Patent Publication Number: US-2011058262-A1

Title: Mini Fixed Focus Lens Module

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Taiwan Patent Application No. 98130377, filed on Sep. 9, 2009, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical device, and in particular relates to a mini fixed focus lens module. 
     2. Description of the Related Art 
     Portable electronic devices with video or picture capture functions are thin, small and light. Thus, fixed focus lens units are utilized therein. For continued application of fixed focus lens units in miniaturized portable electronic devices, total track thereof must be low and optical performance thereof must be high. 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     A mini fixed focus lens module is provided. From an object end to an image end thereof, the mini fixed focus lens module sequentially comprises a first lens, a second lens, a third lens and a fourth lens. The first lens has a positive diopter, the second lens has a negative diopter, the third lens has a positive diopter, the fourth lens has a negative diopter, a combined diopter of the second and third lenses is positive, and the mini fixed focus lens module satisfies the following formula: 
       0.2 &lt;f   23   /f&lt; 1 
     , wherein f 23  is a combined focal length of the second and third lenses, and f is a system focal length of the mini fixed focus lens module. 
     The invention arranges the first, second, third and fourth lenses to control a ratio of the combined focal length to the system focal length to increase viewing angle and to reduce total track length (TTL). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows lens arrangement of a mini fixed focus lens module of a first embodiment of the invention; 
         FIG. 2A  is a ray fan diagram of different wave lengths with different image heights of the first embodiment of the invention; 
         FIG. 2B  is a field curvature diagram of the first embodiment of the invention; 
         FIG. 2C  represents distortion with horizontal magnification corresponding to  FIG. 2B ; 
         FIG. 3  shows lens arrangement of a mini fixed focus lens module of a second embodiment of the invention; and 
         FIG. 4  shows lens arrangement of a mini fixed focus lens module of a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This 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. 
     With reference to  FIG. 1 , a mini fixed focus lens module of an embodiment of the invention is provided. From an object end to an image end thereof, the mini fixed focus lens module sequentially comprises a first lens  1 , an aperture stop  2 , a second lens  3 , a third lens  4  and a fourth lens  5 . A light beam passes through the mini fixed focus lens module, enters a cover glass  6 , and forms an image on an image sensor (for example, CCD or CMOS)  7 . The first lens  1  has a positive diopter. The second lens  3  has a negative diopter. The third lens  4  has a positive diopter. The fourth lens  5  has a negative diopter. The first, second, third and fourth lenses are made of plastic, and are aspheric lenses. The mini fixed focus lens module has a minimum viewing angle equal to 70°. 
     The first lens  1  is a positive meniscus lens. The first lens  1  provides the majority of the refractive power of the mini fixed focus lens module, and reduces distortion via aspheric design thereof. The aperture stop  2  is a central-arranged diaphragm, located between the first lens  1  and the second lens  3  to increase viewing angle. 
     The second lens  3  is close to the third lens  4 . A combined diopter of the second and third lenses is positive. An object surface S 5  of the third lens  4  is concave toward an image surface S 6  thereof to reduce a distance between the second and third lenses, and to decrease a total track length (TTL). The mini fixed focus lens module satisfies the following formula: 
       0.2 &lt;f   23   /f&lt; 1  (1)
 
     , wherein f 23  is a combined focal length of the second and third lenses, and f is a system focal length of the mini fixed focus lens module. When f 23 /f&gt;1, the combined diopter of the second and third lenses is too small, and the total track length (TTL) is increased. When f 23 /f&lt;0.2, radiuses of the second lens  3  and the third lens  4  are too small, and aberration is obvious. The second and third lenses further satisfy the following formula: 
       2 &lt;|f   2   |/|f   3 |&lt;6  (2)
 
