Abstract:
A three-piece optical lens system comprises, in order from the object side to the image side: a first lens element with a positive refractive power having a convex object-side surface, one of the object-side surface and an image-side surface being aspheric; a stop, a second lens element with a positive refractive power having a concave object-side surface, one of the object-side surface and an image-side surface being aspheric; a third lens element with a negative refractive power having a concave image-side surface, one of an object-side surface and the image-side surface being aspheric. A refractive index of the third lens element is N 3 , an Abbe number of the third lens element is V 3 , and they satisfy the relations: N 3 &gt;1.57; V 3 &lt;40. Such arrangements can reduce the volume of the three-piece optical lens system and improve the image quality of the periphery of the image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical lens system, and more particularly to a three-piece optical lens system. 
     2. Description of the Prior Art 
     In recent years, with the popularity of the mobile phone cameras, the optical lens system has become smaller in size, and the electronic 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 sensor has been reduced continuously, and miniaturized optical lens systems have increasingly higher resolution. Therefore, there&#39;s an increasing demand for an imaging lens system with better image quality. 
     Referring to  FIG. 5 , conventional miniaturized lens systems mostly consist of three lens elements, from the object side to the image side: a first lens element  91  with a positive refractive power, a second lens element  92  with a negative refractive power and a third lens element  93  with a positive refractive power. However, the third lens element  93  of the above patent is positive so that the principal point is close to the image side of the system, so the back focal length of such a three-piece lens assembly is relatively long, which results in a relatively large mechanical structure. As a result, the requirement of miniaturization can not be satisfied and the image quality of the periphery of the image will be reduced. 
     Therefore, the present invention is aimed at providing an optical lens system which can satisfy the requirement of miniaturization and improve the image quality of the periphery of the image. 
     The present invention mitigates and/or obviates the aforementioned disadvantages. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a three-piece optical lens system capable of reducing the volume of the optical lens system and effectively flattening the edge of an image, thus improving the image quality of the periphery of the image (namely, the limb darkening) 
     A three-piece optical lens system in accordance with the present invention comprises, in order from the object side to the image side: a first lens element with a positive refractive power having a convex object-side surface, at least one of the object-side and an image-side surfaces of the first lens element being aspheric; a stop; a second lens element with a positive refractive power having a concave object-side surface, at least one of the object-side and an image-side surfaces of the second lens element being aspheric; a third lens element with a negative refractive power having a concave image-side surface, at least one of an object-side and the image-side surfaces of the third lens element being aspheric. In the three-piece optical lens system, the refractive index of the third lens element is N 3 , the Abbe number of the third lens element is V 3 , and they satisfy the relations: N 3 &gt;1.57; V 3 &lt;40. 
     According to one aspect of the present invention three-piece optical lens system, the focal length of the first lens element is f1, the focal length of the second lens element is f2, and they satisfy the relation: 1.4&lt;|f1|/|f2|&lt;3.5. |f1|/|f2| satisfies the above relation, a wide field of view can be provided and the resolution can be improved evidently. Contrarily, If |f1|/|f2| exceeds the above range, the performance and resolution of the optical lens system with a wide field of view will be reduced, and the yield rate will be low. According to another aspect of the present invention three-piece optical lens system, the focal length of the second lens element is f2, the focal length of the third lens element is f3, and they satisfy the relation: 0.3&lt;|f2|/|f3|&lt;1.0. If |f2|/|f3| satisfies the above relation, a wide field of view can be provided and the resolution can be improved evidently. Contrarily, If |f2|/|f3| exceeds the above range, the performance and resolution of the optical lens system with a wide field of view will be reduced, and the yield rate will be low. 
     According to another aspect of the present invention three-piece optical lens system, the focal length of the first lens element and the second lens element combined is f12, the focal length of the third lens element is f3, and they satisfy the relation: 0.3&lt;|f12|/|f3|&lt;1.0. If |f12|/|f3| satisfies the above relation, the volume of the optical lens system can be reduced and the edge of an image can be effectively flattened, thus improving the image quality of the periphery of the image (namely, the limb darkening) In addition, the arrangement of the lens elements can make the optical lens system have a relatively short total track length and provide enough space for the IR cut filter. 
     According to another aspect of the present invention three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the first lens element and the second lens element combined is f12, and they satisfy the relation: 0.5&lt;|f12|/|f|&lt;1.1. If |f12|/|f| satisfies the above relation, the refractive powers of the first and second lens elements will be appropriate, and the requirement of miniaturization can be satisfied. In addition, the arrangement of the lens elements can make the optical lens system have a relatively short total track length and provide enough space for the IR cut filter. 
     According to another aspect of the present invention three-piece optical lens system, the focal length of the first lens element is f1, the focal length of the second lens element and the third lens element combined is f23, and they satisfy the relation: 0.3&lt;|f1|/|f23|&lt;1.8. If |f1|/|f2 3 | satisfies the above relation, a wide field of view can be provided and the resolution can be improved evidently. Contrarily, If |f1|/|f23| exceeds the above range, the performance and resolution of the optical lens system with a wide field of view will be reduced, and the yield rate will be low. 
     According to another aspect of the present invention three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the second lens element and the third lens element combined is f23, and they satisfy the relation: 1.0&lt;|f23|/|f1&lt;2.5. If |f23|/|f| satisfies the above relation, a wide field of view can be provided and the resolution can be improved evidently. Contrarily, If |f23|/|f| exceeds the above range, the performance and resolution of the optical lens system with a wide field of view will be reduced, and the yield rate will be low. According to another aspect of the present invention three-piece optical lens system, the focal length of the three-piece optical lens system is f, the distance between the object-side surface of the first lens element and the image plane is TL, and they satisfy the relation: 0.4&lt;|f/TL|&lt;0.8. If |f/TL| satisfies the above relation, the total track length of the optical lens system can be relatively short, and the requirement of miniaturization can be satisfied. 
     The present invention will be presented in further details from the following descriptions 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. 1A  shows an optical lens system in accordance with a first embodiment of the present invention; 
         FIG. 1B  shows the longitudinal spherical aberration curve, the astigmatic field curve, and the distortion curve of the first embodiment of the present invention; 
         FIG. 2A  shows an optical lens system in accordance with a second embodiment of the present invention; 
         FIG. 2B  shows the longitudinal spherical aberration curve, the astigmatic field curve, and the distortion curve of the second embodiment of the present invention; 
         FIG. 3A  shows an optical lens system in accordance with a third embodiment of the present invention; 
         FIG. 3B  shows the longitudinal spherical aberration curve, the astigmatic field curve, and the distortion curve of the third embodiment of the present invention; 
         FIG. 4A  shows an optical lens system in accordance with a fourth embodiment of the present invention; and 
         FIG. 4B  shows the longitudinal spherical aberration curve, the astigmatic field curve, and the distortion curve of the fourth embodiment of the present invention. 
         FIG. 5  shows a conventional three-piece optical lens system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1A , which shows a three-piece optical lens system in accordance with a first embodiment of the present invention, and  FIG. 1B  shows the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the first embodiment of the present invention. A three-piece optical lens system in accordance with the first embodiment of the present invention comprises, in order from the object side A to the image side B: 
     A first lens element  110  with a positive refractive power made of plastic has a convex object-side surface  111  and a concave image-side surface  112 , and the object-side surface  111  and the image-side surface  112  of the first lens element  110  are aspheric. 
     A Stop  120 . 
     A second lens element  130  with a negative refractive power made of plastic has a concave object-side surface  131  and a convex image-side surface  132 , and the object-side surface  131  and the image-side surface  132  of the second lens element  130  are aspheric. 
     A third lens element  140  with a positive refractive power made of plastic has a convex object-side surface  141  and a concave image-side surface  142 , and the object-side surface  141  and the image-side surface  142  of the third lens element  140  are aspheric. 
     An IR cut filter  150  made of glass is located between the image-side surface  142  of the third lens element  140  and an image plane  160  and has no influence on the focal length of the three-piece optical lens system. 
     The equation for the aspheric surface profiles of the first embodiment is expressed as follows: 
     
