# Patent

## Title
Imaging lens

## Abstract

An imaging lens including, in order from an object side towards an image surface side, a diaphragm, a first lens which is a meniscus lens having a positive power whose convex surface faces the object side, and a second lens which is a lens having a positive power whose convex surface faces the image surface side, wherein conditions expressed by each of following expressions (1)-(6) are to be satisfied; 1.25≧L/fl≧0.8, 0.55≧f1/f2>0, 1.5≧f1/fl≧0.9, 1≧d2/d1≧0.2, 0.35≧d1/fl≧0.1, and 0.27≧d3/fl≧0.1 (where, L: entire length of the imaging lens, fl: focal distance of the entire imaging lens, f1: focal distance of the first lens, f2: focal distance of the second lens, d1: center thickness of the first lens, d2: space between the first lens and the second lens on the optical axis, and d3: center thickness of the second lens).

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging lens. In particular, the present invention relates to an imaging system of a two-lens structure that is capable of reducing size and weight, enhancing optical performance, and improving productivity. The imaging lens is used for an image-taking device that forms an image of an object, such as scenery or a person, on an image-taking surface of a solid image pickup device such as a CCD, CMOS, etc. The solid image pickup device is mounted on a portable computer, a television phone, a portable phone, a digital camera, and the like.
2. Description of the Related Art
Recently, there has been an increasing demand for cameras that utilize a solid image pickup device, such as a CCD, CMOS, or the like, which is mounted on a portable phone, a portable computer, and a television phone, for example. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the imaging lens used for such cameras to be similarly small and light. Conventionally, a single-lens structure lens system using a single lens is used as such imaging lens.
Such a single-lens structure lens system can sufficiently handle being applied to a solid image pickup device called CIF that has a resolution of about 110 thousand pixels. However, in recent years, the use of a solid image pickup device called VGA that has a high resolution of about 300 thousand pixels is being examined. There is a problem in that the conventional single-lens structure lens system cannot sufficiently exert the resolution performance of such high-resolution solid image pickup devices.
Therefore, conventionally, various two-lens structure lens systems and three-lens structure lens systems having an optical performance that is superior to that of the single-lens structure lens system have been proposed.
In this case, in the three-lens structure lens system, each aberration leading to the deterioration of the optical performance can be effectively corrected, thereby allowing an extremely high optical performance to be obtained. However, since the three-lens structure lens system has a large number of parts, it is difficult to reduce the size and weight. Since each component part requires high accuracy, the manufacturing costs increase, as well.
On the other hand, although the two-lens structure lens system cannot achieve an optical performance that is as high as that of the three-lens structure lens system, a higher optical performance than that of the single-lens structure lens system can be acquired. It can be said that the two-lens structure lens system is suitable for a small, high-resolution solid image pickup device.
As such a two-lens structure lens system, conventionally, numerous lens systems called a retro-focus-type in which a negative lens and a positive lens are combined have been proposed. It is possible to reduce costs of such retro-focus-type lens systems through a reduction of the number of parts. However, from this configuration, it is practically impossible to reduce the size and weight to about the size and weight of the single-lens structure lens system since the back focus distance increases.
As another two-lens structure lens system, there is a lens system called a telephoto-type in which a positive lens and a negative lens are combined. However, such telephoto-type lens systems had been developed for silver-salt photography, and therefore, the back focus distance of the telephoto-type lens system is too short. The telephoto-type lens system also has problems regarding telecentricity. It is difficult to apply the telephoto-type lens system as is to the imaging lens for solid image pickup devices.
Furthermore, conventionally, in the two-lens structure lens system or the three-lens structure lens system, a configuration in which a diaphragm is disposed between two lenses that are mutually adjacent in the optical axis direction is mainstream (for example, refer to Patent Literatures 1 and 2).

    [Patent Literature 1] Japanese Patent Unexamined Publication 2004-163850
    [Patent Literature 2] Japanese Patent Unexamined Publication 2004-170460


There is an increasing demand for further improvement of the optical performance, in addition to the reduction in size and weight, of these types of imaging lens. However, in a configuration in which the diaphragm is disposed between the two lenses, as in the imaging lens described in Patent Literatures 1 and 2, it is difficult to achieve both the reduction in size and weight and further improvement in the optical performance. Furthermore, it is difficult to accommodate the sensor characteristics (incident angle to the sensor).
SUMMARY OF THE INVENTION
Therefore, the present invention has been designed in view of the aforementioned problems. The object of the present invention is to provide an imaging lens that can sufficiently meet the demands for the reduction in size and weight and further improvement in the optical performance, and improve productivity.
In the present specifications, productivity means not only the productivity for mass-producing imaging lens (for example, moldability, cost, and the like when imaging lens are mass-produced by injection molding), but also easiness of processing, manufacture, etc. of equipment (for example, easiness of processing and the like of a mold used for injection molding), which is used for manufacturing the imaging lens.
In order to achieve the aforementioned object, the imaging lens according to a first aspect of the present invention is an imaging lens used for forming an image of an object on an image-taking surface of a solid image pickup device, which comprises, in order from an object side towards an image surface side: a diaphragm, a first lens which is a meniscus lens having a positive power whose convex surface faces the object side, and a second lens which is a lens having a positive power whose convex surface faces the image surface side, wherein conditions expressed by each of following expressions (1)-(6) are to be satisfied;

1.25≧L/fl≧0.8  (1)

0.55≧f1/f2>0  (2)

1.5≧f1/fl≧0.9  (3)

1≧d2/d1≧0.2  (4)

0.35≧d1/fl≧0.1  (5)

