Abstract:
A lens assembly includes a first lens, a second lens, a stop, a third lens and a fourth lens, all of which are arranged in sequence from an object side to an image side along an optical axis. The first lens and the fourth lens are with negative refractive power. The second lens and the third lens are biconvex lenses with positive refractive power. The first, third, fourth lens include an object side surface and an image side surface respectively, wherein at least one of the object side surface and the image side surface is an aspheric surface. The lens assembly satisfies: 0&lt;|f 1 /f 2 |&lt;|f 4 /f 3 |&lt;2, f/D 12 &gt;1 wherein f 1 , f 2 , f 3 , f 4 , and f is an effective focal length of the first, second, third, fourth lens and the lens assembly, and D 12  is an interval from the first lens to the second lens along the optical axis.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a lens assembly. 
         [0003]    2. Description of the Related Art 
         [0004]    Nowadays, the development trend of consumer electronic products is continuously toward miniaturization. Therefore, lens assemblies that are used for consumer electronic products also need to be miniaturized. The volume of known lens assembly is relatively large and can&#39;t satisfy requirement of present. Therefore, a lens assembly that is used for consumer electronic products needs a new structure in order to meet the requirements of miniaturization, small F-number and high resolution at the same time. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The invention provides a lens assembly to solve the above problems. The lens assembly of the invention is provided with characteristics of a shortened total lens length and a decreased F-number and still has a good optical performance. 
         [0006]    The lens assembly in accordance with an exemplary embodiment of the invention includes a first lens, a second lens, a stop, a third lens and a fourth lens, all of which are arranged in sequence from an object side to an image side along an optical axis. The first lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The second lens is a biconvex lens with positive refractive power. The third lens is a biconvex lens with positive refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The fourth lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The lens assembly satisfies: 0&lt;|f 1 /f 2 |&lt;|f 4 /f 3 |&lt;2, f/D 12 &gt;1 wherein f 1  is an effective focal length of the first lens, f 2  is an effective focal length of the second lens, f 3  is an effective focal length of the third lens, f 4  is an effective focal length of the fourth lens, f is an effective focal length of the lens assembly and D 12  is an interval from an image side surface of the first lens to an object side surface of the second lens along the optical axis. 
         [0007]    In another exemplary embodiment, the third lens and the fourth lens satisfy 0.1&lt;Vd 4 /f&lt;Vd 3 /f&lt;50, wherein Vd 4  is an Abbe number of the fourth lens, Vd 3  is an Abbe number of the third lens and f is an effective focal length of the lens assembly. 
         [0008]    In yet another exemplary embodiment, the lens assembly satisfies D 34 /D 12 &gt;0.02, wherein D 12  is an interval from an image side surface of the first lens to an object side surface of the second lens along the optical axis, and D 34  is an interval from an image side surface of the third lens to an object side surface of the fourth lens along the optical axis. 
         [0009]    In another exemplary embodiment, the first lens, the third lens and the fourth lens are made of plastic material, and the second lens is made of glass material. 
         [0010]    In yet another exemplary embodiment, the first lens, the second lens, the third lens and the fourth lens are made of plastic material. 
         [0011]    The lens assembly in accordance with another exemplary embodiment of the invention includes a first lens, a second lens, a stop, a third lens and a fourth lens, all of which are arranged in sequence from an object side to an image side along an optical axis. The first lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The second lens is a biconvex lens with positive refractive power. The third lens is a biconvex lens with positive refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The fourth lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The lens assembly satisfies: 0&lt;|f 1 /f 2 |&lt;|f 4 /f 3 |&lt;2, BFL/TTL&lt;0.4 wherein f 1  is an effective focal length of the first lens, f 2  is an effective focal length of the second lens, f 3  is an effective focal length of the third lens, f 4  is an effective focal length of the fourth lens, f is an effective focal length of the lens assembly, BFL is an interval from an image side surface of the fourth lens to an image plane along the optical axis and TTL is an interval from an object side surface of the first lens to the image plane along the optical axis. 
         [0012]    The lens assembly in accordance with another exemplary embodiment of the invention includes a first lens, a second lens, a stop, a third lens and a fourth lens, all of which are arranged in sequence from an object side to an image side along an optical axis. The first lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The second lens is a biconvex lens with positive refractive power. The third lens is a biconvex lens with positive refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The fourth lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The lens assembly satisfies: 0&lt;|f 1 /f 2 |&lt;|f 4 /f 3 |&lt;2, |f 1 /f|≦1.5 wherein f 1  is an effective focal length of the first lens, f 2  is an effective focal length of the second lens, f 3  is an effective focal length of the third lens, f 4  is an effective focal length of the fourth lens and f is an effective focal length of the lens assembly. 