       
         
           
             z 
             = 
             
               
                 
                   ch 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       [ 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               k 
                               + 
                               1 
                             
                             ) 
                           
                           ⁢ 
                           
                             c 
                             2 
                           
                           ⁢ 
                           
                             h 
                             2 
                           
                         
                       
                       ] 
                     
                     0.5 
                   
                 
               
               + 
               
                 Ah 
                 4 
               
               + 
               
                 Bh 
                 6 
               
               + 
               
                 Ch 
                 8 
               
               + 
               
                 Dh 
                 10 
               
               + 
               
                 Eh 
                 12 
               
               + 
               
                 Gh 
                 14 
               
               + 
               … 
             
           
         
       
     
     wherein: 
     z represents the value of a reference position with respect to a vertex of the surface of a lens and a position with a height h along the optical axis  170 ; 
     k represents the conic constant; 
     c represents the reciprocal of the radius of curvature; 
     A            B          C          D          E          G          . . . : represent the high-order aspheric coefficients.
     In the first embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, and it satisfies the relation:
 
f=1.64.
 
     In the first embodiment of the present invention three-piece optical lens system, the f-number of the three-piece optical lens system is Fno, and it satisfies the relation:
 
Fno=2.4.
 
     In the first embodiment of the present invention three-piece optical lens system, the field of view of the three-piece optical lens system is 2ω, and it satisfies the relation:
 
2ω=83:
 
     In the first embodiment of the present invention three-piece optical lens system, the refractive index of the third lens element  140  is N 3 , the Abbe number of the third lens element  140  is V 3 , and they satisfy the relations:
 
N3=1.634;
 
V3=23.8.
 
