Patent Publication Number: US-9405097-B2

Title: Imaging lens, and electronic apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Chinese Application No. 201310746364.X, filed on Dec. 30, 2013. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging lens and an electronic apparatus including the same. 
     2. Description of the Related Art 
     In recent years, as use of portable electronic devices (e.g., mobile phones and digital cameras) becomes ubiquitous, much effort has been put into reducing dimensions of portable electronic devices. Moreover, as dimensions of charged coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) based optical sensors are reduced, dimensions of imaging lenses for use with the optical sensors must be correspondingly reduced without significantly compromising optical performance. 
     Each of U.S. Pat. Nos. 7,848,032, 8,284,502, 8,179,616, and Taiwanese patent publication no. 201109712 discloses a conventional imaging lens that includes four lens elements, and that has a system length of over 8 mm. Particularly, the imaging lens disclosed in U.S. Pat. No. 8,179,616 has a system length of over 11 mm, which disfavors reducing thickness of portable electronic devices, such as mobile phones and digital cameras. 
     Reducing the system length of the imaging lens while maintaining satisfactory optical performance is always a goal in the industry. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an imaging lens having a shorter overall length while maintaining good optical performance. 
     According to one aspect of the present invention, an imaging lens comprises a first lens element, a second lens element, a third lens element and a fourth lens element arranged in order from an object side to an image side along an optical axis of the imaging lens. Each of the first lens element, the second lens element, the third lens element and the fourth lens element has a refractive power, an object-side surface facing toward the object side, and an image-side surface facing toward the image side. 
     The image-side surface of the first lens element has a concave portion in a vicinity of a periphery of the first lens element. 
     The second lens element has a positive refractive power. 
     The image-side surface of the third lens element has a convex portion in a vicinity of the optical axis. 
     The image-side surface of the fourth lens element has a concave portion in a vicinity of the optical axis, and a convex portion in a vicinity of a periphery of the fourth lens element. 
     The imaging lens does not include any lens element with refractive power other than the first lens element, the second lens element, the third lens element and the fourth lens element. 
     The imaging lens satisfies 2.5≦BFL/G 23 , where BFL represents a distance at the optical axis between the image-side surface of the fourth lens element and an image plane at the image side, and G 23  represents an air gap length between the second lens element and the third lens element at the optical axis. 
     Another object of the present invention is to provide an electronic apparatus having an imaging lens with four lens elements. 
     According to another aspect of the present invention, an electronic apparatus includes a housing and an imaging module. The imaging module is disposed in the housing, and includes the imaging lens of the present invention, a barrel on which the imaging lens is disposed, a holder unit on which the barrel is disposed, and an image sensor disposed at the image side of the imaging lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic diagram to illustrate the structure of a lens element; 
         FIG. 2  is a schematic diagram that illustrates the first preferred embodiment of an imaging lens according to the present invention; 
         FIG. 3  shows values of some optical parameters corresponding to the imaging lens of the first preferred embodiment; 
         FIG. 4  shows values of some parameters of an optical relationship corresponding to the imaging lens of the first preferred embodiment; 
         FIGS. 5( a ) to 5( d )  show different optical characteristics of the imaging lens of the first preferred embodiment; 
         FIG. 6  is a schematic diagram that illustrates the second preferred embodiment of an imaging lens according to the present invention; 
         FIG. 7  shows values of some optical parameters corresponding to the imaging lens of the second preferred embodiment; 
         FIG. 8  shows values of some parameters of an optical relationship corresponding to the imaging lens of the second preferred embodiment; 
         FIGS. 9( a ) to 9( d )  show different optical characteristics of the imaging lens of the second preferred embodiment; 
         FIG. 10  is a schematic diagram that illustrates the third preferred embodiment of an imaging lens according to the present invention; 
         FIG. 11  shows values of some optical parameters corresponding to the imaging lens of the third preferred embodiment; 
         FIG. 12  shows values of some parameters of an optical relationship corresponding to the imaging lens of the third preferred embodiment; 
         FIGS. 13( a ) to 13( d )  show different optical characteristics of the imaging lens of the third preferred embodiment; 