     , wherein f 2  is a focal length of the second lens  3 , and f 3  is a focal length of the third lens  4 . The second lens  3  and the third lens  4  are compensating lenses of the first lens  1  to increase focusing power and decrease the total track length (TTL). Meanwhile, by satisfying formulas (1) and (2), viewing angle of the mini fixed focus lens module may be increased to more than 70°. 
     The fourth lens  5  provides a negative diopter refraction to a chief ray, and provides a positive diopter refraction to a marginal ray. The fourth lens  5  balances the positive/negative diopter of the mini fixed focus lens module, and increases viewing angle. 
     The lenses of the embodiment are plastic aspheric lenses made by injection-molding. Therefore, the lenses can be light in weight and mass produced with low cost. The aspheric surface of the lenses can be represented 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 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     With respect to the formula (3), h is the coordinate along the optical axis from an apex of the aspheric surface, z is the vertical distance to the optical axis, k is the conic coefficient, c is the inverse of the radius of curvature, and A to G are aspheric coefficients. Traditional aspheric lenses need more space and long total track length (TTL) to accommodate aberration. An aspheric lens may produce a better image quality than that of a spherical lens. 
     First Embodiment 
     Table 1-1 illustrates the design data of the mini fixed focus lens module of a first embodiment: 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1-1 
               
               
                   
               
               
                   
                   
                   
                 refracting 
                   
               
               
                   
                 curvature radius 
                 thickness 
                 power 
                 Abbe coefficient 
               
               
                 Ser. No. 
                 (mm) 
                 (mm) 
                 N d   
                 ν d   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 S1 
                 0.5039 
                 0.1450 
                 1.5312 
                 56.0438 
               
               
                 S2 
                 2.3382 
                 0.0294 
               
               
                 aperture 
                   
                 0.1969 
               
               
                 stop 
               
               
                 S3 
                 −0.6880 
                 0.0791 
                 1.6142 
                 25.5765 
               
               
                 S4 
                 −1.9900 
                 0.0150 
               
               
                 S5 
                 −1.2172 
                 0.3679 
                 1.5312 
                 56.0438 
               
               
                 S6 
                 −0.2196 
                 0.0139 
               
               
                 S7 
                 0.7291 
                 0.1343 
                 1.5441 
                 56.0936 
               
               
                 S8 
                 0.2014 
                 0.1390 
               
               
                   
               
            
           
         
       
     
     S 1  is an object surface of the first lens  1 , S 2  is an image surface of the first lens  1 , surfaces S 1  to S 8  are arranged from the object end to the image end sequentially, and S 8  is an image surface of the fourth lens  5 . In the first embodiment of the invention, the F-number is 2.8, the system focal length f is 3.5961 mm, the combined focal length f 23  of the second and third lenses is 1.701 mm, the focal length f 2  of the second lens is −6.297 mm, and the focal length f 3  of the third lens is 1.608 mm. The focal length f 2  and the focal length f 3  satisfy formula (1) and (2). Additionally, the Abbe coefficient s of the first, third, and fourth lenses are greater than 56 to reduce aberration. The second lens  3  is a negative meniscus lens. The aspheric coefficients of the first, second, third and fourth lens are shown in Table 1-2: 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1-2 
               
               
                   
               
               
                 Serial No. 
                 k 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 S1 
                 1.711365 
                 −1.54995 
                 −16.1212 
                 31.33032 
                 −5018.87 
                 12253.22 
                 −11720.3 
                 −1992657 
               
               
                 S2 
                 −16.2405 
                 −0.24747 
                 −17.7322 
                 255.1949 
                 −18579.2 
                 204095.5 
                 3355235 
                 −4.8E+07 
               
               
                 S3 
                 5.874722 
                 −5.83776 
                 −27.5103 
                 −328.854 
                 34714.28 
                 −354303 
                 −3001148 
                 62607739 
               
               
                 S4 
                 26.17067 
                 −4.37799 
                 6.652809 
                 151.4094 
                 −890.463 
                 −221.692 
                 150321.3 
                 −783894 
               
               
                 S5 
                 −2.7633 
                 −0.32154 
                 17.61952 
                 −303.496 
                 2629.403 
                 1493.215 
                 −96713.7 
                 288244 
               
               
                 S6 
                 −3.56384 
                 −6.02755 
                 31.57182 
                 −97.9027 
                 65.20729 
                 994.2698 
                 7753.289 
                 −35705.4 
               
               
                 S7 
                 −5.77958 
                 −3.65526 
                 11.72977 
                 −16.2513 
                 8.069578 
                 2.045142 
                 2.909506 
                 −8.24454 
               
