Abstract:
A subminiature imaging optical system including: first, second, third and fourth lenses sequentially positioned from an object side to an image side, the first lens having positive refractivity, the second lens having a concave image-side surface and having negative refractivity, the third lens having negative refractivity, and the fourth lens having positive refractivity, wherein the second lens satisfies following condition 1, and the third lens satisfies following condition 2,
 
20&lt;V2&lt;50  condition 1,
 
20&lt;V3&lt;50  condition 2,
       where V 2  is an Abbe number of the second lens and V 3  is an Abbe number of the third lens.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority of Korean Patent Application No. 2007-12011 filed on Feb. 6, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a subminiature imaging optical system, and more particularly, to a subminiature imaging optical system installed in a mobile telecommunication terminal or a personal digital assistant (PDA), or used in a surveillance camera or a digital camera. 
   2. Description of the Related Art 
   Recently, regarding an image pickup system, studies have been conducted on camera modules for telecommunication terminals, digital still cameras (DSCs), camcorders, and personal computer (PC) cameras which are attached to personal computers an imaging device. Here, an image-forming lens system is the most important component of such an image pickup system to obtain an image. 
   The lens system needs to be high-performing in terms of resolution and image quality, thus complicating a lens configuration. However, such structural and optical complexity leads to increase in size, posing a difficulty to compactness and thinness of the lens system. 
   For example, a camera module should necessarily be miniaturized to be installed in a mobile phone more efficiently. Also, a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) used in the camera module as an image sensor is gradually increased in resolution and reduced in pixel size. In turn, the lens system included in the camera module needs to be smaller-sized and thinner while attaining high resolution and superior optical capabilities. 
   Here, in a case where the CCD or CMOS with 3 million pixels is employed, only three sheets of lenses or less may be arranged to satisfy optical capabilities and miniaturization. However, in a case where three sheets of lenses or less are applied to a high-resolution imaging device such as the CCD or CMOS with at least 5 million pixels, each of the lenses should be increased in refractivity, and thus is hard to be machined. This as a result renders it difficult to achieve both high performance and miniaturization of the lens system. Thus, four sheets of lenses may be employed, but when a spherical lens is utilized in this configuration, the optical lens system is increased in total length, thereby hardly miniaturizable. 
   Therefore there has been a demand for a lens system for a camera module which can be subminiaturized and realize optical capabilities. 
   SUMMARY OF THE INVENTION 
   An aspect of the present invention provides a subminiature lens system for a camera module which adopts only four sheets of lenses to achieve high resolution and subminiaturization, while performing with excellent optical capabilities. 
   An aspect of the present invention also provides a lighter subminiature lens system for a cameral module which employs at least three sheets of plastic lenses thereby to be manufactured in mass production at a low cost. 
   According to an aspect of the present invention, there is provided a subminiature imaging optical system including: first, second, third and fourth lenses sequentially positioned from an object side to an image plane side, the first lens having positive refractivity, the second lens having a concave image-side surface and having negative refractivity, the third lens having negative refractivity, and the fourth lens having positive refractivity, wherein the second lens satisfies following condition 1, and the third lens satisfies following condition 2,
 
20&lt;V2&lt;50  condition 1,
 
20&lt;V3&lt;50  condition 2,
         where V 2  is an Abbe number of the second lens and V 3  is an Abbe number of the third lens.       