         FIG. 14  is a schematic diagram that illustrates the fourth preferred embodiment of an imaging lens according to the present invention; 
         FIG. 15  shows values of some optical parameters corresponding to the imaging lens of the fourth preferred embodiment; 
         FIG. 16  shows values of some parameters of an optical relationship corresponding to the imaging lens of the fourth preferred embodiment; 
         FIGS. 17( a ) to 17( d )  show different optical characteristics of the imaging lens of the fourth preferred embodiment; 
         FIG. 18  is a schematic diagram that illustrates the fifth preferred embodiment of an imaging lens according to the present invention; 
         FIG. 19  shows values of some optical parameters corresponding to the imaging lens of the fifth preferred embodiment; 
         FIG. 20  shows values of some parameters of an optical relationship corresponding to the imaging lens of the fifth preferred embodiment; 
         FIGS. 21( a ) to 21( d )  show different optical characteristics of the imaging lens of the fifth preferred embodiment; 
         FIG. 22  is a schematic diagram that illustrates the sixth preferred embodiment of an imaging lens according to the present invention; 
         FIG. 23  shows values of some optical parameters corresponding to the imaging lens of the sixth preferred embodiment; 
         FIG. 24  shows values of some parameters of an optical relationship corresponding to the imaging lens of the sixth preferred embodiment; 
         FIGS. 25( a ) to 25( d )  show different optical characteristics of the imaging lens of the sixth preferred embodiment; 
         FIG. 26  is a schematic diagram that illustrates the seventh preferred embodiment of an imaging lens according to the present invention; 
         FIG. 27  shows values of some optical parameters corresponding to the imaging lens of the seventh preferred embodiment; 
         FIG. 28  shows values of some parameters of an optical relationship corresponding to the imaging lens of the seventh preferred embodiment; 
         FIGS. 29( a ) to 29( d )  show different optical characteristics of the imaging lens of the seventh preferred embodiment; 
         FIG. 30  is a table that lists values of parameters of other optical relationships corresponding to the imaging lenses of the first to seventh preferred embodiments; 
         FIG. 31  is a schematic partly sectional view to illustrate a first exemplary application of the imaging lens of the present invention; and 
         FIG. 32  is a schematic partly sectional view to illustrate a second exemplary application of the imaging lens of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the present invention is described in greater detail, it should be noted that like elements are denoted by the same reference numerals throughout the disclosure. 
     In the following description, “a lens element has a positive (or negative) refractive power” means the lens element has a positive (or negative) refractive power in a vicinity of an optical axis thereof. “An object-side surface (or image-side surface) has a convex (or concave) portion at a certain area” means that, compared to a radially exterior area adjacent to said certain area, said certain area is more convex (or concave) in a direction parallel to the optical axis. Referring to  FIG. 1  as an example, the lens element is radially symmetrical with respect to an optical axis (I) thereof. The object-side surface of the lens element has a convex portion at an area A, a concave portion at an area B, and a convex portion at an area C. This is because the area A is more convex in a direction parallel to the optical axis (I) in comparison with a radially exterior area thereof (i.e., area B), the area B is more concave in comparison with the area C, and the area C is more convex in comparison with an area E. “In a vicinity of a periphery” refers to an area around a periphery of a curved surface of the lens element for passage of imaging light only, which is the area C in  FIG. 1 . The imaging light includes a chief ray Lc and a marginal ray Lm. “In a vicinity of the optical axis” refers to an area around the optical axis of the curved surface for passage of the imaging light only, which is the area A in  FIG. 1 . In addition, the lens element further includes an extending portion E for installation into an optical imaging lens device. Ideally, the imaging light does not pass through the extending portion E. The structure and shape of the extending portion E are not limited herein. In the following embodiments, the extending portion E is not depicted in the drawings for the sake of clarity. 
     Referring to  FIG. 2 , the first preferred embodiment of an imaging lens  10  according to the present invention includes a first lens element  3 , an aperture stop  2 , a second lens element  4 , a third lens element  5 , a fourth lens element  6 , and an optical filter  7  arranged in the given order along an optical axis (I) from an object side to an image side. The optical filter  7  is an infrared cut filter for selectively absorbing infrared light to thereby reduce imperfection of images formed at an image plane  8 . 
