Patent Publication Number: US-9413936-B2

Title: Imaging lens having six lens elements, and electronic apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Chinese Application No. 201310385419.9, filed on Aug. 29, 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. 
     Many conventional imaging lenses with four lens elements may have shorter system lengths due to fewer lens elements. Increased demand for high-end products has led to growing requirements for pixels and quality of imaging lenses, and to development of further high-end products, such as imaging lenses with six lens elements. Each of U.S. Pat. Nos. 7,663,814 and 8,040,618 discloses an imaging lens that includes six lens elements, and that has a system length of over 21 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 that includes six lens elements, and that has a shorter overall length while maintaining good optical performance. 
     According to one aspect of the present invention, an imaging lens comprises an aperture stop, a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element and a sixth 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, the fourth lens element, the fifth lens element and the sixth lens element has a refractive power, and has an object-side surface facing toward the object side and an image-side surface facing toward the image side. 
     The refractive power of the first lens element is positive. 
     The image-side surface of the second lens element has a convex portion in a vicinity of the optical axis. 
     The image-side surface of the fourth lens element has a convex portion in a vicinity of the optical axis. 
     The object-side surface of the fifth lens element has a concave portion in a vicinity of a periphery of the fifth lens element. 
     The sixth lens element is made of a plastic material, and the image-side surface thereof has a concave portion in a vicinity of 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, the fourth lens element, the fifth lens element and the sixth lens element. 
     Another object of the present invention is to provide an electronic apparatus having an imaging lens with six 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  100  according to the present invention includes an aperture stop  2 , first, second, third, fourth, fifth and sixth lens elements  3 - 8 , and an optical filter  9  arranged in the given order along an optical axis (I) from an object side to an image side. The optical filter  9  is an infrared cut filter for selectively absorbing infrared light to thereby reduce imperfection of images formed at an image plane  10 . 
     Each of the first, second, third, fourth, fifth and sixth lens elements  3 - 8  and the optical filter  9  has an object-side surface  31 ,  41 ,  51 ,  61 ,  71 ,  81 ,  91  facing toward the object side, and an image-side surface  32 ,  42 ,  52 ,  62 ,  72 ,  82 ,  92  facing toward the image side. Light entering the imaging lens  100  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 , the object-side and image-side surfaces  71 ,  72  of the fifth lens element  7 , the object-side and image-side surfaces  81 ,  82  of the sixth lens element  8 , and the object-side and image-side surfaces  91 ,  92  of the optical filter  9 , in the given order, to form an image on the image plane  10 . Each of the object-side surfaces  31 ,  41 ,  51 ,  61 ,  71 ,  81  and the image-side surfaces  32 ,  42 ,  52 ,  62 ,  72 ,  82  is aspherical and has a center point coinciding with the optical axis (I). 
     The lens elements  3 - 8  are made of a plastic material in this embodiment, and at least one of the lens elements  3 - 7  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 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  has a convex 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  has a concave 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 negative refractive power. The object-side surface  51  of the third lens element  5  has a concave portion  511  in a vicinity of the optical axis (I), and a convex 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 concave surface that has a concave portion  523  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 positive refractive power. The object-side surface  61  of the fourth lens element  6  is a concave surface that has a concave 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  is a convex surface that has a convex 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 . 
     The fifth lens element  7  has a positive refractive power. The object-side surface  71  of the fifth lens element  7  is a concave surface that has a concave portion  712  in a vicinity of the optical axis (I) and a concave portion  711  in a vicinity of a periphery of the fifth lens element  7 . The image-side surface  72  of the fifth lens element  7  is a convex surface that has a convex portion  726  in a vicinity of the optical axis (I) and a convex portion  727  in a vicinity of the periphery of the fifth lens element  7 . 
     The sixth lens element  8  has a negative refractive power. The object-side surface  81  of the sixth lens element  8  is a concave surface that has a concave portion  811  in a vicinity of the optical axis (I) and a concave portion  813  in a vicinity of a periphery of the sixth lens element  8 . The image-side surface  82  of the sixth lens element  8  has a concave portion  821  in a vicinity of the optical axis (I), and a convex portion  822  in a vicinity of the periphery of the sixth lens element  8 . 
     Shown in  FIG. 3  is a table that lists values of some optical parameters corresponding to the surfaces  31 - 91 ,  32 - 92  of the first preferred embodiment. The imaging lens  100  has an overall system effective focal length (EFL) of 3.37 mm, a half field-of-view (HFOV) of 34.1°, an F-number of 2.40, and a system length of 4.23 mm. The system length refers to a distance between the object-side surface  31  of the first lens element  3  and the image plane  10 . 
     In this embodiment, each of the object-side surfaces  31 - 81  and the image-side surfaces  32 - 82  is aspherical, and satisfies the optical relationship of 
     