   At least one of respective surfaces of the first, third and fourth lenses may be formed of an aspherical surface. 
   The first, second and third lenses may be formed of plastic and the second lens may be formed of glass. The first to fourth lenses may be formed of plastic. 
   An aperture stop may be disposed in front of an object-side surface of the first lens. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a lens configuration view illustrating a subminiature imaging optical system according to a first embodiment of the invention; 
       FIG. 2  is an aberrational diagram illustrating the embodiment shown in  FIG. 2 , in which A represents spherical aberration, B represents astigmatism and C represents distortion; 
       FIG. 3  is a graph illustrating modulation transfer function (MTF) characteristics of the embodiment shown in  FIG. 1 ; 
       FIG. 4  illustrates lateral chromatic aberration of the embodiment shown in  FIG. 1 ; 
       FIG. 5  is a lens configuration view illustrating a subminiature imaging optical system according to a second embodiment of the invention; 
       FIG. 6  is an aberrational diagram illustrating the embodiment shown in  FIG. 5 , in which A represents spherical aberration, B represents astigmatism and C represents distortion; 
       FIG. 7  is a graph illustrating modulation transfer function (MTF) characteristics of the embodiment shown in  FIG. 4 ; and 
       FIG. 8  illustrates lateral chromatic aberration of the embodiment shown in  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a lens configuration view illustrating a subminiature imaging optical system according to a first embodiment of the invention. In the following lens configuration views, the thicknesses, sizes and shapes of the lenses may be exaggerated for clarity. In particular, the shapes of spherical or aspherical surfaces shown in the views are only exemplary and should not be construed as limiting. 
   In general, a camera module includes at least one lens, a housing having an inner space for housing the lens therein, an image sensor corresponding to an imaging surface formed by the lenses, and a circuit board fixedly installed at another end of the housing and having the image sensor disposed on one surface thereof to process an image sensed by the image sensor. 
   The present invention provides a subminiature imaging optical system for use in a subminiature camera module. 
   As shown in  FIG. 1 , the subminiature imaging optical system includes a first lens L 1  having positive refractivity, a second lens L 2  having a concave image-side surface and having negative refractivity, a third lens L 3  having negative refractivity, a fourth lens L 4  having positive refractivity and an aperture stop S disposed in front of an object-side surface of the first lens L 1 . At least one of respective surfaces of the first, third and fourth lenses L 1 , L 3 , and L 4  may be formed of an aspherical surface. 
   Here, at least three of the first to fourth lenses L 1  to L 4  may be formed of plastic. Particularly, the first, third and fourth lenses L 1 , L 3 , and L 4  having the aspherical surface, respectively may be formed of plastic and the second lens L 2  may be formed of glass. Alternatively, the first to fourth lenses L 1  to L 4  may be formed of plastic. This ensures a smaller and lighter optical system to be manufactured in mass production at a lower cost. Notably, the lenses formed of aspherical surfaces are improved in resolution and reduced in distortion and spherical aberration, thereby realizing a compact optical system excellent in optical properties. 
   Meanwhile, an optical filter such as an infrared ray filter, a cover glass and the like may be provided between the fourth lens LG 4  and an image plane IP. 
   The image plane IP corresponds to an image sensor such as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS). 
   In addition, according to the present embodiment, the aperture stop is located in front of the object-side of the first lens L 1 . Thus the first lens L 1  is exposed in a smallest area to an object side, thereby ensuring less foreign materials to infiltrate thereinto. Also, an angle of view starts from a point close to an upper peripheral portion of the lens, thereby leading to a smaller-sized window of an external instrument for fixing the camera module. Moreover, to attain higher resolution, the image sensor is further decreased in pixel size, accordingly requiring a brighter optical system (lens with a small F number). The aperture stop located in the front of the object side assures the bright optical system to be easily manufactured. 
   With this overall configuration, operational effects of following conditions 1 and 2 will be examined hereunder.
 
20&lt;V2&lt;50  condition 1
 
20&lt;V3&lt;50  condition 2
         wherein V 2  denotes an Abbe number of the second lens L 2  and V 3  denotes an Abbe number of the third lens L 3 .       

   Condition 1 prescribes the Abbe number of the second lens L 2  and condition 2 prescribes the Abbe number of the third lens L 3 . 
   An optical material can be largely broken down into two types. One is a crown material having an Abbe number of at least 50 and the other is a flint material having an Abbe number less than 50. Here, the flint type of optical material, which has an Abbe number less than 50, is greatly chromatically dispersed. According to the present embodiment, the second and third lenses L 2  and L 3  positioned second and third from the object side are formed of the flint material to have negative refractivity, thereby improving chromatic aberration. Moreover, out of known existing optical materials, none has an Abbe number of 20 or less. 
   Now, the present invention will be examined in greater detail through specific numerical examples. 
   As described above, in the following first and second embodiments, a subminiature imaging optical system includes a first lens L 1  having positive refractivity, a second lens L 2  having a concave image-side surface and having negative refractivity, a third lens L 3  having negative refractivity, a fourth lens L 4  having positive refractivity and an aperture stop S disposed in front of an object-side surface of the first lens L 1 . Also, an optical filter such as an infrared ray filter, a cover glass and the like may be provided between the fourth lens LG 4  and an image plane IP. The image plane IP corresponds to an image sensor such as the CCD and CMOS. 
   Each of the aspherical surfaces used in each of the embodiments herein is obtained from following Equation 1: 
   
     
       
         
           
             
               
                 
                   
                     
                       Z 
                       = 
                         
                       ⁢ 
                       
                         
                           
                             cY 
                             2 
                           
                           
                             1 
                             + 
                             
                               
                                 1 
                                 - 
                                 
                                   
                                     ( 
                                     
                                       1 
                                       + 
                                       K 
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     c 
                                     2 
                                   
                                   ⁢ 
                                   
                                     Y 
                                     2 
                                   
                                 
                               
                             
                           
                         
                         + 
                         
                           AY 
                           4 
                         
                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         
                           
                             BY 
                             6 
                           
                           + 
                           
                             CY 
                             8 
                           
                           + 
                           
                             DY 
                             10 
                           
                           + 
                           
                             EY 
                             12 
                           
                           + 
                           
                             FY 
                             14 
                           
                           + 
                           … 
                         
                         ⁢ 
                         
                             
                         
                         , 
                       
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 1 
               
             
           
         
       
     
       
       
         
           where Z is a distance from a vertex of a lens in an optical axis, Y is a distance in a direction perpendicular to the optical axis, C is a reciprocal number of a radius r of curvature at a vertex of the lens, K is a conic constant and A, B, C, D, E and F are aspherical coefficients. 
         