     In the first embodiment of the present invention three-piece optical lens system, the focal length of the first lens element  110  is f1, the focal length of the second lens element  130  is f2, and they satisfy the relation:
 
| f 1|/| f 2|=1.976.
 
     In the first embodiment of the present invention three-piece optical lens system, the focal length of the second lens element  130  is f2, the focal length of the third lens element  140  is f3, and they satisfy the relation:
 
| f 2|/| f 3|=0.586.
 
     In the first embodiment of the present invention three-piece optical lens system, the focal length of the first lens element  110  and the second lens element  130  combined is f12, the focal length of the third lens element  140  is f3, and they satisfy the relation:
 
| f 12|/ |f 3|=0.569.
 
     In the first embodiment of the present invention three-piece optical lens system, the focal length of the first lens element  110  is f1, the focal length of the second lens element  130  and the third lens element  140  combined is f23, and they satisfy the relation:
 
| f 1|/| f 23|=1.032.
 
     In the first embodiment of the present invention three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the first lens element  110  and the second lens element  130  combined is f12, and they satisfy the relation:
 
| f 12|/| f|= 0.81.
 
     In the first embodiment of the present invention three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the second lens element  130  and the third lens element  140  combined is f23, and they satisfy the relation:
 
 |f 23|/| f|= 1.597.
 
     In the first embodiment of the present invention three-piece optical lens system, the focal length of the three-piece optical lens system is f, the distance between the object-side surface  111  of the first lens element  110  and the image plane  160  is TL, and they satisfy the relation:
 
| f/TL|= 0.596.
 
     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. In the tables 1 and 2, the surfaces 1 and 2 represent the object-side surface  111  and the image-side surface  112  of the first lens element  110 , respectively, the surfaces 4 and 5 represent the object-side surface  131  and the image-side surface  132  of the second lens element  130 , respectively, and the surfaces 6 and 7 represent the object-side surface  141  and the image-side surface  142  of the third lens element  140 , respectively. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 (Embodiment 1) 
               
               
                 f(focal length) = 1.64 mm, Fno = 2.4, 2ω = 83°. 
               
             
          
           
               
                 Surface # 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 nd 
                 vd 
               
               
                   
               
             
          
           
               
                 0 
                 Object 
                 Infinity 
                 Infinity 
                   
                   
                   
               
               
                 1 
                 Lens 1 
                 1.207792(ASP) 
                 0.381825 
                 Plastic 
                 1.535 
                 56 
               
               
                 2 
                   
                 6.413388(ASP) 
                 0.124712 
               
               
                 3 
                 Stop 
                 Infinity 
                 0.236605 
               
               
                 4 
                 Lens 2 
                 −0.994232(ASP) 
                 0.689675 
                 Plastic 
                 1.535 
                 56 
               
               
                 5 
                   
                 −0.524721(ASP) 
                 0.082082 
               
               
                 6 
                 Lens 3 
                 1.331715(ASP) 
                 0.348668 
                 Plastic 
                 1.634 
                 23.8 
               
               
                 7 
                   
                 0.631244(ASP) 
                 0.516685 
               
               
                 8 
                 IR-filter 
                 Infinity 
                 0.3 
                 Glass 
                 1.5168  
                 64.167336 
               
               
                 9 
                   
                 Infinity 
                 0.068899 
               
               
                 10 
                 Image 
                 Infinity 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Aspheric Coefficients 
               
             
          
           
               
                   
                 Surface # 
               
             
          
           
               
                   
                 1 
                 2 
                 4 
                 5 
                 6 
                 7 
               
               
                   
                   
               
             
          
           
               
                 k = 
                 1.42846 
                 93.66595 
                 4.589553 
                 −3.30019 
                 0.386114 
                 −5.33595 
               
               
                 A = 
                 0.068543 
                 0.299725 
                 0.734469 
                 −2.17333 
                 −1.60073 
                 −0.53411 
               
               
                 B = 
                 −0.25053 
                 −1.73904 
                 −20.2167 
                 4.239719 
                 3.34629 
                 0.7259 
               
               
                 C = 
                 0.025948 
                 −1.5627 
                 190.0684 
                 −5.18204 
                 −5.60401 
                 −0.78037 
               
               
                 D = 
                 1.383411 
                 8.109434 
                 −1035.59 
                 −11.3807 
                 5.667324 
                 0.493352 
               
