Abstract:
An optical lens system for taking image consisting of: an aperture stop; a first lens element with positive refractive power having a convex object-side surface; a plastic second lens element with positive refractive power having a concave object-side surface and a convex image-side surface, at least one of the object-side and the image-side surfaces of the second lens element being aspheric; and a plastic third lens element with positive refractive power having a convex object-side surface, at least one of the object-side and the image-side surfaces of the third lens element being aspheric. A focal length of the optical lens system for taking image is f, a focal length of the second lens element is f2, a focal length of the third lens element is f3, and at least one of the relations of f/f2 and f/f3 is larger than 0.1.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical lens system for taking image, and more particularly to a miniaturized optical lens system for taking image used in a mobile phone camera. 
   2. Description of the Prior Art 
   In recent years, with the popularity of the mobile phone camera, the optical lens system for taking image has become thinner and thinner, and the electronic imaging sensor of a general digital camera is typically a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) sensor. Due to advances in semiconductor manufacturing, the pixel size of sensors has been reduced continuously, and miniaturized optical lens systems for taking image have increasingly higher resolution. Therefore, there&#39;s increasing demand for image quality. 
   A conventional mobile phone camera, such as the optical lens system for taking image described in U.S. Pat. No. 7,145,736, usually consists of three lens elements, from the object side to the image side: a first lens element with positive refractive power, a second lens element with negative refractive power and a third lens element with positive refractive power. The refractive power arrangement of the lens elements is favorable to correct various aberrations caused by the system, however, the second lens element is a negative lens element and the refractive power of the third lens element is generally small. So in order to provide the refractive power required by the optical lens system, the refractive power of the first lens element must be increased. As a result, the sensitivity of the optical lens system will be increased comparatively, such that the production yield of the optical lens system is reduced. 
   An optical lens system for taking image which uses lens elements made of glass and plastic is described in U.S. Pat. No. 6,490,102, wherein the third lens element is a spherical glass lens element. The use of the spherical lens element reduces the degrees of freedom available for correcting the off-axis aberration of the optical lens system, making it more difficult to control image quality. 
   The present invention has arisen to mitigate and/or obviate the afore-described disadvantages. 
   SUMMARY OF THE INVENTION 
   The primary objective of the present invention is to improve image quality, while considering the ease of manufacture of the lens elements. The present invention provides a three-lens type optical lens system. 
   An optical lens system for taking image in accordance with the present invention comprises: in order from the object side to the image side: 
   a first lens element with positive refractive power having a convex object-side surface; 
   a plastic second lens element with positive refractive power having a concave object-side surface and a convex image-side surface, at least one of the object-side surface and the image-side surface of the second lens element being aspheric; and 
   a third lens element with positive refractive power having a convex object-side surface, at least one of the object-side surface and the image-side surface of the third lens element being aspheric. 
   In the present optical lens system for taking image, the required refractive power to the optical lens system is mainly supplied by the first lens element with positive refractive power. The second and third lens elements serve as correction lens elements to effectively distribute the refractive power of the first lens element, so as to reduce the aberration caused by the optical lens system. 
   The first lens element provides enough positive refractive power, and the aperture stop is located close to the object side, so that a total track length (TTL) of the optical lens system for taking image can be effectively reduced (the total track length here is defined as a distance from the position of the object-side surface of the first lens element on the optical axis to the image plane, which includes a thickness of a flat glass between the image-side surface of the last lens element and the image plane). In addition, the exit pupil of the optical lens system will be far away from the image plane. Therefore, the light will be projected onto the sensor with a relatively small incident angle, this is the telecentric feature of the image side, and this feature is very important to the photosensitive power of the current solid-state sensor, and can improve the photosensitivity of the sensor while reducing the probability of the occurrence of shading. 
   For a wide-angle optical lens system, the correction to the distortion and the chromatic aberration of magnification is very important, and the correction is made by arranging the aperture stop at a balanced position of the refractive power of the optical lens system. 
   If the aperture stop of the present optical lens system is arranged between the first lens element and the second lens element, a balance between the properties of telecentric and wide field of view will be achieved and the sensitivity of the optical lens system can be reduced effectively. 
   With the miniaturization of the optical lens system and the requirement of a large field of view, the focal length of the optical lens system is becoming very short. Therefore, the radius of curvature and the size of the lens elements must be very small, and it is impossible to make such glass lens elements by conventional grind method. Plastic material is introduced to make lens elements by injection molding, using a relatively low cost to produce high precision lens elements. The surfaces of lens element are aspheric, allowing more design parameters (than spherical surfaces), so as to better reduce the aberration and the number of the lens elements. 
   In the present optical lens system for taking image, the first lens element has the convex object-side surface and a convex or concave image-side surface, the second lens element has the concave object-side surface and the convex image-side surface, and the third lens element has the convex object-side surface and a concave image-side surface. Such arrangements can effectively improve image quality of the system. 
   In the present optical lens system for taking image, a focal length of the optical lens system for taking image is f, a focal length of the second lens element is f2, a focal length of the third lens element is f3, at least one of the relations of f/f2 and f/f3 is larger than 0.1. 
   If f/f2 or f/f3 satisfies the above relation, the second and third lens elements can effectively distribute the refractive power of the first lens element. And it will be better if at least one of the relations of f/f2 and f/f3 is larger than 0.2. 
   In the present optical lens system for taking image, the focal length of the first lens element is f1, the focal length of the second lens element is f2, the focal length of the third lens element is f3, and they satisfy the relations:
 