     Each of the first, second, third and fourth lens elements  3 - 6  and the optical filter  7  has an object-side surface  31 ,  41 ,  51 ,  61 ,  71  facing toward the object side, and an image-side surface  32 ,  42 ,  52 ,  62 ,  72  facing toward the image side. Light entering the imaging lens  10  travels through the object-side and image-side surfaces  31 ,  32  of the first lens element  3 , the aperture stop  2 , the object-side and image-side surfaces  41 ,  42  of the second lens element  4 , the object-side and image-side surfaces  51 ,  52  of the third lens element  5 , the object-side and image-side surfaces  61 ,  62  of the fourth lens element  6 , and the object-side and image-side surfaces  71 ,  72  of the optical filter  7 , in the given order, to form an image on the image plane  8 . Each of the object-side surfaces  31 ,  41 ,  51 ,  61  and the image-side surfaces  32 ,  42 ,  52 ,  62  is aspherical and has a center point coinciding with the optical axis (I). 
     It should be noted that the present invention uses an image sensor (not shown) packaged using COB (chip on board) techniques. Compared to the conventional CSP (chip scale package), a cover glass is not required for the COB technique. Hence, the imaging lens of the present invention does not include the cover glass, but the present invention should not be limited in this respect. 
     In the first preferred embodiment, which is depicted in  FIG. 2 , the first lens element  3  has a negative refractive power. The object-side surface  31  of the first lens element  3  is a convex surface that has a convex portion  311  in a vicinity of the optical axis (I) and a convex portion  312  in a vicinity of a periphery of the first lens element  3 . The image-side surface  32  of the first lens element  3  is a concave surface that has a concave portion  322  in a vicinity of the optical axis (I) and a concave portion  321  in a vicinity of a periphery of the first lens element  3 . 
     The second lens element  4  has a positive refractive power. The object-side surface  41  of the second lens element  4  is a convex surface that has a convex portion  411  in a vicinity of the optical axis (I) and a convex portion  412  in a vicinity of a periphery of the second lens element  4 . The image-side surface  42  of the second lens element  4  is a convex surface that has a convex portion  421  in a vicinity of the optical axis (I) and a convex portion  422  in a vicinity of a periphery of the second lens element  4 . 
     The third lens element  5  has a positive refractive power. The object-side surface  51  of the third lens element  5  is a concave surface that has a concave portion  511  in a vicinity of the optical axis (I) and a concave portion  512  in a vicinity of a periphery of the third lens element  5 . The image-side surface  52  of the third lens element  5  is a convex surface that has a convex portion  521  in a vicinity of the optical axis (I) and a convex portion  522  in a vicinity of a periphery of the third lens element  5 . 
     The fourth lens element  6  has a negative refractive power. The object-side surface  61  of the fourth lens element  6  has a convex portion  611  in a vicinity of the optical axis (I), and a concave portion  612  in a vicinity of a periphery of the fourth lens element  6 . The image-side surface  62  of the fourth lens element  6  has a concave portion  621  in a vicinity of the optical axis (I), and a convex portion  622  in a vicinity of the periphery of the fourth lens element  6 . 
     In the first preferred embodiment, the imaging lens  10  does not include any lens element with refractive power other than the aforesaid lens elements  3 - 6 . 
     Shown in  FIG. 3  is a table that lists values of some optical parameters corresponding to the surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  32 ,  42 ,  52 ,  62 ,  72  of the first preferred embodiment. The imaging lens  10  has an overall system effective focal length (EFL) of 2.004 mm, a half field-of-view (HFOV) of 47.138°, an F-number of 2.50, and a system length of 3.88 mm. The system length refers to a distance between the object-side surface  31  of the first lens element  3  and the image plane  8  at the optical axis (I). 
     In this embodiment, each of the object-side surfaces  31 - 61  and the image-side surfaces  32 - 62  is aspherical, and satisfies the relationship of 
     
       
         
           
             
               
                 
                   
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     where: 
     Y represents a perpendicular distance between an arbitrary point on the aspherical surface and the optical axis (I); 
     Z represents a depth of an aspherical surface, which is defined as a perpendicular distance between the arbitrary point on the aspherical surface that is spaced apart from the optical axis (I) by a distance Y, and a tangent plane at a vertex of the aspherical surface at the optical axis (I); 
     R represents a radius of curvature of the aspherical surface; 
     K represents a conic constant; and 
     a 2i  represents a 2i th  aspherical coefficient. 