               
                 S8 
                 −4.46027 
                 −2.18997 
                 5.599761 
                 −11.2108 
                 9.80279 
                 2.872843 
                 −7.40091 
                 0.345971 
               
               
                   
               
            
           
         
       
     
       FIG. 2A  is a ray fan diagram of different wave lengths with different image height. Each image height has two ray fan diagrams responding to coma aberration on tangential planes (PY and EY) and sagittal planes (PX and EX). According to  FIG. 2A , the imaging magnification ratio error is acceptable. 
       FIG. 2B  is a field curvature diagram, showing imaging locations corresponding to different image heights. T and S respectively represent curvature of image fields of the meridional plane and the sagittal plane at different image heights. The horizontal axis represents distance (aberration) from the image point to the ideal image, and the vertical axis represents ideal height of the image.  FIG. 2C  represents distortion with horizontal magnification corresponding to  FIG. 2B , wherein the horizontal axis represents aberration in percentage, and the vertical axis represents the ideal height of the image. As shown in  FIGS. 2B and 2C , distortion and image curvature of image field are not serious. 
     Second Embodiment 
     Table 2-1 illustrates the design data of the mini fixed focus lens module of a second embodiment: 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2-1 
               
               
                   
               
               
                   
                 curvature 
                   
                 refracting 
                   
               
               
                   
                 radius 
                 thickness 
                 power 
                 Abbe coefficient 
               
               
                 Ser. No. 
                 (mm) 
                 (mm) 
                 N d   
                 ν d   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 S1 
                 0.6159 
                 0.1271 
                 1.5312 
                 56.0438 
               
               
                 S2 
                 18.4549 
                 0.0139 
               
               
                 aperture stop 
                   
                 0.2314 
               
               
                 S3 
                 −1.0045 
                 0.0911 
                 1.6142 
                 25.5765 
               
               
                 S4 
                 −6.2687 
                 0.0421 
               
               
                 S5 
                 −0.8088 
                 0.2718 
                 1.5312 
                 56.0438 
               
               
                 S6 
                 −0.2593 
                 0.0139 
               
               
                 S7 
                 0.7315 
                 0.1892 
                 1.5312 
                 56.0438 
               
               
                 S8 
                 0.2563 
                 0.1393 
               
               
                   
               
            
           
         
       
     
     In the second embodiment, the F-number is 2.8, the system focal length f is 3.588 mm, the combined focal length f 23  of the second and third lenses is 2.569 mm, the focal length f 2  of the second lens is −7.035 mm, and the focal length f 3  of the third lens is 2.2015 mm. The focal length f 2  and the focal length f 3  satisfy formula (1) and (2). 
     The aspheric coefficients of the first, second, third and fourth lens are shown in Table 2-2. As shown in  FIG. 3 , lens arrangement and lens shape of the mini fixed focus lens module of the second embodiment can be achieved by referring to Table 2-1 and Table 2-2. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2-2 
               
               
                   
               
               
                 Ser. No. 
                 k 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 S1 
                 −5.65802 
                 1.97776 
                 −18.5922 
                 −204.469 
                 136.7658 
                 −8861.3 
                 −72271.8 
                 −541770 
               
               
                 S2 
                 329.0738 
                 −1.38662 
                 −44.2142 
                 672.0624 
                 −10752.8 
                 13497.69 
                 4751.156 
                 100923.6 
               
               
                 S3 
                 −7.16817 
                 −7.7289 
                 −38.3609 
                 744.1328 
                 −932.655 
                 −12821.8 
                 −342740 
                 3073944 
               
               
                 S4 
                 −81.3991 
                 −2.73534 
                 −20.3732 
                 255.6185 
                 −851.731 
                 −124.91 
                 38022.81 
                 −172036 
               
               
                 S5 
                 −17.4355 
                 −0.3664 
                 0.654212 
                 −124.752 
                 666.641 
                 3264.517 
                 2178.888 
                 −139172 
               
               
                 S6 
                 −3.05581 
                 −4.71099 
                 21.68413 
                 −48.4205 
                 103.1734 
                 1317.1 
                 3612.644 
                 −34463.6 
               
               
                 S7 
                 −3.29441 
                 −3.20689 
                 10.66603 
                 −15.9955 
                 10.49552 
                 −1.11244 
                 0.697235 
                 −3.5186 
               
