Abstract:
A lens assembly comprises sequentially from an object side to an image side along an optical axis a first lens, a second lens, a stop, a third lens, a fourth lens and a fifth lens. The first lens is a biconvex lens with positive refractive power. The second lens is a meniscus lens with negative refractive power and the convex surface of second lens faces the object side. The third lens is a meniscus lens with negative refractive power and the convex surface of third lens faces the object side. The fourth lens is a meniscus lens with positive refractive power and the concave surface of fourth lens faces the object side. The fifth lens is a biconcave lens with negative refractive power. The lens assembly satisfies the following condition: 1.10&lt;D L1 /D ST &lt;10.90, wherein D L1  is an effective diameter of the first lens and D ST  is an effective diameter of the stop.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a lens assembly. 
     Description of the Related Art 
     Digital still cameras and mobile phones have been continually developed toward high pixel number and miniaturization. Therefore, the requirements for lens assemblies with miniaturization and high resolution are greatly increased. The well-known lens assembly with five lens 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a lens assembly to solve the above problems. The lens assembly of the invention, provided with characteristics of a shortened total lens length, still has a good optical performance and can meet a requirement or resolution. 
     The lens assembly in accordance with the invention comprises a first lens, a second lens, a stop, a third lens, a fourth lens and a fifth lens, all of which are arranged in sequence from an object side to an image side along an optical axis. The first lens is a biconvex lens with positive refractive power. The second lens is a meniscus lens with negative refractive power and includes a convex surface facing the object side and concave surface facing the image side. The third lens is a meniscus lens with negative refractive power and includes a convex surface facing the object side and a concave surface facing the image side. The fourth lens is a meniscus lens with positive refractive power and includes a concave surface facing the object side and a convex surface facing the image side. The fifth lens is a biconcave lens with negative refractive power. The lens assembly satisfies the following condition: 1.10&lt;D L1 /D ST &lt;10.90, wherein D L1  is an effective diameter of the first lens and D ST  is an effective diameter of the stop. 
     The lens assembly satisfies 1.10&lt;D L1 /D L2 &lt;1.35, wherein D L1  is an effective diameter of the first lens and D L2  is an effective diameter of the second lens. 
     The lens assembly satisfies Vd 1 &gt;Vd 2 , Vd 1 &gt;Vd 3 , Vd 4 &gt;Vd 2 , Vd 4 &gt;Vd 3 , Vd 5 &gt;Vd 2 , Vd 5 &gt;Vd 3 , wherein Vd 1  is an Abbe number of the first lens, Vd 2  is an Abbe number of the second lens, Vd 3  is an Abbe number of the third lens, Vd 4  is an Abbe number of the fourth lens and Vd 5  is an Abbe number of the fifth lens. 
     The lens assembly satisfies Vd 1 &gt;40, Vd 2 &lt;40, Vd 5 &gt;40, wherein Vd 1  is the Abbe number of the first lens, Vd 2  is the Abbe number of the second lens and Vd 5  is the Abbe number of the fifth lens. 
     The lens assembly satisfies Vd 3 &lt;40, Vd 4 &gt;40, wherein Vd 3  is the Abbe number of the third lens and Vd 4  is the Abbe number of the fourth lens. 
     The third lens and the fourth lens satisfies 15≦Vd 4 −Vd 3 &lt;43.61 , wherein Vd 3  is the Abbe number of the third lens and Vd 4  is the Abbe number of the fourth lens. 
     The first lens, the third lens and the fourth lens satisfies −1.3&lt;f/f 3 +f/f 4 −f/f 1 &lt;−0.1, wherein f is an effective focal length of the lens assembly, f 1  is an effective focal length of the first lens, f 3  is an effective focal length of the third lens and f 4  is an effective focal length of the fourth lens. 
     The lens assembly satisfies 0.6&lt;SL/TTL&lt;0.87 , wherein SL is an interval from the stop to the 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. 
     The fourth lens is made of glass material. 
     The first lens is made of plastic material, the second lens is made of plastic material, the third lens is made of plastic material and the fifth lens is made of plastic material. 
     The stop includes an optical opening, wherein the diameter of the optical opening can be changed so as to make the effective diameter of the stop change. 
     The lens assembly satisfies 1.4≦F≦13, wherein F is a F-number of the lens assembly. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a lens layout and optical path diagram of a lens assembly in accordance with a first embodiment of the invention; 
         FIG. 2A  depicts a field curvature diagram of the lens assembly in accordance with the first embodiment of the invention; 
         FIG. 2B  is a distortion diagram of the lens assembly in accordance with the first embodiment of the invention; 
         FIG. 2C  is a modulation transfer function diagram of the lens assembly in accordance with the first embodiment of the invention; 
         FIG. 3  is a lens layout and optical path diagram of a lens assembly in accordance with a second embodiment of the invention; 
         FIG. 4A  depicts a field curvature diagram of the lens assembly in accordance with the second embodiment of the invention; 
         FIG. 4B  is a distortion diagram of the lens assembly in accordance with the second embodiment of the invention; 
         FIG. 4C  is a modulation transfer function diagram of the lens assembly in accordance with the second embodiment of the invention; and; 
         FIG. 5  is a lens layout and optical path diagram of a lens assembly in accordance with a third embodiment of the invention; 
         FIG. 6A  depicts a field curvature diagram of the lens assembly in accordance with the third embodiment of the invention; 
         FIG. 6B  is a distortion diagram of the lens assembly in accordance with the third embodiment of the invention; and 
         FIG. 6C  is a modulation transfer function diagram of the lens assembly in accordance with the third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Referring to  FIG. 1 ,  FIG. 1  is a lens layout and optical path 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 , a fifth lens L 15  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  1 . The first lens L 11  is with positive refractive power and made of plastic material, wherein the object side surface S 11  is a convex surface, the image side surface S 12  is a convex surface and both of the object side surface S 11  and image side surface  512  are aspheric surfaces. The second lens L 12  is with negative refractive power and made of plastic material, wherein the object side surface S 13  is a convex surface, the image side surface S 14  is a concave surface and both of the object side surface S 13  and image side surface S 14  are aspheric surfaces. The third lens L 13  is with negative refractive power and made of plastic material, wherein the object side surface S 16  is a convex surface, the image side surface S 17  is a concave surface and both of the object side surface S 16  and image side surface S 17  are aspheric surfaces. The fourth lens L 14  is with positive refractive power and made of glass 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. The fifth lens L 15  is with negative refractive power and made of plastic material, wherein the object side surface S 110  is a concave surface, the image side surface S 111  is a concave surface and both of the object side surface S 110  and image side surface S 111  are aspheric surfaces. Both of the object side surface S 112  and image side surface S 113  of the optical filter OF 1  are plane surfaces. In the first embodiment, Abbe number of the first lens L 11 , the fourth lens L 14  and the fifth lens L 15  are greater than Abbe number of the second lens L 12  and the third lens L 13 . 
     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 satisfy the following eleven conditions:
 
