Patent Publication Number: US-9432557-B2

Title: Imaging lens, and electronic apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Chinese Application No. 201310746698.7, 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. patent application publication no. 2011/0242683, and U.S. Pat. Nos. 8,270,097 and 8,379,326 discloses a conventional imaging lens that includes four lens elements, first and second lens elements of which has negative refractive power. However, an air gap between the first and second lens elements is relatively large, resulting in a long system length, and disfavoring miniaturization. 
     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, an aperture stop, 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 an object-side surface facing toward the object side, and an image-side surface facing toward the image side. 
     The object-side surface of the first lens element has a convex portion in a vicinity of the optical axis. The second lens element has a positive refractive power. The third lens element has a positive refractive power. 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 satisfies T 3 /G 12 ≧2.8, where T 3  represents a thickness of the third lens element at the optical axis, and G 12  represents an air gap length between the first lens element and the second 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. 
     According to another aspect of the present invention, an imaging lens comprises a first lens element, an aperture stop, 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 an object-side surface facing toward the object side, and an image-side surface facing toward the image side. 
     The object-side surface of the first lens element has a convex portion in a vicinity of the optical axis. The second lens element has a positive refractive power. The third lens element has a positive refractive power. The fourth lens element has a negative refractive power. 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. 
     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 data corresponding to the imaging lens of the first preferred embodiment; 
         FIG. 4  shows values of some aspherical coefficients 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 data corresponding to the imaging lens of the second preferred embodiment; 
         FIG. 8  shows values of some aspherical coefficients 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 data corresponding to the imaging lens of the third preferred embodiment; 
         FIG. 12  shows values of some aspherical coefficients 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 data corresponding to the imaging lens of the fourth preferred embodiment; 
         FIG. 16  shows values of some aspherical coefficients 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 data corresponding to the imaging lens of the fifth preferred embodiment; 
         FIG. 20  shows values of some aspherical coefficients 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 data corresponding to the imaging lens of the sixth preferred embodiment; 
         FIG. 24  shows values of some aspherical coefficients 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 table that lists values of relationships among some lens parameters corresponding to the imaging lenses of the first to sixth preferred embodiments; 
         FIG. 27  is a schematic partly sectional view to illustrate a first exemplary application of the imaging lens of the present invention; and 
         FIG. 28  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  9 . In other embodiments, the optical filter  7  may be a filter that filters out visible light, and that allows passage of infrared light for use in an infrared sensor. 
     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  9 . In this embodiment, 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). 
     Each of the lens elements  3 - 6  is made of a plastic material and has a refractive power in this embodiment. However, at least one of the lens elements  3 - 6  may be made of other materials in other embodiments. 
     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  321  in a vicinity of the optical axis (I), and a concave portion  322  in a vicinity of the 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 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  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 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 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 data corresponding to the surfaces  31 - 71 ,  32 - 72  of the first preferred embodiment. The imaging lens  10  has an overall system effective focal length (EFL) of 2.407 mm, a half field-of-view (HFOV) of 42.92°, an F-number of 2.4, and a system length of 4.551 mm. The system length refers to a distance between the object-side surface  31  of the first lens element  3  and the image plane  9  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: 
     R represents a radius of curvature of an aspherical surface; 
     Z represents a depth of the aspherical surface, which is defined as a perpendicular distance between an 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); 
     γ represents a perpendicular distance between the arbitrary point on the aspherical surface and the optical axis (I); 
     K represents a conic constant; and 
     a i  represents an i 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 preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the first preferred embodiment are as follows: 
     ALT=2.069; Gaa=0.891; BFL=1.591; 
     T 3 /G 12 =2.801; ALT/BFL=1.300; 
     T 4 /G 12 =0.690; T 3 /G 23 =2.001; 
     ALT/G 12 =5.903; BFL/T 1 =3.571; 
     BFL/Gaa=1.786; T 3 /T 1 =2.203; 
     BFL/G 23 =3.244; T 3 /T 2 =2.455; 
     T 3 /Gaa=1.102; BFL/G 12 =4.540; and 
     ALT/Gaa=2.322 
     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); 
     Gaa represents a sum of air gap widths 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); 
     ALT represents a sum of 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); and 
     BFL represents a back focal length of the imaging lens  10 , i.e., a distance at the optical axis (I) between the image-side surface  62  of the fourth lens element  6  and the image plane  9 . 
       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, 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 ±0.05 mm, the first preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths. Furthermore, since the curves of each wave length 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 4.55 mm, the imaging lens  10  of the first preferred embodiment is still able to achieve a relatively good optical performance. 
     Referring to  FIG. 6 , the differences between the first and second preferred embodiments of the imaging lens  10  of this invention reside in that: the object-side surface  41  of the second lens element  4  has a concave portion  413  in a vicinity of the optical axis (I), and a concave portion  414  in a vicinity of a periphery of the second lens element  4 . 