0.27≧d3/fl≧0.1  (6)

where,

L: entire length of the imaging lens
fl: focal distance of the entire imaging lens
f1: focal distance of the first lens
f2: focal distance of the second lens
d1: center thickness of the first lens
d2: space between the first lens and the second lens on the optical axis
d3: center thickness of the second lens.
In the first aspect of the present invention, the diaphragm is disposed at a position closest to the object side. Thereby, it becomes possible to secure high telecentricity, and the incident angle of a light ray to the sensor of the solid image pickup device can be made more obtuse.
In the present invention, the diaphragm being disposed at a position closest to the object side does not interfere with the diaphragm being disposed in the same position on the optical axis direction as a point on the optical axis on the object side face (convex face) of the first lens, or the object side face of the first lens in the vicinity of the optical axis being positioned closer to the object side than the diaphragm through the diaphragm. Even in this case, the diaphragm is disposed at a position closer to the object side than the entire first lens, as a physical disposition. Therefore this does not go against the descriptions in the claims.
In addition, in the first aspect of the present invention, the first lens is a meniscus lens having a positive power whose convex surface faces the object side, the second lens is a lens having a positive power, and the power of each lens is regulated to satisfy each condition expressed by the expressions (1)-(6) With this, it is possible to improve productivity, while reducing size and weight.
An imaging lens according to a second aspect is the imaging lens according to the first aspect, wherein, further, the second lens is a meniscus lens.
In the second aspect of the present invention, further, it is possible to improve the optical performance of the periphery without placing a load on the shapes of the first lens and the second lens and more effectively use the light ray irradiated on the periphery of the solid image pickup device.
An imaging lens according to a third aspect is the imaging lens according to the first aspect, wherein, further, the object side surface of the second lens is convex towards the object side in the vicinity of the optical axis and is formed into an aspheric surface having an inflection point.
In the third aspect of the present invention, further, it is possible to further reduce the load placed on the shape of each lens and further improve the optical performance of the periphery. In addition, it is possible to more effectively use the light ray irradiated on the periphery of the solid image pickup device.
An imaging lens according to a fourth aspect is the imaging lens according to the third aspect, wherein, further, an outer end section of an effective diameter of the object side face of the second lens is positioned closer to the object side than a point on the optical axis on the object side surface of the second lens.
In the fourth aspect of the present invention, further, it is possible to further improve the optical performance of the periphery. In addition, there are advantages not only when handling the lenses, but also during assembly when the lenses are mounted on a barrel and made into a unit.
An imaging lens according to a fifth aspect is the imaging lens according to any one of aspects 1 to 4, wherein, further, a condition expressed by a following expression (7) is to be satisfied;

f2/fl≧2.3  (7).

In the fifth aspect of the present invention, further, the expression (7) is satisfied. Thereby, by preventing the power of the second lens from becoming too strong, the size can be sufficiently reduced.
An imaging lens according to a sixth aspect is the imaging lens according to aspect 5, wherein, further, the diaphragm satisfies a condition expressed by a following expression (8);

0.2≧S  (8)

where,

S: distance between the diaphragm and the optical surface closest to the object side on the optical axis.
In the sixth aspect of the invention, further, the expression (8) is satisfied. Thereby, telecentricity can be more effectively secured and the size and weight can be further reduced.
An imaging lens according to a seventh aspect is the imaging lens according to aspect 6, wherein, further, a condition expressed by a following expression (9) is to be satisfied;

0.8≧Bfl/fl≧0.4  (9)

where,

Bfl: back focus distance (distance from the last lens surface to the image-taking surface on the optical axis (air reduced length)).
In the seventh aspect of the invention, further, the expression (9) is satisfied. Thereby, the size and weight can be more effectively reduced, and productivity and manageability when assembling can be further improved.
An imaging lens according to an eighth aspect is the imaging lens according to aspect 7, wherein, further, a condition expressed by a following expression (10) is to be satisfied;

2.5≧Bfl≧0.8  (10).

