# Patent

## Title
Image pick-up lens system

## Abstract

An image pick-up lens system includes an aperture stop (10), a biconvex first lens (20), and a meniscus-shaped second lens (30) having a concave surface on a side of an object. The aperture stop, the first lens and the second lens are aligned in that order from the object side to an image side. Each of the lenses has at least one aspheric surface, and the following conditions are satisfied: (1) 0.5<f1/f<0.9, and (2) 1<T/f<1.62, wherein f1 is a focal length of the first lens, f is a focal length of the system, and T is a length from the aperture stop to an image pick-up surface of the image side.

## Background
TECHNICAL FIELD
The present invention relates to an image pick-up lens system which projects an image of an object onto an image pick-up surface, the image pick-up lens system being suitable for use in products such as camera modules.
BACKGROUND
In recent years, camera modules for taking photos have begun to be incorporated in mobile terminals such as mobile phones and lap-top computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be commercialized. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pick-up devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modern image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, and excellent optical performance.
Compactness means in particular that a length from a lens edge of the lens system to an image pick-up surface should be as short as possible.
Low cost means in particular that the lens system should include as few lenses as possible; and that the lenses should be able to be formed from a resin or a plastic and be easily assembled.
Excellent optical performance can be classified into the following four main requirements:
First, a high brightness requirement, which means that the lens system should have a small F number (FNo.) Generally, the FNo. should be 2.8 or less.
Second, a wide angle requirement, which means that half of the field of view of the lens system should be 30° or more.
Third, a uniform illumination on the image surface requirement, which means that the lens system has few eclipses and/or narrows down an angle of incidence onto an image pick-up device.
Fourth, a high resolution requirement, which means that the lens system should appropriately correct fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration.
In a lens system which satisfies the low cost requirement, a single lens made from a resin or a plastic is desired. Typical such lens systems can be found in U.S. Pat. No. 6,297,915B1 and EP Pat. No. 1271215A2. However, even if the lens has two aspheric surfaces, it is difficult to achieve excellent optical performance, especially if a wide angle such as 70° is desired. Thus, the single lens system can generally only be used in a low-resolution image pick-up device such as a CMOS. In addition, a thick lens is generally used for correcting aberrations. Thus, a ratio of a total length of the lens system to a focal length of the lens (L/f) is about 2. In other words, it is difficult to make the lens system compact.
In a lens system which satisfies the excellent optical performance requirement, three lenses are desired. A typical such lens system can be found in U.S. Pat. No. 5,940,219. However, the ratio of a total length of the lens system to a total focal length of the three lenses (L/f) is about 2. It is difficult to make the lens system compact. In addition, the plurality of lenses increases costs.
In order to satisfy all the requirements of compactness, low cost and excellent optical performance, it is commonly believed that a two-lens system is desirable.
A well-known two-lens system is the retro-focus type lens system. A typical such lens system can be found in U.S. Pat. No. 6,449,105B1. The lens system comprises, from an object side to an image side, a first meniscus lens having negative refracting power and a convex surface on the object side, a stop, and a second meniscus lens having positive refracting power and a convex surface on the image side. The lens system helps correct wide angle aberrations. However, a shutter is positioned between the second lens and the image side, which adds to the distance between the second lens and the image side. Thus, the compactness of the lens system is limited.
U.S. Patent Publication No. 2004/0036983 discloses an image pick-up lens which overcomes the above described problems. As represented in FIG. 30 hereof, the image pick-up lens comprises, from an object side to an image side: an aperture stop 1; a biconvex positive lens 2; and a meniscus lens 3 having a concave surface on the object side. When each of the lenses 2, 3 has at least one aspheric surface, the image pick-up lens satisfies the following conditions: 0.3<f1/f<0.9 and T/f<2.4. In these expressions, “f” is an overall focal length of the lens system, “f1” is a focal length of the positive lens 2, and “T” is a length from the aperture stop 1 to an image pick-up surface 5.
However, the ratio of the total length of the lens system to the total focal length of the lenses 2, 3 (L/f) is generally about 2. The smallest ratio obtainable is 1.7, which still constitutes a limitation on the compactness of the lens system. In addition, it is difficult to correct lateral chromatic aberration effectively, and thus the optical performance of the lens system is limited.
Therefore, a low-cost image pick-up lens system which can properly correct aberrations and has a compact configuration is desired.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an image pick-up lens system which has a relatively short total length.
Another object of the present invention is to provide an image pick-up lens system which can optimally correct fundamental aberrations.
To achieve the above-described objects, an image pick-up lens system in accordance with the present invention comprises an aperture stop, a biconvex first lens, and a meniscus-shaped second lens having a concave surface on a side of an object. The aperture stop, the first lens and the second lens are aligned in that order from the object side to an image side. Each of the lenses has at least one aspheric surface. According to a first aspect, the following conditions are satisfied:

