) = + 0.5% The outer diameter of the first lens 1 is 1.76 mm, the outer diameter of the second lens 2 is 2.36 mm, and the outer diameter of the third lens is 3.90 mm.

<Aperture stop SD to first lens 1 to third lens 3 and radius of curvature Ri of glass plates 4 and 5>
R0 = ∞ (aperture stop), R1 = 1.769 mm, R2 = ∞, R3 = −0.899 mm (aspheric surface), R4 = −0.791 mm (aspheric surface), R5 = 3.581 mm (aspheric surface), R6 = 1.166 mm (aspherical surface), R7 = ∞, R8 = ∞, R9 = ∞, R10 = ∞

<Surface spacing on the optical axis (mm)>
D0 = 0.000 mm, D1 = 0.995 mm, D2 = 0.800 mm, D3 = 0.698 mm, D4 = 0.090 mm, D5 = 0.856 mm, D6 = 0.680 mm, D7 = 0.300 mm, D8 = 0.125mm, D9 = 0.400mm, D10 = 0.045mm

<Refractive index Ni (d line) of the first lens 1 to the third lens 3 and the glass plates 4 and 5>
N1 = 1.48700, N2 = 1.52500, N3 = 1.52500, N4 = 1.51700, N5 = 1.51700
<Abbe number νi of the first lens 1 to the third lens 3 and the glass plates 4 and 5>
ν1 = 70.2, ν2 = 56.3, ν3 = 56.3, ν4 = 64.2, ν5 = 64.2

<Numerical data of aspheric coefficient>
<S3 surface>
ε = -1.9074240, D = −3.5933862 × 10 ⁻¹ , E = 1.49112630 × 10 ⁻¹ , F = 2.4173389 × 10 ⁻¹ , G = −1.4869119 × 10 ⁻¹
<S4 surface>
ε = 0.0000000, D = 1.28249935 × 10 ⁻¹ , E = −1.8699142 × 10 ⁻¹ , F = 2.0263375 × 10 ⁻¹ , G = −4.89055687 × 10 ⁻²
<S5 surface>
ε = 1.7455556, D = −1.0798499 × 10 ⁻¹ , E = 5.6726156 × 10 ⁻² , F = −2.0959271 × 10 ⁻² , G = 2.30890388 × 10 ⁻³
<S6 surface>
ε = −6.623133, D = −6.5926906 × 10 ⁻² , E = 2.4144143 × 10 ⁻² , F = −6.4914502 × 10 ⁻³ , G = 5.3024991 × 10 ⁻⁴

  In Example 1, the overall length of the lens system is as short as 5.0 mm (thin), the F value (F number) is as bright as 2.9, the angle of view 2ω is 61.4 °, and as shown in FIG. An imaging lens suitable for a mobile camera mounted on a portable information terminal device such as a mobile phone, a portable personal computer, or a portable music player can be obtained because high optical performance in which various aberrations are well corrected can be secured.

The main specification specifications, various numerical data (setting values), and values of conditional expressions (1) to (5) in Example 2 are as follows. Further, the spherical aberration, astigmatism, distortion, and lateral chromatic aberration in Example 1 are the results shown in FIG. In the astigmatism in FIG. 4, S represents the aberration on the sagittal plane, and M represents the aberration on the meridional plane.
In Example 2, the first lens 1 is made of a glass material, and the second lens 2 and the third lens 3 are made of a resin material (plastic material). The aspheric surfaces are formed on both surfaces S3 and S4 of the second lens 2 and both surfaces S5 and S6 of the third lens 3.

<Value of conditional expression>
(1) | f1 / f2 | = | 3.7 / 3.7 | = 1.0 → 0.9 ≦ 1.0 ≦ 1.0
(2) D2 / f = 0.8 / 3.54 = 0.23 → 0.2 ≦ 0.23 ≦ 0.3
(3) ν1-ν2 = 70.2-56.3 = 13.9 → 13.9 <15
(4) ν1-ν3 = 70.2-56.3 = 13.9 → 13.9 <15
(5) (D2 + D4) / f = (0.8 + 0.08) /3.54=0.25 → 0.25 ≦ 0.25

<Specification specifications>
Object distance = 60 cm, focal length of lens system of first lens 1 to third lens 3 = 3.54 mm, focal length f1 of first lens 1 = 3.7 mm, focal length f2 of second lens 2 = 3.7 mm , F number = 2.9, exit pupil position = -3.6 mm, exit angle of outermost light ray = -22.2 °, lens system total length 4.9 mm, lens total length = 3.5 mm, back focus (air conversion distance) ) = 1.4 mm, angle of view 2ω = 63.9 °, distortion (at 600 mm) = + 0.1%, first lens 1 outer diameter φ = 1.72 mm, second lens 2 outer diameter φ = 2 .32 mm, outer diameter of the third lens 3 = 3.90 mm

<Aperture stop SD to first lens 1 to third lens 3 and radius of curvature Ri of glass plates 4 and 5>
R0 = ∞ (aperture stop), R1 = 1.82 mm, R2 = ∞, R3 = −0.948 mm (aspheric surface), R4 = −0.789 mm (aspheric surface), R5 = 3.734 mm (aspheric surface), R6 = 1.214 mm (aspherical surface), R7 = ∞, R8 = ∞, R9 = ∞, R10 = ∞