               
                 E = 
                 −7.61168 
                 6.90288 
                 3599.381 
                 23.20668 
                 −2.53024 
                 −0.14459 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 2A , which shows a three-piece optical lens system in accordance with a second embodiment of the present invention, and  FIG. 2B  shows the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the second embodiment of the present invention. The second embodiment of the present invention comprises, in order from the object side A to the image side B: 
     A first lens element  210  with a positive refractive power made of plastic has a convex object-side surface  211  and a concave image-side surface  212 , and the object-side surface  211  and the image-side surface  212  of the first lens element  210  are aspheric. 
     A Stop  220 . 
     A second lens element  230  with a negative refractive power made of plastic has a concave object-side surface  231  and a convex image-side surface  232 , and the object-side surface  231  and the image-side surface  232  of the second lens element  230  are aspheric. 
     A third lens element  240  with a positive refractive power made of plastic has a convex object-side surface  241  and a concave image-side surface  242 , and the object-side surface  241  and the image-side surface  242  of the third lens element  240  are aspheric. 
     An IR cut filter  250  made of glass is located between the image-side surface  242  of the third lens element  240  and an image plane  260  and has no influence on the focal length of the three-piece optical lens system. 
     The equation for the aspheric surface profiles of the second embodiment is expressed as follows: 
     
       
         
           
             z 
             = 
             
               
                 
                   ch 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       [ 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               k 
                               + 
                               1 
                             
                             ) 
                           
                           ⁢ 
                           
                             c 
                             2 
                           
                           ⁢ 
                           
                             h 
                             2 
                           
                         
                       
                       ] 
                     
                     0.5 
                   
                 
               
               + 
               
                 Ah 
                 4 
               
               + 
               
                 Bh 
                 6 
               
               + 
               
                 Ch 
                 8 
               
               + 
               
                 Dh 
                 10 
               
               + 
               
                 Eh 
                 12 
               
               + 
               
                 Gh 
                 14 
               
               + 
               … 
             
           
         
       
     
     wherein: 
     z represents the value of a reference position with respect to a vertex of the surface of a lens and a position with a height h along the optical axis  270 ; 
     k represents the conic constant; 
     c represents the reciprocal of the radius of curvature; 
     A            B          C          D          E          G          . . . : represent the high-order aspheric coefficients. In the second embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, and it satisfies the relation:
 
f=1.53.

     In the second embodiment of the present three-piece optical lens system, the f-number of the three-piece optical lens system is Fno, and it satisfies the relation:
 
Fno=2.0.
 
     In the second embodiment of the present three-piece optical lens system, the field of view of the three-piece optical lens system is 2ω, and it satisfies the relation:
 
2ω=79:
 
     In the second embodiment of the present three-piece optical lens system, the refractive index of the third lens element  240  is N 3 , the Abbe number of the third lens element  240  is V 3 , and they satisfy the relations:
 
N3=1.632;
 
V3=23.
 
     In the second embodiment of the present three-piece optical lens system, the focal length of the first lens element  210  is f1, the focal length of the second lens element  230  is f2, and they satisfy the relation:
 
 |f 1|/| f 2|=1.802.
 
     In the second embodiment of the present three-piece optical lens system, the focal length of the second lens element  230  is f2, the focal length of the third lens element  240  is f3, and they satisfy the relation:
 
 |f 2|/| f 3|=0.724.
 
     In the second embodiment of the present three-piece optical lens system, the focal length of the first lens element  210  and the second lens element  230  combined is f12, the focal length of the third lens element  240  is f3, and they satisfy the relation:
 
 |f 12|/| f 3|=0.709.
 
     In the second embodiment of the present three-piece optical lens system, the focal length of the second lens element  230  and the third lens element  240  combined is f23, the focal length of the first lens element  210  is f1, and they satisfy the relation:
 
 |f 1|/| f 23|=0.634.
 
     In the second embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the first lens element  210  and the second lens element  230  combined is f12, and they satisfy the relation:
 
 f 12|/| f|= 0 . 745 .  
 
     In the second embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the second lens element  230  and the third lens element  240  combined is f23, and they satisfy the relation:
 
 |f 23|/| f|= 2.163.
 
     In the second embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the distance between the object-side surface  211  of the first lens element  210  and the image plane  260  is TL, and they satisfy the relation:
 
 |f/TL|= 0.652.
 
     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. In the tables 3 and 4, the surfaces 1 and 2 represent the object-side surface  211  and the image-side surface  212  of the first lens element  210 , respectively, the surfaces 4 and 5 represent the object-side surface  231  and the image-side surface  232  of the second lens element  230 , respectively, and the surfaces 6 and 7 represent the object-side surface  241  and the image-side surface  242  of the third lens element  240 , respectively. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 (Embodiment 2) 
               
               
                 f(focal length) = 1.53 mm, Fno = 2.0, 2ω = 79°. 
               
