Patent Publication Number: US-9411129-B2

Title: Imaging lens, and electronic apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Chinese Application No. 201410331107.4, filed on Jul. 11, 2014. 
     FIELD OF THE INVENTION 
     The present invention relates to an imaging lens and an electronic apparatus including the same. 
     BACKGROUND OF THE INVENTION 
     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. Image quality and size are two of the most important characteristics for an imaging lens. 
     Taiwanese patent No. I254140 discloses a conventional imaging lens that includes four lens elements and having a F-number of 4.0. However, the amount of light entering such imaging lens is insufficient to obtain a satisfactory imaging quality and the system length of such imaging lens is up to 12 mm, which disfavors reducing the thickness of portable electronic devices such as mobile phones with a slim profile. 
     Therefore, technical difficulties of a miniaturized imaging lens are higher than those of traditional imaging lenses. Producing an imaging lens that meets requirements of consumer electronic products with satisfactory optical performance is always a goal in the industry. 
     SUMMARY OF THE INVENTION 
     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 includes an aperture stop, 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 first lens element has a positive refractive power, and the image-side surface of the first lens element has a convex portion in a vicinity of the optical axis and a convex portion in a vicinity of a periphery of the first lens element. The second lens element has a negative refractive power, and the object-side surface of the second lens element has a concave portion in a vicinity of the optical axis and a concave portion in a vicinity of a periphery of the second lens element. The third lens element has a positive refractive power, the object-side surface of the third lens element has a concave portion in a vicinity of the optical axis and a concave portion in a vicinity of a periphery of the third lens element, and the image-side surface of the third lens element has a convex portion in a vicinity of the optical axis. The fourth lens element has a negative refractive power and is made of a plastic material. The object-side surface of the fourth lens element has a convex portion in a vicinity of the optical axis, and the image-side surface of the fourth lens element has a convex portion in a vicinity of a periphery of the fourth lens element. 
     The imaging lens satisfies 6≦EFL/T2≦11; 4≦ALT/G23≦18; 1.4≦Gaa/T2≦2.11; 5≦ALT/T2≦7.2; and T4/G23≧1, where EFL represents a system effective focal length of the imaging lens, T2 represents a thickness of the second lens element at the optical axis, T4 represents a thickness of the fourth lens element at the optical axis, ALT represents a sum of thicknesses of the first lens element, the second lens element, the third lens element, and the fourth lens element at the optical axis, Gaa represents a sum of three air gap lengths among the first lens element, the second lens element, the third lens element, and the fourth lens element, and G23 represents the air gap length between the second lens element and the third lens element at the optical axis. 
     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. 
     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 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 embodiment of an imaging lens according to the present invention; 
         FIG. 3  shows values of some optical data corresponding to the imaging lens of the first embodiment; 
         FIG. 4  shows values of some aspherical coefficients corresponding to the imaging lens of the first embodiment; 
         FIGS. 5( a ) to 5( d )  show different optical characteristics of the imaging lens of the first embodiment; 
         FIG. 6  is a schematic diagram that illustrates the second embodiment of an imaging lens according to the present invention; 
         FIG. 7  shows values of some optical data corresponding to the imaging lens of the second embodiment; 
         FIG. 8  shows values of some aspherical coefficients corresponding to the imaging lens of the second embodiment; 
         FIGS. 9( a ) to 9( d )  show different optical characteristics of the imaging lens of the second embodiment; 
         FIG. 10  is a schematic diagram that illustrates the third embodiment of an imaging lens according to the present invention; 
         FIG. 11  shows values of some optical data corresponding to the imaging lens of the third embodiment; 
         FIG. 12  shows values of some aspherical coefficients corresponding to the imaging lens of the third embodiment; 
         FIGS. 13( a ) to 13( d )  show different optical characteristics of the imaging lens of the third embodiment; 
         FIG. 14  is a schematic diagram that illustrates the fourth embodiment of an imaging lens according to the present invention; 
         FIG. 15  shows values of some optical data corresponding to the imaging lens of the fourth embodiment; 
         FIG. 16  shows values of some aspherical coefficients corresponding to the imaging lens of the fourth embodiment; 