In the eighth aspect of the invention, further, the expression (10) is satisfied. Thereby, the size and weight can be more effectively reduced, and productivity and manageability when assembling can be further improved.
With the imaging lens according to the first aspect of the present invention, it is possible to achieve an imaging lens that has reduced size and weight, superior optical performance, and excellent productivity.
Further, in addition to the effects of the imaging lens according to the first aspect, the imaging lens according to the second aspect can achieve a small imaging lens that has a better improved optical performance, while maintaining excellent productivity, and can more effectively use the light ray irradiated on the periphery of the solid image pickup device.
Further, in addition to the effects of the imaging lens according to the first aspect, the imaging lens according to the third aspect can achieve a small imaging lens that has more superior optical performance, while maintaining excellent productivity, and can more effectively use the light ray irradiated on the periphery of the solid image pickup device.
Further, in addition to the effects of the imaging lens according to the third aspect, the imaging lens according to the fourth aspect can achieve an imaging lens that has more superior optical performance, while maintaining excellent productivity, and can more effectively use the light ray irradiated on the periphery of the solid image pickup device.
Further, in addition to the effects of the imaging lens according to any one of aspects 1 to 4, the imaging lens according to the fifth aspect can achieve an imaging lens that has more reduced size and weight.
Further, in addition to the effects of the imaging lens according to any one of aspects 1 to 5, the imaging lens according to the sixth aspect can achieve an imaging lens that more effectively secures telecentricity and has more reduced size and weight.
Further, in addition to the effects of the imaging lens according to any one of aspects 1 to 6, the imaging lens according to the seventh aspect can achieve an imaging lens that has more reduced size and weight and more excellent productivity.
Further, in addition to the effects of the imaging lens according to any one of aspects 1 to 7, the imaging lens according to the eighth aspect can achieve an imaging lens that is suitable for further reduction in size and weight and further improvement in productivity.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram for showing an embodiment of the imaging lens according to the present invention;
FIG. 2 is a schematic diagram for showing FIRST EXAMPLE of the imaging lens according to the present invention;
FIG. 3 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 2;
FIG. 4 is a schematic diagram for showing SECOND EXAMPLE of the imaging lens according to the present invention;
FIG. 5 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 4;
FIG. 6 is a schematic diagram for showing THIRD EXAMPLE of the imaging lens according to the present invention;
FIG. 7 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 6;
FIG. 8 is a schematic diagram for showing FOURTH EXAMPLE of the imaging lens according to the present invention;
FIG. 9 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 8;
FIG. 10 is a schematic diagram for showing FIFTH EXAMPLE of the imaging lens according to the present invention;
FIG. 11 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 10;
FIG. 12 is a schematic diagram for showing SIXTH EXAMPLE of the imaging lens according to the present invention;
FIG. 13 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 12;
FIG. 14 is a schematic diagram for showing SEVENTH EXAMPLE of the imaging lens according to the present invention;
FIG. 15 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 14;
FIG. 16 is a schematic diagram for showing EIGHTH EXAMPLE of the imaging lens according to the present invention;
FIG. 17 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 16;
FIG. 18 is a schematic diagram for showing NINTH EXAMPLE of the imaging lens according to the present invention;
FIG. 19 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 18;
FIG. 20 is a schematic diagram for showing TENTH EXAMPLE of the imaging lens according to the present invention;
FIG. 21 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 20;
FIG. 22 is a schematic diagram for showing ELEVENTH EXAMPLE of the imaging lens according to the present invention;
FIG. 23 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 22;
FIG. 24 is a schematic diagram for showing TWELFTH EXAMPLE of the imaging lens according to the present invention; and
FIG. 25 shows graphs for describing the spherical aberration, astigmatism, and distortion of the imaging lens shown in FIG. 24.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the imaging lens according to the present invention will be described hereinafter by referring to FIG. 1.
As shown in FIG. 1, an imaging lens 1 of the embodiment comprises, in order from the object side towards the image surface side, a diaphragm 2, a resin-type first lens 3 which is a meniscus lens having a positive power with its convex surface facing the object side, and a resin-type second lens 4 which is a lens having a positive power with its convex surface facing the image surface side.
Herein, each of the lens surfaces on the object side and the image surface side of the first lens 3 and the second lens 4 is respectively referred to as a first face and a second face.
On the second face side of the second lens 4, there are disposed various filters 6 such as a cover glass, an IR cut filter, and a low pass filter, and an image-taking surface 7 which is a light-receiving surface of an image sensor element such as a CCD or a CMOS, respectively. The various filters 6 may be omitted as required.
The closer the position of the diaphragm 2 is to the image surface side, the closer the position of the exit pupil is to the image surface side. As a result, it becomes difficult to secure telecentricity. An off-axis light emitted from the imaging lens 1 is irradiated at an angle to the sensor of the solid image pickup device.
On the other hand, in the embodiment, the exit pupil can be positioned at a position far from the sensor face (image-taking surface) of the solid image pickup device, by the diaphragm 2 being disposed at a position closest to the object side.
Therefore, in the embodiment, high telecentricity can be secured and the incident angle of the light ray in relation to the sensor of the solid image pickup device can be made more obtuse.
In addition, in the embodiment, the diaphragm 2 is disposed on the object side of the first lens 3, and the first lens 3 has a meniscus-shape of which its convex surface faces the object side. Thereby, the second face of the first lens 3 can be effectively used.
Further, in the embodiment, the shape of the second face of the second lens 4 is convex towards the image surface side. Thereby, a higher telecentricity can be secured, and the incident angle to the sensor of the solid image pickup device can be more effectively controlled. Still further, the shape of the second face of the second lens 4 is more preferably an aspherical surface in which the curvature becomes larger the farther away from the optical axis 8 it is. With this, a higher telecentricity can be secured and the incident angle to the sensor of the solid image pickup device can be more effectively controlled.
Further, in the embodiment, the imaging lens 1 is to satisfy each condition expressed by the following expressions (1)-(6).

1.25≧L/fl≧0.8  (1)

0.55≧f1/f2>0  (2)

1.5≧f1/fl≧0.9  (3)

1≧d2/d1≧0.2  (4)

0.35≧d1/fl≧0.1  (5)

0.27≧d3/fl≧0.1  (6)

where, L in the expression (1) is the entire length of the lens system i.e., the optical distance between the surface physically closest to the object side and the image-taking surface. More specifically, when the first surface of the first lens 3 in the vicinity of the optical axis 8 is positioned closer to the image surface side than the diaphragm 2, the distance from the diaphragm 2 to the image-taking surface is L. At the same time, as described above, when the first surface of the first lens 3 in the vicinity of the optical axis 8 is positioned closer to the object side than the diaphragm 2 through the diaphragm 2, the distance from the first surface of the first lens 3, rather than the diaphragm 2, to the image-taking surface becomes L. In addition, when the diaphragm 2 is disposed at the same position in the optical axis 8 direction as the point on the optical axis 8 of the first surface of the first lens 3, the distance from the diaphragm 2 and the first surface of the first lens 3 to the image-taking surface becomes L. fl in the expressions (1), (3), (5), and (6) is the focal distance of the entire lens system. f1 in the expressions (2) and (3) is the focal distance of the first lens 3. f2 in the expressions (2) and (4) is the focal distance of the second lens 4. d1 in the expressions (4) and (5) is the center thickness of the first lens 3. d2 in the expression (4) is the space between the first lens 3 and the second lens 4 on the optical axis 8. d3 in the expression (6) is the center thickness of the second lens 4.