         [0013]    The lens assembly in accordance with another exemplary embodiment of the invention includes a first lens, a second lens, a stop, a third lens and a fourth lens, all of which are arranged in sequence from an object side to an image side along an optical axis. The first lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The second lens is a biconvex lens with positive refractive power. The third lens is a biconvex lens with positive refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The fourth lens is with negative refractive power and includes an object side surface and an image side surface, wherein at least one of the object side surface and the image side surface is an aspheric surface. The lens assembly satisfies: 0&lt;|f 1 /f 2 |&lt;|f 4 /f 3 |&lt;2, 0.1&lt;f/Nd 4 &lt;f/Nd 3 &lt;3 wherein f 1  is an effective focal length of the first lens, f 2  is an effective focal length of the second lens, f 3  is an effective focal length of the third lens, f 4  is an effective focal length of the fourth lens, f is an effective focal length of the lens assembly, Nd 3  is an index of refraction of the third lens and Nd 4  is an index of refraction of the fourth lens. 
         [0014]    The lens assembly in accordance with another exemplary embodiment of the invention includes a first lens, a second lens, a stop, a third lens and a fourth lens, all of which are arranged in sequence from an object side to an image side along an optical axis. The first lens is with negative refractive power. The second lens is with positive refractive power. The third lens is with positive refractive power. The fourth lens is with negative refractive power. The lens assembly satisfies: 0&lt;|f 1 /f 2 |&lt;|f 4 /f 3 |&lt;2, BFL/TTL&lt;0.4, |f 1 /f|≦1.5, f/D 12 &gt;1, 0.1&lt;Vd 4 /f&lt;Vd 4 |f&lt;Vd 3 /f&lt;50, 0.1&lt;f/Nd 4 &lt;f/Nd 3 &lt;3, D 34 /D 12 &gt;0.02 wherein f 1  is an effective focal length of the first lens, f 2  is an effective focal length of the second lens, f 3  is an effective focal length of the third lens, f 4  is an effective focal length of the fourth lens, f is an effective focal length of the lens assembly, BFL is an interval from an image side surface of the fourth lens to an image plane along the optical axis, TTL is an interval from an object side surface of the first lens to the image plane along the optical axis, D 12  is an interval from an image side surface of the first lens to an object side surface of the second lens along the optical axis, D 34  is an interval from an image side surface of the third lens to an object side surface of the fourth lens along the optical axis, Vd 3  is an Abbe number of the third lens, Vd 4  is an Abbe number of the fourth lens, Nd 3  is an index of refraction of the third lens and Nd 4  is an index of refraction of the fourth lens. 
         [0015]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0017]      FIG. 1  is a lens layout diagram of a lens assembly in accordance with a first embodiment of the invention; 
           [0018]      FIG. 2A  depicts a longitudinal aberration diagram of the lens assembly in accordance with the first embodiment of the invention; 
           [0019]      FIG. 2B  is a field curvature diagram of the lens assembly in accordance with the first embodiment of the invention; 
           [0020]      FIG. 2C  is a distortion diagram of the lens assembly in accordance with the first embodiment of the invention; 
           [0021]      FIG. 2D  is a lateral color diagram of the lens assembly in accordance with the first embodiment of the invention; 
           [0022]      FIG. 3  is a lens layout diagram of a lens assembly in accordance with a second embodiment of the invention; 
           [0023]      FIG. 4A  depicts a longitudinal aberration diagram of the lens assembly in accordance with the second embodiment of the invention; 
           [0024]      FIG. 4B  is a field curvature diagram of the lens assembly in accordance with the second embodiment of the invention; 
           [0025]      FIG. 4C  is a distortion diagram of the lens assembly in accordance with the second embodiment of the invention; and 
           [0026]      FIG. 4D  is a lateral color diagram of the lens assembly in accordance with the second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The following description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0028]    Referring to  FIG. 1 ,  FIG. 1  is a lens layout diagram of a lens assembly in accordance with a first embodiment of the invention. The lens assembly  1  includes a first lens L 11 , a second lens L 12 , a stop ST 1 , a third lens L 13 , a fourth lens L 14  and an optical filter OF 1 , all of which are arranged in sequence from an object side to an image side along an optical axis OA 1 . In operation, an image of light rays from the object side is formed at an image plane IMA 1 . The first lens L 11  is a biconcave lens with negative refractive and made of plastic material, wherein both of the object side surface S 11  and image side surface S 12  are aspheric surfaces. The second lens L 12  is a biconvex lens with positive refractive power and made of glass material, wherein both of the object side surface S 13  and image side surface S 14  are spherical surfaces. The third lens L 13  is a biconvex lens with positive refractive power and made of plastic material, wherein both of the object side surface S 16  and image side surface S 17  are aspheric surfaces. The fourth lens L 14  is a concave-convex lens with negative refractive power and made of plastic material, wherein the object side surface S 18  is a concave surface, the image side surface S 19  is a convex surface and both of the object side surface S 18  and image side surface S 19  are aspheric surfaces. There is an air space between the third lens L 13  and the fourth lens L 14 . Both of the object side surface S 110  and image side surface S 111  of the optical filter OF 1  are plane surfaces. 
         [0029]    In order to maintain excellent optical performance of the lens assembly in accordance with the first embodiment of the invention, the lens assembly  1  must satisfies the following seven conditions: 
         [0000]      0&lt;| f 1 1   /f 1 2   |&lt;|f 1 4   /f 1 3 |&lt;2  (1)
 