             
          
           
               
                 Surface # 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 nd 
                 vd 
               
               
                   
               
             
          
           
               
                 0 
                 Object 
                 Infinity 
                 Infinity 
                   
                   
                   
               
               
                 1 
                 Lens 1 
                 1.018658(ASP) 
                 0.368546 
                 Plastic 
                 1.535 
                 56 
               
               
                 2 
                   
                 9.162535(ASP) 
                 0.061668 
               
               
                 3 
                 Stop 
                 Infinity 
                 0.311096 
               
               
                 4 
                 Lens 2 
                 −0.81862(ASP) 
                 0.513284 
                 Plastic 
                 1.535 
                 56 
               
               
                 5 
                   
                 −0.43252(ASP) 
                 0.044321 
               
               
                 6 
                 Lens 3 
                 1.451375(ASP) 
                 0.294357 
                 Plastic 
                 1.632 
                 23 
               
               
                 7 
                   
                 0.554059(ASP) 
                 0.236829 
               
               
                 8 
                 IR-filter 
                 Infinity 
                 0.21 
                 Glass 
                 1.5168 
                 64.167336 
               
               
                 9 
                   
                 Infinity 
                 0.308183 
               
               
                 10 
                 Image 
                 Infinity 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Aspheric Coefficients 
               
             
          
           
               
                   
                 Surface # 
               
             
          
           
               
                   
                 1 
                 2 
                 4 
                 5 
                 6 
                 7 
               
               
                   
                   
               
             
          
           
               
                 k = 
                 0.872352 
                 −118.1303 
                 2.323299 
                 −2.837341 
                 0.887843 
                 −6.182162 
               
               
                 A = 
                 −0.09754 
                 0.084314 
                 0.325379 
                 −2.16915 
                 −1.68209 
                 −0.6683 
               
               
                 B = 
                 −0.25811 
                 −1.17987 
                 −17.0153 
                 3.735106 
                 3.780999 
                 0.936757 
               
               
                 C = 
                 −0.08734 
                 1.327168 
                 183.4481 
                 −5.52424 
                 −6.09281 
                 −0.97563 
               
               
                 D = 
                 −0.5966 
                 −7.86312 
                 −1145.87 
                 −11.6855 
                 6.231298 
                 0.569988 
               
               
                 E = 
                 −4.40427 
                 −138.915 
                 4330.385 
                 30.29379 
                 −3.49097 
                 −0.13994 
               
               
                 F = 
                 −14.4538 
                 775.38 
                 −4622.8 
                 44.80149 
                 0.202775 
                 −0.04572 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 3A , which shows a three-piece optical lens system in accordance with a third embodiment of the present invention, and  FIG. 3B  shows the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the third embodiment of the present invention. The third embodiment of the present invention comprises, in order from the object side A to the image side B: 
     A first lens element  310  with a positive refractive power made of plastic has a convex object-side surface  311  and a concave image-side surface  312 , and the object-side surface  311  and the image-side surface  312  of the first lens element  310  are aspheric. 
     A Stop  320 . 
     A second lens element  330  with a negative refractive power made of plastic has a concave object-side surface  331  and a convex image-side surface  332 , and the object-side surface  331  and the image-side surface  332  of the second lens element  330  are aspheric. 
     A third lens element  340  with a positive refractive power made of plastic has a convex object-side surface  341  and a concave image-side surface  342 , and the object-side surface  341  and the image-side surface  342  of the third lens element  340  are aspheric. 
     An IR cut filter  350  made of glass is located between the image-side surface  342  of the third lens element  340  and an image plane  360  and has no influence on the focal length of the optical lens system. 
     A cover glass  380  made of glass is located between the IR cut filter  350  and the image plane  360  to protect a sensor (not shown), and has no influence on the focal length of the three-piece optical lens system. 
     The equation for the aspheric surface profiles of the third embodiment is expressed as follows: 
     
       
         
           
             z 
             = 
             
               
                 
                   ch 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       [ 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               k 
                               + 
                               1 
                             
                             ) 
                           
                           ⁢ 
                           
                             c 
                             2 
                           
                           ⁢ 
                           
                             h 
                             2 
                           
                         
                       
                       ] 
                     
                     0.5 
                   
                 
               
               + 
               
                 Ah 
                 4 
               
               + 
               
                 Bh 
                 6 
               
               + 
               
                 Ch 
                 8 
               
               + 
               
                 Dh 
                 10 
               
               + 
               
                 Eh 
                 12 
               
               + 
               
                 Gh 
                 14 
               
               + 
               … 
             
           
         
       
     
     wherein: 
     z represents the value of a reference position with respect to a vertex of the surface of a lens and a position with a height h along the optical axis  370   
     k represents the conic constant; 
     c represents the reciprocal of the radius of curvature; 
     A            B          C          D          E          G          . . . : represent the high-order aspheric coefficients.
     In the third embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, and it satisfies the relation:
 
f=1.82.
 
     In the third embodiment of the present three-piece optical lens system, the f-number of the three-piece optical lens system is Fno, and it satisfies the relation:
 
Fno=2.4.
 
     In the third embodiment of the present three-piece optical lens system, the field of view of the three-piece optical lens system is 2ω, and it satisfies the relation:
 
2ω=82:
 
     In the third embodiment of the present three-piece optical lens system, the refractive index of the third lens element  340  is N 3 , the Abbe number of the third lens element  340  is V 3 , and they satisfy the relations:
 
N3=1.612;
 
V3=26.
 