 f 3/ f 1&gt;3;
 
 f 2/ f 1&gt;2.
 
   If f3/f1 and f2/f1 satisfy the above relations, the second and third lens elements serve as correction lens elements to effectively reduce the aberration caused by the optical lens system. And it will be better if f3/f1 satisfies the relation:
 
 f 3/ f 1&gt;6.
 
   In the present optical lens system for taking image, an Abbe number of the second lens element is V2, and it satisfies the relation:
 
V2&gt;40.
 
   If V2 satisfies the above relation, it is favorable to correct the coma aberration of the system. And it will be better if the Abbe number V1 of the first lens element, the Abbe number V2 of the second lens element and the Abbe number V3 of the third lens element V3 satisfy the relations:
 
52&lt;V1&lt;62;
 
52&lt;V2&lt;62;
 
52&lt;V3&lt;62.
 
   In the present optical lens system for taking image, the refractive index of the first lens element is N1, the refractive index of the second lens element is N2, the refractive index of the third lens element is N3, and they satisfy the relations:
 
1.52&lt;N1&lt;1.60;
 
1.52&lt;N2&lt;1.60;
 
1.52&lt;N3&lt;1.60.
 
   If N1, N2 and N3 satisfy the above relations, it is easy to find suitable plastic material to match the optical lens system. 
   In the present optical lens system for taking image, the focal length of the optical lens system for taking image is f, the focal length of the first lens element is f1, and they satisfy the relation:
 
0.7 &lt;f/f 1&lt;1.05.
 
   If the value of f/f1 is smaller than the above lower limit, the refractive power of the system will be weak, the total track length of the system will be too long, and it will be difficult to suppress the incident angle of the light with respect to the sensor. And if the value of f/f1 is greater than the above upper limit, the high order aberration of the system will be too large. 
   In the present optical lens system for taking image, the radius of curvature of the object-side surface of the first lens element is R1, the radius of curvature of the image-side surface of the first lens element is R2, and they satisfy the relation:
 
0.25 &lt;R 1/ R 2&lt;0.7.
 
   If the value of R1/R2 is smaller than the above lower limit, it will be difficult to correct the astigmatism caused by the system. And if the value of R1/R2 is greater than the above upper limit, it will be difficult to correct the spherical aberration caused by the system. 
   In the present optical lens system for taking image, the edge thickness of the third lens element is ET3, the center thickness of the third lens element is CT3, and they satisfy the relation:
 
0.8 &lt;ET 3/ CT 3&lt;1.4.
 