0.5<f1/f<0.9, and  (1)

1<T/f<1.62,  (2)

wherein, f1 is a focal length of the first lens, f is a focal length of the system, and T is a length from the aperture stop to an image pick-up surface of the image side.

According to a second aspect, preferably, both a first surface on the object side and a second surface on the image side of the first lens are aspheric, and the following conditions are satisfied:

0.2<R2/R1<1, and  (3)

1.2<d/R2<2.1,  (4)

wherein, R1 is an absolute value of a radius of curvature of the first surface, R2 is an absolute value of a radius of curvature of a second surface, and d is a thickness of the first lens.

Further, to correct field curvature, each of the first and second lenses is aspheric on both surfaces thereof, and the following condition is satisfied:

0.5<(1/R3)/(1/R1+1/R2+1/R4)<1,  (5)

wherein, R3 is an absolute value of a radius of curvature of a third surface of the second lens on the object side, and R4 is an absolute value of a radius of curvature of a fourth surface of the second lens on the image side.

Further still, the two lenses are made from a resin or a plastic. To correct chromatic aberration, the Abbe constant ν1 of the first lens and the Abbe constant ν2 of the second lens preferably satisfy the following condition:

ν1−ν2>20.  (6)

Because the first lens is positioned adjacent the aperture stop and has at least one aspheric surface, the image pick-up lens system can appropriately correct spherical and coma aberrations. In addition, because the second lens is positioned away from the aperture stop and has at least one aspheric surface, different chief rays of different field angle can have very different corresponding projection heights at the second lens. Therefore the system can appropriately correct astigmatism, field curvature and distortion, all of which are related to the field angle. Furthermore, the fourth surface of the second lens has a gradually varying refraction from a central portion thereof near an optical axis of the system to a peripheral edge portion thereof. Thus, a central portion of the second lens diverges light rays and a peripheral edge portion of the second lens converges light rays, so that the meridional/sagittal sections easily coincide. For all the above reasons, the optical image performance in wide angles of the system is enhanced. Moreover, because the first and second lenses can be made from a resin or a plastic, the system is relatively easy and inexpensive to mass manufacture.
Other objects, advantages and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an image pick-up lens system in accordance with the present invention;
FIGS. 2-5 are graphs respectively showing transverse ray fan plots, field curvature and distortion, longitudinal spherical aberration, and lateral chromatic aberration curves for an image pick-up lens system in accordance with a first exemplary embodiment of the present invention;
FIGS. 6-9 are graphs respectively showing transverse ray fan plots, field curvature and distortion, longitudinal spherical aberration, and lateral chromatic aberration curves for an image pick-up lens system in accordance with a second exemplary embodiment of the present invention;
FIGS. 10-13 are graphs respectively showing transverse ray fan plots, field curvature and distortion, longitudinal spherical aberration, and lateral chromatic aberration curves for an image pick-up lens system in accordance with a third exemplary embodiment of the present invention;
FIGS. 14-17 are graphs respectively showing transverse ray fan plots, field curvature and distortion, longitudinal spherical aberration, and lateral chromatic aberration curves for an image pick-up lens system in accordance with a fourth exemplary embodiment of the present invention;
FIGS. 18-21 are graphs respectively showing transverse ray fan plots, field curvature and distortion, longitudinal spherical aberration, and lateral chromatic aberration curves for an image pick-up lens system in accordance with a fifth exemplary embodiment of the present invention;
FIGS. 22-25 are graphs respectively showing transverse ray fan plots, field curvature and distortion, longitudinal spherical aberration, and lateral chromatic aberration curves for an image pick-up lens system in accordance with a sixth exemplary embodiment of the present invention;
FIGS. 26-29 are graphs respectively showing transverse ray fan plots, field curvature and distortion, longitudinal spherical aberration, and lateral chromatic aberration curves for an image pick-up lens system in accordance with a seventh exemplary embodiment of the present invention; and
FIG. 30 is a schematic representation of an image pick-up lens in accordance with a prior publication.



DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
FIG. 1 shows a schematic configuration of an image pick-up lens system in accordance with the present invention. The system comprises an aperture stop 10, a biconvex first lens 20, and a meniscus-shaped second lens 30 having a concave surface on a side of an object. The aperture stop 10, the first lens 20 and the second lens 30 are aligned in that order from the object side to an image side. The first and the second lenses 20, 30 each have at least one aspheric surface. The first and second lenses 20, 30 can be made from a resin or a plastic, which makes their manufacture relatively easy and inexpensive.
The aperture stop 10 is arranged closest to the object in order to narrow down an incident angle of chief rays onto an image pick-up surface 40 located at the image side. In addition, this arrangement of the aperture stop 10 helps shorten a total length of the system. For further cost reduction, the aperture stop 10 is preferably formed directly on a first surface (not labeled) of the first lens 20 on the object side. In practice, a portion of the first surface of the first lens 20 through which light rays are not transmitted is coated with a black material, which functions as the aperture stop 10.
In order to provide compactness and excellent optical performance, the first and second lenses 20, 30 satisfy the following conditions:

1<T/f<1.62, and  (1)

(2)0.5<f1/f<0.9,  (2)

wherein, f1 is a focal length of the first lens 20, f is a focal length of the system, and T is a length from the aperture stop 10 to the image pick-up surface 40. The first condition (1) is for limiting the total length of the system. The second condition (2) is for correcting monochromatic aberrations, and providing both compactness and a desirable distribution of refracting power. In one aspect, when the ratio f1/f is above the lower limit of 0.5, the system provides satisfactory total refracting power and keeps high-order spherical aberration, high-order coma and lateral chromatic aberration of the system in a controlled range. In another aspect, when the ratio f1/f is below the upper limit of 0.9, the system is compact and provides satisfactory total refracting power.

The surfaces of the first and second lenses 20, 30 are appropriately aspheric, which enables this small number of lenses to satisfy many if not all of the above-described requirements of compactness, low cost, and excellent optical performance.
In addition, preferably, both the first surface and a second surface (not labeled) of the first lens 20 on the image side are aspheric, and the following conditions are satisfied:

0.2<R2/R1<1, and  (3)

1.2<d/R2<2.1,  (4)

wherein, R1 is an absolute value of a radius of curvature of the first surface, R2 is an absolute value of a radius of curvature of the second surface, and d is a thickness of the first lens 20. The third condition (3) governs a distribution of refracting power for the first lens 20, in order to correct monochromatic aberrations. The fourth condition (4) is for lessening an incident angle of the second surface of the first lens 20, to further correct high-order aberrations.

The concave surface of the second lens 30 is defined as a third surface (not labeled). The first lens 20 and the second lens 30 satisfy the following condition:

0.5<(1/R3)/(1/R1+1/R2+1/R4)<1,  (5)

wherein, R3 is an absolute value of a radius of curvature of the third surface of the second lens 30, and R4 is an absolute value of a radius of curvature of a fourth surface (not labeled) of the second lens 30 on the image side.