1.10&lt; D 1 L11   /D 1 ST1 &lt;10.90   (1)
 
1.10 &lt;D 1 L11   /D 1 L12 &lt;1.35   (2)
 
Vd1 1 &gt;40   (3)
 
Vd1 2 &lt;40   (4)
 
Vd1 3 &lt;40   (5)
 
Vd1 4 &gt;40   (6)
 
Vd1 5 &gt;40   (7)
 
−1.3&lt; f 1 /f 1 3   +f 1 /f 1 4   −f 1 /f 1 1 &lt;−0.1   (8)
 
15 ≦Vd 1 4   −Vd 1 3 &lt;43.61   (9)
 
0.6&lt; SL 1 /TTL 1&lt;0.87   (10)
 
1.4≦F1≦13   (11)
 
     wherein D 1   L11  is an effective diameter of the first lens L 11 , D 1   L12  is an effective diameter of the second lens L 12 , D 1   ST1  is an effective diameter of the stop ST 1 . A straight length of the effective diameter D 1   L11  of the first lens L 11  means from an edge of the first lens L 11  through a center point of the first lens L 11  to the other edge. A straight length of the effective diameter D 1   L12  of the second lens L 12  means from an edge of the second lens L 12  through a center point of the second lens L 12  to the other edge. The effective diameter D 1   ST1  of the stop ST 1  means a diameter of optical opening of the stop ST 1 . Vd 1   1  is an Abbe number of first lens L 11 , Vd 1   2  is an Abbe number of second lens L 12 , Vd 1   3  is an Abbe number of third lens L 13 , Vd 1   4  is an Abbe number of fourth lens L 14 , Vd 1   5  is an Abbe number of fifth lens L 15 , f 1  is an effective focal length of the lens assembly  1 , f 1   1  is an effective focal length of the first lens L 11 , 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 , SL 1  is an interval from the stop ST 1  to the image plane IMA 1  along the optical axis OA 1 , TTL 1  is an interval from an object side surface S 11  of the first lens to the image plane IMA 1  along the optical axis OA 1 . 
     By the above design of the lenses and stop ST 1 , the lens assembly  1  is provided with a shortened total lens length, an effective corrected aberration and an increased resolution. 
     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 in mm, thickness between adjacent surface in mm, refractive index of each lens and Abbe number of each lens. Table 1 shows that the effective focal length is equal to 4.914 mm, F-number is equal to 1.6, total lens length is equal to 5.515 mm, field of view is equal to 120°, effective diameter of the first lens L 11  is equal to 2.68 mm, effective diameter of the second lens L 12  is equal to 2.030 mm and effective diameter of the stop ST 1  is equal to 1.998 mm for the lens assembly  1  of the first embodiment of the invention. 
     
       
         
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Effective Focal Length = 4.914 mm 
                 F-number = 1.6 
               
               
                 Total Lens Length = 5.515 mm 
                 field of view = 120° 
               
               
                 effective diameter of the first lens L11 = 2.68 mm 
                   
               
               
                 effective diameter of the second lens L12 = 2.030 mm 
                   
               
               
                 effective diameter of the stop ST1 = 1.998 mm 
               
               
                   
               
             
          
           
               
                   
                 Radius of 
                   
                   
                   
                   
               
               
                 Surface 
                 Curvature 
                 Thickness 
                   
                   
                   
               
               
                 Number 
                 (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 Remark 
               
               
                   
               
               
                 S11 
                 1.768392 
                 0.88 
                 1.53 
                 56.1 
                 The First Lens L11 
               
               
                 S12 
                 −15.8212 
                 0.016 
                   
                   
                   
               
               
                 S13 
                 78.15854 
                 0.302841 
                 1.65 
                 21.5 
                 The Second Lens L12 
               
               
                 S14 
                 3.647587 
                 0.538276 
                   
                   
                   
               
               
                 S15 
                 ∞ 
                 0.339278 
                   
                   
                 Stop ST1 
               
               
                 S16 
                 69.30394 
                 0.310414 
                 1.65 
                 21.5 
                 The Third Lens L13 
               
               
                 S17 
                 8.059585 
                 0.298384 
                   
                   
                   
               
               
                 S18 
                 −59.6721 
                 0.758629 
                 1.85 
                 40.3 
                 The Fourth Lens L14 
               
               
                 S19 
                 −3.19383 
                 0.672237 
                   
                   
                   
               
               
                 S110 
                 −2.52244 
                 0.499967 
                 1.53 
                 56.1 
                 The Fifth Lens L15 
               
               
                 S111 
                 6.620541 
                 0.24 
                   
                   
                   
               
               
                 S112 
                 ∞ 
                 0.145 
                   
                   
                 Optical Filter OF1 
               
               
                 S113 
                 ∞ 
                 0.514626 
               
               
                   
               
             
          
         
       
     
     The aspheric surface sag z of each lens in table 1 can be calculated by the following formula:
 
 z=ch   2 /{1+[1−( k+ 1) c   2   h   2 ] 1/2   }+Ah   4   +Bh   6   +Ch   8   +Dh   10   +Eh   12   +Fh   14   +Gh   16  
 
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, D, E, F and G are aspheric coefficients.
 