     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 preferred embodiment. The imaging lens  10  has an overall system focal length of 2.180 mm, an HFOV of 45.69°, an F-number of 2.4, and a system length of 4.125 mm. 
     Shown in  FIG. 8  is a table that lists values of some aspherical coefficients 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 as follows: 
     ALT=2.326; Gaa=0.535; BFL=1.264; 
     T 3 /G 12 =6.783; ALT/BFL=1.840; 
     T 4 /G 12 =1.629; T 3 /G 23 =3.626; 
     ALT/G 12 =13.769; BFL/T 1 =2.306; 
     BFL/Gaa=2.364; T 3 /T 1 =2.090; 
     BFL/G 23 =4.001; T 3 /T 2 =3.213; 
     T 3 /Gaa=2.142; BFL/G 12 =7.485; and 
     ALT/Gaa=4.348. 
       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 a third preferred embodiment of an imaging lens  10  according to the present invention, which has a configuration similar to that of the first preferred embodiment. The differences between the first and third preferred 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 . 
     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 preferred embodiment. The imaging lens  10  has an overall system focal length of 2.256 mm, an HFOV of 44.39°, an F-number of 2.4, and a system length of 4.601 mm. 
     Shown in  FIG. 12  is a table that lists values of some aspherical coefficients 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 as follows: 
     ALT=2.693; Gaa=0.721; BFL=1.187; 
     T 3 /G 12 =4.050; ALT/BFL=2.269; 
     T 4 /G 12 =1.034; T 3 /G 23 =5.182; 
     ALT/G 12 =7.252; BFL/T 1 =2.789; 
     BFL/Gaa=1.645; T 3 /T 1 =3.544; 
     BFL/G 23 =4.090; T 3 /T 2 =3.947; 
     T 3 /Gaa=2.084; BFL/G 12 =3.197; 
     and ALT/Gaa=3.733. 
       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. 
     Referring to  FIG. 14 , a fourth preferred embodiment of the imaging lens  10  of this invention is shown. The differences between the first and fourth preferred embodiments reside in modifications of some optical data, aspherical coefficients and lens parameters of the lens elements  3 - 6 . 
     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 preferred embodiment. The imaging lens  10  has an overall system focal length of 1.878 mm, an HFOV of 49.44°, an F-number of 2.4, and a system length of 4.066 mm. 
     Shown in  FIG. 16  is a table that lists values of some aspherical coefficients of the aforementioned relationship (1) corresponding to the fourth preferred embodiment. 
     Relationships among some of the aforementioned lens parameters corresponding to the fourth preferred embodiment are as follows: 
     ALT=2.850; Gaa=0.510; BFL=0.707; 
     T 3 /G 12 =7.806; ALT/BFL=4.032; 
     T 4 /G 12 =4.136; T 3 /G 23 =4.546; 
     ALT/G 12 =16.843; BFL/T 1 =1.578; 
     BFL/Gaa=1.387; T 3 /T 1 =2.949; 
     BFL/G 23 =2.433; T 3 /T 2 =3.465; 
     T 3 /Gaa=2.591; BFL/G 12 =4.177; 
     and ALT/Gaa=5.591. 
       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. 
     Referring to  FIG. 18 , a fifth preferred embodiment of the imaging lens  10  of this invention is shown. The differences between the first and fifth preferred embodiments of the imaging lens  10  reside in modifications of some optical data, aspherical coefficients and lens parameters of the lens elements  3 - 6 . 
     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 preferred embodiment. The imaging lens  10  has an overall system focal length of 1.989 mm, an HFOV of 47.41°, an F-number of 2.4, and a system length of 3.930 mm. 
     Shown in  FIG. 20  is a table that lists values of some aspherical coefficients 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 as follows: 
     ALT=2.193; Gaa=0.599; BFL=1.138; 
     T 3 /G 12 =5.214; ALT/BFL=1.928; 
     T 4 /G 12 =1.644; T 3 /G 23 =3.543; 
     ALT/G 12 =10.141; BFL/T 1 =3.242; 
     BFL/Gaa=1.899; T 3 /T 1 =3.214; 
     BFL/G 23 =3.574; T 3 /T 2 =3.139; 
     T 3 /Gaa=1.882; BFL/G 12 =5.260; 
     and ALT/Gaa=3.661. 
       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 ) 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. The differences between the first and sixth preferred embodiments of the imaging lens  10  of this invention reside in modifications of some optical data, aspherical coefficients and lens parameters of the lens elements  3 - 6 . 
     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 preferred embodiment. The imaging lens  10  has an overall system focal length of 1.850 mm, an HFOV of 49.58°, an F-number of 2.4, and a system length of 3.571 mm. 