       
         
           
             
               
                 
                   
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     where: 
     R represents a radius of curvature of the aspherical surface; 
     Z represents a depth of an 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); 
     Y represents a perpendicular distance between the arbitrary point on the aspherical surface and the optical axis (I); 
     K represents a conic constant; and 
     a 21  represents a 2i th  aspherical coefficient. 
     Shown in  FIG. 4  is a table that lists values of some optical parameters such as the conic constant and the aspherical coefficients of the aforementioned optical relationship (1) corresponding to the first preferred embodiment. 
     Relationships among some of the aforementioned optical parameters corresponding to the first preferred embodiment are as follows: 
     ALT/T2=4.44 
     ALT/AGmax=4.83 
     ALT/T4=4.68 
     EFL/T1=8.02 
     T5/T2=0.61 
     EFL/T2=6.88 
     EFL/T5=11.23 
     EFL/AGmax=7.49 
     EFL/T4=7.25 
     Gaa/AGmax=1.58 
     CTmax/T4=1.05 
     where: 
     T1 represents a distance between the object-side surface  31  and the image-side surface  32  of the first lens element  4  at the optical axis (I); 
     T2 represents a distance between the object-side surface  41  and the image-side surface  42  of the second lens element  4  at the optical axis (I); 
     T4 represents a distance between the object-side surface  61  and the image-side surface  62  of the fourth lens element  6  at the optical axis (I); 
     T5 represents a distance between the object-side surface  71  and the image-side surface  72  of the fifth lens element  7  at the optical axis (I); 
     Gaa represents a sum of a distance between the image-side surface  32  of the first lens element  3  and the object-side surface  41  of the second lens element  4  at the optical axis (I), a distance between the image-side surface  42  of the second lens element  4  and the object-side surface  51  of the third lens element  5  at the optical axis (I), a distance between the image-side surface  52  of the third lens element  5  and the object-side surface  61  of the fourth lens element  6  at the optical axis (I), a distance between the image-side surface  62  of the fourth lens element  6  and the object-side surface  71  of the fifth lens element  7  at the optical axis (I), and a distance between the image-side surface  72  of the fifth lens element  7  and the object-side surface  81  of the sixth lens element  8  at the optical axis (I); 
     ALT represents a sum of the distance between the object-side surface  31  and the image-side surface  32  of the first lens element  3  at the optical axis (I), the distance between the object-side surface  41  and the image-side surface  42  of the second lens element  4  at the optical axis (I), a distance between the object-side surface  51  and the image-side surface  52  of the third lens element  5  at the optical axis (I), the distance between the object-side surface  61  and the image-side surface  62  of the fourth lens element  6  at the optical axis (I), the distance between the object-side surface  71  and the image-side surface  72  of the fifth lens element  7  at the optical axis (I), and a distance between the object-side surface  81  and the image-side surface  82  of the sixth lens element  8  at the optical axis (I); 
     AGmax represents a maximum among the distance between the image-side surface  32  of the first lens element  3  and the object-side surface  41  of the second lens element  4  at the optical axis (I), the distance between the image-side surface  42  of the second lens element  4  and the object-side surface  51  of the third lens element  5  at the optical axis (I), the distance between the image-side surface  52  of the third lens element  5  and the object-side surface  61  of the fourth lens element  6  at the optical axis (I), the distance between the image-side surface  62  of the fourth lens element  6  and the object-side surface  71  of the fifth lens element  7  at the optical axis (I), and the distance between the image-side surface  72  of the fifth lens element  7  and the object-side surface  81  of the sixth lens element  8  at the optical axis (I); 