         [0000]        BFL 1/ TTL 1&lt;0.4  (2)
 
         [0000]      | f 1 1   /f 1 1 |≦1.5  (3)
 
         [0000]        f 1/ D 1 12 &gt;1  (4)
 
         [0000]      0.1&lt; Vd 1 4   /f 1&lt; Vd 1 3   /f 1&lt;50  (5)
 
         [0000]      0.1&lt; f 1/ Nd 1 4   &lt;f 1/ Nd 1 3 &lt;3  (6)
 
         [0000]        D 1 34   /D 1 12 &gt;0.02  (7)
 
         [0000]    wherein f 1   1  is an effective focal length of the first lens L 11 , f 1   2  is an effective focal length of the second lens L 12 , f 1   3  is an effective focal length of the third lens L 13 , f 1   4  is an effective focal length of the fourth lens L 14 , f 1  is an effective focal length of the lens assembly  1 , BFL 1  is an interval from the image side surface S 19  of the fourth lens L 14  to the image plane IMA 1  along the optical axis OA 1 , TTL 1  is an interval from the object side surface S 11  of the first lens L 11  to the image plane IMA 1  along the optical axis OA 1 , D 1   12  is an interval from the image side surface S 12  of the first lens L 11  to the object side surface S 13  of the second lens L 12  along the optical axis OA 1 , D 1   34  is an interval from the image side surface S 17  of the third lens L 13  to the object side surface S 18  of the fourth lens L 14  along the optical axis OA 1 , Vd 1   3  is an Abbe number of the third lens L 13 , Vd 1   4  is an Abbe number of the fourth lens L 14 , Nd 1   3  is an index of refraction of the third lens L 13  and Nd 1   4  is an index of refraction of the fourth lens L 14 . 
         [0030]    By the above design of the lenses and stop ST 1 , the lens assembly  1  is provided with a shortened total lens length, a decreased F-number and an effective corrected aberration. 
         [0031]    In order to achieve the above purposes and effectively enhance the optical performance, the lens assembly  1  in accordance with the first embodiment of the invention is provided with the optical specifications shown in Table 1, which include the effective focal length, F-number, total lens length, radius of curvature of each lens surface, thickness between adjacent surface, refractive index of each lens and Abbe number of each lens. Table 1 shows that the effective focal length is equal to 3.3127 mm, F-number is equal to 2.0 and total lens length is equal to 11.501 mm for the lens assembly  1  of the first embodiment of the invention. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Effective Focal Length = 3.3127 mm F-number = 2.0  
               