     In the first embodiment, the conic constant k and the aspheric coefficients A, B, C, D, E, F, G of each surface are shown in Table 2. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Surface 
                 k 
                 A 
                 B 
                 C 
               
               
                 Number 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 S11 
                 −0.54105 
                 0.009829374 
                 0.000971779 
                 −0.006244744 
               
               
                   
                 0.007262122 
                 −0.003886691 
                   2.14E−05 
                 −6.99E−07 
               
               
                 S12 
                 107.373 
                 0.024638449 
                 0.004349484 
                 −0.0178282 
               
               
                   
                 −0.000432988 
                 0.006673496 
                 −2.17E−03 
                 −1.47E−04 
               
               
                 S13 
                 4125.063 
                 0.02456465 
                 0.023273928 
                 −0.028320209 
               
               
                   
                 0.006128956 
                 0.009453012 
                 −3.86E−03 
                 −2.17E−04 
               
               
                 S14 
                 −14.9028 
                 0.058170471 
                 0.10004734 
                 0.000307198 
               
               
                   
                 −0.021863276 
                 0.04726975 
                 −2.38E−02 
                   6.65E−04 
               
               
                 S16 
                 −7265.04 
                 −0.10306565 
                 0.003871945 
                 0.025943356 
               
               
                   
                 −0.023903474 
                 0.006864957 
                   1.71E−03 
                 −2.69E03 
               
               
                 S17 
                 −217.534 
                 −0.067730931 
                 −0.020575026 
                 0.021656606 
               
               
                   
                 −0.002067265 
                 −0.001974986 
                   7.07E−04 
                 −5.54E−05 
               
               
                 S18 
                 1046.437 
                 −0.01043783 
                 −0.015395534 
                 0.003496637 
               
               
                   
                 0.000720169 
                 −0.000453211 
                   4.89E−05 
                   5.44E−06 
               
               
                 S19 
                 −0.23313 
                 0.016003498 
                 −0.006712571 
                 0.002465054 
               
               
                   
                 −0.000428693 
                 −8.63E−05 
                   2.71E−05 
                 −1.38E−06 
               
               
                 S110 
                 −10.535 
                 −0.11112052 
                 0.05001558 
                 −0.011027879 
               
               
                   
                 0.001436059 
                 −0.000102669 
                   3.13E−06 
                 −5.44E−09 
               
               
                 S111 
                 −49.8499 
                 −0.046132544 
                 0.013207205 
                 −0.00262408 
               
               
                   
                 0.00027185 
                 −1.12E−05 
                 −1.97E−08 
                   8.06E−09 
               
               
                   
               
             
          
         
       
     
     For the lens assembly  1  of the first embodiment, the effective diameter D 1   L11  of the first lens L 11  is equal to 2.68 mm, the effective diameter D 1   L12  of the second lens L 12  is equal to 2.030 mm, the effective diameter of the stop ST 1  is equal to 1.998 mm, the Abbe number Vd 1   1  of the first lens L 11  is equal to 56.1, the Abbe number Vd 1   2  of the second lens L 12  is equal to 21.5, the Abbe number Vd 1   3  of the third lens L 13  is equal to 21.5, the Abbe number Vd 1   4  of the fourth lens L 14  is equal to 40.3, the Abbe number Vd 1   5  of the fifth lens L 15  is equal to 56.1, the effective focal length f 1  of the lens assembly  1  is equal to 4.914 mm, the effective focal length f 1   1  of the first lens L 11  is equal to 3.0183 mm, the effective focal length f 1   3  of the third lens L 13  is equal to −13.9211, the effective focal length f 1   4  of the fourth lens L 14  is equal to 3.9326 mm, the interval SL 1  from the stop ST 1  to image plane IMA 1  along the optical axis OA 1  is equal to 3.778 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 5.515 mm. According to the above data, the following values can be obtained:
 
 D 1 L11   /D 1 ST1 =1.34,
 
 D 1 L11   /D 1 L12 =1.32,
 
Vd1 1 =56.1,
 
Vd1 2 =21.5,
 
Vd1 3 =21.5,
 
Vd1 4 =40.3,
 
Vd1 5 =56.1,
 
 f 1 /f 1 3   +f 1 /f 1 4   −f 1 /f 1 1 =−0.7314,
 
 Vd 1 4   −Vd 1 3 −18.8,
 
 SL 1 /TTL 1=0.685,
 
F1=1.6
 
which respectively satisfy the above conditions (1)-(11).
 