When the value of the L/fl exceeds the value (1.25) shown in the expression (1), the entire optical system becomes too large and goes against the demand for reduction in size and weight. At the same time, when the L/fl becomes below the value (0.8) shown in the expression (1), the entire optical system becomes small. Thus, productivity is deteriorated and it becomes difficult to maintain the optical performance. In addition, it becomes difficult to secure the necessary back focus distance.
Therefore, by further setting the value of L/fl to satisfy the expression (1) in the embodiment, the size and weight of the imaging lens system can be sufficiently reduced, while maintaining the required back focus distance, and an excellent optical performance can be maintained.
It is more preferable for the relation between L and fl to satisfy an expression 1.2≧L/fl≧1.05.
Further, when the value of f1/f2 exceeds the value (0.55) shown in the expression (2), the power of the first lens 2 becomes so week. In addition, the back focus distance becomes too long, making it difficult to reduce the size of the entire optical system. At the same time, when the value of f1/f2 becomes less or equal the value (0) shown in the expression (2), the power of the second lens 4 becomes too week, making it difficult to correct the various aberrations. In addition, it becomes difficult to secure the necessary back focus distance.
Therefore, by further setting the f1/f2 to satisfy the expression (2) in the embodiment, the size and weight of the entire optical system can be sufficiently reduced, while more effectively securing the necessary back focus distance. In addition, the various aberrations can be more favorably corrected.
It is more preferable for the relation between f1 and f2 to satisfy an expression 0.2>f1/f2>0.
Further, when the value of f1/fl exceeds the value (1.5) shown in the expression (3), the back focus distance becomes too long, thereby making it difficult to reduce the size and weight. At the same time, when the value of f1/fl becomes below the value (0.9) shown in the expression (3), the productivity of the first lens is deteriorated and it becomes difficult to secure the necessary back focus distance.
Therefore, by further setting the value of f1/fl to satisfy the expression (3) in the embodiment, the size and weight can be further reduced and productivity can be improved, while securing the necessary back focus distance.
It is more preferable for the relation between f1 and fl to satisfy an expression 1.5≧f1/fl≧1.1.
Still further, when the value of d2/d1 exceeds the value (1) shown in the expression (4), the power of the first lens 3 and the second lens 4 is required to be increased, thereby making it difficult to manufacture each lens 3 and 4. In addition, the height of the light ray passing through the image surface side face of the second lens 4 is high. Therefore, the power of the aspherical surface increases and manufacturing becomes more difficult. At the same time, when the value of d2/d1 is below the value (0.2) shown in the expression (4), the center thickness of the first lens 3 becomes relatively too thick. Thus, it becomes difficult to secure the back focus distance and insert a diaphragm that effectively limits the amount of light.
Therefore, by further setting the value of d2/d1 to satisfy the expression (4) in the embodiment, productivity can be further improved, the necessary back focus distance can be more appropriately secured, and more excellent optical performance can be maintained.
It is more preferable for the relation between d2 and d1 to satisfy an expression 0.9≧d2/d1≧0.3.
Still further, when the value of d1/fl exceeds the value (0.35) shown in the expression (5), the entire length of the optical system becomes too long, thereby making it difficult to reduce the size and weight. At the same time, when the value of d1/fl is below the value (0.1) shown in the expression (5), it becomes difficult to manufacture the first lens 3.
Therefore, by further setting the value of d1/fl to satisfy the expression (5) in the embodiment, the size and weight can be further reduced and productivity can be improved.
It is more preferable for the relation between d1 and fl to satisfy an expression 0.3≧d1/fl≧0.15.
Still further, when the value of d3/fl exceeds the value (0.27) shown in the expression (6), the entire length of the optical system becomes too long, thereby making it difficult to reduce the size and weight. At the same time, when the value of d3/fl is below the value (0.1) shown in the expression (6), it becomes difficult to manufacture the second lens 4.
Therefore, by further setting the value of d3/fl to satisfy the expression (6) in the embodiment, the size and weight of the entire optical system can be further reduced and productivity can be improved.
It is more preferable for the relation between d3 and fl to satisfy an expression 0.25≧d3/fl≧0.15.
In addition to the above-described structures, the second lens 4 is preferably a meniscus lens.
In this case, the optical performance of the periphery can be improved without placing a load on the shapes of the first lens 3 and the second lens 4 and the light ray irradiated on the periphery of the solid image pickup device can be more effectively used.
Furthermore, the first surface of the second lens 4 in the vicinity of the optical axis 8 preferably has a convex surface facing the object side and is formed into an aspherical surface having an inflection point.
Herein, the inflection point of the first surface of the second lens 4 is a point on a cross-section of the second lens 4 that includes the optical axis 8, in which a tangent contacting a curved line (a curved line on the cross-section) on the first surface of the second lens 4 changes the symbol of its angle.
Therefore, as described above, when the center section of the first surface of the second lens 4 has a convex surface facing the object side, the surface shape of the peripheral section surrounding the center section of the first surface changes to a concave surface facing the object side with the inflection point as the boundary.
As a result, the optical performance of the periphery can be improved without placing a load on the shapes of the first lens 3 and the second lens 4 and the light rays respectively passing through the lenses 3 and 4 can be more effectively used.
The first face of the second lens 4 can have a surface shape in which a plurality of inflection points appears toward the periphery from the optical axis 8. In this case, the various aberrations can be more favorably corrected.
Further, in addition to the above-described structures, the outer end section of the effective diameter of the object side face of the second lens is preferably positioned closer to the object side than the point on the optical axis on the object side surface of the second lens 4.
As a result, the optical performance of the periphery can be further improved. In addition, there are advantages not only when handling the lenses, but also during assembly when the lenses are mounted on a barrel and made into a unit.
Further, in addition to the above-described structures, it is more desirable to satisfy the following expression (7).

f2/fl≧2.3  (7)

Herein, when the value of f2/fl becomes below the value (2.3) shown in the expression (7), the power of the second lens 4 becomes too strong. In addition, the back focus distance becomes so long that it becomes difficult to be reduced the size and weight.
Therefore, by further setting the value of f2/fl to satisfy the expression (7) in the embodiment, the size and weight can be further reduced.
It is more preferable for the relation between f2 and fl to satisfy an expression 10≧f2/fl>4.
Further, in addition to the above-described structures, it is more desirable if the diaphragm 2 satisfies the following expression (8).
S in the expression (8) is the distance between the diaphragm 2 and the optical surface closest to the object side on the optical axis 8. In other words, S is the distance between the diaphragm 2 and the first face of the first lens 3 on the optical axis 8. In addition, S is a physical distance. The diaphragm 2 can be positioned closer to either the object side or the image surface side than the point on the optical axis 8 on the first face of the first lens 3.

0.2≧S  (8)

When S=0, the position of the diaphragm in the optical axis 8 direction is the same position as the point on the optical axis 8 on the first face of the first lens 3.
As a result, telecentricity can be further effectively maintained and the size and weight can be further reduced.
It is more preferable that S is 0.15≧S.
Further, in addition to the above-described structures, it is more desirable to satisfy the following expression (9).
However, Bfl in the expression (9) is the back focus distance or, in other words, the distance from the last lens surface (second face of the second lens 4) to the image-taking surface 7 on the optical axis 8 [air reduced length].

0.8≧Bfl/fl≧0.4  (9)

As a result, the size and weight can be more effectively reduced, and productivity and manageability when assembling can be further improved.
It is more preferable for the relation between Bfl and fl to satisfy an expression 0.7≧Bfl/fl≧0.5.
Further, in addition to the above-described structures, it is more desirable to satisfy the following expression (10).