         FIGS. 17( a ) to 17( d )  show different optical characteristics of the imaging lens of the fourth embodiment; 
         FIG. 18  is a schematic diagram that illustrates the fifth embodiment of an imaging lens according to the present invention; 
         FIG. 19  shows values of some optical data corresponding to the imaging lens of the fifth embodiment; 
         FIG. 20  shows values of some aspherical coefficients corresponding to the imaging lens of the fifth embodiment; 
         FIGS. 21( a ) to 21( d )  show different optical characteristics of the imaging lens of the fifth embodiment; 
         FIG. 22  is a schematic diagram that illustrates the sixth embodiment of an imaging lens according to the present invention; 
         FIG. 23  shows values of some optical data corresponding to the imaging lens of the sixth embodiment; 
         FIG. 24  shows values of some aspherical coefficients corresponding to the imaging lens of the sixth embodiment; 
         FIGS. 25( a ) to 25( d )  show different optical characteristics of the imaging lens of the sixth embodiment; 
         FIG. 26  is a schematic diagram that illustrates the seventh embodiment of an imaging lens according to the present invention; 
         FIG. 27  shows values of some optical data corresponding to the imaging lens of the seventh embodiment; 
         FIG. 28  shows values of some aspherical coefficients corresponding to the imaging lens of the seventh embodiment; 
         FIGS. 29( a ) to 29( d )  show different optical characteristics of the imaging lens of the seventh embodiment; 
         FIG. 30  is a schematic diagram that illustrates the eighth embodiment of an imaging lens according to the present invention; 
         FIG. 31  shows values of some optical data corresponding to the imaging lens of the eighth embodiment; 
         FIG. 32  shows values of some aspherical coefficients corresponding to the imaging lens of the eighth embodiment; 
         FIGS. 33( a ) to 33( d )  show different optical characteristics of the imaging lens of the eighth embodiment; 
         FIGS. 34 to 37  are tables each listing values of relationships among some lens parameters corresponding to the imaging lenses of the first to eight embodiments; 
         FIG. 38  is a schematic partly sectional view to illustrate a first exemplary application of the imaging lens of the present invention; and 
         FIG. 39  is a schematic partly sectional view to illustrate a second exemplary application of the imaging lens of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE 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 embodiment of an imaging lens  10  according to the present invention includes an aperture stop  2 , a first lens element  3 , 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  100 . 
     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 aperture stop  2 , the object-side and image-side surfaces  31 ,  32  of the first lens element  3 , 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  100 . 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). 
     The lens elements  3 - 6  are made of a plastic material in this embodiment, and at least one of the lens elements  3 - 5  may be made of other materials in other embodiments. In addition, each of the lens elements  3 - 6  has a refractive power. 
     In the first embodiment, which is depicted in  FIG. 2 , the first lens element  3  has a positive 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 convex surface that has a convex portion  321  in a vicinity of the optical axis (I), and a convex portion  322  in a vicinity of the periphery of the first lens element  3 . 
     The second lens element  4  has a negative refractive power. The object-side surface  41  of the second lens element  4  is a concave surface that has a concave portion  411  in a vicinity of the optical axis (I), and a concave 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 the 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  has a convex portion  521  in a vicinity of the optical axis (I), and a concave portion  522  in a vicinity of the 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 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 - 71 ,  32 - 72  of the first embodiment. The imaging lens  10  has an overall system effective focal length (EFL) of 2.333 mm, a half field-of-view (HFOV) of 37.033°, an F-number of 2.219, and a system length of 3.341 mm. The system length refers to a distance between the object-side surface  31  of the first lens element  3  and the image plane  100  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 an aspherical surface and the optical axis (I); 
     Z represents a depth of the 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 coefficients of the aforementioned relationship (1) corresponding to the first embodiment. Note that in  FIG. 4 , the row under “ 31 ” represents aspherical coefficients of the object-side surface  31  of the first lens element  3  and the values listed in the other rows correspond to other surfaces of the lens elements  3 - 6 . 