     In the third embodiment of the present three-piece optical lens system, the focal length of the first lens element  310  is f1, the focal length of the second lens element  330  is f2, and they satisfy the relation:
 
 |f 1|/| f 2|=1.963.
 
     In the third embodiment of the present three-piece optical lens system, the focal length of the second lens element  330  is f2, the focal length of the third lens element  340  is f3, and they satisfy the relation:
 
 |f 2|/| f 3|=0.753.
 
     In the third embodiment of the present three-piece optical lens system, the focal length of the first lens element  310  and the second lens element  330  combined is f12, the focal length of the third lens element  340  is f3, and they satisfy the relation:
 
 |f 12|/| f 3|=0.73.
 
     In the third embodiment of the present three-piece optical lens system, the focal length of the second lens element  330  and the third lens element  340  combined is f23, the focal length of the first lens element  310  is f1, and they satisfy the relation:
 
 |f 1|/| f 23|=0.672.
 
     In the third embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the first lens element  310  and the second lens element  330  combined is f12, and they satisfy the relation:
 
 |f 12|/| f|= 0.714.
 
     In the third embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the second lens element  330  and the third lens element  340  combined is f23, and they satisfy the relation:
 
 |f 23|/| f|= 2.152.
 
     In the third embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the distance between the object-side surface  311  of the first lens element  310  and the image plane  360  is TL, and they satisfy the relation:
 
 |f/TL|= 0.654.
 
     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. In the tables 5 and 6, the surfaces 1 and 2 represent the object-side surface  311  and the image-side surface  312  of the first lens element  310 , respectively, the surfaces 4 and 5 represent the object-side surface  331  and the image-side surface  332  of the second lens element  330 , respectively, and the surfaces 6 and 7 represent the object-side surface  341  and the image-side surface  342  of the third lens element  340 , respectively. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 (Embodiment 3) 
               
               
                 f(focal length) = 1.82 mm, Fno = 2.4, 2ω = 82°. 
               
             
          
           
               
                 Surface # 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 nd 
                 vd 
               
               
                   
               
             
          
           
               
                 0 
                 Object 
                 Infinity 
                 Infinity 
                   
                   
                   
               
               
                 1 
                 Lens 1 
                 1.157237(ASP) 
                 0.375208 
                 Plastic 
                 1.535 
                 56 
               
               
                 2 
                   
                 5.679283(ASP) 
                 0.086905 
               
               
                 3 
                 Stop 
                 Infinity 
                 0.408079 
               
               
                 4 
                 Lens 2 
                 −0.980386(ASP) 
                 0.48273 
                 Plastic 
                 1.535 
                 56 
               
               
                 5 
                   
                 −0.486358(ASP) 
                 0.042475 
               
               
                 6 
                 Lens 3 
                 2.052499(ASP) 
                 0.351495 
                 Plastic 
                 1.612 
                 26 
               
               
                 7 
                   
                 0.682256(ASP) 
                 0.16907 
               
               
                 8 
                 IR-filter 
                 Infinity 
                 0.21 
                 Glass 
                 1.5168 
                 64.167336 
               
               
                 9 
                   
                 Infinity 
                 0.2 
               
               
                 10 
                 Cover 
                 Infinity 
                 0.4 
                 Glass 
                 1.5168 
                 64.167336 
               
               
                 11 
                 Glass 
                 Infinity 
                 0.045 
               
               
                 12 
                 Image 
                 Infinity 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Aspheric Coefficients 
               
             
          
           
               
                   
                 Surface # 
               
             
          
           
               
                   
                 1 
                 2 
                 4 
                 5 
                 6 
                 7 
               
               
                   
                   
               
             
          
           
               
                 k = 
                 0.815092 
                 15.62383 
                 2.215985 
                 −3.13406 
                 2.200667 
                 −7.3661 
               
               
                 A = 
                 −0.0475 
                 0.047567 
                 0.237145 
                 −1.52237 
                 −0.85334 
                 −0.3253 
               
               
                 B = 
                 −0.02503 
                 −0.37735 
                 −7.90144 
                 1.93684 
                 1.386517 
                 0.315208 
               
               
                 C = 
                 −0.24343 
                 −0.19218 
                 47.12193 
                 −2.20156 
                 −1.51974 
                 −0.25094 
               
               
                 D = 
                 −0.01857 
                 −6.42683 
                 −180.392 
                 −2.50398 
                 0.980679 
                 0.126773 
               
               
                 E = 
                 −1.43995 
                 22.44807 
                 377.0114 
                 12.72607 
                 −0.31327 
                 −0.03353 
               
               
                 F = 
                 −0.16459 
                 11.38928 
                 227.0883 
                 1.357769 
                 −0.85334 
                 −0.3253 
               