       
     
  
   Here, the MTF depends on a spatial frequency of a cycle per millimeter and is defined by the following Equation 2 between a maximum intensity and a minimum intensity of light. 
   
     
       
         
           
             
               
                 MTF 
                 = 
                 
                   
                     Max 
                     - 
                     Min 
                   
                   
                     Max 
                     + 
                     Min 
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 2 
               
             
           
         
       
     
   
   That is, MTF is most ideal when 1 and a smaller MTF deteriorates a resolution. 
   First Embodiment 
   Table 1 below shows numerical values according to a first embodiment of the present invention.  FIG. 1  is a view illustrating a lens arrangement of the subminiature imaging optical system according to the first embodiment of the present invention, and  FIGS. 2A to 2C  show aberrations of the embodiment shown in Table 1 and  FIG. 1 .  FIG. 3  is a graph illustrating MTF characteristics shown in Table 1 and  FIG. 1 , and  FIG. 4  shows chromatic aberration of the subminiature imaging optical system shown in Table 1 and  FIG. 1 . 
   In the first embodiment, an F number Fno is 2.85, an angle of view is 60 degrees, a total length TL from the object-side surface  2  of the first lens L 1  to the image plane  12  is 5.95 mm, and an effective focal length f is 4.8 mm. Also, the first lens L 1 , the third lens L 3  and the fourth lens L 4  are formed of a plastic material and the second lens L 2  is formed of a glass material. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Radius of 
                 
                 
               Abbe 
                 
             
             
               Surface 
               curvature 
               Thickness or 
               Refractive 
               number 
                 
             
             
               No. 
               (R) 
               distance (t) 
               index N d   
               V d   
               Remark 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                1 
               ∞ 
               0.100000 
                 
                 
               Aperture stop 
             
             
               *2 
               2.15644 
               0.963121 
               1.53 
               55.6 
               First lens 
             
             
               *3 
               −3.86786 
               0.100000 
             
             
                4 
               −23.86737 
               0.341086 
               1.755 
               27.38 
               Second lens 
             
             
                5 
               4.27838 
               0.676298 
             
             
               *6 
               −1.58816 
               0.410158 
               1.6127 
               27.3 
               Third lens 
             
             
               *7 
               −2.35679 
               0.524788 
             
             
               *8 
               1.85880 
               1.294468 
               1.53 
               55.6 
               Fourth lens 
             
             
               *9 
               2.04959 
               0.458366 
             
             
               10 
               ∞ 
               0.300000 
               1.517 
               64.2 
               Optical filter 
             
             
               11 
               ∞ 
               0.781682 
             
             
               12 
               ∞ 
               0.000000 
                 
                 
               Image plane 
             
             
                 
             
           
        
       
     
   
   In Table 1, * represents an aspherical surface, and in the first embodiment, refractive surfaces  2  and  3  of the first lens L 1  at the object side and the image side, refractive surfaces  6  and  7  of the third lens L 3  at the object side and the image plane side and refractive surfaces  8  and  9  of the fourth lens L 4  at the object side and the image side are aspherical. 
   Values of aspherical coefficients in the first embodiment according to Equation 1 are noted in Table 2 below. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Surface 
                 
                 
                 
                 
                 
                 
             
             
               No. 
               K 
               A 
               B 
               C 
               D 
               E 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               2 
               −0.062193 
               −.162552E−01 
                −.552733E−02 
               −.251800E−01 
               0.276464E−01 
               −.236928E−01 
             
             
               3 
               0.000000 
               −.183882E−01 
                −.754492E−02 
               −.264668E−01 
               0.110351E−01 
               −.551816E−02 
             
             
               6 
               −0.468375 
               −.625088E−01 
                −.860660E−01 
               0.184654 
               −.147045 
               0.529697E−01 
             
             
               7 
               0.680310 
               −.146192 
                0.962949E−01 
               −.306657E−01 
               0.180470E−01 
               −.187781E−02 
             
             
               8 
               −6.312525 
               −.651373E−01 
               0.2011113E−01 
               −.276819E−02 
               0.766500E−05 
               0.211259E−04 
             
             
               9 
               −3.741125 
               −.397781E−01 
                0.618371E−02 
               −.497931E−03 
               −.136965E−04 
               0.202135E−05 
             
             
                 
             
           
        
       
     
   