     Relationships among some of the lens parameters corresponding to the first embodiment are listed in columns of  FIGS. 34 and 36  corresponding to the first embodiment, where: 
     T1 represents the thickness of the first lens element  3  at the optical axis (I); 
     T2 represents the thickness of the second lens element  4  at the optical axis (I); 
     T3 represents the thickness of the third lens element  5  at the optical axis (I); 
     T4 represents the thickness of the fourth lens element  6  at the optical axis (I); 
     G12 represents an air gap length between the first lens element  3  and the second lens element  4  at the optical axis (I); 
     G23 represents an air gap length between the second lens element  4  and the third lens element  5  at the optical axis (I); 
     G34 represents an air gap length between the third lens element  5  and the fourth lens element  6  at the optical axis (I); 
     Gaa represents a sum of the three 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), i.e., the sum of G12, G23, and G34; 
     ALT represents a sum of the thicknesses of 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), i.e., a sum of T1, T2, T3, and T4; 
     TTL represents a distance at the optical axis (I) between the object-side surface  31  of the first lens element  3  and the image plane  100  at the image side; 
     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  100 ; and 
     EFL represents a system effective focal length of the imaging lens  10 . 
     In addition, some referenced terminologies are defined herein, where: 
     G4F represents an air gap length between the fourth lens element  6  and the optical filter  7  at the optical axis (I); 
     TF represents a thickness of the optical filter  7  at the optical axis (I); and 
     GFP represents an air gap length between the optical filter  7  and the image plane  100  at the optical axis (I). 
       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 embodiment. In each of the simulation results, curves corresponding respectively to wavelengths of 470 nm, 555 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 ±0.03 mm, the first embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths. Furthermore, since a deviation in focal length among the curves at each of the wavelengths of 470 nm, 555 nm, and 650 nm does not exceed the range of 0.2 mm, the first 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 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 ±0.8%, the first 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 3.341 mm, the imaging lens  10  of the first embodiment is still able to achieve a relatively good optical performance. 
       FIG. 6  illustrates the second embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first embodiment. The differences between the first and second embodiments of the imaging lens  10  of this invention reside in modifications of some optical data, aspherical coefficients and the lens parameters of the lens elements  3 - 6 . It should be noted that the reference numerals of the concave portions and the convex portions in the following embodiments that are the same as those indicated in the first embodiment are omitted in the drawings of the following embodiments for the sake of clarity. 
     Shown in  FIG. 7  is a table that lists values of some optical data corresponding to the surfaces  31 - 71 ,  32 - 72  of the second embodiment. The imaging lens  10  has an overall system effective focal length of 2.196 mm, an HFOV of 38.832°, an F-number of 2.208, and a system length of 3.112 mm. 
     Shown in  FIG. 8  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the second embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the second embodiment are listed in columns of  FIGS. 34 and 36  corresponding to the second embodiment. 
       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 embodiment. It can be understood from  FIGS. 9( a ) to 9( d )  that the second embodiment is able to achieve a relatively good optical performance. 
     In comparison to the first embodiment, the second embodiment has a shorter system length, a greater HFOV, and better imaging quality. Additionally, manufacture of the second embodiment is relatively easier as compared to the first embodiment, such that yield rate of the second embodiment may be greater than that of the first embodiment. 
       FIG. 10  illustrates the third embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first embodiment. The differences between the first and third embodiments of the imaging lens  10  of this invention reside in that: in the third embodiment, the image-side surface  42  of the second lens element  4  has a concave portion  423  in a vicinity of the optical axis (I), and the object-side surface  61  of the fourth lens element  6  has a convex portion  613  in a vicinity of the periphery of the fourth lens element  6 . In  FIG. 10 , the reference numerals of the concave portions and the convex portions that are the same as those of the first embodiment are omitted for the sake of clarity. 
     Shown in  FIG. 11  is a table that lists values of some optical data corresponding to the surfaces  31 - 71 ,  32 - 72  of the third embodiment. The imaging lens  10  has an overall system effective focal length of 2.294 mm, an HFOV of 37.552°, an F-number of 2.202, and a system length of 3.350 mm. 
     Shown in  FIG. 12  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the third embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the third embodiment are listed in columns of  FIGS. 34 and 36  corresponding to the third embodiment. 
       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 embodiment. It can be understood from  FIGS. 13( a ) to 13( d )  that the third embodiment is able to achieve a relatively good optical performance. 