               
                 Total Lens Length = 11.501 mm 
               
             
          
           
               
                   
                 Radius of 
                   
                   
                   
                   
               
               
                 Surface 
                 Curvature 
                 Thickness 
                   
                   
                   
               
               
                 Number 
                 (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 Remark 
               
               
                   
               
             
          
           
               
                 S11 
                 −9.555 
                 0.5 
                 1.5346 
                 56.07 
                 The First  
               
               
                   
                   
                   
                   
                   
                 Lens L11 
               
               
                 S12 
                 2.594 
                 1.976 
                   
                   
                   
               
               
                 S13 
                 6.167 
                 1.968 
                 1.4875 
                 70.24 
                 The Second  
               
               
                   
                   
                   
                   
                   
                 Lens L12 
               
               
                 S14 
                 −3.616 
                 0 
                   
                   
                   
               
               
                 S15 
                 ∞ 
                 0.844 
                   
                   
                 Stop ST1 
               
               
                 S16 
                 2.927 
                 1.981 
                 1.5346 
                 56.07  
                 The Third  
               
               
                   
                   
                   
                   
                   
                 Lens L13 
               
               
                 S17 
                 −2.234 
                 0.1 
                   
                   
                   
               
               
                 S18 
                 −1.66 
                 0.606 
                 1.6142 
                 25.58 
                 The Fourth  
               
               
                   
                   
                   
                   
                   
                 Lens L14 
               
               
                 S19 
                 −17.286 
                 2.688 
                   
                   
                   
               
               
                 S110 
                 ∞ 
                 0.735 
                 1.5168 
                 64.16  
                 Optical Filter  
               
               
                   
                   
                   
                   
                   
                 OF1 
               
               
                 S111 
                 ∞ 
                 0.104 
               
               
                   
               
             
          
         
       
     
         [0032]    The aspheric surface sag z of each lens in table 1 can be calculated by the following formula: 
         [0000]        z=ch   2 /{1+[1−( k+ 1) c   2   h   2 ] 1/2   }+Ah   4   +Bh   6   +Ch   8   +Dh   10  
 
         [0000]    where c is curvature, h is the vertical distance from the lens surface to the optical axis, k is conic constant and A, B, C and D are aspheric coefficients. 
         [0033]    In the first embodiment, the conic constant k and the aspheric coefficients A, B, C, D of each surface are shown in Table 2. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Surface 
                   
                   
                   
                   
                   
               
               
                 Number 
                 k 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 S11 
                 7.1967 
                 3.881E−4  
                 2.969E−4 
                 −1.044E−5 
                 0 
               
               
                 S12 
                 0.1471 
                  7.79E−3 
                 −3.024E−4  
                  6.836E−4 
                 0 
               
               
                 S16 
                 0.051 
                 −4.225E−4  
                 1.756E−4 
                 −3.422E−4 
                 0 
               
               
                 S17 
                 −2.5091 
                 1.039E−2 
                 1.169E−3 
                  −7.28E−4 
                 0 
               
               
                 S18 
                 −3.7936 
                 1.336E−2  
                 5.753E−4 
                  −4.55E−4 
                 0 
               
               
                 S19 
                 0 
                 6.805E−2  
                 −1.624E−2   
                  4.816E−3 
                 −5.549E−4 
               