     CTmax represents a maximum among the distance between the object-ide surface  31  and the image-side surface  32  of the first lens element  3  at the optical axis (I), the distance between the object-side surface  41  and the image-side surface  42  of the second lens element  4  at the optical axis (I), the distance between the object-side surface  51  and the image-side surface  52  of the third lens element  5  at the optical axis (I), the distance between the object-side surface  61  and the image-side surface  62  of the fourth lens element  6  at the optical axis (I), the distance between the object-side surface  71  and the image-side surface  72  of the fifth lens element  7  at the optical axis (I), and the distance between the object-side surface  81  and the image-side surface  82  of the sixth lens element  8  at the optical axis (I); and 
     EFL represents a system focal length of the imaging lens  100 . 
       FIGS. 5( a ) to 5( d )  show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the first preferred embodiment, respectively. 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 ±0.025 mm, the first preferred 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 field of view does not exceed the range of ±0.015 mm, 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 ±1.6%, 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 4.23 mm, the imaging lens  100  of the first preferred embodiment is still able to achieve a relatively good optical performance. 
       FIG. 6  illustrates the second preferred embodiment of an imaging lens  100  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 - 91 ,  32 - 92  of the second preferred embodiment. The imaging lens  100  has an overall system focal length of 3.36 mm, an HFOV of 34.3°, an F-number of 2.40, and a system length of 4.44 mm. 
     Shown in  FIG. 8  is a table that lists values of some optical parameters of the aforementioned optical relationship (1) corresponding to the second preferred embodiment. 
     Relationships among some of the aforementioned optical parameters corresponding to the second preferred embodiment are as follows: 
     ALT/T2=9.04 
     ALT/AGmax=4.98 
     ALT/T4=2.57 
     EFL/T1=8.00 
     T5/T2=1.11 
     EFL/T2=12.44 
     EFL/T5=11.20 
     EFL/AGmax=6.86 
     EFL/T4=3.54 
     Gaa/AGmax=1.53 
     CTmax/T4=1.00 
       FIGS. 9( a ) to 9( d )  show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the second preferred embodiment, respectively. It can be understood from  FIG. 9( a )  that, since the curves corresponding to longitudinal spherical aberration are close to each other, the second preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths, and has a relatively low chromatic aberration. Referring to  FIGS. 9( b ), 9( c ) and 9( d ) , since the curves corresponding to astigmatism aberrations fall within the range of ±0.09 mm of focal length, and the curves corresponding to distortion aberration fall within the range of ±1.8%, the second preferred embodiment is able to achieve a relatively good optical performance even with the system length reduced down to 4.44 mm. 
     Referring to  FIG. 10 , the differences between the first and third preferred embodiments of the imaging lens  100  of this invention reside in that: the image-side surface  32  of the first lens element  3  has a concave portion  323  in a vicinity of the optical axis (I), and a convex portion  324  in a vicinity of a periphery of the first lens element  3 ; the second lens element  4  has a negative refractive power, and the object-side surface  41  thereof is a concave surface; the third lens element  5  has a positive refractive power, the object-side surface  51  thereof is a concave surface, and the image-side surface  52  thereof is a convex surface; the fifth lens element  7  has a negative refractive power; and the object-side surface  81  of the sixth lens element  8  has a concave portion  811  in a vicinity of the optical axis (I), and a convex portion  812  in a vicinity of a periphery of the sixth lens element  8 . 
     Shown in  FIG. 11  is a table that lists values of some optical parameters corresponding to the surfaces  31 - 91 ,  32 - 92  of the third preferred embodiment. The imaging lens  100  has an overall system focal length of 4.17 mm, an HFOV of 35.6°, an F-number of 2.20, and a system length of 5.28 mm. 
     Shown in  FIG. 12  is a table that lists values of some optical parameters of the aforementioned optical relationship (1) corresponding to the third preferred embodiment. 
     Relationships among some of the aforementioned optical parameters corresponding to the third preferred embodiment are as follows: 
     ALT/T2=9.50 
     ALT/AGmax=5.00 
     ALT/T4=5.50 
     EFL/T1=4.92 
     T5/T2=1.55 
     EFL/T2=13.48 
     EFL/T5=8.70 
     EFL/AGmax=7.09 
     EFL/T4=7.81 