     In comparison to the first embodiment, the third embodiment has a greater HFOV, and may have a higher yield rate since the third embodiment is relatively easier to fabricate. 
       FIG. 14  illustrates the fourth embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first embodiment. The differences between the first and fourth embodiments reside in modifications of some optical data, aspherical coefficients and the lens parameters of the lens elements  3 - 6 . In  FIG. 14 , the reference numerals of the concave portions and the convex portions that are the same as those of the first embodiment are omitted for the sake of clarity. 
     Shown in  FIG. 15  is a table that lists values of some optical data corresponding to the surfaces  31 - 71 ,  32 - 72  of the fourth embodiment. The imaging lens  10  has an overall system effective focal length of 2.308 mm, an HFOV of 37.465°, an F-number of 2.213, and a system length of 3.299 mm. 
     Shown in  FIG. 16  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the fourth embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the fourth embodiment are listed in columns of  FIGS. 34 and 36  corresponding to the fourth embodiment. 
       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 embodiment. It can be understood from  FIGS. 17( a ) to 17( d )  that the fourth embodiment is able to achieve a relatively good optical performance. 
     In comparison to the first embodiment, the fourth embodiment has a shorter system length, a greater HFOV, better imaging quality, and may have a higher yield rate since the fourth embodiment is relatively easier to fabricate. 
       FIG. 18  illustrates the fifth embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first embodiment. The differences between the first and fifth embodiments of the imaging lens  10  of this invention reside in that: in the fifth embodiment, the image-side surface  42  of the second lens element  4  has a concave portion  423  in a vicinity of the optical axis (I). In  FIG. 18 , the reference numerals of the concave portions and the convex portions that are the same as those of the first embodiment are omitted for the sake of clarity. 
     Shown in  FIG. 19  is a table that lists values of some optical data corresponding to the surfaces  31 - 71 ,  32 - 72  of the fifth embodiment. The imaging lens  10  has an overall system effective focal length of 2.242 mm, an HFOV of 38.252°, an F-number of 2.215, and a system length of 3.202 mm. 
     Shown in  FIG. 20  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the fifth embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the fifth embodiment are listed in columns of  FIGS. 35 and 37  corresponding to the fifth embodiment. 
       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 embodiment. It can be understood from  FIGS. 21( a ) to 21( d )  that the fifth embodiment is able to achieve a relatively good optical performance. 
     In comparison to the first embodiment, the fifth embodiment has a shorter system length, a greater HFOV, better image quality, and may have a higher yield rate since the fifth embodiment is relatively easier to fabricate. 
       FIG. 22  illustrates the sixth embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first embodiment. The differences between the first and sixth embodiments of the imaging lens  10  of this invention reside in that the image-side surface  42  of the second lens element  4  has a concave portion  423  in a vicinity of the optical axis (I). In  FIG. 22 , the reference numerals of the concave portions and the convex portions that are the same as those of the first embodiment are omitted for the sake of clarity. 
     Shown in  FIG. 23  is a table that lists values of some optical data corresponding to the surfaces  31 - 71 ,  32 - 72  of the sixth embodiment. The imaging lens  10  has an overall system effective focal length of 2.307 mm, an HFOV of 37.485°, an F-number of 2.213, and a system length of 3.225 mm. 
     Shown in  FIG. 24  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the sixth embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the sixth embodiment are listed in columns of  FIGS. 35 and 37  corresponding to the sixth embodiment. 
       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 embodiment. It can be understood from  FIGS. 25( a ) to 25( d )  that the sixth embodiment is able to achieve a relatively good optical performance. 
     In comparison to the first embodiment, the sixth embodiment has a shorter system length, a greater HFOV, better imaging quality, and may have a higher yield rate since the sixth embodiment is relatively easier to fabricate. 
       FIG. 26  illustrates the seventh embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first embodiment. The differences between the first and seventh embodiments of the imaging lens  10  of this invention reside in that: in the seventh embodiment, the image-side surface  42  of the second lens element  4  has a concave portion  423  in a vicinity of the optical axis (I), and the object-side surface  61  of the fourth lens element  6  has a convex portion  613  in a vicinity of the periphery of the fourth lens element  6 . In  FIG. 26 , the reference numerals of the concave portions and the convex portions that are the same as those of the first embodiment are omitted for the sake of clarity. 