The fifth condition (5) is for correcting field curvature and obtaining a flat field. In one aspect, when the ratio (1/R3)/(1/R1+1/R2+1/R4) is above the lower limit of 0.5, the negative Petzval's Sum produced by the third surface of the second lens 30 can compensate the total positive Petzval's Sum produced by the first and second surfaces of the first lens 20 and the fourth surface of the second lens 30. Thus, it is relatively easy to correct field curvature of the system. In another aspect, when the ratio (1/R3)/(1/R1+1/R2+1/R4) is below the upper limit of 1, the negative refracting power produced by the third surface of the second lens 30 can effectively compensate the positive coma and lateral chromatic aberration produced by the first lens 20. Meanwhile, the radius of curvature R3 of the third surface of the second lens 30 is not so small that increases the high-order aberrations of the system, and the negative refractive power provide by R3 can correct the lateral chromatic aberration of Lens 20. Furthermore, the radius of curvature R3 of the third surface of the second lens 30 has the smallest absolute value among the four absolute values of radiuses of curvature R1, R2, R3, R4 of the first and second lenses 20, 30. Thus in order to correct field curvature without producing high-order aberrations, the third surface of the second lens 30 is concave to the aperture stop 10.
Also, in order to appropriately correct the chromatic aberration of the system, the Abbe constant ν1 of the first lens 20 and the Abbe constant ν2 of the second lens 30 preferably satisfy the following condition:

ν1−ν2>20.  (6)

Further, the fourth surface of the second lens 30 preferably has a gradually varying refraction from a central portion thereof near an optical axis of the system to a peripheral edge portion thereof. Thus, a central portion of the second lens 30 diverges light rays and a peripheral edge portion of the second lens 30 converges light rays, so that meridional/sagittal sections easily coincide. This feature further enhances the optical image performance in wide angles of the system.
The above explanations outline fundamental constituent features of the system of the present invention. Examples of the system will be described below with reference to FIGS. 2-29. It is to be understood that the invention is not limited to these examples. The following are symbols used in each exemplary embodiment.

    T: length from the aperture stop to the image pick-up surface
    f: total length of the system
    FNo: F number
    ω: half field angle
    2ω: field angle
    R: radius of curvature
    d: distance between surfaces on the optical axis of the system
    Nd: refractive index of lens
    ν: Abbe constant


In each example, the first and second surfaces of the first lens 20 and the third and fourth surfaces of the second lens 30 are aspheric. The shape of each aspheric surface is provided by expression 1 below. Expression 1 is based on a Cartesian coordinate system, with the vertex of the surface being the origin, and the optical axis extending from the vertex being the x-axis.
Expression 1:




  x
  =
  
    
      
        ch
        2
      
      
        1
        +
        
          
            1
            -
            
              
                (
                
                  k
                  +
                  1
                
                )
              
              ⁢
              
                c
                2
              
              ⁢
              
                h
                2
              
            
          
        
      
    
    +
    
      ∑
      
        
          A
          i
        
        ⁢
        
          h
          i
        
      
    
  




wherein, h is a height from the optical axis to the surface, c is a vertex curvature, k is a conic constant, and Ai are i-th order correction coefficients of the aspheric surfaces.

EXAMPLE 1
Tables 1 and 2 show lens data of Example 1.







TABLE 1







f = 3.21 mm T = 4.00 mm FNo = 2.83 ω = 35°












Surface No.
R (mm)
D (mm)
Nd
ν
k















Stop 10
infinite
−0.08


0


1st surface
1.908685
1.949767
1.492
57.4
0.3969374


2nd surface
−1.386019
0.8410808


0.4322471


3rd surface
−0.6313817
0.9296156
1.585
29.9
−0.6737203


4th surface
−1.140475
0.2679967


−0.9677394


















TABLE 2









Surface No.