2.5≧Bfl≧0.8  (10)

As a result, the size and weight can be more effectively reduced, and productivity and manageability when assembling can be further improved.
It is more preferable that Bfl is 2.0≧Bfl≧1.0.
Further, in addition to the above-described structures, it is more desirable that fl satisfies 5≧fl≧1 (more preferably 3.5≧fl≧1.5).
As a result, a configuration that is more suitable for a lens used in a camera module that is mounted on a portable terminal and the like can be achieved.
Moreover, examples of a resin material used for molding the first lens 3 and the second lens 4 may be materials of various compositions with transparency, such as acryl, polycarbonate, amorphous polyolefin resin, etc. However, from the perspective of further improving the manufacturing efficiency and further reducing the manufacturing costs, it is preferable that the resin materials of both lenses 3 and 4 are unified and are the same resin material.
EXAMPLES
Next, EXAMPLES of the present invention will be described by referring to FIG. 2 or FIG. 25.
In the EXAMPLES, F no denotes F number and r denotes the curvature radius of the optical surface (the center radius curvature in the case of a lens) Further, d denotes a distance to the next optical surface, nd denotes the index of refraction when the d line (yellow) is irradiated, and νd denotes the Abbe number of each optical system also when the d line is irradiated.
k, A, B, C, and D denote each coefficient in a following expression (11). Specifically, the shape of the aspherical surface of the lens is expressed by the following expression provided that the direction of the optical axis 8 is taken as the Z axis, the direction orthogonal to the optical axis 8 as the X axis, the traveling direction of light is positive, k is the constant of cone, A, B, C, and Dare the aspherical coefficients, and r is the curvature radius.

Z(X)=r−1X2/[1+{1−(k+1)r−2X2}1/2]+AX4+BX6+CX8+DX10  (11)

In the following EXAMPLES, reference code E used for a numerical value denoting the constant of cone and the aspherical coefficient indicates that the numerical value following E is an exponent having 10 as the base and that the numerical value before E is multiplied by the numerical value denoted by the exponent having 10 as the base. For example, −1.48E−1 denotes −1.48×10−1.
First Example
FIG. 2 shows a FIRST EXAMPLE of the present invention. In the FIRST EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the FIRST EXAMPLE was set under the following condition.










Lens Data


L = 2.09 mm, fl = 1.86 mm, f1 = 2.37 mm, f2 = 4.55 mm, d1 = 0.5 mm,


d2 = 0.18 mm, d3 = 0.4 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
0.625
0.50
1.525
56.0


3(Second Face of First Lens)
0.909
0.18


4(First Face of Second Lens)
−20.000
0.40
1.525
56.0


5(Second Face of Second Lens)
−2.151
0.00


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−1.48E−1
1.81E−1
−2.67E−1
−2.74E+1
3.22E+2


3
−1.72
5.93E−1
2.97
1.23E+1
−8.24


4
−2.43E+5
−1.86
7.95
−5.31E+1
0


5
7.93E−1
−2.85E−1
−2.43
1.05E+1
−2.38E+1









Under such conditions, L/fl=1.12 was achieved, thereby satisfying the expression (1). f1/f2=0.52 was achieved, thereby satisfying the expression (2). f1/fl=1.27 was achieved, thereby satisfying the expression (3). d2/d1=0.36 was achieved, thereby satisfying the expression (4). d1/fl=0.27 was achieved, thereby satisfying the expression (5). d3/fl=0.22 was achieved, thereby satisfying the expression (6). f2/fl=2.45 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.54 was achieved, thereby satisfying the expression (9). Bfl=1.01 mm was achieved, thereby satisfying the expression (10).
FIG. 3 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the FIRST EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Second Example
FIG. 4 shows a SECOND EXAMPLE of the present invention. In the SECOND EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the SECOND EXAMPLE was set under the following condition.










Lens Data


L = 2.26 mm, fl = 1.99 mm, f1 = 2.59 mm, f2 = 4.68 mm, d1 = 0.55 mm,


d2 = 0.15 mm, d3 = 0.45 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
0.690
0.55
1.531
56.0


3(Second Face of First Lens)
1.000
0.15


4(First Face of Second Lens)
−33.333
0.45
1.531
56.0


5(Second Face of Second Lens)
−2.326
0.00


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−1.48E−1
1.47E−1
−1.49E−1
−1.40E+1
1.38E+2


3
−1.71
4.46E−1
1.86
6.34
−3.49


4
−2.43E+5
−1.44
4.59
−3.00E+1
0


5
−5.22E−1
−1.97E−1
−1.50
5.30
−1.00E+1









Under such conditions, L/fl=1.14 was achieved, thereby satisfying the expression (1). f1/f2=0.55 was achieved, thereby satisfying the expression (2). f1/fl=1.30 was achieved, thereby satisfying the expression (3). d2/d1=0.27 was achieved, thereby satisfying the expression (4). d1/fl=0.28 was achieved, thereby satisfying the expression (5). d3/fl=0.23 was achieved, thereby satisfying the expression (6). f2/fl=2.35 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.56 was achieved, thereby satisfying the expression (9). Bfl=1.11 mm was achieved, thereby satisfying the expression (10).
FIG. 5 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the SECOND EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Third Example
FIG. 6 shows a THIRD EXAMPLE of the present invention. In the THIRD EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the THIRD EXAMPLE was set under the following condition.