     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-2C , wherein  FIG. 2A  shows a field curvature diagram of the lens assembly  1  in accordance with the first embodiment of the invention,  FIG. 2B  shows a distortion diagram of the lens assembly  1  in accordance with the first embodiment of the invention,  FIG. 2C  shows a modulation transfer function diagram of the lens assembly  1  in accordance with the first embodiment of the invention. 
     It can be seen from  FIG. 2A  that the field curvature of tangential direction and sagittal direction in the lens assembly  1  of the first embodiment ranges from −0.16 mm to 0.06 mm for the wavelength of 0.435 μm, 0.555 μm and 0.650 μm. It can be seen from  FIG. 2B  that the distortion in the lens assembly  1  of the first embodiment ranges from 0.0% to 1.4% for the wavelength of 0.435 μm, 0.555 μm, and 0.650 μm. It can be seen from  FIG. 2C  that the modulation transfer function of tangential direction and sagittal direction in the lens assembly  1  of the first embodiment ranges from 0.0 to 1.0 when the wavelength ranges from 0.435 μm to 0.650 μm, the fields respectively are 0.0000 mm, 0.6864 mm, 1.3728 mm, 2.4024 mm and 3.4320 mm, and the spatial frequency ranges from 0 lp/mm to 446 lp/mm. It is obvious that the field curvature and the distortion of the lens assembly  1  of the first embodiment can be corrected effectively, and the resolution of the lens assembly  1  of the first embodiment can meet the requirement. Therefore, the lens assembly  1  of the first embodiment is capable of good optical performance. 
     In the above embodiment, when the effective diameter of the stop ST 1  can be respectively adjusted to 2.348 mm, 1.458 mm, 0.954 mm and 0.246 mm, F-number of the lens assembly  1  can be respectively changed to 1.4, 2.4, 3.4 and 13, the maximum value of D 1   L11 /D 1   ST1  is equal to 10.894(2.68/0.246) and the minimum value of D 1   L11 /D 1   ST1  is equal to 1.141(2.68/2.348), which satisfy the above condition (1). By changing the effective diameter of the stop ST 1 , which can control the amount of the incident light of the lens assembly  1  and change illuminance of image plane IMA 1 . On the other hand, changing the effective diameter of the stop ST 1  can control depth of Field, when the greater effective diameter of the stop ST 1 , the shorter depth of Field. When the smaller effective diameter of the stop ST 1 , the longer depth of Field. 
     Referring to  FIG. 3 ,  FIG. 3  is a lens layout and optical path 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 , a fifth lens L 25  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 with positive refractive power and made of plastic material, wherein the object side surface S 21  is a convex surface, the image side surface S 22  is a convex surface and both of the object side surface S 21  and image side surface S 22  are aspheric surfaces. The second lens L 22  is with negative refractive power and made of plastic material, wherein the object side surface S 23  is a convex surface, the image side surface S 24  is a concave surface and both of the object side surface S 23  and image side surface S 24  are aspheric surfaces. The third lens L 23  is with negative refractive power and made of plastic material, wherein the object side surface S 26  is a convex surface, the image side surface S 27  is a concave surface and both of the object side surface S 26  and image side surface S 27  are aspheric surfaces. The fourth lens L 24  is with positive refractive power and made of glass 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. The fifth lens L 25  is with negative refractive power and made of plastic material, wherein the object side surface S 210  is a concave surface, the image side surface S 211  is a concave surface and both of the object side surface S 210  and image side surface S 211  are aspheric surfaces. Both of the object side surface S 212  and image side surface S 213  of the optical filter OF 2  are plane surfaces. In the second embodiment, Abbe number of the first lens L 21 , the fourth lens L 24  and the fifth lens L 25  are greater than Abbe number of the second lens L 22  and the third lens L 23 . 
     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 satisfy the following eleven conditions:
 
1.10 &lt;D 2 L21   /D 2 ST2 &lt;10.90   (12)
 
1.10 &lt;D 2 L21   /D 2 L22 &lt;1.35   (13)
 
Vd2 1 &gt;40   (14)
 
Vd2 2 &lt;40   (15)
 
Vd2 3 &lt;40   (16)
 
Vd2 4 &gt;40   (17)
 
Vd2 5 &gt;40   (18)
 
−1.3&lt; f 2 /f 2 3   +f 2 /f 2 4   −f 2 /f 2 1 &lt;−0.1   (19)
 
15≦ Vd 2 4   −Vd 2 3 &lt;43.61   (20)
 
0.6&lt; SL 2 /TTL 2&lt;0.87   (21)
 
1.4≦F2≦13   (22)
 
     wherein D 2   L21  is an effective diameter of the first lens L 21 , D 2   L22  is an effective diameter of the second lens L 22 , D 2   ST2  is an effective diameter of the stop ST 2 . A straight length of the effective diameter D 2   L21  of the first lens L 21  means from an edge of the first lens L 21  through a center point of the first lens L 21  to the other edge. A straight length of the effective diameter D 2   L22  of the second lens L 22  means from an edge of the second lens L 22  through a center point of the second lens L 22  to the other edge. The effective diameter D 2   ST2  of the stop ST 2  means a diameter of optical opening of the stop ST 2 . Vd 2   1  is an Abbe number of first lens L 21 , Vd 2   2 is an Abbe number of second lens L 22 , Vd 2   3  is an Abbe number of third lens L 23 , Vd 2   4  is an Abbe number of fourth lens L 24 , Vd 2   5  is an Abbe number of fifth lens L 25 , f 2  is an effective focal length of the lens assembly  2 , f 2   1  is an effective focal length of the first lens L 21 , 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 , SL 2  is an interval from the stop ST 2  to the image plane IMA 2  along the optical axis OA 2 , TTL 2  is an interval from an object side surface S 21  of the first lens to the image plane IMA 2  along the optical axis OA 2 . 
     By the above design of the lenses and stop ST 2 , the lens assembly  2  is provided with a shortened total lens length, an effective corrected aberration and an increased resolution. 
     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 in mm, thickness between adjacent surface in mm, refractive index of each lens and Abbe number of each lens. Table 3 shows that the effective focal length is equal to 4.837 mm, F-number is equal to 1.6, total lens length is equal to 5.493 mm, field of view is equal to 120°, effective diameter of the first lens L 21  is equal to 2.74 mm, effective diameter of the second lens L 22  is equal to 2.314 mm and effective diameter of the stop ST 2  is equal to 2.052 mm for the lens assembly  2  of the second embodiment of the invention. 
     