               
                   
               
             
          
         
       
     
         [0034]    For the lens assembly  1  of the first embodiment, the effective focal length f 1   1  of the first lens L 11  is equal to −3.746 mm, the effective focal length f 1   2  of the second lens L 12  is equal to 4.99 mm, the effective focal length f 1   3  of the third lens L 13  is equal to 2.725 mm, the effective focal length f 1   4  of the fourth lens L 14  is equal to −3.004 mm, the effective focal length f 1  of the lens assembly  1  is equal to 3.3127 mm, the interval BFL 1  from the image side surface S 19  of the fourth lens L 14  to the image plane IMA 1  along the optical axis OA 1  is equal to 3.527 mm, the interval TTL 1  from the object side surface S 11  of the first lens L 11  to the image plane IMA 1  along the optical axis OA 1  is equal to 11.5 mm, the interval D 1   12  from the image side surface S 12  of the first lens L 11  to the object side surface S 13  of the second lens L 12  along the optical axis OA 1  is equal to 1.976 mm, the interval D 1   34  from the image side surface S 17  of the third lens L 13  to the object side surface S 18  of the fourth lens L 14  along the optical axis OA 1  is equal to 0.1 mm, the Abbe number Vd 1   3  of the third lens L 13  is equal to 56.07, the Abbe number Vd 1   4  of the fourth lens L 14  is equal to 25.58, the index of refraction Nd 1   3  of the third lens L 13  is equal to 1.5346 and the index of refraction Nd 1   4  of the fourth lens L 14  is equal to 1.6142. According to the above data, the following values can be obtained: 
         [0000]      | f 1 1   /f 1 2 |=0.751, 
         [0000]      | f 1 4   /f 1 3 |=1.102, 
         [0000]        BFL 1/ TTL 1=0.307, 
         [0000]      | f 1 1   /f 1|=1.131, 
         [0000]        f 1/ D 1 12 =1.676, 
         [0000]        Vd 1 4   /f 1=7.721, 
         [0000]        Vd 1 3   /f 1=16.926, 
         [0000]        f 1/ Nd 1 4 =2.052, 
         [0000]        f 1/ Nd 1 3 =2.159, 
         [0000]        D 1 34   /D 1 12 =0.051 
         [0035]    which respectively satisfy the above conditions (1)-(7). 
         [0036]    By the above arrangements of the lenses and stop ST 1 , the lens assembly  1  of the first embodiment can meet the requirements of optical performance as seen in  FIGS. 2A-2D , wherein  FIG. 2A  shows the longitudinal aberration diagram of the lens assembly  1  in accordance with the first embodiment of the invention,  FIG. 2B  shows the field curvature diagram of the lens assembly  1  in accordance with the first embodiment of the invention,  FIG. 2C  shows the distortion diagram of the lens assembly  1  in accordance with the first embodiment of the invention and  FIG. 2D  shows the lateral color diagram of the lens assembly  1  in accordance with the first embodiment of the invention. 
         [0037]    It can be seen from  FIG. 2A  that the longitudinal aberration in the lens assembly  1  of the first embodiment ranges from 0.006 mm to 0.015 mm for the wavelength of 0.436 μm, 0.486 μm, 0.546 μm, 0.587 μm and 0.656 μm. It can be seen from  FIG. 2B  that the field curvature of tangential direction and sagittal direction in the lens assembly  1  of the first embodiment ranges from −0.02 mm to 0.09 mm for the wavelength of 0.546 μm. It can be seen from  FIG. 2C  that the distortion in the lens assembly  1  of the first embodiment ranges from −25% to 0% for the wavelength of 0.546 μm. It can be seen from  FIG. 2D  that the lateral color in the lens assembly  1  of the first embodiment ranges from 0 μm to 3.5 μm for the wavelength ranges from 0.436 μm to 0.656 μm, with field ranged from 0 mm to 2.419 mm. It is obvious that the longitudinal aberration, the field curvature, the distortion and the lateral color of the lens assembly  1  of the first embodiment can be corrected effectively. Therefore, the lens assembly  1  of the first embodiment is capable of good optical performance. 
         [0038]    Referring to  FIG. 3 ,  FIG. 3  is a lens layout diagram of a lens assembly in accordance with a second embodiment of the invention. The lens assembly  2  includes a first lens L 21 , a second lens L 22 , a stop ST 2 , a third lens L 23 , a fourth lens L 24  and an optical filter OF 2 , all of which are arranged in sequence from an object side to an image side along an optical axis OA 2 . In operation, an image of light rays from the object side is formed at an image plane IMA 2 . The first lens L 21  is a biconcave lens with negative refractive and made of plastic material, wherein both of the object side surface S 21  and image side surface S 22  are aspheric surfaces. The second lens L 22  is a biconvex lens with positive refractive power and made of plastic material, wherein both of the object side surface S 23  and image side surface S 24  are aspheric surfaces. The third lens L 23  is a biconvex lens with positive refractive power and made of plastic material, wherein both of the object side surface S 26  and image side surface S 27  are aspheric surfaces. The fourth lens L 24  is a concave-convex lens with negative refractive power and made of plastic material, wherein the object side surface S 28  is a concave surface, the image side surface S 29  is a convex surface and both of the object side surface S 28  and image side surface S 29  are aspheric surfaces. There is an air space between the third lens L 23  and the fourth lens L 24 . Both of the object side surface S 210  and image side surface S 211  of the optical filter OF 2  are plane surfaces. 
         [0039]    In order to maintain excellent optical performance of the lens assembly in accordance with the second embodiment of the invention, the lens assembly  2  must satisfies the following seven conditions: 
         [0000]      0&lt;| f 2 1   /f 2 2   |&lt;|f 2 4   /f 2 3 |&lt;2  (8)
 