   The edge thickness is: the length projected on an optical axis by the distance between the positions of the effective diameter of the object-side and the image-side surfaces of the lens. 
   The above relation can effectively correct the off-axis aberration of the system, so as to improve the image quality. And it will be better if the value of ET3/CT3 satisfies the relation:
 
0.9 &lt;ET 3/ CT 3&lt;1.1.
 
   In the present optical lens system for taking image, making the periphery of the third lens element have a negative refractive power can effectively flatten the periphery of the image, so as to improve the image quality of the periphery of the image. 
   In the present optical lens system for taking picture, an object to be photographed is imaged on an electronic imaging sensor, the total track length of the system is TTL, the image height of the system is ImgH, and they satisfy the relation:
 
 TTL/ImgH&lt; 2.35.
 
   The above relation can maintain the objective of miniaturization of the optical lens system for taking image. 
   The present invention will become more obvious from the following description when taken in connection with the accompanying drawings, which show, for purpose of illustrations only, the preferred embodiments in accordance with the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an optical lens system for taking image in accordance with a first embodiment of the present invention; 
       FIG. 2  shows the aberration curve of the first embodiment of the present invention; 
       FIG. 3  shows an optical lens system for taking image in accordance with a second embodiment of the present invention; 
       FIG. 4  shows the aberration curve of the second embodiment of the present invention; 
       FIG. 5  shows an optical lens system for taking image in accordance with a third embodiment of the present invention; and 
       FIG. 6  shows the aberration curve of the third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , which shows an optical lens system for taking image in accordance with a first embodiment of the present invention, and  FIG. 2  shows the aberration curve of the first embodiment of the present invention. An optical lens system for taking image in accordance with a first embodiment of the present invention comprises: in order from the object side to the image side: 
   An aperture stop  40 . 
   A plastic first lens element  10  with positive refractive power has a convex object-side surface  11  and a concave image-side surface  12 , and the object-side surface  11  and the image-side surface  12  of the first lens element  10  are aspheric. 
   A plastic second lens element  20  with positive refractive power has a concave object-side surface  21  and a convex image-side surface  22 , and the object-side surface  21  and the image-side surface  22  of the second lens element  20  are aspheric. 
   A plastic third lens element  30  with positive refractive power has a convex object-side surface  31  and a concave image-side surface  32 , and the object-side surface  31  and the image-side surface  32  of the third lens element  30  are aspheric. 
   An IR cut filter  50  is located behind the third lens element  30  and has no influence on the focal length of the optical lens system. 
   A sensor cover glass  60  is located behind the IR cut filter  60  and has no influence on the focal length of the optical lens system. 
   An image plane  70  is located behind the sensor cover glass  60 . 
   The equation for the aspheric surface profiles of the first embodiment is expressed as follows: 
   
     
       
         
           
             X 
             ⁡ 
             
               ( 
               Y 
               ) 
             
           
           = 
           
             
               
                 ( 
                 
                   
                     Y 
                     2 
                   
                   / 
                   R 
                 
                 ) 
               
               / 
               
                 ( 
                 
                   1 
                   + 
                   
                     sqrt 
                     ⁡ 
                     
                       ( 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               1 
                               + 
                               k 
                             
                             ) 
                           
                           * 
                           
                             
                               ( 
                               
                                 Y 
                                 / 
                                 R 
                               
                               ) 
                             
                             2 
                           
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
             
             + 
             
               
                 ∑ 
                 i 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   ( 
                   
                     A 
                     i 
                   
                   ) 
                 
                 * 
                 
                   ( 
                   
                     Y 
                     i 
                   
                   ) 
                 
               
             
           
         
       
     
   
   wherein: 
   X: the height of a point on the aspheric lens surface at a distance Y from the optical axis relative to the tangential plane of the aspheric surface vertex; 
   Y: the distance from the point on the curve of the aspheric surface to the optical axis; 
   k: the conic coefficient; 
   Ai: the aspheric surface coefficient of order i. 
   In the first embodiment of the present optical lens system for taking image, the focal length of the optical lens system for taking image is f, the focal length of the first lens element is f1, the focal length of the second lens element is f2, the focal length of the third lens element is f3, and they satisfy the relations:
 
f=2.62 mm;
 
 f/f 1=0.86;
 
 f/f 2=0.04
 
 f/f 3=0.22;
 
 f 2 /f 1=19.43;
 
 f 3/ f 1=3.88.
 