       
         
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                 Effective Focal Length = 4.837 mm 
                 F-number = 1.6 
               
               
                 Total Lens Length = 5.493 mm 
                 field of view = 120° 
               
               
                 effective diameter of the first lens L21 = 2.74 mm 
                   
               
               
                 effective diameter of the second lens L22 = 2.314 mm 
                   
               
               
                 effective diameter of the stop ST2 = 2.052 mm 
               
               
                   
               
             
          
           
               
                   
                 Radius of 
                   
                   
                   
                   
               
               
                 Surface 
                 Curvature 
                 Thickness 
                   
                   
                   
               
               
                 Number 
                 (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 Remark 
               
               
                   
               
               
                 S21 
                 1.770213 
                 0.875511 
                 1.53 
                 56.1 
                 The First Lens L21 
               
               
                 S22 
                 −15.6246 
                 0.016305 
                   
                   
                   
               
               
                 S23 
                 77.46229 
                 0.302447 
                 1.65 
                 21.5 
                 The Second Lens L22 
               
               
                 S24 
                 3.653142 
                 0.39944 
                   
                   
                   
               
               
                 S25 
                 ∞ 
                 0.469051 
                   
                   
                 Stop ST2 
               
               
                 S26 
                 68.47689 
                 0.310352 
                 1.65 
                 35 
                 The Third Lens L23 
               
               
                 S27 
                 8.225647 
                 0.310045 
                   
                   
                   
               
               
                 S28 
                 −59.9334 
                 0.748543 
                 1.85 
                 50 
                 The Fourth Lens L24 
               
               
                 S29 
                 −3.18744 
                 0.672541 
                   
                   
                   
               
               
                 S210 
                 −2.56721 
                 0.499967 
                 1.53 
                 56.1 
                 The Fifth Lens L25 
               
               
                 S211 
                 6.809095 
                 0.24 
                   
                   
                   
               
               
                 S212 
                 ∞ 
                 0.145 
                   
                   
                 Optical Filter OF2 
               
               
                 S213 
                 ∞ 
                 0.504791 
               
               
                   
               
             
          
         
       
     
     The aspheric surface sag z of each lens in table 3 can be calculated by the following formula:
 
 z=ch   2 /{1+[1−( k+ 1) c   2   h   2 ] 1/2   }+Ah   4   +Bh   6   +Ch   8   +Dh   10   +Eh   12   +Fh   14   +Gh   16  
 
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, D, E, F and G are aspheric coefficients.
 
     In the second embodiment, the conic constant k and the aspheric coefficients A, B, C, D, E, F, G of each surface are shown in Table 4. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Surface 
                 k 
                 A 
                 B 
                 C 
               
               
                 Number 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 S21 
                 −0.53631 
                 0.009886469 
                 0.000977292 
                 −0.006377328 
               
               
                   
                 0.007205064 
                 −0.003904845 
                   1.57E−05 
                   3.78E−06 
               
               
                 S22 
                 107.7376 
                 0.02458309 
                 0.004359863 
                 −0.01793564 
               
               
                   
                 −0.000517998 
                 0.006683355 
                 −2.20E−03 
                 −1.33E−04 
               
               
                 S23 
                 4349.852 
                 0.024255909 
                 0.023224891 
                 −0.028221819 
               
               
                   
                 0.00603599 
                 0.009384775 
                 −3.91E−03 
                 −2.80E−04 
               
               
                 S24 
                 −14.6512 
                 0.058017374 
                 0.009487751 
                 0.000117773 
               
               
                   
                 −0.02067217 
                 0.046531098 
                 −2.39E−02 
                   1.12E−04 
               
               
                 S26 
                 −5619.73 
                 −0.095569255 
                 −0.003050543 
                 0.024097369 
               
               
                   
                 −0.017237898 
                 0.008476985 
                 −3.84E−03 
                 −9.15E−04 
               
               
                 S27 
                 −137.244 
                 −0.066642389 
                 −0.021435231 
                 0.023104931 
               
               
                   
                 −0.001598446 
                 −0.002293318 
                   5.25E−04 
                 −1.43E−05 
               
               
                 S28 
                 982.5376 
                 −0.01043783 
                 −0.015395534 
                 0.003496637 
               
               
                   
                 0.000720169 
                 −0.000453211 
                   4.89E−05 
                   5.44E−06 
               
               
                 S29 
                 0.143144 
                 −0.012751779 
                 −0.016881917 
                 0.004023966 
               
               
                   
                 0.000693383 
                 −4.49E−04 
                   6.42E−05 
                   1.46E−06 
               
               
                 S210 
                 −7.95772 
                 0.011901475 
                 0.049934571 
                 −0.011071364 
               
               
                   
                 0.001432537 
                 −0.000102839 
                   3.14E−06 
                 −1.73E−09 
               
               
                 S211 
                 −92.1371 
                 −0.045853456 
                 0.013576947 
                 −0.003674994 
               
               
                   
                 0.000266902 
                 −1.07E−05 
                   3.66E−08 
                   1.05E−09 
               
               
                   
               
             
          
         
       
     
     For the lens assembly  2  of the second embodiment, the effective diameter D 2   L21  of the first lens L 21  is equal to 2.74 mm, the effective diameter D 2   L22  of the second lens L 22  is equal to 2.314 mm, the effective diameter of the stop ST 2  is equal to 2.052 mm, the Abbe number Vd 2   1  of the first lens L 21  is equal to 56.1, the Abbe number Vd 2   2  of the second lens L 22  is equal to 21.5, the Abbe number Vd 2   3  of the third lens L 23  is equal to 35, the Abbe number Vd 2   4  of the fourth lens L 24  is equal to 50, the Abbe number Vd 2   5  the fifth lens L 25  is equal to 56.1, the effective focal length f 2  of the lens assembly  2  is equal to 4.837 mm, the effective focal length f 2   1  of the first lens L 21  is equal to 3.0152 mm, the effective focal length f 2   3  of the third lens L 23  is equal to −14.3156, the effective focal length f 2   4  of the fourth lens L 24  is equal to 3.9271 mm, the interval SL 2  from the stop ST 2  to image plane IMA 2  along the optical axis OA 2  is equal to 3.897 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 5.493 mm. According to the above data, the following values can be obtained:
 
 D 2 L21   /D 2 ST2 =1.34,
 
 D 2 L21   /D 2 L22 =1.18,
 
Vd2 1 =56.1,
 
Vd2 2 =21.5,
 
Vd2 3 =35,
 
Vd2 4 =50,
 
Vd2 5 =56.1,
 
 f 2/ f 2 3   +f 2 /f 2 4   −f 2/ f 2 1 =−0.71,
 
 Vd 2 4   −Vd 2 3 =15,
 
 SL 2 /TTL 2=0.709,
 
F2=1.6
 
which respectively satisfy the above conditions (12)-(22).
 