<Surface spacing on the optical axis (mm)>
D0 = 0.000 mm, D1 = 0.900 mm, D2 = 0.800 mm, D3 = 0.672 mm, D4 = 0.080 mm, D5 = 0.830 mm, D6 = 0.725 mm, D7 = 0.300 mm, D8 = 0.125mm, D9 = 0.400mm, D10 = 0.045mm
<Refractive index Ni (d line) of the first lens 1 to the third lens 3 and the glass plates 4 and 5>
N1 = 1.48700, N2 = 1.52500, N3 = 1.52500, N4 = 1.51700, N5 = 1.51700
<Abbe number νi of the first lens 1 to the third lens 3 and the glass plates 4 and 5>
ν1 = 70.2, ν2 = 56.3, ν3 = 56.3, ν4 = 64.2, ν5 = 64.2

<Numerical data of aspheric coefficient>
<S3 surface>
ε = −2.2952600, D = −3.431010 × 10 ⁻¹ , E = 1.5735900 × 10 ⁻¹ , F = 2.4010000 × 10 ⁻¹ , G = −1.5386100 × 10 ⁻¹
<S4 surface>
ε = 0.0000000, D = 1.3217300 × 10 ⁻¹ , E = −1.8339800 × 10 ⁻¹ , F = 2.0368500 × 10 ⁻¹ , G = −4.823000 × 10 ⁻²
<S5 surface>
ε = 0.5872280, D = −1.0651500 × 10 ⁻² , E = 5.6279000 × 10 ⁻² , F = −2.2516000 × 10 ⁻² , G = 2.5190990 × 10 ⁻³
<S6 surface>
ε = −6.804830, D = −6.8247000 × 10 ⁻² , E = 2.3780000 × 10 ⁻² , F = −6.283737 × 10 ⁻³ , G = 4.6664320 × 10 ⁻⁴

  In the second embodiment, the total length of the lens system is as short as 4.9 mm (thin), the F value (F number) is as bright as 2.9, the field angle 2ω is 63.9 °, as shown in FIG. An imaging lens suitable for a mobile camera mounted on a portable information terminal device such as a mobile phone, a portable personal computer, or a portable music player can be obtained because high optical performance in which various aberrations are well corrected can be secured.

  As described above, the imaging lens according to the present invention achieves reductions in size, thickness, weight, cost, and the like, can correct various aberrations favorably, and is bright (small F value). Therefore, it can be applied to a mobile camera mounted on a portable information terminal device such as a mobile phone, a portable personal computer, a portable music player, etc. It is also useful as an imaging lens or in other optical systems that are not particularly required to be downsized.

It is a block diagram which shows one Embodiment of the imaging lens which concerns on this invention. FIG. 2 is an optical path diagram of the imaging lens shown in FIG. 1. FIG. 3 is an aberration diagram illustrating various aberrations of spherical aberration, astigmatism, distortion, and lateral chromatic aberration in Example 1. FIG. 6 is an aberration diagram showing various aberrations of spherical aberration, astigmatism, distortion, and lateral chromatic aberration in Example 2.

Explanation of symbols

L Optical axis SD Aperture stop P Image plane 1 First lens 2 Second lens 3 Third lens 4, 5 Glass plate f Focal length f1 of first to third lens systems Focal length f2 of first lens Second Lens focal length ν1 First lens Abbe number ν2 Second lens Abbe number ν3 Third lens Abbe number D2 Distance on optical axis between first lens and second lens D4 Optical axis of second lens and third lens Spacing above

Claims (5)

Arranged in order from the object side to the image plane side,
An aperture stop having a predetermined aperture;
A plano-convex first lens having positive refractive power;
A second lens having a meniscus-shaped positive refractive power;
A third lens having a negative refractive power formed on an aspherical surface having an inflection point where the direction of curvature changes within the effective diameter range of the surface on the image surface side,
The focal length of the lens system of the first lens to the third lens is f, the focal length of the first lens is f1, the focal length of the second lens is f2, and on the optical axis of the first lens and the second lens. When the interval is D2, conditional expressions (1) and (2)
(1) 0.9 ≦ | f1 / f2 | ≦ 1.0
(2) 0.2 ≦ D2 / f ≦ 0.3
Satisfied, consisting of 3 pieces in 3 groups,
An imaging lens characterized by the above. The first lens, the second lens, and the third lens are formed with at least one aspheric surface in addition to the aspheric surface on the image plane side of the third lens.
The imaging lens according to claim 1. The first lens, the second lens, and the third lens include at least one plastic lens.
The imaging lens according to claim 1 or 2, wherein Conditional expressions (3) and (4) where the Abbe number of the first lens is ν1, the Abbe number of the second lens is ν2, and the Abbe number of the third lens is ν3.
(3) ν1-ν2 <15
(4) ν1-ν3 <15
Satisfy,
The imaging lens according to any one of claims 1 to 3. The focal length of the lens system of the first lens to the third lens is f, the distance on the optical axis between the first lens and the second lens is D2, and the distance on the optical axis between the second lens and the third lens is D4. When conditional expression (5)
(5) (D2 + D4) /f≦0.25
Satisfy,
The imaging lens according to claim 1, wherein:

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