   Second Embodiment 
   Table 3 below shows numerical values according to a second embodiment of the present invention.  FIG. 5  is a view illustrating a lens arrangement of the subminiature imaging optical system according to the second embodiment of the present invention, and  FIGS. 6A to 6C  show aberrations of the embodiment shown in Table 3 and  FIG. 4 .  FIG. 7  is a graph illustrating MTF characteristics shown in Table 3 and  FIG. 5 , and  FIG. 8  shows chromatic aberration of the subminiature imaging optical system shown in Table 3 and  FIG. 5 . 
   In the second embodiment, an F number Fno is 2.8, an angle of view is 61.5 degrees, a total length TL from the object-side surface  2  of the first lens L 1  to the image plane  12  is 5.83 mm, and an effective focal length f is 4.81 mm. Also, in the second embodiment, the first to fourth lenses L 1  to L 4  are formed of a plastic material. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
                 
               Radius of 
                 
                 
               Abbe 
                 
             
             
               Surface 
               curvature 
               Thickness or 
               Refractive 
               number 
                 
             
             
               No. 
               (R) 
               distance (t) 
               index N d   
               V d   
               Remark 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                1 
               ∞ 
               0.100000 
                 
                 
               Aperture stop 
             
             
               *2 
               2.06743 
               0.791628 
               1.53 
               55.6 
               First lens 
             
             
               *3 
               −6.86313 
               0.100000 
             
             
                4 
               60.71127 
               0.292945 
               1.63898 
               23.42 
               Second lens 
             
             
                5 
               3.96951 
               0.795435 
             
             
               *6 
               −2.07202 
               0.450870 
               1.6127 
               27.3 
               Third lens 
             
             
               *7 
               −3.37783 
               0.543693 
             
             
               *8 
               1.91572 
               1.271693 
               1.53 
               55.6 
               Fourth lens 
             
             
               *9 
               2.03199 
               0.482887 
             
             
               10 
               ∞ 
               0.300000 
               1.517 
               64.2 
               Optical filter 
             
             
               11 
               ∞ 
               0.700000 
             
             
               12 
               ∞ 
               0.000000 
                 
                 
               Image plane 
             
             
                 
             
           
        
       
     
   
   In Table 3, * represents an aspherical surface, and in the second embodiment, refractive surfaces  2  and  3  of the first lens L 1  at the object side and the image side, refractive surfaces  6  and  7  of the third lens L 3  at the object side and the image side and refractive surfaces  8  and  9  of the fourth lens L 4  at the object side and the image side are aspherical. 
   Values of aspherical coefficients in the second embodiment according to Equation 1 are noted in Table 4 below. 
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
               Surface 
                 
                 
                 
                 
                 
                 
             
             
               No. 
               K 
               A 
               B 
               C 
               D 
               E 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               2 
               0.019324 
               −.150962E−01 
               −.344869E−02 
               −.278714E−01 
               0.288479E−01 
               −.229876E−01 
             
             
               3 
               0.000000 
               −.186248E−01 
               −.104370E−01 
               −.230823E−01 
               0.116908E−01 
               −.881911E−02 
             
             
               6 
               −2.072020 
               −.679264E−01 
               −.992324E−01 
               0.184327 
               −.141165 
               0.452324E−01 
             
             
               7 
               −3.377830 
               −.158237 
               0.945185E−01 
               −.345190E−01 
               0.155615E−01 
               −.128546E−02 
             
             
               8 
               1.915720 
               −.779236E−01 
               0.222125E−01 
               −.279536E−02 
               −.132569E−04 
               0.250379E−04 
             
             
               9 
               2.031990 
               −.364748E−01 
               0.535216E−02 
               −.524543E−03 
               0.419584E−05 
               0.101173E−05 
             
             
                 
             
           
        
       
     
   
   As can be seen from the above embodiments, the subminiature imaging optical system with excellent aberrational characteristics as shown in  FIGS. 2 and 6  is obtained according to the present invention. 
   In the meantime, values of conditions 1 to 5 for the above first and second embodiments are noted in Table 5. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
               First embodiment 
               Second embodiment 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Condition 1 
               27.38 
               23.42 
             
             
                 
               Condition 2 
               27.3 
               27.3 
             
             
                 
                 
             
           
        
       
     
   
   As noted in Table 5 above, the first and second embodiments of the present invention satisfy conditions 1 to 5. 
   As set forth above, according to exemplary embodiments of the invention, a subminiature imaging optical system is suitably applicable to subminiature optical instruments such as a mobile phone camera using an image sensor such as a CCD and CMOS. Moreover, lenses of the optical system are adjusted in radius of curvature, respectively and adopt an aspherical surface, thereby minimizing various aberrations and attaining an image with high resolution. 
   In addition, a number of lenses are formed of plastic to easily manufacture a lighter and lower-cost subminiature imaging optical system in mass production. 
   While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.