     Shown in  FIG. 27  is a table that lists values of some optical data corresponding to the surfaces  31 - 71 ,  32 - 72  of the seventh embodiment. The imaging lens  10  has an overall system effective focal length of 2.307 mm, an HFOV of 37.466°, an F-number of 2.221, and a system length of 3.273 mm. 
     Shown in  FIG. 28  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the seventh embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the seventh embodiment are listed in columns of  FIGS. 35 and 37  corresponding to the seventh embodiment. 
       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 embodiment. It can be understood from  FIGS. 29( a ) to 29( d )  that the seventh embodiment is able to achieve a relatively good optical performance. 
     In comparison to the first embodiment, the seventh embodiment has a shorter system length, a greater HFOV, and may have a higher yield rate since the seventh embodiment is relatively easier to fabricate. 
       FIG. 30  illustrates the eighth embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first embodiment. The differences between the first and eighth embodiments of the imaging lens  10  of this invention reside in that: the image-side surface  42  of the second lens element  4  has a concave portion  423  in a vicinity of the optical axis (I), and the image-side surface  52  of the third lens element  5  is a convex surface that has a convex portion  523  in a vicinity of the periphery of the third lens element  5 . The object-side surface  61  of the fourth lens element  6  is a convex surface that has a convex portion  613  in a vicinity of the periphery of the fourth lens element  6 . In  FIG. 30 , the reference numerals of the concave portions and the convex portions that are the same as those of the first embodiment are omitted for the sake of clarity. 
     Shown in  FIG. 31  is a table that lists values of some optical data corresponding to the surfaces  31 - 71 ,  32 - 72  of the eighth embodiment. The imaging lens  10  has an overall system effective focal length of 2.307 mm, an HFOV of 37.488°, an F-number of 2.207, and a system length of 3.186 mm. 
     Shown in  FIG. 32  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the eighth embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the eighth embodiment are listed in columns of  FIGS. 35 and 37  corresponding to the eighth embodiment. 
       FIGS. 33( a ) to 33( d )  respectively show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the eighth embodiment. It can be understood from  FIGS. 33( a ) to 33( d )  that the eighth embodiment is able to achieve a relatively good optical performance. 
     In comparison to the first embodiment, the eighth embodiment has a shorter system length, a greater HFOV, and better imaging quality. Additionally, manufacture of the eighth embodiment is relatively easier as compared to the first embodiment, such that yield rate of the eighth embodiment may be greater than that of the first embodiment. 
     Shown in  FIGS. 34 to 37  are tables each listing the aforesaid relationships among some of the aforementioned lens parameters corresponding to the eighth embodiments for comparison. It should be noted that the values of the lens parameters and the relationships listed in  FIGS. 34 to 37  are rounded off to the third 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: 
     6≦EFL/T2≦11; 1.4≦Gaa/T2≦2.11; 5≦ALT/T2≦7.2; and 4.6≦BFL/T2: The second lens element  4  has a relatively small effective optical diameter, and the reducible ratio of T2 is relatively large. Although reduction in EFL, Gaa and ALT favors reduction of the system length of the imaging lens  10 , ELF/T2, Gaa/T2, and ALT/G2 should be designed to be within proper ranges for ease of manufacture and a relatively simple assembling process. Design of BFL should be sufficient for accommodating the optical filter  7  and other elements. Thus, design of BFL should tend to be large. Preferably, 4.6≦BFL/T2≦7. 