     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-4C , wherein  FIG. 4A  shows a field curvature diagram of the lens assembly  2  in accordance with the second embodiment of the invention,  FIG. 4B  shows a distortion diagram of the lens assembly  2  in accordance with the second embodiment of the invention,  FIG. 4C  shows a modulation transfer function diagram of the lens assembly  2  in accordance with the second embodiment of the invention. 
     It can be seen from  FIG. 4A  that the field curvature of tangential direction and sagittal direction in the lens assembly  2  of the second embodiment ranges from −0.020 mm to 0.035 mm for the wavelength of 0.470 μm, 0.555 μm and 0.650 μm. It can be seen from  FIG. 4B  that the distortion in the lens assembly  2  of the second embodiment ranges from 0.0% to 0.6% for the wavelength of 0.470 μm, 0.555 μm, and 0.650 μm. It can be seen from  FIG. 4C  that the modulation transfer function of tangential direction and sagittal direction in the lens assembly  2  of the second embodiment ranges from 0.0 to 1.0 when the wavelength ranges from 0.470 μm to 0.650 μm, the fields respectively are 0.0000 mm, 0.6864 mm, 1.3728 mm, 2.4024 mm and 3.4320 mm, and the spatial frequency ranges from 0 lp/mm to 446 lp/mm. It is obvious that the field curvature and the distortion of the lens assembly  2  of the second embodiment can be corrected effectively, and the resolution of the lens assembly  2  of the second embodiment can meet the requirement. Therefore, the lens assembly  2  of the second embodiment is capable of good optical performance. 
     In another embodiment described above, when the effective diameter of the stop ST 2  can be respectively adjusted to 2.222 mm, 1.41 mm, 0.98 mm and 0.252 mm, F-number of the lens assembly  2  can be respectively changed to 1.4, 2.4, 3.4 and 13, the maximum value of D 2   L21 /D 2   ST2  is equal to 10.873(2.74/0.252) and the minimum value of D 2   L21 /D 2   ST2  is equal to 1.233(2.74/2.222), which satisfy the above condition (12). By changing the effective diameter of the stop ST 2 , which can control the amount of the incident light of the lens assembly  2  and change illuminance of image plane IMA 2 . On the other hand, changing the effective diameter of the stop ST 2  can control depth of Field, when the greater effective diameter of the stop ST 2 , the shorter depth of Field. When the smaller effective diameter of the stop ST 2 , the longer depth of Field. 
     Referring to  FIG. 5 ,  FIG. 5  is a lens layout and optical path diagram of a lens assembly in accordance with a third embodiment of the invention. The lens assembly  3  includes a first lens L 31 , a second lens L 32 , a stop ST 3 , a third lens L 33 , a fourth lens L 34 , a fifth lens L 35  and an optical filter OF 3 , all of which are arranged in sequence from an object side to an image side along an optical axis OA 3 . In operation, an image of light rays from the object side is formed at an image plane IMA 3 . The first lens L 31  is with positive refractive power and made of plastic material, wherein the object side surface S 31  is a convex surface, the image side surface S 32  is a convex surface and both of the object side surface S 31  and image side surface S 32  are aspheric surfaces. The second lens L 32  is with negative refractive power and made of plastic material, wherein the object side surface S 33  is a convex surface, the image side surface S 34  is a concave surface and both of the object side surface S 33  and image side surface S 34  are aspheric surfaces. The third lens L 33  is with negative refractive power and made of plastic material, wherein the object side surface S 36  is a convex surface, the image side surface S 37  is a concave surface and both of the object side surface S 36  and image side surface S 37  are aspheric surfaces. The fourth lens L 34  is with positive refractive power and made of glass material, wherein the object side surface S 38  is a concave surface, the image side surface S 39  is a convex surface and both of the object side surface S 38  and image side surface S 39  are aspheric surfaces. The fifth lens L 35  is with negative refractive power and made of plastic material, wherein the object side surface S 310  is a concave surface, the image side surface S 311  is a concave surface and both of the object side surface S 310  and image side surface S 311  are aspheric surfaces. Both of the object side surface S 312  and image side surface S 313  of the optical filter OF 3  are plane surfaces. In the third embodiment, Abbe number of the first lens L 31 , the fourth lens L 34  and the fifth lens L 35  are greater than Abbe number of the second lens L 32  and the third lens L 33 . 
     In order to maintain excellent optical performance of the lens assembly in accordance with the third embodiment of the invention, the lens assembly  3  must satisfy the following eleven conditions:
 
1.10 &lt;D 3 L31   /D 3 ST2 &lt;10.90   (23)
 
1.10 &lt;D 3 L31   /D 3 L32 &lt;1.35   (24)
 
Vd3 1 &gt;40   (25)
 
Vd3 2 &lt;40   (26)
 
Vd3 3 &lt;40   (27)
 
Vd3 4 &gt;40   (28)
 
Vd3 5 &gt;40   (29)
 
−1.3&lt; f 3 /f 3 3   +f 3/ f 3 4   −f 3 /f 3 1 &lt;−0.1   (30)
 
15≦ Vd 3 4   −Vd 3 3 &lt;43.61   (31)
 
0.6&lt; SL 3 /TTL 3&lt;0.87   (32)
 