   In the first embodiment of the present optical lens system for taking image, the refractive index of the first lens element is N1, the refractive index of the second lens element is N2, the refractive index of the third lens element is N3, and they satisfy the relations:
 
N1=1.544;
 
N2=1.544;
 
N3=1.544.
 
   In the first embodiment of the present optical lens system for taking image, the Abbe number of the first lens element is V1 the Abbe number of the second lens element is V2, the Abbe number of the third lens element is V3, and they satisfy the relations:
 
V1=55.9;
 
V2=55.9;
 
V3=55.9.
 
   In the first embodiment of the present optical lens system for taking image, the radius of curvature of the object-side surface of the first lens element is R1, the radius of curvature of the image-side surface of the first lens element is R2, and they satisfy the relation:
 
 R 1/ R 2=0.34.
 
   In the first embodiment of the present optical lens system for taking image, the edge thickness of the third lens element is ET3, the center thickness of the third lens element is CT3, and they satisfy the relation:
 
 ET 3/ CT 3=0.65.
 
   In the first embodiment of the present optical lens system for taking image, the total track length of the optical lens system for taking image is TTL, the maximum image height of the optical lens system for taking image is lmgh, and they satisfy the relation:
 
 TTL/ImgH= 2.11.
 
   The detailed optical data of the first embodiment is shown in table 1, and the aspheric surface data is shown in table 2, wherein the units of the radius of curvature, the thickness and the focal length are expressed in mm, and HFOV is half of the maximal field of view. 
   
     
       
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               (Embodiment 1) 
             
             
               f(focal length) = 2.62 mm, Fno = 2.83, HFOV(half of field of view) = 32.9 deg. 
             
           
        
         
             
                 
                 
               Curvature 
                 
                 
                 
                 
               Focal 
             
             
               Surface # 
                 
               Radius 
               Thickness 
               Material 
               Index 
               Abbe # 
               length 
             
             
                 
             
           
        
         
             
               0 
               Object 
               Plano 
               Infinity 
                 
                 
                 
                 
             
             
               1 
               Aperture 
               Plano 
               −0.097 
             
             
                 
               Stop 
             
             
               2 
               Lens 1 
                1.15742(ASP) 
               0.324 
               Plastic 
               1.544 
               55.9 
               3.06 
             
             
               3 
                 
                3.42870(ASP) 
               0.845 
             
             
               4 
               Lens 2 
               −0.54216(ASP) 
               0.300 
               Plastic 
               1.544 
               55.9 
               59.40 
             
             
               5 
                 
               −0.63719(ASP) 
               0.050 
             
             
               6 
               Lens 3 
                1.36830(ASP) 
               0.625 
               Plastic 
               1.544 
               55.9 
               11.87 
             
             
               7 
                 
                1.45676(ASP) 
               0.250 
             
             
               8 
               IR-filter 
               Plano 
               0.300 
               Glass 
               1.517 
               64.2 
             
             
               9 
                 
               Plano 
               0.050 
             
             
               10 
               Sensor 
               Plano 
               0.400 
               Glass 
               1.517 
               64.2 
             
             
                 
               Cover 
             
             
                 
               Glass 
             
             
               11 
                 
               Plano 
               0.407 
             
             
               12 
               Image 
               Plano 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Aspheric Coefficients 
             
           
        
         
             
               Surface # 
               2 
               3 
               4 
               5 
               6 
               7 
             
             
                 