     4≦ALT/G23≦18; 1≦T4/G23; Gaa/G23≦2.8; T3/G23≦1.9; 2.1≦Gaa/(G12+G34); and T1/G23≦0.5: By virtue of configurations of the convex portions  321 ,  322  of the first lens element  3  and the concave portions  411 ,  412  of the second lens element  4 , G12 may be made smaller without causing interference between the first and second lens elements  3 ,  4 . By virtue of configurations of the convex portion  521  of the third lens element  5  and the convex portion  611  of the fourth lens element  6 , and the larger difference in optical effective diameter between the third and fourth lens elements  5 ,  6 , G34 may be made smaller without causing interference between the third and fourth lens elements  5 ,  6 . Since G23 should be designed to be within a proper range to allow light entering the third lens element  5  at an appropriate height for convergence, reducible ratio thereof is relatively small. In order to reduce the system length of the imaging lens  10 , except for design of the fourth lens element  6  that has a relatively large effective optical diameter should tend to be large, designs of the remaining lens element should tend to be thinner. ALT/G23 should be designed to be within a proper range for ease of manufacture. Designs of Gaa/G23, T3/G23, and T1/G23 should tend to be small, whereas designs of Gaa/(G12+G34) and T4/G23 should tend to be large. Preferably, 1≦T4/G23≦3.5; 1≦Gaa/G23≦2.8; 0.5≦T3/G23≦1.9; 2.1≦Gaa/(G12+G34)≦4; and 1≦T1/G23≦2.5. 
     1.9≦T3/(G12+G34); EFL/T3≦6.5; 3.55≦ALT/T3; 2.65≦BFL/T3; and ALT/T4≦4.2: Although the thickness of the third lens element  5  should tend to be small, reducible ratio of T3 is limited by current technology, and is relatively small compared to G12 and G34. Thus design of T3/(G12+G34) should tend to be large. As mentioned above, since design of BFL should be within a proper range, design of BFL/T3 should tend to be large. Since ALT includes T4 which has a relatively small reducible ratio as compared to that of T3, design of ALT/T3 should tend to be large whereas design of ALT/T4 should tend to be small. EFL is related to T3, the air gap lengths among lens elements, the thickness of the lens elements, and the material for manufacturing the lens element. When EFL/T3≦6.5 is satisfied, better imaging quality and yield rate can be obtained. Preferably, 1.9≦T3/(G12+G34)≦3.5; 3≦EFL/T3≦6.5; 3.55≦ALT/T3≦5; 2.65≦BFL/T3≦3.8; and 2.5≦ALT/T4≦4.2. 
     7≦TTL/T3; TTL/T1≦7.15 and TTL/T4≦9.1: A relatively large reducible ratio of TTL represents reduction in total length of the imaging lens  10 . As mentioned above, design of T4 should tend to be large and design of T3 should tend to be small. Since the first lens element  3  has a positive refractive power, if the radius of curvature of each of the surfaces  31 ,  32  and the material of the first lens element  3  are not variables, the thickness of the first lens element  3  should be designed to be within a proper range to achieve a larger positive refractive power for focusing of the imaging lens  10 . As a result, design of TTL/T1 should tend to be small. Preferably, 7≦TTL/T3≦10; 5≦TTL/T1≦7.15; and 4.5≦TTL/T4≦9.1. 
     Although the design of an optical system is generally associated with unpredictability, satisfaction of the aforementioned relationships may enable the imaging lens  10  to have reductions in the system length and the F-number, to have wider field of view, to have enhancement of imaging quality, or to have a relatively higher yield rate of assembly, thereby alleviating at least one drawback of the prior art. 
     To sum up, effects and advantages of the imaging lens  10  according to the present invention are described hereinafter. 
     1. By virtue of cooperation among the convex portions  321 ,  322 , the concave portions  411 ,  412 , the concave portions  511 ,  512 , the convex portion  521 , and the convex portions  611 ,  622 , optical aberration of the imaging may be corrected, thereby improving the image quality of the imaging lens  10 . In addition, configuration of the concave portion  613  favors a better yield rate. 
     2. Configurations of the first lens element  3  having a positive refractive power, the second lens element  4  having a negative refractive power, the third lens element  5  having a positive refractive power and the fourth lens element  6  having a negative refractive power favor aberration correction for the imaging lens  10 . 
     3. Since the fourth lens element  6  is made of a plastic material, it is advantageous for reducing lens weight and fabrication cost, and may be easily made to be aspherical. 
     4. Through design of the relevant lens parameters, 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. 
     5. Through the aforesaid eight embodiments, it is evident that the system length of this invention may be reduced down to below 3.5 mm, so as to facilitate developing thinner relevant products with economic benefits. 
     Shown in  FIG. 38  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  100  (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. 39  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.