     Shown in  FIG. 4  is a table that lists values of some aspherical parameters of the aforementioned relationship ( 1 ) corresponding to the first preferred embodiment. 
     Relationships among some lens parameters corresponding to the first preferred embodiment are listed in  FIG. 30 , where: 
     T 1  represents a thickness of the first lens element  3  at the optical axis (I); 
     T 2  represents a thickness of the second lens element  4  at the optical axis (I); 
     T 3  represents a thickness of the third lens element  5  at the optical axis (I); 
     T 4  represents a thickness of the fourth lens element  6  at the optical axis (I); 
     G 12  represents an air gap length between the first lens element  3  and the second lens element  4  at the optical axis (I); 
     G 23  represents an air gap length between the second lens element  4  and the third lens element  5  at the optical axis (I); 
     TTL represents a distance between the object-side surface  31  of the first lens element  3  and the image plane  8  at the optical axis (I); 
     BFL represents a distance at the optical axis (I) between the image-side surface  62  of the fourth lens element  6  and the image plane  8 ; 
     ALT represents a sum of thicknesses of the lens elements  3 - 6  at the optical axis (I); 
     Gaa represents a sum of air gap lengths among the first lens element  3 , the second lens element  4 , the third lens element  5  and the fourth lens element  6  at the optical axis (I); and 
     EFL represents a system focal length of the imaging lens  10 . 
       FIGS. 5( a ) to 5( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the first preferred embodiment. In each of the simulation results, curves corresponding respectively to wavelengths of 470 nm, 550 nm, and 650 nm are shown. 
     It can be understood from  FIG. 5( a )  that, since each of the curves corresponding to longitudinal spherical aberration has a focal length at each field of view (indicated by the vertical axis) that falls within the range of ±0.1 mm, the first preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths. Furthermore, since the curves at each field of view are close to each other, the first preferred embodiment has a relatively low chromatic aberration. 
     It can be understood from  FIGS. 5( b ) and 5( c )  that, since each of the curves falls within the range of ±0.1 mm of focal length, the first preferred embodiment has a relatively low optical aberration. 
     Moreover, as shown in  FIG. 5( d ) , since each of the curves corresponding to distortion aberration falls within the range of ±2%, the first preferred embodiment is able to meet requirements in imaging quality of most optical systems. 
     In view of the above, even with the system length reduced down to 3.88 mm, the imaging lens  10  of the first preferred embodiment is still able to achieve a relatively good optical performance. 
       FIG. 6  illustrates the second preferred embodiment of the imaging lens  10  according to the present invention, which has a configuration similar to that of the first preferred embodiment. 
     Shown in  FIG. 7  is a table that lists values of some optical parameters corresponding to the surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  32 ,  42 ,  52 ,  62 ,  72  of the second preferred embodiment. The imaging lens  10  has an overall system focal length of 2.090 mm, an HFOV of 45.880°, an F-number of 2.49, and a system length of 4.11 mm. 
     Shown in  FIG. 8  is a table that lists values of some aspherical parameters of the aforementioned relationship ( 1 ) corresponding to the second preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the second preferred embodiment are listed in  FIG. 30 . 
       FIGS. 9( a ) to 9( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the second preferred embodiment. It can be understood from  FIGS. 9( a ) to 9( d )  that the second preferred embodiment is able to achieve a relatively good optical performance. 
       FIG. 10  illustrates the third preferred embodiment of the imaging lens  10  according to the present invention, which has a configuration similar to that of the first preferred embodiment. 
     Shown in  FIG. 11  is a table that lists values of some optical parameters corresponding to the surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  32 ,  42 ,  52 ,  62 ,  72  of the third preferred embodiment. The imaging lens  10  has an overall system focal length of 1.929 mm, an HFOV of 49.294°, an F-number of 2.48, and a system length of 3.81 mm. 
     Shown in  FIG. 12  is a table that lists values of some aspherical parameters of the aforementioned relationship ( 1 ) corresponding to the third preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the third preferred embodiment are listed in  FIG. 30 . 