             
             
               k = 
               −1.81882E−01 
                3.60023E+01 
               −6.10384E−01 
               −9.24869E−01 
               −7.88241E+00 
               −6.45929E+00 
             
             
               A4 = 
                8.60233E−02 
                5.25618E−03 
                4.00551E−01 
               −6.63351E−02 
               −1.03925E−01 
               −1.87211E−01 
             
             
               A6 = 
               −1.50472E−02 
               −5.02195E−01 
                4.75880E−01 
                2.30581E−01 
                9.66128E−02 
                1.10788E−01 
             
             
               A8 = 
                4.75950E−01 
                2.79215E+00 
               −4.43560E−00 
                5.25284E−01 
               −4.34932E−02 
               −4.33451E−02 
             
             
               A10 = 
                 
               −7.90842E+00 
                8.16882E+00 
                5.46476E−01 
                1.44641E−03 
                6.10503E−03 
             
             
               A12 = 
                 
                 
               −8.56322E+00 
               −8.42492E−01 
                1.80807E−03 
               −3.48397E−04 
             
             
                 
             
           
        
       
     
   
   Referring to  FIG. 3 , which shows an optical lens system for taking image in accordance with a second embodiment of the present invention, and  FIG. 4  shows the aberration curve of the second embodiment of the present invention. The second embodiment of the present invention comprises: in order from the object side to the image side: 
   An aperture stop  40 . 
   A plastic first lens element  10  with positive refractive power has a convex object-side surface  11  and a concave image-side surface  12 , and the object-side surface  11  and the image-side surface  12  of the first lens element  10  are aspheric. 
   A plastic second lens element  20  with positive refractive power has a concave object-side surface  21  and a convex image-side surface  22 , and the object-side surface  21  and the image-side surface  22  of the second lens element  20  are aspheric. 
   A plastic third lens element  30  with positive refractive power has a convex object-side surface  31  and a concave image-side surface  32 , the object-side surface  31  and the image-side surface  32  of the third lens element  30  are aspheric, and the peripheral edge of the third lens element  30  has a negative refractive power. 
   An IR cut filter  50  is located behind the third lens element  30  and has no influence on the focal length of the optical lens system. 
   A sensor cover glass  60  is located behind the IR cut filter  60  and has no influence on the focal length of the optical lens system. 
   An image plane  70  is located behind the sensor cover glass  60 . 
   The equation for the aspheric surface profiles of the second embodiment has the same form as that of the first embodiment. 
   In the second embodiment of the present optical lens system for taking image, the focal length of the optical lens system for taking image is f, the focal length of the first lens element is f1, the focal length of the second lens element is f2, the focal length of the third lens element is f3, and they satisfy the relations:
 
F=3.04 mm;
 
 f/f 1=0.75;
 
 f/f 2=0.26;
 
 f/f 3=0.06;
 
 f 2/ f 1=2.90;
 
 f 3 /f 1=12.98.
 
   In the second embodiment of the present optical lens system for taking image, the refractive index of the first lens element is N1, the refractive index of the second lens element is N2, the refractive index of the third lens element is N3, and they satisfy the relations:
 
N1=1.544;
 
N2=1.544;
 
N3=1.544.
 
   In the second embodiment of the present optical lens system for taking image, the Abbe number of the first lens element is V1, the Abbe number of the second lens element is V2, the Abbe number of the third lens element is V3, and they satisfy the relations:
 
V1=55.9;
 
V2=55.9;
 
V3=55.9.
 
   In the second embodiment of the present optical lens system for taking image, the radius of curvature of the object-side surface of the first lens element is R1, the radius of curvature of the image-side surface of the first lens element is R2, and they satisfy the relation:
 
 R 1/ R 2=0.47.
 
   In the second embodiment of the present optical lens system for taking image, the edge thickness of the third lens element is ET3, the center thickness of the third lens element is CT3, and they satisfy the relation:
 
 ET 3/ CT 3=1.06.
 