1st surface
2nd surface
3rd surface
4th surface















Aspherical
A2 = 0
A2 = 0
A2 = 0
A2 = 0


coefficient
A4 = −0.037981674
A4 = 0.040516016
A4 = −0.04769116
A4 = −0.0026459285



A6 = −0.11117147
A6 = −0.0040262084
A6 = 0.05474841
A6 = 0.0088382976



A8 = 0.25717755
A8 = −0.0022363842
A8 = 0.018494473
A8 = 0.0026971968



A10 = −0.44540332
A10 = 0.0040769403
A10 = −0.0063129926
A10 = 0.00071976951



A12 = 0.1964647
A12 = −0.0011660921
A12 = −0.002450423
A12 = −0.00036281554



A14 = 0
A14 = 0
A14 = 0
A14 = 0



A16 = 0
A16 = 0
A16 = 0
A16 = 0









FIGS. 2-5 are graphs of aberrations (transverse ray fan plots, field curvature/distortion, longitudinal spherical aberration, and lateral chromatic aberration) of the system of Example 1. FIGS. 2A-2D respectively show aberration curves of meridional/sagittal sections in 0°, 15°, 25° and 35° field angles. FIGS. 3A and 3B respectively show field curvature and distortion curves. The first lens 20 is made from polymethyl methacrylate (PMMA), and the second lens 30 is made from a polycarbonate.
EXAMPLE 2
Lens data of Example 2 are shown in tables 3 and 4.







TABLE 3







f = 3.19 mm T = 3.99 mm FNo = 2.80 ω = 35°












Surface No.
R (mm)
D (mm)
Nd
ν
k















Stop 10
infinite
−0.08


0


1st surface
1.895722
1.934008
1.492
57.4
−1.031395


2nd surface
−1.376186
0.8348153


0.441094


3rd surface
−0.6242678
0.9118045
1.585
29.9
−0.673748


4th surface
−1.126245
0.3122


−0.9667288


















TABLE 4









Surface No.












1st surface
2nd surface
3rd surface
4th surface















Aspherical
A2 = 0
A2 = 0
A2 = 0
A2 = 0


coefficient
A4 = −0.0086994569
A4 = 0.041163675
A4 = −0.048693006
A4 = −0.0027907875



A6 = −0.1161784
A6 = −0.0042075403
A6 = 0.057214163
A6 = 0.0092363559



A8 = 0.27353815
A8 = −0.002378654
A8 = 0.019671017
A8 = 0.0028687816



A10 = −0.48215993
A10 = 0.0044133871
A10 = −0.0068339682
A10 = 0.00077916802



A12 = 0.21645868
A12 = −0.0012847639
A12 = −0.0026997998
A12 = −0.00039973886



A14 = 0
A14 = 0
A14 = 0
A14 = 0



A16 = 0
A16 = 0
A16 = 0
A16 = 0









FIGS. 6-9 are graphs of aberrations (transverse ray fan plots, field curvature/distortion, longitudinal spherical aberration, and lateral chromatic aberration) of the system of Example 2. FIGS. 6A-6D respectively show aberrations curves of meridional/sagittal sections in 0°, 15°, 25° and 35° field angles. FIGS. 7A and 7B respectively show field curvature and distortion curves. The first lens 20 is made from polymethyl methacrylate (PMMA), and the second lens 30 is made from a polycarbonate.
EXAMPLE 3
Lens data of Example 3 are shown in tables 5 and 6. In the lens data shown below, E shows powers of 10; that is, for example, 2.5E-0.3 means 2.5×10−3.







TABLE 5







f = 3.21 mm T = 4.05 mm FNo = 2.83 ω = 35°












Surface No.
R (mm)
D (mm)
Nd
ν
k















Stop 10
infinite
−0.0798


0


1st surface
1.937576
1.951123
1.492
57.4
−1.026366


2nd surface
−1.395695
0.842203


0.3649843


3rd surface
−0.6367427
0.9198735
1.585
29.9
−0.6782141


4th surface
−1.111585
0.3119658


−4.560295


















TABLE 6









Surface No.