     Shown in  FIG. 24  is a table that lists values of some aspherical coefficients 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 as follows: 
     ALT=1.963; Gaa=0.620; BFL=0.987; 
     T 3 /G 12 =3.915; ALT/BFL=1.988; 
     T 4 /G 12 =1.764; T 3 /G 23 =2.218; 
     ALT/G 12 =10.112; BFL/T 1 =2.377; 
     BFL/Gaa=1.591; T 3 /T 1 =1.830; 
     BFL/G 23 =2.882; T 3 /T 2 =1.707; 
     T 3 /Gaa=1.225; BFL/G 12 =5.085; 
     and ALT/Gaa=3.164 
       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. 
     Shown in  FIG. 26  is a table that lists the aforesaid relationships among some of the aforementioned lens parameters corresponding to the six preferred embodiments for comparison. When each of the lens parameters of the imaging lens  10  according to this invention satisfies the following relationships, the optical performance is still relatively good even with the reduced system length: 
     (1) T 3 /G 12 ≧2.8: Although reductions of both T 3  and G 12  may facilitate developing a thinner imaging lens  10 , reduction of T 3  is limited due to manufacturing techniques, so that T 3  should have a suitable value. On the other hand, G 12  does not have such limitations, and design thereof tends to be small. Better arrangement may be achieved when this relationship is satisfied. Preferably, 2.8≦T 3 /G 12 ≦9.0 
     (2) T 3 /G 12 ≧2.0, ALT/G 12 ≧5.0, T 3 /Gaa≧1.05, and ALT/Gaa≧3.0: ALT is limited due to manufacturing techniques like T 3 , and both G 12  and Gaa should be designed to have relatively large reducible ratios due to the same reason as G 12 . Thus, T 3 /G 23 , ALT/G 12 , T 3 /Gaa and ALT/Gaa tend to be large. Better arrangement may be achieved when these relationships are satisfied. Preferably, 2.0≦T 3 /G 23 ≦6.0, 5.0≦ALT/G 12 ≦18.0, 1.05≦T 3 /Gaa≦3.0, and 3.0≦ALT/Gaa≦7.0. 
     (3) BFL/G 23 ≧2.4, BFL/G 12 ≧4.0, and BFL/Gaa≧1.2: Adjustment of BFL is limited due to specifications of peripheral elements of the imaging lens  10 , while G 12 , G 23  and Gaa may be reduced properly for reduction of the system length of the imaging lens  10 . Therefore, designs of BFL/G 12 , BFL/G 23  and BFL/Gaa tend to be large. Better arrangement may be achieved when these relationships are satisfied. Preferably, 2.4≦BFL/G 23 ≦5.0, 4.0≦BFL/G 12 ≦8.0, and 1.2≦BFL/Gaa≦3.0. 
     (4) ALT/BFL≧1.3, BFL/T 1 ≧1.0 (for instance, 1.0≦BFL/T 1 ≦4.0), T 3 /T 1 ≧1.5, and T 3 /T 2 ≧2.4: Ratios among ALT, BFL, T 1 , T 2  and T 3  should be proper to avoid any one of these parameters being too large, resulting in a long system length of the imaging lens  10 , and/or to avoid any one of these parameters being too small, resulting in difficulty in manufacturing the imaging lens  10 . Better arrangement may be achieved when these relationships are satisfied. Preferably, 1.3≦ALT/BFL≦5.0, 2.5≦BFL/T 1 ≦4.0, 1.5≦T 3 /T 1 ≦4.0, and 2.4≦T 3 /T 2 ≦4.5. 
     (5) T 4 /G 12 ≦5.0: Maintaining a proper ratio between T 4  and G 12  may enhance optical performance of the imaging lens  10 . 
     To sum up, effects and advantages of the imaging lens  10  according to the present invention are described hereinafter. 
     Longitudinal spherical aberration, astigmatism aberration, and distortion aberration of the first to sixth preferred embodiments do not exceed the range of +0.05 mm, the range of ±0.1 mm, and the range of +2%, respectively. The off-axis rays corresponding respectively to wavelengths of 470 nm (blue ray), 555 nm (green ray), and 650 nm (red ray) are around the imaging point. It is evident from the deviation range of each of the curves that deviations of the imaging points of the off-axis rays with different height are well controlled so that the imaging lens  10  has good performance in terms of in spherical aberration, astigmatism aberration and distortion aberration at each of the wavelengths. Furthermore, since the curves with different wavelengths that respectively represent red, green, and blue rays are close to each other, the imaging lens  10  has a relatively low chromatic aberration. As a result, by virtue of the abovementioned design of the lens elements, good image quality may be achieved. 
     In addition, through the aforesaid six preferred embodiments, it is known that the system length of this invention may be reduced down to below 5.1 mm while maintaining good optical performance. 
     Shown in  FIG. 27  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  9  (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. 28  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.