               
                   
               
             
          
         
       
     
     Referring to  FIG. 4A , which shows a three-piece optical lens system in accordance with a fourth embodiment of the present invention, and  FIG. 4B  shows the longitudinal spherical aberration curves, the astigmatic field curves, and the distortion curve of the fourth embodiment of the present invention. The fourth embodiment of the present invention comprises, in order from the object side A to the image side B: 
     A first lens element  410  with a positive refractive power made of plastic has a convex object-side surface  411  and a concave image-side surface  412 , and the object-side surface  411  and the image-side surface  412  of the first lens element  410  are aspheric. 
     A Stop  420 . 
     A second lens element  430  with a negative refractive power made of plastic has a concave object-side surface  431  and a convex image-side surface  432 , and the object-side surface  431  and the image-side surface  432  of the second lens element  430  are aspheric. 
     A third lens element  440  with a positive refractive power made of plastic has a convex object-side surface  441  and a concave image-side surface  442 , and the object-side surface  441  and the image-side surface  442  of the third lens element  440  are aspheric. 
     An IR cut filter  450  made of glass is located between the image-side surface  442  of the third lens element  440  and an image plane  460  and has no influence on the focal length of the three-piece optical lens system. 
     The equation for the aspheric surface profiles of the fourth embodiment is expressed as follows: 
     
       
         
           
             z 
             = 
             
               
                 
                   ch 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       [ 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               k 
                               + 
                               1 
                             
                             ) 
                           
                           ⁢ 
                           
                             c 
                             2 
                           
                           ⁢ 
                           
                             h 
                             2 
                           
                         
                       
                       ] 
                     
                     0.5 
                   
                 
               
               + 
               
                 Ah 
                 4 
               
               + 
               
                 Bh 
                 6 
               
               + 
               
                 Ch 
                 8 
               
               + 
               
                 Dh 
                 10 
               
               + 
               
                 Eh 
                 12 
               
               + 
               
                 Gh 
                 14 
               
               + 
               … 
             
           
         
       
     
     wherein: 
     z represents the value of a reference position with respect to a vertex of the surface of a lens and a position with a height h along the optical axis  470   
     k represents the conic constant; 
     c represents the reciprocal of the radius of curvature; 
     A            B          C          D          E          G          . . . : represent the high-order aspheric coefficients. In the fourth embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, and it satisfies the relation:
 
 f= 1.67.

     In the fourth embodiment of the present three-piece optical lens system, the f-number of the three-piece optical lens system is Fno, and it satisfies the relation:
 
Fno=2.4.
 
     In the fourth embodiment of the present three-piece optical lens system, the field of view of the three-piece optical lens system is 2ω, and it satisfies the relation:
 
2ω=81:
 
     In the fourth embodiment of the present three-piece optical lens system, the refractive index of the third lens element  440  is N 3 , the Abbe number of the third lens element  440  is V 3 , and they satisfy the relations:
 
N3=1.607
 
V3=27.
 
     In the fourth embodiment of the present three-piece optical lens system, the focal length of the first lens element  410  is f1, the focal length of the second lens element  430  is f2, and they satisfy the relation:
 
 |f 1|/| f 2|=3.089
 
     In the fourth embodiment of the present three-piece optical lens system, the focal length of the second lens element  430  is f2, the focal length of the third lens element  440  is f3, and they satisfy the relation:
 
 |f 2|/| f 3|=0.551
 
     In the fourth embodiment of the present three-piece optical lens system, the focal length of the first lens element  410  and the second lens element  430  combined is f12, the focal length of the third lens element  440  is f3, and they satisfy the relation:
 
 |f 12|/| f 3|=0.572.
 
     In the fourth embodiment of the present three-piece optical lens system, the focal length of the first lens element  410  is f1, the focal length of the second lens element  430  and the third lens element  440  combined is f23, and they satisfy the relation:
 
 |f 1|/| f 23|=1.527.
 
     In the fourth embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the first lens element  410  and the second lens element  430  combined is f12, and they satisfy the relation:
 
 |f 12|/| f| =0.68.
 
     In the fourth embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the focal length of the second lens element  430  and the third lens element  440  combined is f23, and they satisfy the relation:
 
 |f 23|/| f|= 1.324.
 
     In the fourth embodiment of the present three-piece optical lens system, the focal length of the three-piece optical lens system is f, the distance between the object-side surface  411  of the first lens element  410  and the image plane  460  is TL, and they satisfy the relation:
 
 |f/TL|= 0.569.
 
     The detailed optical data of the fourth embodiment is shown in table 7, and the aspheric surface data is shown in table 8, wherein the units of the radius of curvature, the thickness and the focal length are expressed in mm. In the tables 7 and 8, the surfaces 1 and 2 represent the object-side surface  411  and the image-side surface  412  of the first lens element  410 , respectively, the surfaces 4 and 5 represent the object-side surface  431  and the image-side surface  432  of the second lens element  430 , respectively, and the surfaces 6 and 7 represent the object-side surface  441  and the image-side surface  442  of the third lens element  440 , respectively. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 (Embodiment 4) 
               
               
                 f(focal length) = 1.67 mm, Fno = 2.4, 2ω = 81°. 
               