1.4≦F3≦13   (33)
 
     wherein D 3   L31  is an effective diameter of the first lens L 31 , D 3   L32  is an effective diameter of the second lens L 32 , D 3   ST3  is an effective diameter of the stop ST 3 . A straight length of the effective diameter D 3   L31  of the first lens L 31  means from an edge of the first lens L 31  through a center point of the first lens L 31  to the other edge. A straight length of the effective diameter D 3   L32  of the second lens L 32  means from an edge of the second lens L 32  through a center point of the second lens L 32  to the other edge. The effective diameter D 2   ST3  of the stop ST 3  means a diameter of optical opening of the stop ST 3 . Vd 3   1  is an Abbe number of first lens L 31 , Vd 3   2  is an Abbe number of second lens L 32 , Vd 3   3  is an Abbe number of third lens L 33 , Vd 3   4  is an Abbe number of fourth lens L 34 , Vd 3   5  is an Abbe number of fifth lens L 35 , f 3  is an effective focal length of the lens assembly  3 , f 3   1  an effective focal length of the first lens L 31 , f 3   3  is an effective focal length of the third lens L 33 , f 3   4  is an effective focal length of the fourth lens L 34 , SL 3  is an interval from the stop ST 3  to the image plane IMA 3  along the optical axis OA 3 , TTL 3  is an interval from an object side surface S 31  of the first lens to the image plane IMA 3  along the optical axis OA 3 . 
     By the above design of the lenses and stop ST 3 , the lens assembly  3  is provided with a shortened total lens length, an effective corrected aberration and an increased resolution. 
     In order to achieve the above purposes and effectively enhance the optical performance, the lens assembly  3  in accordance with the third embodiment of the invention is provided with the optical specifications shown in Table 5, which include the effective focal length, F-number, total lens length, radius of curvature of each lens surface in mm, thickness between adjacent surface in mm, refractive index of each lens and Abbe number of each lens. Table 5 shows that the effective focal length is equal to 4.885 mm, F-number is equal to 1.6, total lens length is equal to 5.494 mm, field of view is equal to 120°, effective diameter of the first lens L 31  is equal to 2.59 mm, effective diameter of the second lens L 32  is equal to 2.268 mm and effective diameter of the stop ST 3  is equal to 2.084 mm for the lens assembly  3  of the third embodiment of the invention. 
     
       
         
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
             
             
               
                 Effective Focal Length = 4.885 mm 
                 F-number = 1.6 
               
               
                 Total Lens Length = 5.494 mm 
                 field of view = 120° 
               
               
                 effective diameter of the first lens L31 = 2.59 mm 
                   
               
               
                 effective diameter of the second lens L32 = 2.268 mm 
                   
               
               
                 effective diameter of the stop ST3 = 2.084 mm 
               
               
                   
               
             
          
           
               
                   
                 Radius of 
                   
                   
                   
                   
               
               
                 Surface 
                 Curvature 
                 Thickness 
                   
                   
                   
               
               
                 Number 
                 (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 Remark 
               
               
                   
               
               
                 S31 
                 1.771681 
                 0.871357 
                 1.53 
                 56.1 
                 The First Lens L31 
               
               
                 S32 
                 −15.6329 
                 0.016305 
                   
                   
                   
               
               
                 S33 
                 76.71141 
                 0.301682 
                 1.65 
                 21.5 
                 The Second Lens L32 
               
               
                 S34 
                 3.653628 
                 0.390248 
                   
                   
                   
               
               
                 S35 
                 ∞ 
                 0.475287 
                   
                   
                 Stop ST3 
               
               
                 S36 
                 54.90513 
                 0.317373 
                 1.65 
                 21.5 
                 The Third Lens L33 
               
               
                 S37 
                 8.028085 
                 0.326353 
                   
                   
                   
               
               
                 S38 
                 −60.1783 
                 0.732153 
                 1.85 
                 60 
                 The Fourth Lens L34 
               
               
                 S39 
                 −3.17972 
                 0.676934 
                   
                   
                   
               
               
                 S310 
                 −2.58237 
                 0.499967 
                 1.53 
                 56.1 
                 The Fifth Lens L35 
               
               
                 S311 
                 5.601191 
                 0.24 
                   
                   
                   
               
               
                 S312 
                 ∞ 
                 0.145 
                   
                   
                 Optical Filter OF3 
               
               
                 S313 
                 ∞ 
                 0.501848 
               
               
                   
               
             
          
         
       
     
     The aspheric surface sag z of each lens in table 5 can be calculated by the following formula:
 
 z=ch   2 /{1+[1−( k+ 1) c   2   h   2 ] 1/2   }+Ah   4   +Bh   6   +Ch   8   +Dh   10   +Eh   12   +Fh   14   +Gh   16  
 
     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, D, E, F and G are aspheric coefficients. 
     In the third embodiment, the conic constant k and the aspheric coefficients A, B, C, D, E, F, G of each surface are shown in Table 6. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Surface 
                 k 
                 A 
                 B 
                 C 
               
               
                 Number 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 S31 
                 −0.54305 
                 0.009688492 
                 0.001055272 
                 −0.006371799 
               
               
                   
                 0.007246844 
                 −0.003896498 
                   9.48E−06 
                 −1.65E−05 
               
               
                 S32 
                 106.7435 
                 0.024704209 
                 0.004421365 
                 −0.017974575 
               
               
                   
                 −0.000557139 
                 0.006669641 
                 −2.20E−03 
                 −1.20E−04 
               
               
                 S33 
                 4404.847 
                 0.024268271 
                 0.02319228 
                 −0.028077951 
               
               
                   
                 0.006114599 
                 0.009445244 
                 −3.91E−03 
                 −3.39E−04 
               
               
                 S34 
                 −14.9208 
                 0.057718415 
                 0.009933214 
                 0.000310022 
               
               
                   
                 −0.02034366 
                 0.046576848 
                 −2.39E−02 
                   4.57E−04 
               
               
                 S36 
                 −8515.43 
                 −0.094921996 
                 −0.002853276 
                 0.023489048 
               
               
                   
                 −0.01757771 
                 0.008593427 
                 −3.70E−03 
                 −1.15E−03 
               
               
                 S37 
                 −150.284 
                 −0.067431765 
                 −0.021110609 
                 0.023090302 
               
               
                   
                 −0.001762208 
                 −0.002359591 
                   5.24E−04 
                   2.18E−08 
               
               
                 S38 
                 1012.33 
                 −0.011840386 
                 −0.016768142 
                 0.004149594 
               
               
                   
                 0.000725294 
                 −0.000447794 
                   6.23E−05 
                   4.39E−07 
               
               
                 S39 
                 0.121642 
                 0.011829762 
                 −0.006379843 
                 0.002670135 
               
               
                   
                 −0.000376206 
                 −9.04E−05 
                   2.14E−05 
                 −2.56E−07 
               
               
                 S310 
                 −6.59419 
                 −0.10856299 
                 0.049904696 
                 −0.011065644 
               
               
                   
                 0.00143304 
                 −0.000102805 
                   3.14E−06 
                 −1.76E−09 
               
               
                 S311 
                 −95.3522 
                 −0.046252171 
                 0.013398786 
                 −0.002672975 
               
               
                   
                 0.000267844 
                 −1.06E−05 
                   3.02E−08 
                 −6.14E −10 
               
               
                   
               
             
          
         
       
     
     For the lens assembly  3  of the third embodiment, the effective diameter D 3   L31  of the first lens L 31  is equal to 2.59 mm, the effective diameter D 3   L32  of the second lens L 32  is equal to 2.268 mm, the effective diameter of the stop ST 3  is equal to 2.084 mm, the Abbe number Vd 3   1  of the first lens L 31  is equal to 56.1, the Abbe number Vd 3   2  of the second lens L 32  is equal to 21.5, the Abbe number Vd 3   3  of the third lens L 33  is equal to 21.5, the Abbe number Vd 3   4  of the fourth lens L 34  is equal to 60, the Abbe number Vd 3   5  of the fifth lens L 35  is equal to 56.1, the effective focal length f 3  of the lens assembly  3  is equal to 4.885 mm, the effective focal length f 3   1  of the first lens L 31  is equal to 3.017 mm, the effective focal length f 3   3  of the third lens L 33  is equal to −14.362, the effective focal length f 3   4  of the fourth lens L 34  is equal to 3.913 mm, the interval SL 3  from the stop ST 3  to image plane IMA 3  along the optical axis OA 3  is equal to 3.911 mm, the interval TTL 3  from the object side surface S 31  of the first lens L 31  to the image plane IMA 3  along the optical axis OA 3  is equal to 5.494 mm. According to the above data, the following values can be obtained:
 
 D 3 L31   /D 3 ST3 =1.24,
 
 D 3 L31   /D 3 L32 =1.14,
 
Vd3 1 =56.1,
 
Vd3 2 =21.5,
 
Vd3 3 =21.5,
 
Vd3 4 =60,
 
Vd3 5 =56.1,
 
 f 3/ f 3 3   +f 3 /f 3 4   −f 3 /f 3 1 =−0.7111,
 
 Vd 3 4   −Vd 3 3 =38.5,
 
 SL 3 /TTL 3=0.711,
 
F3=1.6
 
which respectively satisfy the above conditions (23)-(33).
 
     By the above arrangements of the lenses and stop ST 3 , the lens assembly  3  of the third embodiment can meet the requirements of optical performance as seen in  FIGS. 6A-6C , wherein  FIG. 6A  shows a field curvature diagram of the lens assembly  3  in accordance with the third embodiment of the invention,  FIG. 6B  shows a distortion diagram of the lens assembly  3  in accordance with the third embodiment of the invention,  FIG. 6C  shows a modulation transfer function diagram of the lens assembly  3  in accordance with the third embodiment of the invention. 
     It can be seen from  FIG. 6A  that the field curvature of tangential direction and sagittal direction in the lens assembly  3  of the third embodiment ranges from −0.04 mm to 0.06 mm for the wavelength of 0.470 μm, 0.555 μm and 0.650 μm. It can be seen from  FIG. 6B  that the distortion in the lens assembly  3  of the third embodiment ranges from −0.2% to 0.4% for the wavelength of 0.470 μm, 0.555 μm, and 0.650 μm. It can be seen from  FIG. 6C  that the modulation transfer function of tangential direction and sagittal direction in the lens assembly  3  of the third embodiment ranges from 0.0 to 1.0 when the wavelength ranges from 0.470 μm to 0.650 μm, the fields respectively are 0.0000 mm, 0.6864 mm, 1.3728 mm, 2.4024 mm and 3.4320 mm, and the spatial frequency ranges from 0 lp/mm to 446 lp/mm. It is obvious that the field curvature and the distortion of the lens assembly  3  of the third embodiment can be corrected effectively, and the resolution of the lens assembly  3  of the third embodiment can meet the requirement. Therefore, the lens assembly  3  of the third embodiment is capable of good optical performance. 
     In another embodiment described above, when the effective diameter of the stop ST 3  can be respectively adjusted to 2.258 mm, 1.434 mm, 0.996 mm and 0.256 mm, F-number of the lens assembly  3  can be respectively changed to 1.4, 2.4, 3.4 and 13, the maximum value of D 3   L31 /D 3   ST3  is equal to 10.117(2.59/0.256) and the minimum value of D 3   L31 /D 3   ST3  is equal to 1.147(2.59/2.258), which satisfy the above condition (23). By changing the effective diameter of the stop ST 3 , which can control the amount of the incident light of the lens assembly  3  and change illuminance of image plane IMA 3 . On the other hand, changing the effective diameter of the stop ST 3  can control depth of Field, when the greater effective diameter of the stop ST 3 , the shorter depth of Field. When the smaller effective diameter of the stop ST 3 , the longer depth of Field.