     Gaa/AGmax=2.16 
     CTmax/T4=1.59 
       FIGS. 13( a ) to 13( d )  show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the third preferred embodiment, respectively. It can be understood from  FIG. 13 ( a )  that, since the curves corresponding to longitudinal spherical aberration are close to each other, the third preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths, and has a relatively low chromatic aberration. Referring to  FIGS. 13( b ), 13( c ) and 13( d ) , since the curves corresponding to astigmatism aberrations fall within the range of ±0.1 mm of focal length, and the curves corresponding to distortion aberration fall within the range of ±1.8%, the third preferred embodiment is able to achieve a relatively good optical performance even with the system length reduced down to 5.28 mm. 
     Referring to  FIG. 14 , the differences between the third and fourth preferred embodiments of the imaging lens  100  of this invention reside in that: the image-side surface  72  of the fifth lens element  7  has a concave portion  721  in a vicinity of the optical axis (I), and a convex portion  722  in a vicinity of a periphery of the fifth lens element  7 . 
     Shown in  FIG. 15  is a table that lists values of some optical parameters corresponding to the surfaces  31 - 91 ,  32 - 92  of the fourth preferred embodiment. The imaging lens  100  has an overall system focal length of 4.04 mm, an HFOV of 36.8°, an F-number of 2.20, and a system length of 5.39 mm. 
     Shown in  FIG. 16  is a table that lists values of some optical parameters of the aforementioned optical relationship (1) corresponding to the fourth preferred embodiment. 
     Relationships among some of the aforementioned optical parameters corresponding to the fourth preferred embodiment are as follows: 
     ALT/T2=4.14 
     ALT/AGmax=4.77 
     ALT/T4=3.83 
     EFL/T1=6.93 
     T5/T2=0.36 
     EFL/T2=5.53 
     EFL/T5=15.50 
     EFL/AGmax=6.38 
     EFL/T4=5.11 
     Gaa/AGmax=2.07 
     CTmax/T4=1.00 
       FIGS. 17( a ) to 17( d )  show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the fourth preferred embodiment, respectively. It can be understood from  FIG. 17 ( a )  that, since the curves corresponding to longitudinal spherical aberration are close to each other, the fourth preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths, and has a relatively low chromatic aberration. Referring to  FIGS. 17( b ), 17( c ) and 17( d ) , since the curves corresponding to astigmatism aberrations fall within the range of ±0.1 mm of focal length, and the curves corresponding to distortion aberration fall within the range of ±1.8%, the fourth preferred embodiment is able to achieve a relatively good optical performance even with the system length reduced down to 5.39 mm. 
     Referring to  FIG. 18 , the differences between the third and fifth preferred embodiments of the imaging lens  100  of this invention reside in that: the image-side surface  72  of the fifth lens element  7  has a concave portion  721  in a vicinity of the optical axis (I), and a convex portion  722  in a vicinity of a periphery of the fifth lens element  7 . 
     Shown in  FIG. 19  is a table that lists values of some optical parameters corresponding to the surfaces  31 - 91 ,  32 - 92  of the fifth preferred embodiment. The imaging lens  100  has an overall system focal length of 4.09 mm, an HFOV of 36.4°, an F-number of 2.20, and a system length of 5.39 mm. 
     Shown in  FIG. 20  is a table that lists values of some optical parameters of the aforementioned optical relationship (1) corresponding to the fifth preferred embodiment. 
     Relationships among some of the aforementioned optical parameters corresponding to the fifth preferred embodiment are as follows: 
     ALT/T2=4.12 
     ALT/AGmax=4.80 
     ALT/T4=3.75 
     EFL/T1=6.82 
     T5/T2=0.33 
     EFL/T2=5.64 
     EFL/T5=17.10 
     EFL/AGmax=6.57 
     EFL/T4=5.14 
     Gaa/AGmax=2.10 
     CTmax/T4=1.00 
       FIGS. 21( a ) to 21( d )  show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the fifth preferred embodiment, respectively. It can be understood from  FIG. 21( a )  that, since the curves corresponding to longitudinal spherical aberration are close to each other, the fifth preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths, and has a relatively low chromatic aberration. Referring to  FIGS. 21( b ), 21( c ) and 21( d ) , since the curves corresponding to astigmatism aberrations fall within the range of ±0.2 mm of focal length, and the curves corresponding to distortion aberration fall within the range of ±1.8%, the fifth preferred embodiment is able to achieve a relatively good optical performance even with the system length reduced down to 5.39 mm. 