   In the second embodiment of the present optical lens system for taking image, the total track length of the optical lens system for taking image is TTL, the maximum image height of the optical lens system for taking image is ImgH, and they satisfy the relation:
 
 TTL/ImgH= 2.31.
 
   The detailed optical data of the second embodiment is shown in table 3, and the aspheric surface data is shown in table 4, wherein the units of the radius of curvature, the thickness and the focal length are expressed in mm, and HFOV is half of the maximal field of view. 
   
     
       
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               (Embodiment 2) 
             
             
               f(focal length) = 3.04 mm, Fno = 3.2, HFOV(half of field of view) = 28.9 deg. 
             
           
        
         
             
                 
                 
               Curvature 
                 
                 
                 
                 
               Focal 
             
             
               Surface # 
                 
               Radius 
               Thickness 
               Material 
               Index 
               Abbe # 
               length 
             
             
                 
             
           
        
         
             
               0 
               Object 
               Plano 
               Infinity 
                 
                 
                 
                 
             
             
               1 
               Aperture 
               Plano 
               −0.09 
             
             
                 
               Stop 
             
             
               2 
               Lens 1 
                1.27257(ASP) 
               0.355 
               Plastic 
               1.544 
               55.9 
               4.04 
             
             
               3 
                 
                2.72255(ASP) 
               0.692 
             
             
               4 
               Lens 2 
               −1.35999(ASP) 
               0.424 
               Plastic 
               1.544 
               55.9 
               11.75 
             
             
               5 
                 
               −1.24471(ASP) 
               0.050 
             
             
               6 
               Lens 3 
                1.04656(ASP) 
               0.350 
               Plastic 
               1.544 
               55.9 
               52.46 
             
             
               7 
                 
                0.95836(ASP) 
               0.800 
             
             
               8 
               IR-filter 
               Plano 
               0.300 
               Glass 
               1.517 
               64.2 
             
             
               9 
                 
               Plano 
               0.100 
             
             
               10 
               Sensor 
               Plano 
               0.400 
               Glass 
               1.517 
               64.2 
             
             
                 
               Cover 
             
             
                 
               Glass 
             
             
               11 
                 
               Plano 
               0.401 
             
             
               12 
               Image 
               Plano 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               Aspheric Coefficients 
             
           
        
         
             
               Surface # 
               2 
               3 
               4 
               5 
               6 
               7 
             
             
                 
             
             
               k = 
               −3.89305E−01 
                2.08228E+01 
               −3.78636E−01 
               −2.89933E+00 
               −1.29472E+00 
               −1.91756E+00 
             
             
               A4 = 
                6.14447E−02 
               −1.07163E−03 
                3.70316E−01 
                6.58893E−02 
               −2.76837E−01 
               −2.50759E−01 
             
             
               A6 = 
                7.71456E−02 
               −5.47057E−01 
                1.42616E−01 
               −4.88203E−03 
                3.34275E−02 
                1.03315E−01 
             
             
               A8 = 
               −1.13448E−02 
                2.93527E+00 
               −2.20086E+00 
               −2.83119E−02 
               −1.27310E−04 
               −4.40820E−02 
             
             
               A10 = 
                 
               −9.19340E+00 
                5.62154E+00 
                3.78583E−01 
                4.82019E−02 
                1.64415E−03 
             
             
               A12 = 
                 
                 
               −4.81422E+00 
               −3.71796E−01 
               −6.91957E−02 
               −1.82834E−03 
             
             
                 
             
           
        
       
     
   