1st surface
2nd surface
3rd surface
4th surface















Aspherical
A2 = 0
A2 = 0
A2 = 0
A2 = 0


coefficient
A4 = −0.0090154464
A4 = 0.047546265
A4 = −0.067457258
A4 = −0.17525752



A6 = −0.027257396
A6 = −0.067428703
A6 = 0.048702325
A6 = 0.053841628



A8 = −0.31450985
A8 = 0.088613357
A8 = −0.0016956663
A8 = 0.0007158852



A10 = 0.60707428
A10 = −0.04786747
A10 = −8.414046E−005
A10 = −0.001238689



A12 = 0
A12 = 0
A12 = 0
A12 = 0



A14 = 0
A14 = 0
A14 = 0
A14 = 0



A16 = 0
A16 = 0
A16 = 0
A16 = 0









FIGS. 10-13 are graphs of aberrations (transverse ray fan plots, field curvature/distortion, longitudinal spherical aberration, and lateral chromatic aberration) of the system of Example 3. FIGS. 10A-10D respectively show aberrations curves of meridional/sagittal sections in 0°, 15°, 25° and 35° field angles. FIGS. 11A and 11B respectively show field curvature and distortion curves. The first lens 20 is made from polymethyl methacrylate (PMMA), and the second lens 30 is made from a polycarbonate.
EXAMPLE 4
Lens data of Example 4 are shown in tables 7 and 8. In the lens data shown below, E shows powers of 10.







TABLE 7







f = 3.26 mm T = 4.22 mm FNo = 2.80 ω = 35°












Surface No.
R (mm)
D (mm)
Nd
ν
k















Stop 10
infinite
−0.067878


0


1st surface
2.124272
1.991005
1.492
57.4
−12.41067


2nd surface
−1.327932
0.7080908


−0.1739528


3rd surface
−0.6807691
0.6568542
1.585
29.9
−0.9940377


4th surface
−1.337036
0.861976


−3.860014


















TABLE 8









Surface No.












1st surface
2nd surface
3rd surface
4th surface















Aspherical
A2 = 0
A2 = 0
A2 = 0
A2 = 0


coefficient
A4 = 0.10925473
A4 = 0.011162728
A4 = −0.22040448
A4 = −0.14881856



A6 = −0.13095248
A6 = −0.066705593
A6 = 0.019200829
A6 = 0.051775666



A8 = 0
A8 = 0.068492417
A8 = −0.023114479
A8 = 0.0011871839



A10 = 0
A10 = −0.04357828
A10 = −9.0582917E−005
A10 = −0.0013271067



A12 = 0
A12 = 0
A12 = 0
A12 = 0



A14 = 0
A14 = 0
A14 = 0
A14 = 0



A16 = 0
A16 = 0
A16 = 0
A16 = 0









FIGS. 14-17 are graphs of aberrations (transverse ray fan plots, field curvature/distortion, longitudinal spherical aberration, and lateral chromatic aberration) of the system of Example 4. FIGS. 14A-14D respectively show aberrations curves of meridional/sagittal sections in 0°, 15°, 25° and 35° field angles. FIGS. 15A and 15B respectively show field curvature and distortion curves. The first lens 20 is made from polymethyl methacrylate (PMMA), and the second lens 30 is made from a polycarbonate.
EXAMPLE 5
Lens data of Example 5 are shown in tables 9 and 10.







TABLE 9







f = 3.22 mm T = 4.99 mm FNo = 2.80 ω = 35°












Surface No.
R (mm)
D (mm)
Nd
ν
k















Stop 10
infinite
−0.03


0


1st surface
3.567241
2.042576
1.492
57.4
−0.9115067


2nd surface
−1.204826
0.8256124


−0.1979544


3rd surface
−0.5674448
0.8186415
1.585
29.9
−0.9227495


4th surface
−0.8538844



−1.068108


















TABLE 10









Surface No.