     Referring to  FIG. 22 , the differences between the third and sixth preferred embodiments of the imaging lens  100  of this invention reside in that: 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 a periphery of the third lens element  5 ; and the object-side surface  81  of the sixth lens element  8  is a concave surface. 
     Shown in  FIG. 23  is a table that lists values of some optical parameters corresponding to the surfaces  31 - 91 ,  32 - 92  of the sixth preferred embodiment. The imaging lens  100  has an overall system focal length of 4.18 mm, an HFOV of 35.5°, an F-number of 2.20, and a system length of 5.24 mm. 
     Shown in  FIG. 24  is a table that lists values of some optical parameters of the aforementioned optical relationship (1) corresponding to the sixth preferred embodiment. 
     Relationships among some of the aforementioned optical parameters corresponding to the sixth preferred embodiment are as follows: 
     ALT/T2=8.39 
     ALT/AGmax=3.40 
     ALT/T4=4.21 
     EFL/T1=5.27 
     T5/T2=0.78 
     EFL/T2=13.18 
     EFL/T5=16.80 
     EFL/AGmax=5.34 
     EFL/T4=6.61 
     Gaa/AGmax=1.76 
     CTmax/T4=1.25 
       FIGS. 25( a ) to 25( d )  show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the sixth preferred embodiment, respectively. It can be understood from  FIG. 25 ( a )  that, since the curves corresponding to longitudinal spherical aberration are close to each other, the sixth preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths, and has a relatively low chromatic aberration. Referring to  FIGS. 25( b ), 25( c ) and 25( d ) , since the curves corresponding to astigmatism aberrations fall within the range of ±0.09 mm of focal length, and the curves corresponding to distortion aberration fall within the range of ±1.2%, the sixth preferred embodiment is able to achieve a relatively good optical performance even with the system length reduced down to 5.24 mm. 
     Referring to  FIG. 26 , the differences between the third and seventh preferred embodiments of the imaging lens  100  of this invention reside in that: the image-side surface  72  of the fifth lens element  7  has a first convex portion  723  in a vicinity of the optical axis (I), a second convex portion  724  in a vicinity of a periphery of the fifth lens element  7 , and a concave portion  725  between the first and second convex portions  723 ,  724 . 
     Shown in  FIG. 27  is a table that lists values of some optical parameters corresponding to the surfaces  31 - 91 ,  32 - 92  of the seventh preferred embodiment. The imaging lens  100  has an overall system focal length of 3.94 mm, an HFOV of 37.3°, an F-number of 2.20, and a system length of 5.35 mm. 
     Shown in  FIG. 28  is a table that lists values of some optical parameters of the aforementioned optical relationship (1) corresponding to the seventh preferred embodiment. 
     Relationships among some of the aforementioned optical parameters corresponding to the seventh preferred embodiment are as follows: 
     ALT/T2=8.87 
     ALT/AGmax=5.00 
     ALT/T4=5.45 
     EFL/T1=5.62 
     T5/T2=1.59 
     EFL/T2=11.41 
     EFL/T5=7.17 
     EFL/AGmax=6.42 
     EFL/T4=7.01 
     Gaa/AGmax=2.11 
     CTmax/T4=1.25 
       FIGS. 29( a ) to 29( d )  show simulation results corresponding to longitudinal spherical aberration, sagittal astigmatism aberration, tangential astigmatism aberration, and distortion aberration of the seventh preferred embodiment, respectively. It can be understood from  FIG. 29( a )  that, since the curves corresponding to longitudinal spherical aberration are close to each other, the seventh preferred embodiment is able to achieve a relatively low spherical aberration at each of the wavelengths, and has a relatively low chromatic aberration. Referring to  FIGS. 29( b ), 29( c ) and 29( d ) , since the curves corresponding to astigmatism aberrations fall within the range of ±0.16 mm of focal length, and the curves corresponding to distortion aberration fall within the range of ±1.8%, the seventh preferred embodiment is able to achieve a relatively good optical performance even with the system length reduced down to 5.35 mm. 
     Shown in  FIG. 30  is a table that lists the aforesaid relationships among some of the aforementioned optical parameters corresponding to the seven preferred embodiments for comparison. When each of the optical 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:
 