   Referring to  FIG. 5 , which shows an optical lens system for taking image in accordance with a third embodiment of the present invention, and  FIG. 6  shows the aberration curve of the third embodiment of the present invention. The third embodiment of the present invention comprises: in order from the object side to the image side: 
   A plastic first lens element  10  with positive refractive power has a convex object-side surface  11  and a convex image-side surface  12 , and the object-side surface  11  and the image-side surface  12  of the first lens element  10  are aspheric. 
   An aperture stop  40 . 
   A plastic second lens element  20  with positive refractive power has a concave object-side surface  21  and a convex image-side surface  22 , and the object-side surface  21  and the image-side surface  22  of the second lens element  20  are aspheric. 
   A plastic third lens element  30  with positive refractive power has a convex object-side surface  31  and a concave image-side surface  32 , the object-side surface  31  and the image-side surface  32  of the third lens element  30  are aspheric, and the peripheral edge of the third lens element  30  has a negative refractive power. 
   An IR cut filter  50  is located behind the third lens element  30  and has no influence on the focal length of the optical lens system. 
   A sensor cover glass  60  is located behind the IR cut filter  60  and has no influence on the focal length of the optical lens system. 
   An image plane  70  is located behind the sensor cover glass  60 . 
   The equation for the aspheric surface profiles of the third embodiment has the same form as that of the first embodiment. 
   In the third embodiment of the present optical lens system for taking image, the focal length of the optical lens system for taking image is f, the focal length of the first lens element is f1, the focal length of the second lens element is f2, the focal length of the third lens element is f3, and they satisfy the relations:
 
f=2.56 mm;
 
 f/f 1=0.98;
 
 f/f 2=2=0.06;
 
 f/f 3=0.11;
 
 f 2 /f 1=17.80;
 
 f 3 /f 1=8.89.
 
   In the third embodiment of the present optical lens system for taking image, the refractive index of the first lens element is N1, the refractive index of the second lens element is N2, the refractive index of the third lens element is N3, and they satisfy the relations:
 
N1=1.544;
 
N2=1.544;
 
N3=1.544.
 
   In the third embodiment of the present optical lens system for taking image, the Abbe number of the first lens element is V1, the Abbe number of the second lens element is V2, the Abbe number of the third lens element is V3, and they satisfy the relations:
 
V1=55.9;
 
V2=55.9;
 
V3=55.9.
 
   In the third embodiment of the present optical lens system for taking image, the radius of curvature of the object-side surface of the first lens element is R1, the radius of curvature of the image-side surface of the first lens element is R2, and they satisfy the relation:
 
 R 1/ R 2=−0.12.
 
   In the third embodiment of the present optical lens system for taking image, the edge thickness of the third lens element is ET3, the center thickness of the third lens element is CT3, and they satisfy the relation:
 
 ET 3/ CT 3=0.79.
 
   In the third embodiment of the present optical lens system for taking image, the total track length of the optical lens system for taking image is TTL, the maximum image height of the optical lens system for taking image is ImgH, and they satisfy the relation:
 
 TTL/ImgH= 2.26.
 
   The detailed optical data of the third embodiment is shown in table 5, and the aspheric surface data is shown in table 6, wherein the units of the radius of curvature, the thickness and the focal length are expressed in mm, and HFOV is half of the maximal field of view. 
   In the present optical lens system for taking image, the second lens element is made of plastic, and the first and third lens elements can be made of glass or plastic. If the lens elements are made of glass, the freedom of distributing the refractive power of the optical lens system will be improved. If the lens elements are made of plastic, the cost will be effectively reduced. 
   
     
       
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 5 
             
           
           
             
                 
             
             
               (Embodiment 3) 
             
             
               f(focal length) = 2.56 mm, Fno = 2.85, HFOV(half of field of view) = 32.6 deg. 
             
           
        
         
             
                 
                 
               Curvature 
                 
                 
                 
                 
               Focal 
             
             
               Surface # 
                 
               Radius 
               Thickness 
               Material 
               Index 
               Abbe # 
               length 
             
             
                 
             
           
        
         
             
               0 
               Object 
               Plano 
               Infinity 
                 
                 
                 
                 
             
             
               1 
               Lens 1 
                1.54804(ASP) 
               0.701 
               Plastic 
               1.544 
               55.9 
               2.59 
             
             
               2 
                 
               −13.38270(ASP) 
               0.085 
             
             
               3 
               Aperture 
               Plano 
               0.526 
             
             
                 
               Stop 
             
             
               4 
               Lens 2 
                −0.56535(ASP) 
               0.409 
               Plastic 
               1.544 
               55.9 
               46.16 
             