1st surface
2nd surface
3rd surface
4th surface















Aspherical
A2 = 0
A2 = 0
A2 = 0
A2 = 0


coefficient
A4 = −0.038187626
A4 = 0.046356766
A4 = 0.040616649
A4 = 0.0185752



A6 = 0.025227628
A6 = −0.0032353324
A6 = 0.085273579
A6 = 0.0030064393



A8 = 0.05774558
A8 = −0.0028835816
A8 = −0.079657862
A8 = 0.002911957



A10 = −0.44540332
A10 = 0.0019185201
A10 = 0.050834821
A10 = 0.00065240269



A12 = 0.1964647
A12 = 0.00024573464
A12 = −0.016829857
A12 = −0.00033965939



A14 = 0
A14 = 0
A14 = 0
A14 = 0



A16 = 0
A16 = 0
A16 = 0
A16 = 0









FIGS. 18-21 are graphs of aberrations (transverse ray fan plots, field curvature/distortion, longitudinal spherical aberration, and lateral chromatic aberration) of the system of Example 5. FIGS. 18A-18D respectively show aberrations curves of meridional/sagittal sections in 0°, 15°, 25° and 35° field angles. FIGS. 19A and 19B respectively show field curvature and distortion curves. The first lens 20 is made from polymethyl methacrylate (PMMA), and the second lens 30 is made from a polycarbonate.
EXAMPLE 6
Lens data of Example 6 are shown in tables 11 and 12.







TABLE 11







f = 3.19 mm T = 4.32 mm FNo = 2.8 ω = 35°












Surface No.
R (mm)
D (mm)
Nd
ν
k















Stop 10
infinite
−0.05


0


1st surface
2.704951
1.934008
1.531
56.0
−19.70274


2nd surface
−1.264784
0.8348153


−0.3060202


3rd surface
−0.60228
0.9118045
1.585
29.9
−0.9132682


4th surface
−1.056885
0.640254


−2.01608


















TABLE 12









Surface No.












1st surface
2nd surface
3rd surface
4th surface















Aspherical
A2 = 0
A2 = 0
A2 = 0
A2 = 0


coefficient
A4 = 0.078640248
A4 = 0.043790817
A4 = 0.086709045
A4 = 0.028145024



A6 = −0.1986909
A6 = −0.092545745
A6 = 0.049096719
A6 = −0.00012293722



A8 = 0.27353815
A8 = 0.12998134
A8 = −0.10131265
A8 = −0.0034547048



A10 = −0.48215993
A10 = −0.11896713
A10 = −0.14111931
A10 = −0.00054609844



A12 = 0.21645868
A12 = 0.038733469
A12 = 0.13145728
A12 = 0.00054942201



A14 = 0
A14 = 0
A14 = 0
A14 = 0



A16 = 0
A16 = 0
A16 = 0
A16 = 0









FIGS. 22-25 are graphs of aberrations (transverse ray fan plots, field curvature/distortion, longitudinal spherical aberration, and lateral chromatic aberration) of the system of Example 6. FIGS. 22A-22D respectively show aberrations curves of meridional/sagittal sections in 0°, 15°, 25° and 35° field angles. FIGS. 23A and 23B respectively show field curvature and distortion curves. The first lens 20 is made from a cyclo-olefin polymer, and the second lens 30 is made from a polycarbonate.
EXAMPLE 7
Lens data of Example 7 are shown in tables 13 and 14.







TABLE 13







f = 3.19 mm T = 5.12 mm FNo = 2.73 ω = 35°












Surface No.
R (mm)
D (mm)
Nd
ν
k















Stop 10
infinite
−0.04


0


1st surface
3.73331
1.934008
1.531
56.0
7.649853


2nd surface
−0.9675068
0.3958819


−0.5408035


3rd surface
−0.5563445
0.9118045
1.585
29.9
−0.7896022


4th surface
−1.039531



−1.222231


















TABLE 14









Surface No.