             
          
           
               
                 Surface # 
                   
                 Curvature Radius 
                 Thickness 
                 Material 
                 nd 
                 vd 
               
               
                   
               
             
          
           
               
                 0 
                 Object 
                 Infinity 
                 Infinity 
                   
                   
                   
               
               
                 1 
                 Lens 1 
                 1.532591(ASP) 
                 0.454031 
                 Plastic 
                 1.535 
                 56 
               
               
                 2 
                   
                 8.719898(ASP) 
                 0.170321 
               
               
                 3 
                 Stop 
                 Infinity 
                 0.138891 
               
               
                 4 
                 Lens 2 
                 −0.81759(ASP) 
                 0.638601 
                 Plastic 
                 1.535 
                 56 
               
               
                 5 
                   
                 −0.43579(ASP) 
                 0.043507 
               
               
                 6 
                 Lens 3 
                 3.104954(ASP) 
                 0.38279 
                 Plastic 
                 1.607 
                 27 
               
               
                 7 
                   
                 0.858346(ASP) 
                 0.474653 
               
               
                 8 
                 IR-filter 
                 Infinity 
                 0.21 
                 Glass 
                 1.5168 
                 64.167336 
               
               
                 9 
                   
                 Infinity 
                 0.421493 
               
               
                 10 
                 Image 
                 Infinity 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Aspheric Coefficients 
               
             
          
           
               
                   
                 Surface # 
               
             
          
           
               
                   
                 1 
                 2 
                 4 
                 5 
                 6 
                 7 
               
               
                   
                   
               
             
          
           
               
                 k = 
                 −0.51484 
                 198.1856 
                 2.538317 
                 −2.17241 
                 12.14719 
                 −11.1481 
               
               
                 A = 
                 0.218489 
                 0.334491 
                 0.195327 
                 −1.47945 
                 −0.95368 
                 −0.41778 
               
               
                 B = 
                 −0.67673 
                 −2.46745 
                 −17.2444 
                 −0.00932 
                 3.52412 
                 0.927894 
               
               
                 C = 
                 3.219872 
                 −1.93595 
                 150.4772 
                 8.6889 
                 −10.2511 
                 −1.64519 
               
               
                 D = 
                 −7.51231 
                 23.63663 
                 −2038.48 
                 −52.7293 
                 14.47767 
                 1.41453 
               
               
                 E = 
                 4.80667 
                 −34.7034 
                 10407.42 
                 56.92328 
                 −7.80196 
                 −0.43665 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Embodi- 
               
               
                   
                 Embodiment 1 
                 Embodiment 2 
                 Embodiment 3 
                 ment 4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 f 
                 1.64 
                 1.53 
                 1.82 
                 1.67 
               
               
                 Fno 
                 2.4 
                 2.0 
                 2.4 
                 2.4 
               
               
                 2ω 
                 83 
                 79 
                 82 
                 81 
               
               
                 N3 
                 1.634 
                 1.632 
                 1.612 
                 1.607 
               
               
                 V3 
                 23.8 
                 23 
                 26 
                 27 
               
               
                 |f1|/|f2| 
                 1.976 
                 1.802 
                 1.963 
                 3.089 
               
               
                 |f2|/|f3| 
                 0.586 
                 0.724 
                 0.753 
                 0.551 
               
               
                 |f12|/|f3| 
                 0.569 
                 0.709 
                 0.73 
                 0.572 
               
               
                 |f1|/|f23| 
                 1.032 
                 0.634 
                 0.672 
                 1.527 
               
               
                 |f12|/|f| 
                 0.81 
                 0.745 
                 0.714 
                 0.68 
               
               
                 |f23|/|f| 
                 1.597 
                 2.163 
                 2.152 
                 1.324 
               
               
                 |f/TL| 
                 0.596 
                 0.652 
                 0.654 
                 0.569 
               
               
                   
               
             
          
         
       
     
     It is to be noted that the tables 1-8 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 9 lists the relevant data for the various embodiments of the present invention. 
     In the present three-piece optical lens system, the lens elements can be made of glass or plastic. If the lens elements are made of glass, there is more freedom in distributing the refractive power of the three-piece optical lens system. If the lens elements are made of plastic, the cost will be effectively reduced. 
     In the present three-piece optical lens system, if the object-side or the image-side surface of the lens elements is convex, the object-side or the image-side surface of the lens elements in proximity of the optical axis is convex. If the object-side or the image-side surface of the lens elements is concave, the object-side or the image-side surface of the lens elements in proximity of the optical axis is concave. 
     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.