       FIGS. 13( a ) to 13( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the third preferred embodiment. It can be understood from  FIGS. 13( a ) to 13( d )  that the third preferred embodiment is able to achieve a relatively good optical performance. 
       FIG. 14  illustrates the fourth preferred embodiment of the imaging lens  10  according to the present invention, which has a configuration similar to that of the first preferred embodiment. 
     Shown in  FIG. 15  is a table that lists values of some optical parameters corresponding to the surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  32 ,  42 ,  52 ,  62 ,  72  of the fourth preferred embodiment. The imaging lens  10  has an overall system focal length of 2.128 mm, an HFOV of 46.406°, an F-number of 2.42, and a system length of 4.25 mm. 
     Shown in  FIG. 16  is a table that lists values of some aspherical parameters of the aforementioned optical relationship ( 1 ) corresponding to the fourth preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the fourth preferred embodiment are listed in  FIG. 30 . 
       FIGS. 17( a ) to 17( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the fourth preferred embodiment. It can be understood from  FIGS. 17( a ) to 17( d )  that the fourth preferred embodiment is able to achieve a relatively good optical performance. 
       FIG. 18  illustrates the fifth preferred embodiment of the imaging lens  10  according to the present invention, which has a configuration similar to that of the first preferred embodiment. 
     Shown in  FIG. 19  is a table that lists values of some optical parameters corresponding to the surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  32 ,  42 ,  52 ,  62 ,  72  of the fifth preferred embodiment. The imaging lens  10  has an overall system focal length of 2.361 mm, an HFOV of 43.78°, an F-number of 2.60, and a system length of 3.88 mm. 
     Shown in  FIG. 20  is a table that lists values of some aspherical parameters of the aforementioned relationship ( 1 ) corresponding to the fifth preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the fifth preferred embodiment are listed in  FIG. 30 . 
       FIGS. 21( a ) to 21( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the fifth preferred embodiment. It can be understood from  FIGS. 21( a ) to 21( d )  that the fifth preferred embodiment is able to achieve a relatively good optical performance. 
       FIG. 22  illustrates the sixth preferred embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first preferred embodiment. 
     Shown in  FIG. 23  is a table that lists values of some optical parameters corresponding to the surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  32 ,  42 ,  52 ,  62 ,  72  of the sixth preferred embodiment. The imaging lens  10  has an overall system focal length of 2.077 mm, an HFOV of 47.748°, an F-number of 2.42, and a system length of 3.84 mm. 
     Shown in  FIG. 24  is a table that lists values of some aspherical parameters of the aforementioned relationship ( 1 ) corresponding to the sixth preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the sixth preferred embodiment are listed in  FIG. 30 . 
       FIGS. 25( a ) to 25( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the sixth preferred embodiment. It can be understood from  FIGS. 25( a ) to 25( d )  that the sixth preferred embodiment is able to achieve a relatively good optical performance. 
     Referring to  FIG. 26 , the differences between the first and seventh preferred embodiments of the imaging lens  10  of this invention reside in that: the aperture stop  2  is arranged between the first lens element  3  and the to-be-captured object; the first lens element  3  has a positive refractive power; and the image-side surface  42  of the second lens element  4  has a concave portion  421 ′ in a vicinity of the optical axis (I), and a convex portion  422  in a vicinity of a periphery of the second lens element  4 . 
     Shown in  FIG. 27  is a table that lists values of some optical parameters corresponding to the surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  32 ,  42 ,  52 ,  62 ,  72  of the seventh preferred embodiment. The imaging lens  10  has an overall system focal length of 2.464 mm, an HFOV of 42.503°, an F-number of 2.61, and a system length of 3.64 mm. 
     Shown in  FIG. 28  is a table that lists values of some aspherical parameters of the aforementioned relationship ( 1 ) corresponding to the seventh preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the seventh preferred embodiment are listed in  FIG. 30 . 
       FIGS. 29( a ) to 29( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the seventh preferred embodiment. It can be understood from  FIGS. 29( a ) to 29( d )  that the seventh preferred embodiment is able to achieve a relatively good optical performance. 