Lens Data


L = 2.25 mm, fl = 2.05 mm, f1 = 2.67 mm, f2 = 7.83 mm, d1 = 0.3 mm,


d2 = 0.2 mm, d3 = 0.45 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.224
0.30
1.531
56.0


3(Second Face of First Lens)
0.598
0.20


4(First Face of Second Lens)
0.428
0.45
1.531
56.0


5(Second Face of Second Lens)
0.202
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face Number
k
A
B
C
D





2
−2.15
−7.3E−1
1.10E+1
−1.67E+2
4.08E+2


3
−7.93
−2.36
−1.61
−8.83E+1
4.55E+1


4
0.00
−2.63
2.04E+1
−6.00E+2
5.63E+3


5
0.00
−4.57E−1
1.71
−1.58E+1
3.98E+1









Under such conditions, L/fl=1.10 was achieved, thereby satisfying the expression (1). f1/f2=0.34 was achieved, thereby satisfying the expression (2). f1/fl=1.30 was achieved, thereby satisfying the expression (3). d2/d1=0.67 was achieved, thereby satisfying the expression (4). d1/fl=0.15 was achieved, thereby satisfying the expression (5). d3/fl=0.22 was achieved, thereby satisfying the expression (6). f2/fl=3.82 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.63 was achieved, thereby satisfying the expression (9). Bfl=1.298 mm was achieved, thereby satisfying the expression (10).
FIG. 7 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the THIRD EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Fourth Example
FIG. 8 shows a FOURTH EXAMPLE of the present invention. In the FOURTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the FOURTH EXAMPLE was set under the following condition.










Lens Data


L = 2.25 mm, fl = 2.05 mm, f1 = 2.15 mm, f2 = 27.53 mm, d1 = 0.3 mm,


d2 = 0.25 mm, d3 = 0.40 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.215
0.30
1.531
56.0


3(Second Face of First Lens)
0.394
0.25


4(First Face of Second Lens)
0.018
0.40
1.531
56.0


5(Second Face of Second Lens)
−0.050
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−1.70
−6.55E−1
1.21E+1
−2.01E+2
6.85E+2


3
3.75E+1
−2.17
−3.46
−8.54E+1
3.93E+1


4
0.00
−2.58
2.23E+1
−6.39E+2
6.00E+3


5
0.00
−5.83E−1
2.17
−1.66E+1
3.62E+1









Under such conditions, L/fl=1.10 was achieved, thereby satisfying the expression (1). f1/f2=0.08 was achieved, thereby satisfying the expression (2). f1/fl=1.05 was achieved, thereby satisfying the expression (3). d2/d1=0.83 was achieved, thereby satisfying the expression (4). d1/fl=0.15 was achieved, thereby satisfying the expression (5). d3/fl=0.20 was achieved, thereby satisfying the expression (6). f2/fl=13.43 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.63 was achieved, thereby satisfying the expression (9). Bfl=1.3 mm was achieved, thereby satisfying the expression (10).
FIG. 9 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the FOURTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Fifth Example
FIG. 10 shows a FIFTH EXAMPLE of the present invention. In the FIFTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the FIFTH EXAMPLE was set under the following condition.










Lens Data


L = 2.25 mm, fl = 1.99 mm, f1 = 2.4 mm, f2 = 8.4 mm, d1 = 0.3 mm,


d2 = 0.22 mm, d3 = 0.43 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.234
0.30
1.531
56.0


3(Second Face of First Lens)
0.519
0.22


4(First Face of Second Lens)
0.100
0.43
1.531
56.0


5(Second Face of Second Lens)
−0.125
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−1.35
−5.83E−1
1.23E+1
−2.36E+2
1.14E+3


3
2.43E+1
−2.14
−4.92
−7.50E+1
−9.04E+1


4
0.00
−2.32
2.46E+1
−6.49E+2
5.97E+3


5
0.00
−3.51E−1
1.97
−1.63E+1
3.62E+1









Under such conditions, L/fl=1.13 was achieved, thereby satisfying the expression (1). f1/f2=0.29 was achieved, thereby satisfying the expression (2). f1/fl=1.21 was achieved, thereby satisfying the expression (3). d2/d1=0.72 was achieved, thereby satisfying the expression (4). d1/fl=0.15 was achieved, thereby satisfying the expression (5). d3/fl=0.22 was achieved, thereby satisfying the expression (6). f2/fl=4.22 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.65 was achieved, thereby satisfying the expression (9). Bfl=1.3 mm was achieved, thereby satisfying the expression (10).
FIG. 11 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the FIFTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Sixth Example
FIG. 12 shows a SIXTH EXAMPLE of the present invention. In the SIXTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the SIXTH EXAMPLE was set under the following condition.










Lens Data


L = 2.23 mm, fl = 2.05 mm, f1 = 2.4 mm, f2 = 8.4 mm, d1 = 0.42 mm,


d2 = 0.20 mm, d3 = 0.33 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.215
0.42
1.531
56.0


3(Second Face of First Lens)
0.394
0.20


4(First Face of Second Lens)
0.015
0.33
1.531
56.0


5(Second Face of Second Lens)
−0.020
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−1.51
−7.61E−1
2.04E+1
−2.64E+2
9.09E+2


3
2.24E+1
−2.30
−1.92
−1.10E+2
1.54E+2


4
0.00
−3.40
2.11E+1
−8.31E+2
9.36E+3


5
0.00
−1.03
1.95
−1.41E+1
1.44E+1









Under such conditions, L/fl=1.09 was achieved, thereby satisfying the expression (1). f1/f2=0.29 was achieved, thereby satisfying the expression (2). f1/fl=1.17 was achieved, thereby satisfying the expression (3). d2/d1=0.48 was achieved, thereby satisfying the expression (4). d1/fl=0.21 was achieved, thereby satisfying the expression (5). d3/fl=0.16 was achieved, thereby satisfying the expression (6). f2/fl=4.10 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.62 was achieved, thereby satisfying the expression (9). Bfl=1.28 mm was achieved, thereby satisfying the expression (10).
FIG. 13 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the SIXTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Seventh Example
FIG. 14 shows a SEVENTH EXAMPLE of the present invention. In the SEVENTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the SEVENTH EXAMPLE was set under the following condition.