             
               5 
                 
                −0.69386(ASP) 
               0.050 
             
             
               6 
               Lens 3 
                1.46087(ASP) 
               0.604 
               Plastic 
               1.544 
               55.9 
               23.05 
             
             
               7 
                 
                1.41254(ASP) 
               0.200 
             
             
               8 
               IR-filter 
               Plano 
               0.300 
               Glass 
               1.517 
               64.1 
             
             
               9 
                 
               Plano 
               0.200 
             
             
               10 
               Sensor 
               Plano 
               0.400 
               Glass 
               1.517 
               64.1 
             
             
                 
               Cover 
             
             
                 
               Glass 
             
             
               11 
                 
               Plano 
               0.229 
             
             
               12 
               Image 
               Plano 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 6 
             
           
           
             
                 
             
             
               Aspheric Coefficients 
             
           
        
         
             
               Surface # 
               2 
               3 
               4 
               5 
               6 
               7 
             
             
                 
             
             
               k = 
                1.34048E−01 
               −7.48681E+02 
               −1.06097E+00 
               −7.38290E−01 
               −2.59384E+01 
               −1.08384E+01 
             
             
               A4 = 
               −4.70237E−02 
               −1.72547E−01 
               −6.52384E−02 
               −3.28052E−01 
               −4.90865E−02 
               −1.73119E−01 
             
             
               A6 = 
               −4.80440E−02 
               −4.13931E−01 
               −2.87295E+00 
                9.84203E−01 
               −1.19674E−01 
                1.24236E−01 
             
             
               A8 = 
               −8.68674E−02 
                2.27129E+00 
                1.77466E+01 
                6.02267E−01 
               −9.66213E−02 
               −6.80533E−02 
             
             
               A10 = 
               −3.80478E−02 
               −4.47306E+00 
               −4.06040E+01 
               −2.57130E+00 
                2.81994E−02 
                2.88669E−02 
             
             
               A12 = 
                 
                 
               −4.82153E+01 
                5.10888E+00 
               −2.23770E−03 
               −1.10489E−02 
             
             
               A14 = 
                 
                 
                 
               −3.38609E+00 
                 
                1.86325E−03 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 7 
             
             
                 
                 
             
             
                 
               Embodiment 1 
               Embodiment 2 
               Embodiment 3 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               f 
               2.62 
               3.04 
               2.56 
             
             
               Fno 
               2.83 
               3.20 
               2.85 
             
             
               HFOV 
               32.9 
               28.9 
               32.6 
             
             
               V1 
               55.9 
               55.9 
               55.9 
             
             
               V2 
               55.9 
               55.9 
               55.9 
             
             
               V3 
               55.9 
               55.9 
               55.9 
             
             
               N1 
               1.544 
               1.544 
               1.544 
             
             
               N2 
               1.544 
               1.544 
               1.544 
             
             
               N3 
               1.544 
               1.544 
               1.544 
             
             
               f/f1 
               0.86 
               0.75 
               0.98 
             
             
               f/f2 
               0.04 
               0.26 
               0.06 
             
             
               f/f3 
               0.22 
               0.06 
               0.11 
             
             
               f2/f1 
               19.43 
               2.90 
               17.80 
             
             
               F3/f1 
               3.88 
               12.98 
               8.89 
             
             
               R1/R2 
               0.34 
               0.47 
               −0.12 
             
             
               ET3/CT3 
               0.65 
               1.06 
               0.79 
             
             
               TTL/ImgH 
               2.11 
               2.31 
               2.26 
             
             
                 
             
           
        
       
     
   
   It is to be noted that the tables 1-6 show different data from the different embodiments, however, the data of the different embodiments is obtained from experiments. Therefore, any product of the same structure is deemed to be within the scope of the present invention even if it uses different data. Table 7 lists the relevant data for the various embodiments of the present invention. 
   While we have shown and described various embodiments in accordance with the present invention, it should be clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention.