TECHNICAL FIELD 
   The present invention relates to a lens system, and particularly to a lens system for use in electronic devices. 
   DESCRIPTION OF RELATED ART 
   In recent years, camera modules using an image sensor like a charged coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) for capturing images have begun to be incorporated in portable electronic devices such as mobile phones and laptop computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these electronic devices. 
   Therefore, it is desirable to provide a lens system with short length, compact size, excellent optical performance, and great image quality. Obtaining a short length requires very short distance between the first lens and the image plane of the lens system. 
   SUMMARY 
   In accordance with a present embodiment, a lens system comprising a first lens positive in power, a second lens positive in power, a third lens negative in power, a fourth lens positive in power, and a fifth lens negative in power is shown. The lens system meets a criteria of 1.6&lt;TT/f&lt;1.8, 1.6&lt;f/f 2 &lt;1.8, and −0.6&lt;f/f 5 &lt;−0.4, wherein TT denotes a distance between an object-side surface of the first lens and an image plane of the lens system; f 2  denoting an effective focal length of the second lens; f 5  denoting an effective focal length of the second lens; and f denoting an effective focal length of the entire lens system. 
   Other advantages and novel features will be drawn from the following detailed description of at least one preferred embodiment, when considered conjunction with the attached drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the present lens system can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present lens system. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is a schematic drawing of a lens system according to a first present embodiment. 
       FIG. 2  is a spherical aberration diagram of the lens system in  FIG. 1 . 
       FIG. 3  is a field curvature diagram of the lens system in  FIG. 1 . 
       FIG. 4  is a distortion diagram of the lens system in  FIG. 1 . 
       FIG. 5  is a spherical aberration diagram of the lens system according to a second present embodiment. 
       FIG. 6  is a field curvature diagram of the lens system according to the second present embodiment. 
       FIG. 7  is a distortion diagram of the lens system according to the second present embodiment. 
       FIG. 8  is a spherical aberration diagram of the lens system according to a third present embodiment. 
       FIG. 9  is a field curvature diagram of the lens system according to the third present embodiment. 
       FIG. 10  is a distortion diagram of the lens system according to the third present embodiment. 
   


   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Embodiments will now be described in detail below, with reference to the drawings. 
   Referring to  FIG. 1 , a lens system  100  according to a present embodiment is shown. The lens system includes, in order from an object-side to an image-side, a first lens  10  positive in power, a second lens  20  positive in power, a third lens  30  negative in power, a fourth lens  40  positive in power, and a fifth lens  50  negative in power. 
   Light coming from the object-side of the lens system  100  transmits through the first lens  10 , the second lens  20 , the third lens  30 , the fourth lens  40 , the fifth lens  50  and ultimately projects onto an image plane  80 . A CCD or CMOS can be disposed in the image plane  80  for capturing an image in view of the lens system. 
   In order to provide the lens system  100  with a compact size and excellent optical performance, the lens system  100  meets the following conditions:
 
1.6 &lt;TT/f&lt; 1.8  (1)
 
1.6 &lt;f/f   2 &lt;1.8; and  (2)
 
−0.6 &lt;f/f   5 &lt;−0.4  (3)
 
wherein TT denotes a distance between an object-side surface of the first lens and the image plane  80 ; f 5  denotes an effective focal length of the fifth lens  50 ; and f denotes an effective focal length of the entire lens system  100 . Condition (1) defines a compact total length of the lens system  100  and condition (2) provides balance between total length, spherical aberration and chromatic aberration of the lens system  100 . Condition (3) provides balance between total length and distortion of the lens system  100 .
 
   Preferably, the lens system  100  further meets the following condition
 
1 &lt;f/f   4 &lt;1.3,  (4)
 
wherein f 4  denotes the effective focal length of the fourth lens  40 . Condition (4) ensures the lens system  100  possesses a qualified dioptric strength and is nearly a telecentric optic system for providing uniform illumination of the image plane.
 
   In order to correct chromatic aberrations of the lens system  100 , especially the chromatic difference of magnification, the lens system  100  further meet the following condition:
 
 Vd   2   −Vd   3 &gt;15  (5)
 
wherein Vd 2  denotes the Abbe number of the second lens  20  and Vd 3  denotes the Abbe number of the third lens  30 . Condition (5) keeps the lens system  100  a balance between total length and the chromatic difference.
 
   In order to install an aperture between the first lens  10  and the second lens  20 , the lens system  100  further meet the following condition:
 
0.24 &lt;DA   1-2   /f&lt; 0.3  (6)
 
wherein DA 1-2  denotes a distance between the image-side surface of the first lens  10  and the object-side surface of the second lens  20 ; f denotes the focal length of the entire lens system  100 .
 
   The fifth lens  50  further meets the following condition:
 
 FBL   5   /f&lt; 0.46  (7)
 
wherein FBL 5  denotes a distance between an image-side surface of the fifth lens  50  and the image plane  80 
 
   The lens system  100  can further include an aperture stop  60  and a filter  70 . The aperture stop  60  is installed between the first lens  10  and the second lens  20  and is configured for adjusting the light flux transmitted through the first lens  10  to the second lens  20 . The aperture  70  ensures uniform light transmission through the first lens  10  to correct chromatic aberration of the lens system  100 . To save on production cost and downsize the lens system  100 , the aperture  60  can be formed by applying an opaque material on an outer rim of the object-side surface of the second lens  20 . The filter  70  is installed between the fifth lens  50  and the image plane  80  and is configured for filtering out IR light that enters the lens system  100 . 
   Preferably, the first lens  10 , the second lens  20 , and the third lens  30  in the lens system  100  can be made of glass. Meanwhile, the fourth lens  40  and the fifth lens  50  can be made of plastic to achieve better image quality but lower production cost. 
   Referring to  FIGS. 2˜10 , embodiments of the lens system  100  are shown in the following. The first lens  10 , the second lens  20 , and the third lens  30  as shown in the following embodiments are all spherical lenses. The fourth lens  40  and the fifth lens  50  are aspheric lenses. By setting the center of the lens surface as an origin point, structure of the aspheric lenses can be expressed as: 
           x   =         ch   2       1   +       1   -       (     k   +   1     )     ⁢     c   2     ⁢     h   2               +     ∑       A   i     ⁢     h   i                 
wherein x is the length of a line drawn from a point on the aspheric lens surface at a distance h; h=√{square root over (Y 2 +Z 2 )} represents a distance from the optical axis to the tangential plane of the aspheric lens surface; c is the curvature of the aspheric lens surface; k represents a coefficient of the second-order term in the equation of the quadric surface, and A i  is the aspheric surface coefficient of i-th order term for the surface.
 
   In these embodiments, “f” represents a focal length of the lens system  100 ; “FNo” represents the aperture F-number; “2ω” represents a field angle. 
   Embodiment 1 
   The following table 1 and table 2 show specifications of a first embodiment of the lens system  100 , wherein TT=11.64 mm; f=6.86 mm; f 2 =3.918 mm; f 5 =−13.262 mm; FNo=3.2; 2ω=58°. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Radius of 
                 
                 
                 
             
             
                 
               curvature 
               Thickness 
               Refractive 
               Abbe 
             
             
               Lens system 100 
               (mm) 
               (mm) 
               index 
               number 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Object-side surface of the 
               20 
               0.6 
               1.847 
               23.8 
             
             
               first lens 10 
             
             
               Image-side surface of the 
               40 
               0.68 
               — 
               — 
             
             
               first lens 10 
             
             
               Aperture stop 60 
               ∞ 
               0.9848579 
               — 
               — 
             
             
               Object-side surface of the 
               4.660256 
               1.218602 
               1.835 
               42.7 
             
             
               second lens 20 
             
             
               Image-side surface of the 
               −9.673459 
               0.1706635 
               — 
               — 
             
             
               second lens 20 
             
             
               Object-side surface of the 
               −6.827207 
               0.35 
               1.805 
               25.4 
             
             
               third lens 30 
             
             
               Image-side surface of the 
               6.277945 
               1.372414 
               — 
               — 
             
             
               third lens 30 
             
             
               Object-side surface of the 
               −3.574036 
               1.519003 
                1.5253 
                55.95 
             
             
               fourth lens 40 
             
             
               Image-side surface of the 
               −1.894176 
               0.1 
               — 
               — 
             
             
               fourth lens 40 
             
             
               Object-side surface of the 
               4.335789 
               1.274355 
                1.5253 
                55.95 
             
             
               fifth lens 50 
             
             
               Image-side surface of the 
               2.402021 
               1.872747 
               — 
               — 
             
             
               fifth lens 50 
             
             
               Object-side surface of the 
               ∞ 
               0.8 
               1.516 
               64.1 
             
             
               filter 60 
             
             
               Image-side surface of the 
               ∞ 
               0.5 
               — 
               — 
             
             
               filter 60 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
               Aspheric surface 
             
             
               Surface 
               coefficients of the surface equation 
             
             
                 
             
           
           
             
               Object-side surface of the 
               K = −0.629145; A4 = 0.000283365; 
             
             
               fourth lens 40 
               A6 = −0.007896287; A8 = 3.68026E−03; 
             
             
                 
               A10 = −4.64809E−04; A12 = 1.78941E−05 
             
             
               Image-side surface of the 
               K = −0.8988602; A4 = 0.00281701; 
             
             
               fourth lens 40 
               A6 = 0.00117827; A8 = −8.30719E−04; 
             
             
                 
               A10 = 2.66314E−04; A12 = −1.86077E−05 
             
             
               Object-side surface of the 
               K = −19.6134; A4 = −0.00440008; 
             
             
               fifth lens 50 
               A6 = 0.00107469; A8 = −8.65362E−05; 
             
             
                 
               A10 = 2.07529E−06; A12 = 8.19722E−09 
             
             
               Image-side surface of the 
               K = −7.431847; A4 = −0.001150747; 
             
             
               fourth lens 50 
               A6 = 0.00064682; A8 = −2.24282E−06; 
             
             
                 
               A10 = −2.36763E−06; A12 = 7.82124E−08 
             
             
                 
             
           
        
       
     
   
   The spherical aberration diagram, the field curvature diagram, and the distortion diagram of the lens system  100  are respectively shown in  FIGS. 2˜4 . Spherical aberrations of line g (λ=435.8 nm) and line d (λ=587.6 nm) and line c (λ=656.3 nm) are shown in  FIG. 2 . Generally, spherical aberration of visible light (with a wavelength between 400˜700 nm) of the lens system  100  in the first embodiment is within a range of −70 μm to 70 μm. The sagittal field curvature and tangential field curvature shown in  FIG. 3  are kept within a range of −50 μm to 50 μm. The distortion in  FIG. 4  falls within a range of −2% to 2%. Obviously, spherical aberration, field curvature, and distortion are well controlled in the first embodiment of the lens system  100 . 
   Embodiment 2 
   The following table 3 and table 4 show specifications of a second embodiment of the lens system  100 , wherein TT=11.675 mm; f=6.67 mm; f 2 =3.785 mm; f 5 =−16.293 mm; F NO =3.2; 2ω=59.18°. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
                 
               Radius of 
                 
                 
                 
             
             
                 
               curvature 
               Thickness 
               Refractive 
               Abbe 
             
             
               Lens system 100 
               (mm) 
               (mm) 
               index 
               number 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Object-side surface of the 
               32.26686 
               0.6123 
               1.8322  
               25.14  
             
             
               first lens 10 
             
             
               Image-side surface of the 
               64.64433 
               0.6654 
               — 
               — 
             
             
               first lens 10 
             
             
               Aperture stop 60 
               ∞ 
               0.9775 
               — 
               — 
             
             
               Object-side surface of the 
               4.734222 
               1.248031 
               1.8315  
               43.1699 
             
             
               second lens 20 
             
             
               Image-side surface of the 
               −8.26632 
               0.1923626 
               — 
               — 
             
             
               second lens 20 
             
             
               Object-side surface of the 
               −5.695776 
               0.35 
               1.735201 
               27.5795 
             
             
               third lens 30 
             
             
               Image-side surface of the 
               6.476238 
               1.338504 
               — 
               — 
             
             
               third lens 30 
             
             
               Object-side surface of the 
               −3.53409 
               1.546352 
               1.48749  
               70.4058 
             
             
               fourth lens 40 
             
             
               Image-side surface of the 
               −1.848934 
               0.1 
               — 
               — 
             
             
               fourth lens 40 
             
             
               Object-side surface of the 
               3.87294 
               1.245206 
               1.503667 
               68.6849 
             
             
               fifth lens 50 
             
             
               Image-side surface of the 
               2.34782 
               1.899395 
               — 
               — 
             
             
               fifth lens 50 
             
             
               Object-side surface of the 
               ∞ 
               0.8 
               1.516   
               64.1   
             
             
               filter 60 
             
             
               Image-side surface of the 
               ∞ 
               0.5 
               — 
               — 
             
             
               filter 60 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
                 
               Aspheric surface 
             
             
               Surface 
               coefficients of the surface equation 
             
             
                 
             
           
           
             
               Object-side surface of the 
               K = −0.6734973; A4 = 0.000522885; 
             
             
               fourth lens 40 
               A6 = −0.007918341; A8 = 0.003667617; 
             
             
                 
               A10 = −0.000465353; A12 = 1.86E−05 
             
             
               Image-side surface of the 
               K = −0.8867163; A4 = 0.002429206; 
             
             
               fourth lens 40 
               A6 = 0.001176508; A8 = −0.000848205; 
             
             
                 
               A10 = 0.000266894; A12 = −1.86E−05 
             
             
               Object-side surface of the 
               K = −13.44202; A4 = −0.004722538; 
             
             
               fifth lens 50 
               A6 = 0.001088901; A8 = −8.58E−05; 
             
             
                 
               A10 = 2.07E−06; A12 = 3.62E−09 
             
             
               Image-side surface of the 
               K = −6.658952; A4 = −0.011869003; 
             
             
               fourth lens 50 
               A6 = 0.000685275; A8 = −2.41E−06; 
             
             
                 
               A10 = −2.40E−06; A12 = 7.92E−08 
             
             
                 
             
           
        
       
     
   
   The spherical aberration diagram, the field curvature diagram, and the distortion diagram of the second embodiment in the lens system  100  are respectively shown in  FIGS. 5˜7 . Spherical aberrations of line g (λ=435.8 nm) and line d (λ=587.6 nm) and line c (λ=656.3 nm) are shown in  FIG. 5 . Generally, the spherical aberration of visible light (with a wavelength between 400˜700 nm) of the lens system  100  in the second embodiment is kept within a range of −70 μm to 70 μm. The sagittal field curvature and tangential field curvature shown in  FIG. 6  are kept within a range of −50 μm to 50 μm. The distortion shown in  FIG. 7  falls within a range of −2% to 2%. Obviously, spherical aberration, field curvature, and distortion are well controlled in the second embodiment of the lens system  100 . 
   Embodiment 3 
   The following table 5 and table 6 show specifications of a third embodiment of the lens system  100 , wherein TT=11.47 mm; f=6.51 mm; f 2 =3.968 mm; f 5 =−11.485 mm; F No =3.2; 2ω=60.4°. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 5 
             
             
                 
             
             
                 
               Radius of 
                 
                 
                 
             
             
                 
               curvature 
               Thickness 
               Refractive 
               Abbe 
             
             
               Lens system 100 
               (mm) 
               (mm) 
               index 
               number 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Object-side surface of the 
               18.888 
               0.6421 
               1.8123  
               25.5689 
             
             
               first lens 10 
             
             
               Image-side surface of the 
               37.776 
               0.7 
               — 
               — 
             
             
               first lens 10 
             
             
               Aperture stop 60 
               ∞ 
               0.952 
               — 
               — 
             
             
               Object-side surface of the 
               4.662901 
               1.207266 
               1.805   
               44.7  
             
             
               second lens 20 
             
             
               Image-side surface of the 
               −8.971283 
               0.1724069 
               — 
               — 
             
             
               second lens 20 
             
             
               Object-side surface of the 
               −6.399854 
               0.55 
               1.8123  
               25.5689 
             
             
               third lens 30 
             
             
               Image-side surface of the 
               6.999723 
               1.363556 
               — 
               — 
             
             
               third lens 30 
             
             
               Object-side surface of the 
               −3.73191 
               1.476334 
               1.594884 
               61.7144 
             
             
               fourth lens 40 
             
             
               Image-side surface of the 
               −1.933912 
               0.1 
               — 
               — 
             
             
               fourth lens 40 
             
             
               Object-side surface of the 
               3.975914 
               1.107187 
               1.623906 
               47.358  
             
             
               fifth lens 50 
             
             
               Image-side surface of the 
               2.283559 
               1.903675 
               — 
               — 
             
             
               fifth lens 50 
             
             
               Object-side surface of the 
               ∞ 
               0.8 
               1.516   
               64.1   
             
             
               filter 60 
             
             
               Image-side surface of the 
               ∞ 
               0.5 
               — 
               — 
             
             
               filter 60 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
           
         
             
               TABLE 6 
             
             
                 
             
             
                 
               Aspheric surface 
             
             
               Surface 
               coefficients of the surface equation 
             
             
                 
             
           
           
             
               Object-side surface of the 
               K = −0.4709513; A4 = −0.00016586; 
             
             
               fourth lens 40 
               A6 = −0.007898677; A8 = 0.003663979; 
             
             
                 
               A10 = −0.00046653; A12 = 1.87E−05 
             
             
               Image-side surface of the 
               K = −0.8916619; A4 = 0.002874988; 
             
             
               fourth lens 40 
               A6 = 0.001026954; A8 = −0.00086664; 
             
             
                 
               A10 = 0.000266443; A12 = −1.83E−03 
             
             
               Object-side surface of the 
               K = −16.81341; A4 = −0.005188043; 
             
             
               fifth lens 50 
               A6 = 0.001120208; A8 = −8.48E−05; 
             
             
                 
               A10 = 2.09E−06; A12 = 1.29E−09 
             
             
               Image-side surface of the 
               K = −7.204009; A4 = −0.011436668; 
             
             
               fourth lens 50 
               A6 = 0.000691118; A8 = −2.77E−06; 
             
             
                 
               A10 = −2.39E−06; A12 = 8.23E−08 
             
             
                 
             
           
        
       
     
   
   The spherical aberration diagram, the field curvature diagram, and the distortion diagram of the third embodiment in the lens system  100  are respectively shown in  FIGS. 8˜10 . Spherical aberrations of line g (λ=435.8 nm) and line d (λ=587.6 nm) and line c (λ=656.3 nm) are shown in  FIG. 8 . Generally, the spherical aberration of visible light (with a wavelength between 400˜700 nm) of the lens system  100  in the third embodiment is kept within a range of −70 μm to 70 μm. The sagittal field curvature and tangential field curvature shown in  FIG. 9  are kept within a range of −50 μm to 50 μm. The distortion shown in  FIG. 10  falls within a range of −2% to 2%. Obviously, spherical aberration, field curvature, and distortion are well controlled in the third embodiment of the lens system  100 . 
   The aforementioned lens system  100  has a compact size and excellent optical performance to meet the trend toward miniature lens systems. The lens system  100  works to keep chromatic aberrations at a minimum while reducing the total length of the lens system. 
   While certain embodiments have been described and exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is not limited to the particular embodiments described and exemplified but is capable of considerable variation and modification without departure from the scope of the appended claims.
A lens system comprises a first lens positive in power, a second lens positive in power, a third lens negative in power, a fourth lens positive in power, and a fifth lens negative in power. The lens system meets a criteria of 1.6<TT/f<1.8, 1.6<f/f 2 <1.8, and -0.6<f/f 5 <-0.4, wherein TT denotes a distance between an object-side surface of the first lens and an image plane of the lens system; f 2 denoting an effective focal length of the second lens; f denoting an effective focal length of the entire lens system.