Lens Data


L = 2.25 mm, fl = 2.05 mm, f1 = 2.16 mm, f2 = 27.63 mm, d1 = 0.3 mm,


d2 = 0.25 mm, d3 = 0.4 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.215
0.30
1.531
56.0


3(Second Face of First Lens)
0.395
0.25


4(First Face of Second Lens)
0.018
0.40
1.531
56.0


5(Second Face of Second Lens)
−0.050
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−1.88
−6.61E−1
1.49E+1
−2.59E+2
1.05E+3


3
4.37E+1
−2.08
−8.48
−2.37E+1
−2.64E+2


4
0.00
−2.46
1.90E+1
−5.91E+2
5.65E+3


5
0.00
−5.63E−1
1.95
−1.63E+1
3.73E+1









Under such conditions, L/fl=1.10 was achieved, thereby satisfying the expression (1). f1/f2=0.08 was achieved, thereby satisfying the expression (2). f1/fl=1.05 was achieved, thereby satisfying the expression (3). d2/d1=0.83 was achieved, thereby satisfying the expression (4). d1/fl=0.15 was achieved, thereby satisfying the expression (5). d3/fl=0.20 was achieved, thereby satisfying the expression (6). f2/fl=13.48 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.63 was achieved, thereby satisfying the expression (9). Bfl=1.3 mm was achieved, thereby satisfying the expression (10).
FIG. 15 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the SEVENTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Eighth Example
FIG. 16 shows an EIGHTH EXAMPLE of the present invention. In the EIGHTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the EIGHTH EXAMPLE was set under the following condition.










Lens Data


L = 2.16 mm, fl = 1.9 mm, f1 = 2.26 mm, f2 = 8.4 mm, d1 = 0.35 mm,


d2 = 0.2 mm, d3 = 0.4 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.230
0.35
1.531
56.0


3(Second Face of First Lens)
0.468
0.20


4(First Face of Second Lens)
0.100
0.40
1.531
56.0


5(Second Face of Second Lens)
−0.125
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−2.75
−8.57E−1
4.45E+1
−8.59E+2
5.41E+3


3
3.20E+1
−2.12
−5.11
−8.95E+1
2.53


4
0.00
−2.00
9.07
−5.02E+2
5.76E+3


5
0.00
−2.96E−1
−1.34E−1
−3.50
−3.89









Under such conditions, L/fl=1.14 was achieved, thereby satisfying the expression (1). f1/f2=0.27 was achieved, thereby satisfying the expression (2). f1/fl=1.19 was achieved, thereby satisfying the expression (3). d2/d1=0.57 was achieved, thereby satisfying the expression (4). d1/fl=0.18 was achieved, thereby satisfying the expression (5). d3/fl=0.21 was achieved, thereby satisfying the expression (6). f2/fl=4.42 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.64 was achieved, thereby satisfying the expression (9). Bfl=1.21 mm was achieved, thereby satisfying the expression (10).
FIG. 17 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the EIGHTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Ninth Example
FIG. 18 shows a NINTH EXAMPLE of the present invention. In the NINTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the NINTH EXAMPLE was set under the following condition.










Lens Data


L = 2.42 mm, fl = 2.2 mm, f1 = 2.44 mm, f2 = 13.94 mm, d1 = 0.42 mm,


d2 = 0.2 mm, d3 = 0.4 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.200
0.42
1.531
56.0


3(Second Face of First Lens)
0.520
0.20


4(First Face of Second Lens)
−0.005
0.40
1.531
56.0


5(Second Face of Second Lens)
−0.140
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−2.53E−1
−7.44E−1
1.28E+1
−1.64E+2
5.84E+2


3
1.84E+1
−1.99
−3.47
−7.95E+1
6.17E+1


4
0.00
−2.14
6.52
−4.35E+2
4.72E+3


5
0.00
−4.69E−1
6.41E−1
−1.00E+1
2.02E+1









Under such conditions, L/fl=1.10 was achieved, thereby satisfying the expression (1). f1/f2=0.18 was achieved, thereby satisfying the expression (2). f1/fl=1.11 was achieved, thereby satisfying the expression (3). d2/d1=0.48 was achieved, thereby satisfying the expression (4). d1/fl=0.19 was achieved, thereby satisfying the expression (5). d3/fl=0.18 was achieved, thereby satisfying the expression (6). f2/fl=6.34 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.64 was achieved, thereby satisfying the expression (9). Bfl=1.4 mm was achieved, thereby satisfying the expression (10).
FIG. 19 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the NINTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Tenth Example
FIG. 20 shows a TENTH EXAMPLE of the present invention. In the TENTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of TENTH EXAMPLE was set under the following condition.










Lens Data


L = 2.31 mm, fl = 2.08 mm, f1 = 2.33 mm, f2 = 12.61 mm, d1 = 0.42 mm,


d2 = 0.2 mm, d3 = 0.4 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1(Diaphragm)
0.000
0.00


2(First Face of First Lens)
1.200
0.42
1.531
56.0


3(Second Face of First Lens)
0.475
0.20


4(First Face of Second Lens)
0.050
0.40
1.531
56.0


5(Second Face of Second Lens)
−0.100
0.30


6(First Face of Cover Glass)
0.000
0.30
1.516
64.0


7(Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−8.59E−1
−9.37E−1
2.63E+1
−3.98E+2
1.95E+3


3
2.16E+1
−2.04
−1.04
−1.05E+2
1.80E+2


4
0.00
−1.88
−1.75
−3.20E+2
4.19E+3


5
0.00
−3.70E−1
−9.48E−1
1.26
−1.45E+1









Under such conditions, L/fl=1.11 was achieved, thereby satisfying the expression (1). f1/f2=0.18 was achieved, thereby satisfying the expression (2). f1/fl=1.12 was achieved, thereby satisfying the expression (3). d2/d1=0.48 was achieved, thereby satisfying the expression (4). d1/fl=0.20 was achieved, thereby satisfying the expression (5). d3/fl=0.19 was achieved, thereby satisfying the expression (6). f2/fl=6.06 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.62 was achieved, thereby satisfying the expression (9). Bfl=1.29 mm was achieved, thereby satisfying the expression (10).
FIG. 21 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the TENTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Eleventh Example
FIG. 22 shows an ELEVENTH EXAMPLE of the present invention. In the ELEVENTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the ELEVENTH EXAMPLE was set under the following condition.