         [0000]        BFL 2/ TTL 2&gt;0.4  (9)
 
         [0000]      | f 2 1   /f 2|≦1.5  (10)
 
         [0000]        f 2/ D 2 12 &gt;1  (11)
 
         [0000]      0.1&lt; Vd 2 4   /f 2&lt; Vd 2 3   /f 2&lt;50  (12)
 
         [0000]      0.1&lt; f 2/ Nd 2 4   &lt;f 2/ Nd 2 3 &lt;3  (13)
 
         [0000]        D 2 34   /D 2 12 &gt;0.02  (14)
 
         [0000]    wherein f 2   1  is an effective focal length of the first lens L 21 , f 2   2  is an effective focal length of the second lens L 22 , f 2   3  is an effective focal length of the third lens L 23 , f 2   4  is an effective focal length of the fourth lens L 24 , f 2  is an effective focal length of the lens assembly  2 , BFL 2  is an interval from the image side surface S 29  of the fourth lens L 24  to the image plane IMA 2  along the optical axis OA 2 , TTL 2  is an interval from the object side surface S 21  of the first lens L 21  to the image plane IMA 2  along the optical axis OA 2 , D 2   12  is an interval from the image side surface S 22  of the first lens L 21  to the object side surface S 23  of the second lens L 22  along the optical axis OA 2 , D 2   34  is an interval from the image side surface S 27  of the third lens L 23  to the object side surface S 28  of the fourth lens L 24  along the optical axis OA 2 , Vd 2   3  is an Abbe number of the third lens L 23 , Vd 2   4  is an Abbe number of the fourth lens L 24 , Nd 2   3  is an index of refraction of the third lens L 23  and Nd 2   4  is an index of refraction of the fourth lens L 24 . 
         [0040]    By the above design of the lenses and stop ST 2 , the lens assembly  2  is provided with a shortened total lens length, a decreased F-number and an effective corrected aberration. 
         [0041]    In order to achieve the above purposes and effectively enhance the optical performance, the lens assembly  2  in accordance with the second embodiment of the invention is provided with the optical specifications shown in Table 3, which include the effective focal length, F-number, total lens length, radius of curvature of each lens surface, thickness between adjacent surface, refractive index of each lens and Abbe number of each lens. Table 3 shows that the effective focal length is equal to 3.3124 mm, F-number is equal to 2.0 and total lens length is equal to 11.497 mm for the lens assembly  2  of the second embodiment of the invention. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Effective Focal Length = 3.3124 mm F-number = 2.0  
               