               
                 S8 
                 −4.66174 
                 −1.71743 
                 4.291583 
                 −9.16498 
                 9.691347 
                 1.079264 
                 −8.8016 
                 2.694083 
               
               
                   
               
            
           
         
       
     
     Third Embodiment 
     Table 3-1 illustrates the design data of the mini fixed focus lens module of a third embodiment: 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3-1 
               
               
                   
               
               
                   
                   
                   
                 refracting 
                   
               
               
                   
                 curvature radius 
                 thickness 
                 power 
                 Abbe coefficient 
               
               
                 Ser. No. 
                 (mm) 
                 (mm) 
                 N d   
                 ν d   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 S1 
                 0.6463 
                 0.1261 
                 1.5312 
                 56.0438 
               
               
                 S2 
                 15.4876 
                 0.0181 
               
               
                 S3 
                 −3.3367 
                 0.0791 
                 1.6142 
                 25.5765 
               
               
                 S4 
                 3.0495 
                 0.0764 
               
               
                 S5 
                 −0.8738 
                 0.3221 
                 1.5312 
                 56.0438 
               
               
                 S6 
                 −0.248 
                 0.0139 
               
               
                 S7 
                 0.508 
                 0.1421 
                 1.5855 
                 29.9092 
               
               
                 S8 
                 0.2057 
                 0.2784 
               
               
                   
               
            
           
         
       
     
     In the third embodiment, the F-number is 2.8, the system focal length f is 3.592 mm, the combined focal length f 23  of the second and third lenses is 2.157 mm, the focal length f 2  of the second lens is −9.275 mm, and the focal length f 3  of the third lens is 1.987 mm. The focal length f 2  and the focal length f 3  satisfy formula (1) and (2). The second lens  3  is a biconcave lens. 
     The aspheric coefficients of the first, second, third and fourth lens are shown in Table 3-2. As shown in  FIG. 4 , lens arrangement and lens shape of the mini fixed focus lens module of the second embodiment can be achieved by referring to Table 3-1 and Table 3-2. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3-2 
               
               
                   
               
               
                 Serial No. 
                 k 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 S1 
                 −6.23379 
                 1.839228 
                 −16.588 
                 −241.848 
                 642.9472 
                 14433.07 
                 −415932 
                 837637.6 
               
               
                 S2 
                 0 
                 −1.3206 
                 −44.0317 
                 770.2913 
                 −12677.9 
                 37322.72 
                 6516.022 
                 157340 
               
               
                 S3 
                 94.27759 
                 −4.56351 
                 −54.3325 
                 843.0139 
                 −2021.37 
                 −8872.37 
                 −228426 
                 1383509 
               
               
                 S4 
                 −284.866 
                 −0.90906 
                 −38.4953 
                 364.0487 
                 −721.944 
                 −2266.04 
                 −2542.95 
                 38517.98 
               
               
                 S5 
                 −19.2101 
                 −0.91086 
                 6.507327 
                 −131.909 
                 163.7292 
                 2520.77 
                 10155.23 
                 −87427.9 
               
               
                 S6 
                 −3.54404 
                 −4.70474 
                 19.45443 
                 −39.2818 
                 10.57235 
                 326.0565 
                 1019.118 
                 −1790.81 
               
               
                 S7 
                 −4.79393 
                 −2.74903 
                 10.28662 
                 −17.4072 
                 12.73998 
                 −0.32582 
                 −0.84166 
                 −3.42917 
               
               
                 S8 
                 −3.9064 
                 −1.72367 
                 4.259129 
                 −6.12457 
                 3.421952 
                 1.451318 
                 −0.49915 
                 −2.22007 
               
               
                   
               
            
           
         
       
     
     As mentioned above, in the embodiment, the ratio of the combined focal length to the system focal length satisfies formula (1), and the focal length ratio of the second to third lenses satisfies formula (2). The second lens  3  is disposed close to the third lens  4 . The aperture stop  2  is disposed between the first lens  1  and the second lens  3  to increase the viewing angle of the mini fixed focus lens module, and to reduce total track. Additionally, the first, second, third and fourth lenses are aspheric plastic lenses, which can reduce aberration, weight and cost. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.