     Shown in  FIG. 30  is a table that lists the aforesaid relationships among some of the aforementioned lens parameters corresponding to the seven preferred embodiments for comparison, and it should be noted that the values of the lens parameters and the relationships listed in  FIG. 34  are rounded off to the second decimal place. When each of the lens parameters of the imaging lens  10  according to this invention satisfies the following optical relationships, the optical performance is still relatively good even with the reduced system length: 
     (1) 2.5≦BFL/G 23 : One of the objects of the present invention is to achieve a large field of view. Shorter EFL favors not only enlargement of the field of view, but also reduction of BFL. In order to reduce the system length, all of the lens parameters are preferred to be reduced. Furthermore, since the image-side surface  42  and the object-side surface  51  are not limited in surface shapes, G 23  may have effective reduction to meet requirements of a wide field of view and a short system length. Preferably, 2.5≦BFL/G 23 ≦6.5. 
     (2) EFL/T 3 ≦5: Shorter EFL favors enlargement of the field of view. Since the image-side surface  52  of the third lens element  5  has the convex portion  521  in a vicinity of the optical axis (I), T 3  has a relatively small reducible ratio. Therefore, the design of EFL/T 3  tends to be small. Preferably, 1.3≦EFL/T 3 ≦5. 
     (3) 2.15≦BFL/T 1 : As mentioned above, BFL may be reduced. In addition, since the first lens element  3  has no limitation in the area near the optical axis (I), a reducible ratio of T 1  is not limited. When this relationship is satisfied, T 1  may be effectively reduced, favoring reduction of the imaging lens  10 . Preferably, 2.15≦BFL/T 1 ≦7.5. 
     (4) 3.9≦BFL/G 12 ≦17: As mentioned above, BFL may be reduced. On the other hand, G 12  should have sufficient length to allow light entering the second lens element  4  at a suitable height. Considering optical performance and manufacturing ability, better arrangements of BFL and G 12  may be achieved when this relationship is satisfied. 
     (5) ALT/G 23 ≦11: As mentioned above, G 23  may have an effective reduction. In addition, ALT is large compared to other lens parameters. It further favors reduction of the system length of the imaging lens  10  if reduction of ALT is greater than that of G 23 . Preferably, 2≦ALT/G 23 ≦11. 
     (6) 7.8≦ALT/G 12 ≦17.8: As mentioned above, ALT may have a relatively large reducible ratio, and G 12  should have sufficient length to allow light entering the second lens element  4  at a suitable height. Considering optical performance and manufacturing ability, better arrangements of ALT and G 12  may be achieved when this relationship is satisfied. 
     (7) 1.15≦T 3 /Gaa: Since the image-side surface  52  of the third lens element  5  has the convex portion  521  in a vicinity of the optical axis (I), T 3  has a relatively small reducible ratio. 
     Aside from that G 12  should have a sufficient length, G 23  may be reduced with less limitation, so that Gaa has a relative large reducible ratio. Preferably, 1.15≦T 3 /Gaa≦2.8. 
     (8) 2.85≦T 3 /G 23 : As mentioned above, since T 3  has a relatively small reducible ratio, while G 23  has a relatively large reducible ratio, design of T 3 /G 23  tends to be large. Preferably, 2.85≦T 3 /G 23 ≦6.5. 
     (9) 4.4≦ALT/T 2 : As mentioned above, ALT has a large reducible ratio. Since the second lens element  4  has the positive refractive power, T 2  may be made thicker. However, it favors reduction of the system total length of the imaging lens  10  if T 2  also has effective reduction. The system length may be effectively reduced when ALT/T 2  is designed to be large. Preferably, 4.4≦ALT/T 2 ≦8. 
     (10) 5.1≦ALT/T 1 : As mentioned above, ALT has a large reducible ratio, and T 1  may be effective reduced when ALT/T 1  is designed to be large. Preferably, 5.1≦ALT/T 1 ≦9. 
     (11) 3≦T 3 /G 12 : Since the image-side surface  52  of the third lens element  5  has the convex portion  521  in a vicinity of the optical axis (I), T 3  has a relatively small reducible ratio. On the other hand, G 12  should have sufficient length to allow light entering the second lens element  4  at a suitable height. Thus, both of T 3  and G 12  are limited in amount of reduction. However, since adjacent surfaces of G 12  do not have limitations in shape, G 12  has a relatively large reducible ratio compared to T 3 , so that design of T 3 /G 12  tends to be large. Preferably, 3≦T 3 /G 12 ≦10. 