               
                 Total Lens Length = 11.497 mm 
               
             
          
           
               
                   
                 Radius of 
                   
                   
                   
                   
               
               
                 Surface 
                 Curvature 
                 Thickness 
                   
                   
                   
               
               
                 Number 
                 (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 Remark 
               
               
                   
               
             
          
           
               
                 S21 
                 −9.555 
                 0.5 
                 1.5346 
                 56.07 
                 The First  
               
               
                   
                   
                   
                   
                   
                 Lens L21 
               
               
                 S22 
                 2.594 
                 1.805 
                   
                   
                   
               
               
                 S23 
                 6.52 
                 2.363 
                 1.5346 
                 56.07 
                 The Second  
               
               
                   
                   
                   
                   
                   
                 Lens L22 
               
               
                 S24 
                 −3.956 
                 0 
                   
                   
                   
               
               
                 S25 
                 ∞ 
                 0.62 
                   
                   
                 Stop ST2 
               
               
                 S26 
                 2.927 
                 1.981 
                 1.5346 
                 56.07 
                 The Third  
               
               
                   
                   
                   
                   
                   
                 Lens L23 
               
               
                 S27 
                 −2.234 
                 0.1 
                   
                   
                   
               
               
                 S28 
                 −1.66 
                 0.606 
                 1.6142 
                 25.58 
                 The Fourth  
               
               
                   
                   
                   
                   
                   
                 Lens L24 
               
               
                 S29 
                 −17.286 
                 2.688 
                   
                   
                   
               
               
                 S210 
                 ∞ 
                 0.735 
                 1.5168 
                 64.16 
                 Optical Filter  
               
               
                   
                   
                   
                   
                   
                 OF2 
               
               
                 S211 
                 ∞ 
                 0.1 
               
               
                   
               
             
          
         
       
     
         [0042]    The aspheric surface sag z of each lens in table 3 can be calculated by the following formula: 
         [0000]        z=ch   2 /{1+[1−( k+ 1) c   2   h   2 ] 1/2   }+Ah   4   +Bh   6   +Ch   8   +Dh   10  
 
         [0000]    where c is curvature, h is the vertical distance from the lens surface to the optical axis, k is conic constant and A, B, C and D are aspheric coefficients. 
         [0043]    In the second embodiment, the conic constant k and the aspheric coefficients A, B, C, D of each surface are shown in Table 4. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Surface 
                   
                   
                   
                   
                   
               
               
                 Number 
                 k 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 S21 
                 7.1967 
                 3.881E−4  
                 2.969E−4 
                 −1.044E−5 
                 0 
               
               
                 S22 
                 0.1471 
                  7.79E−3 
                 −3.024E−4  
                  6.836E−4 
                 0 
               
               
                 S23 
                 9.396 
                 −3.374E−3  
                 −8.706E−4  
                  1.726E−4 
                 −8.132E−5 
               
               
                 S24 
                 1.0842 
                 5.767E−4  
                 2.122E−3 
                 −1.058E−3 
                   2.13E−4 
               
               
                 S26 
                 0.051 
                 −4.225E−4  
                 1.756E−4 
                 −3.422E−4 
                 0 
               
               
                 S27 
                 −2.5091 
                 1.039E−2 
                 1.169E−3 
                  −7.28E−4 
                 0 
               
               
                 S28 
                 −3.7936 
                 1.336E−2 
                 5.753E−4 
                  −4.55E−4 
                 0 
               
               
                 S29 
                 0 
                 6.805E−2 
                 −1.624E−2   
                  4.816E−3 
                 −5.549E−4 
               
               
                   
               
             
          
         
       
     