Lens Data


L = 2.36 mm, fl = 2.13 mm, f1 = 2.33 mm, f2 = 15.7 mm, d1 = 0.42 mm,


d2 = 0.2 mm, d3 = 0.4 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1 (Diaphragm)
0.000
0.00


2 (First Face of First Lens)
1.200
0.42
1.531
56.0


3 (Second Face of First Lens)
0.475
0.20


4 (First Face of Second Lens)
−0.005
0.40
1.531
56.0


5 (Second Face of Second Lens)
−0.125
0.30


6 (First Face of Cover Glass)
0.000
0.30
1.516
64.0


7 (Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−5.60E−1
−8.50E−1
1.90E+1
−2.62E+2
1.09E+3


3
 1.94E+1
−2.01
−2.20
−9.72E+1
1.49E+2


4
0.00
−1.99
−1.68
−3.26E+2
4.18E+3


5
0.00
−4.10E−1
−5.40E−1
−1.98
−4.20









Under such conditions, L/fl=1.11 was achieved, thereby satisfying the expression (1). f1/f2=0.15 was achieved, thereby satisfying the expression (2). f1/fl=1.09 was achieved, thereby satisfying the expression (3). d2/d1=0.48 was achieved, thereby satisfying the expression (4). d1/fl=0.20 was achieved, thereby satisfying the expression (5). d3/fl=0.19 was achieved, thereby satisfying the expression (6). f2/fl=7.37 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.63 was achieved, thereby satisfying the expression (9). Bfl=1.34 mm was achieved, thereby satisfying the expression (10).
FIG. 23 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the ELEVENTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
Twelfth Example
FIG. 24 shows a TWELFTH EXAMPLE of the present invention. In the TWELFTH EXAMPLE, like the imaging lens with the structure of FIG. 1, a diaphragm 2 is disposed on the object side of the first face of the first lens 3 and a cover glass is disposed between the second face of the second lens 4 and the image-taking surface 7, as a filter 6. The diaphragm 2 is disposed at the same position on the optical axis 8 as the point on the optical axis 8 on the first face of the first lens 3.
The imaging lens 1 of the TWELFTH EXAMPLE was set under the following condition.










Lens Data


L = 2.41 mm, fl = 2.18 mm, f1 = 2.44 mm, f2 = 12.98 mm, d1 = 0.42 mm,


d2 = 0.2 mm, d3 = 0.4 mm, Fno = 4.0



















Face Number
r
d
nd
νd





(Object Point)


1 (Diaphragm)
0.000
0.00


2 (First Face of First Lens)
1.200
0.42
1.531
56.0


3 (Second Face of First Lens)
0.520
0.20


4 (First Face of Second Lens)
−0.005
0.40
1.531
56.0


5 (Second Face of Second Lens)
−0.150
0.30


6 (First Face of Cover Glass)
0.000
0.30
1.516
64.0


7 (Second Face of Cover Glass)
0.000


(Image Surface)















Face







Number
k
A
B
C
D





2
−2.70E−1
−7.56E−1
1.38E+1
−1.84E+2
7.11E+2


3
 1.82E+1
−1.96
−3.28
−8.03E+1
6.90E+1


4
0.00
−2.08
5.78
−4.16E+2
4.54E+3


5
0.00
−4.48E−1
4.91E−1
−8.86
1.58E+1









Under such conditions, L/fl=1.11 was achieved, thereby satisfying the expression (1). f1/f2=0.19 was achieved, thereby satisfying the expression (2). f1/fl=1.12 was achieved, thereby satisfying the expression (3). d2/d1=0.48 was achieved, thereby satisfying the expression (4). d1/fl=0.19 was achieved, thereby satisfying the expression (5). d3/fl=0.18 was achieved, thereby satisfying the expression (6). f2/fl=5.95 was achieved, thereby satisfying the expression (7). S=0 mm was achieved, thereby satisfying the expression (8). Bfl/fl=0.64 was achieved, thereby satisfying the expression (9). Bfl=1.39 mm was achieved, thereby satisfying the expression (10).
FIG. 25 shows the spherical aberration, astigmatism, and distortion of the imaging lens 1 of the TWELFTH EXAMPLE.
According to the result, each of the spherical aberration, astigmatism, and distortion was almost satisfied. It can be seen from the result that a sufficient optical property can be obtained.
The present invention is not limited to the above-described embodiments and EXAMPLES, and various modifications are possible as required.
For example, a light amount limiting board can be disposed between the second face of the first lens 3 and the first face of the second lens 4 as required.

## Claims

1. An imaging lens used for forming an image of an object on an image-taking surface of a solid image pickup device, comprising:
in order from an object side towards an image surface side, a diaphragm, a first lens which is a meniscus lens having a positive power whose convex surface faces said object side, and a second lens which is a lens having a positive power whose convex surface faces said image surface side, wherein conditions expressed by each of following expressions (1)-(6) are to be satisfied;


1.25≧L/fl≧0.8  (1)



0.2>f1/f2>0  (2)



1.5≧f1/fl≧0.9  (3)



1≧d2/d1≧0.2  (4)



0.35≧d1/fl≧0.1  (5)



0.27≧d3/fl≧0.1  (6)


where,
L: entire length of said imaging lens
fl: focal distance of entire imaging lens
f1: focal distance of said first lens
f2: focal distance of said second lens
d1: center thickness of said first lens
d2: space between said first lens and said second lens on an optical axis
d3: center thickness of said second lens.

2. The imaging lens according to claim 1, wherein, said second lens is a meniscus lens.

3. The imaging lens according to claim 1, wherein an object side face of said second lens is convex towards said object side in the vicinity of said optical axis and is formed into an aspheric surface having an inflection point.

4. The imaging lens according to claim 3, wherein
an outer end section of an effective diameter of said object side face of said second lens is positioned closer to said object side than a point on said optical axis on said object side face of said second lens.

5. The imaging lens according to any one of claims 1 to 4, wherein
a condition expressed by a following expression (7) is to be satisfied;


f2/fl≧2.3  (7).

6. The imaging lens according to claim 5, wherein
said diaphragm satisfies a condition expressed by a following expression (8);


0.2≧S  (8)


where,
S: distance between said diaphragm and said optical surface closest to said object side on said optical axis.

7. The imaging lens according to claim 6, wherein
a condition expressed by a following expression (9) is to be satisfied;


0.8≧Bfl/fl≧0.4  (9)


where,
Bfl: back focus distance (distance from a last lens surface to said image-taking surface on said optical axis).

8. The imaging lens according to claim 7, wherein
a condition expressed by a following expression (10) is to be satisfied;


2.5≧Bfl≧0.8  (10).