     (12) 1.55≦T 3 /T 4 : Since the object of the present invention is to achieve a wide field of view and to reduce the system length of the imaging lens  10 , light should be converged within a shorter distance. In addition, since the image-side surface  52  of the third lens element  5  has the convex portion  521  in a vicinity of the optical axis (I), light has a shorter distance to be converged after refraction at the convex portion  521 , so that the fourth lens element  6  should be made thinner in order to favor arrangement of BFL. Therefore, design of T 3 /T 4  tends to be large. Preferably, 1.55≦T 3 /T 4 ≦4.2. 
     To sum up, effects and advantages of the imaging lens  10  according to the present invention are described hereinafter. 
     1. The second lens element  4  with the positive refractive power may provide a portion of required positive refractive power of the imaging lens  10 . In addition, by virtue of the concave portion  321 , the convex portion  521 , the concave portion  621  and the convex portion  622 , optical aberration of the image may be corrected. Furthermore, when the aperture stop  2  is arranged between the first lens element  3  and the second lens element  4 , field of view may be enlarged. If the image-side surface  42  of the second lens element  4  has the convex portion  421  in a vicinity of the optical axis (I), correction of optical aberration may be further enhanced. 
     2. Through design of the relevant optical parameters, such as BFL/G 23 , EFL/T 3 , BFL/T 1 , BFL/G 12 , ALT/G 23 , ALT/G 12 , T 3 /Gaa, T 3 /G 23 , ALT/G 12 , T 3 /Gaa, T 3 /G 23 , ALT/T 2 , ALT/T 1 , T 3 /G 12  and T 3 /T 4 , optical aberrations, such as spherical aberration, may be reduced or even eliminated. Further, through surface design and arrangement of the lens elements  3 - 6 , even with the system length reduced, optical aberrations may still be reduced or even eliminated, resulting in relatively good optical performance. 
     3. Through the aforesaid seven preferred embodiments, it is known that the system length of this invention may be reduced down to below 4.25 mm, so as to facilitate developing thinner relevant products with economic benefits. 
     Shown in  FIG. 31  is a first exemplary application of the imaging lens  10 , in which the imaging lens  10  is disposed in a housing  11  of an electronic apparatus  1  (such as a mobile phone, but not limited thereto), and forms a part of an imaging module  12  of the electronic apparatus  1 . The imaging module  12  includes a barrel  21  on which the imaging lens  10  is disposed, a holder unit  120  on which the barrel  21  is disposed, and an image sensor  130  disposed at the image plane  8  (see  FIG. 2 ). 
     The holder unit  120  includes a first holder portion  121  in which the barrel  21  is disposed, and a second holder portion  122  having a portion interposed between the first holder portion  121  and the image sensor  130 . The barrel  21  and the first holder portion  121  of the holder unit  120  extend along an axis (II), which coincides with the optical axis (I) of the imaging lens  10 . 
     Shown in  FIG. 32  is a second exemplary application of the imaging lens  10 . The differences between the first and second exemplary applications reside in that, in the second exemplary application, the holder unit  120  is configured as a voice-coil motor (VCM), and the first holder portion  121  includes an inner section  123  in which the barrel  21  is disposed, an outer section  124  that surrounds the inner section  123 , a coil  125  that is interposed between the inner and outer sections  123 ,  124 , and a magnetic component  126  that is disposed between an outer side of the coil  125  and an inner side of the outer section  124 . 
     The inner section  123  and the barrel  21 , together with the imaging lens  10  therein, are movable with respect to the image sensor  130  along an axis (III), which coincides with the optical axis (I) of the imaging lens  10 . The optical filter  7  of the imaging lens  10  is disposed at the second holder portion  122 , which is disposed to abut against the outer section  124 . Configuration and arrangement of other components of the electronic apparatus  1  in the second exemplary application are identical to those in the first exemplary application, and hence will not be described hereinafter for the sake of brevity. 
     By virtue of the imaging lens  10  of the present invention, the electronic apparatus  1  in each of the exemplary applications may be configured to have a relatively reduced overall thickness with good optical and imaging performance, so as to reduce cost of materials, and satisfy requirements of product miniaturization. 
     While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.