         [0044]    For the lens assembly  2  of the second embodiment, the effective focal length f 2   1  of the first lens L 21  is equal to −3.746 mm, the effective focal length f 2   2  of the second lens L 22  is equal to 4.978 mm, the effective focal length f 2   3  of the third lens L 23  is equal to 2.725 mm, the effective focal length f 2   4  of the fourth lens L 24  is equal to −3.044 mm, the effective focal length f 2  of the lens assembly  2  is equal to 3.3124 mm, the interval BFL 2  from the image side surface S 29  of the fourth lens L 24  to the image plane IMA 2  along the optical axis OA 2  is equal to 3.523 mm, the interval TTL 2  from the object side surface S 21  of the first lens L 21  to the image plane IMA 2  along the optical axis OA 2  is equal to 11.497 mm, the interval D 2   12  from the image side surface S 22  of the first lens L 21  to the object side surface S 23  of the second lens L 22  along the optical axis OA 2  is equal to 1.805 mm, the interval D 2   34  from the image side surface S 27  of the third lens L 23  to the object side surface S 28  of the fourth lens L 24  along the optical axis OA 2  is equal to 0.1 mm, the Abbe number Vd 2   3  of the third lens L 23  is equal to 56.07, the Abbe number Vd 2   4  of the fourth lens L 24  is equal to 25.58, the index of refraction Nd 2   3  of the third lens L 23  is equal to 1.5346 and the index of refraction Nd 2   4  of the fourth lens L 24  is equal to 1.6142. According to the above data, the following values can be obtained: 
         [0000]      | f 2 1   /f 2 2 1=0.752, 
         [0000]      | f 2 4   /f 2 3 |=1.102, 
         [0000]        BFL 2/ TTL 2=0.306, 
         [0000]      | f 2 1   /f 2|=1.131, 
         [0000]        f 2/ D 2 12 =1.835, 
         [0000]        Vd 2 4   /f 2=7.721, 
         [0000]        Vd 2 3   /f 2=16.928, 
         [0000]        f 2/ Nd 2 4 =2.051, 
         [0000]        f 2/ Nd 2 3 =2.158, 
         [0000]        D 2 34   /D 2 12 =0.055 
         [0045]    which respectively satisfy the above conditions (8)-(14). 
         [0046]    By the above arrangements of the lenses and stop ST 2 , the lens assembly  2  of the second embodiment can meet the requirements of optical performance as seen in  FIGS. 4A-4D , wherein  FIG. 4A  shows the longitudinal aberration diagram of the lens assembly  2  in accordance with the second embodiment of the invention,  FIG. 4B  shows the field curvature diagram of the lens assembly  2  in accordance with the second embodiment of the invention,  FIG. 4C  shows the distortion diagram of the lens assembly  2  in accordance with the second embodiment of the invention and  FIG. 4D  shows the lateral color diagram of the lens assembly  2  in accordance with the second embodiment of the invention. 
         [0047]    It can be seen from  FIG. 4A  that the longitudinal aberration in the lens assembly  2  of the second embodiment ranges from 0.003 mm to 0.045 mm for the wavelength of 0.436 nm, 0.486 nm, 0.546 nm, 0.587 nm and 0.656 nm. It can be seen from  FIG. 4B  that the field curvature of tangential direction and sagittal direction in the lens assembly  2  of the second embodiment ranges from 0.00 mm to 0.12 mm for the wavelength of 0.546 nm. It can be seen from  FIG. 4C  that the distortion in the lens assembly  2  of the second embodiment ranges from 30% to 0% for the wavelength of 0.546 nm. It can be seen from  FIG. 4D  that the lateral color in the lens assembly  2  of the second embodiment ranges from 0.5 nm to 1.5 nm for the wavelength ranges from 0.436 nm to 0.656 nm, with field ranged from 0 mm to 2.419 mm. It is obvious that the longitudinal aberration, the field curvature, the distortion and the lateral color of the lens assembly  2  of the second embodiment can be corrected effectively. Therefore, the lens assembly  2  of the second embodiment is capable of good optical performance.