1st surface
2nd surface
3rd surface
4th surface















Aspherical
A2 = 0
A2 = 0
A2 = 0
A2 = 0


coefficient
A4 = −0.036296997
A4 = 0.1129865
A4 = 0.31675922
A4 = 0.021944232



A6 = −0.14080751
A6 = −0.084130154
A6 = 0.1861908
A6 = 0.0334663164



A8 = 0.27353815
A8 = 0.12343708
A8 = −0.20723569
A8 = −0.0087931231



A10 = −0.48215993
A10 = −0.11896713
A10 = 0.0707272
A10 = −0.002238773



A12 = 0.21645868
A12 = 0.038733447
A12 = 0.032847897
A12 = 0.001090651



A14 = 0
A14 = 0
A14 = 0
A14 = 0



A16 = 0
A16 = 0
A16 = 0
A16 = 0









FIGS. 26-29 are graphs of aberrations (transverse ray fan plots, field curvature/distortion, longitudinal spherical aberration, and lateral chromatic aberration) of the system of Example 7. FIGS. 26A-26D respectively show aberrations curves of meridional/sagittal sections in 0°, 15°, 25° and 35° field angles. FIGS. 27A and 27B respectively show field curvature and distortion curves. The first lens 20 is made from a cyclo-olefin polymer, and the second lens 30 is made from a polycarbonate.
Table 15 compares focal lengths and other parameters across Examples 1 through 7.









TABLE 15









Example















1
2
3
4
5
6
7


















FNo
2.83
2.8
2.83
2.8
2.8
2.8
2.73


2ω (°)
70
70
70
70
70
70
70


T (mm)
4
3.99
4.05
4.22
4.99
4.32
5.12


f (mm)
3.21
3.19
3.21
3.26
3.22
3.19
3.19


T/f
1.25
1.24
1.26
1.29
1.55
1.35
1.61


f1/f
0.63
0.63
0.62
0.63
0.66
0.61
0.53


R2/R1
0.73
0.73
0.72
0.62
0.34
0.47
0.26


d/R2
1.4
1.41
1.4
1.49
1.69
1.53
1.99


(1/R3)/(1/R1 + 1/R2 +
0.75
0.75
0.74
0.75
0.77
0.79
0.79


1/R4)


ν1-ν2
27.5
27.5
27.5
27.5
27.5
26.1
26.1









As seen in the above-described examples, the present invention provides a low-cost image pick-up lens system with a field angle of at least 70°. The total length of the system is small, and the system appropriately corrects fundamental aberrations.
It is to be understood that the invention may be embodied in other forms without departing from the spirit thereof. Thus, the described exemplary embodiments are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein.

## Claims

1. An image pick-up lens system comprising:
an aperture stop;
a biconvex first lens; and
a meniscus-shaped second lens having a concave surface on a side of an object;
wherein the aperture stop, the first lens and the second lens are aligned in that order from the object side to an image side, the first lens is aspheric on both convex surfaces thereof, the second lens has at least one aspheric surface, and the following conditions are satisfied;


0.6<f1/f<0.7,  (1)



1<T/f<1.56,  (2)



0.45<R2/R1<1; and  (3)



1.2<d/R2<2.1,   (4)



wherein f1 is a focal length of the first lens, f is a focal length of the system, T is a length from the aperture stop to an image pick-up surface of the image side, R1 is an absolute value of a radius of curvature of the first lens on the object side, R2 is an absolute value of a radius of curvature of the first lens on the image side, and d is a thickness of the first lens.

2. The image pick-up lens system in accordance with claim 1, wherein the second lens is aspheric on both surfaces and the following condition is satisfied;


0.5<(1/R3)/(1/R1+1/R2+1/R4)<1,  (5)

where R3 is an absolute value of a radius of curvature of the second lens on the object side, and R4 is an absolute value of a radius of curvature of the second lens on the image side.

3. The image pick-up lens system in accordance with claim 1, wherein the following condition is satisfied:


ν1-ν2<20,  (6)

where ν1 is the Abbe constant of the first lens, and ν2 is the Abbe constant of the second lens.

4. The image pick-up lens system in accordance with claim 1, wherein the first and second lenses are made from a resin or a plastic material respectively.

5. The image pick-up lens system in accordance with claim 4, wherein the first and second lenses are made from materials selected from polymethyl methacrylate, a polycarbonate, and a cyclo-olefin polymer.

6. The image pick-up lens system in accordance with claim 1, wherein the aperture stop is formed on the first lens.