 ALT/T 2≦9.5  (2)
 
 ALT/AG max≦5.0  (3)
 
 ALT/T 4≦5.5  (4)
 
 EFL/T 1≦8.5  (5)
 
 T 5/ T 2≦1.6  (6)
 
 EFL/T 2≦13.5  (7)
 
 EFL/AG max≦7.5  (8)
 
7.0≦ EFL/T 5  (9)
 
 EFL/T 4≦8.0  (10)
 
 Gaa/AG max≦2.2  (11)
 
 CT max/ T 4≦1.6  (12)
 
     Reduction of the system length of the imaging lens  100  may result in reduced ALT and T2. Since ALT is the sum of thicknesses of the lens elements  3 - 8 , reducible ratio of ALT is greater than that of T2. Better arrangement may be achieved when the relationship (2) is satisfied. Preferably, the relationship (2) may have a lower limit: 3.7≦ALT/T2≦9.5, so that ALT falls within a suitable range. 
     Reduction of the system length of the imaging lens  100  may result in reduced ALT. Considering optical properties (e.g., light incident location/angle, etc.) and manufacturing capability (such as assembly), reduction of AGmax is limited. Therefore, reducible ratio of ALT is greater than that of AGmax according to the present invention. Better arrangement may be achieved when the relationship (3) is satisfied. Preferably, the relationship (3) may have a lower limit: 3.0≦ALT/AGmax≦5.0, so that ALT falls within a suitable range. 
     Reduction of the system length of the imaging lens  100  may result in reduced ALT and T4. Since ALT is the sum of thicknesses of the lens elements  3 - 8  and the fourth lens element  6  has a greater effective optical radius and may be made thicker to facilitate fabrication, reducible ratio of T4 is smaller than that of ALT. Better arrangement may be achieved when the relationship (4) is satisfied. Preferably, the relationship (4) may have a lower limit: 2.0≦ALT/T4≦5.5, so that T4 falls within a suitable range. 
     Reduction of the system length of the imaging lens  100  may result in reduced EFL. Since the first lens element  3  has the positive refractive power, it may be made thicker. Better arrangement may be achieved when the relationship (5) is satisfied. Preferably, the relationship (5) may have a lower limit: 4.5≦EFL/T1≦8.5, so that T1 falls within a suitable range. 
     Due to the greater effective optical radius of the fifth lens element  7 , thicker T5 facilitates manufacturing. Since the image-side surface  42  of the second lens element  4  has the convex portion  421  in the vicinity of the optical axis (I), reduction of T2 has a relatively serious limitation. Accordingly, reducible ratio of T2 is smaller than that of T5. Better arrangement may be achieved when the relationship (6) is satisfied. Preferably, the relationship (6) may have a lower limit: 0.1≦T5/T2≦1.6, so that T5 falls within a suitable range. 
     Reduction of the system length of the imaging lens  100  may result in reduced EFL. Since the image-side surface  42  of the second lens element  4  has the convex portion  421  in the vicinity of the optical axis (I), reduction of T2 has a relatively serious limitation. Accordingly, reducible ratio of T2 is smaller than that of EFL. Better arrangement may be achieved when the relationship (7) is satisfied. In addition, when EFL/T2≦11.5, thicker T2 facilitates manufacturing. Preferably, the relationship (7) may have a lower limit: 5.0≦EFL/T2≦13.5, so that T2 falls within a suitable range. 
     Reduction of the system length of the imaging lens  100  may result in reduced EFL. Considering optical properties (e.g., light incident location/angle, etc.) and manufacturing capability (such as assembly), reduction of AGmax is limited. Therefore, reducible ratio of EFL is greater than that of AGmax according to the present invention. Better arrangement may be achieved when the relationship (8) is satisfied. Preferably, the relationship (8) may have a lower limit: 5.0≦EFL/AGmax≦7.5, so that AGmax falls within a suitable range. 
     Reduction of the system length of the imaging lens  100  may result in reduced EFL. In this invention, the fifth lens element  7  may be designed to be thinner, so that the reducible ratio of T5 is greater than that of EFL. Better arrangement may be achieved when the relationship (9) is satisfied. Preferably, the relationship (9) may have an upper limit: 7.0≦EFL/T5≦18.0, so that T5 falls within a suitable range. 
     Reduction of the system length of the imaging lens  100  may result in reduced EFL. Since the fourth lens element  6  has a greater effective optical radius and may be made thicker to facilitate fabrication, reducible ratio of T4 is smaller than that of EFL. Better arrangement may be achieved when the relationship (10) is satisfied. Preferably, the relationship (10) may have a lower limit: 3.0≦EFL/T4≦8.0, so that T4 falls within a suitable range. 
     Reduction of Gaa favors reduction of the system length of the imaging lens  100 . Since reducible ratio of AGmax is smaller than that of Gaa in the present invention, better arrangement may be achieved when the relationship (11) is satisfied. Preferably, the relationship (11) may have a lower limit: 1.0≦Gaa/AGmax≦2.2, so that Gaa falls within a suitable range. 
     Since reduction of the system length of the imaging lens  100  may result in reduced thicknesses of most of the lens elements  3 - 8 , CTmax may be reduced. Since the fourth lens element  6  has a greater effective optical radius and may be made thicker to facilitate fabrication, reducible ratio of T4 is smaller than that of CTmax. Better arrangement may be achieved when the relationship (12) is satisfied. Preferably, the relationship (12) may have a lower limit: 0.8≦CTmax/T4≦1.6, so that T4 falls within a suitable range. 
     To sum up, effects and advantages of the imaging lens  100  according to the present invention are described hereinafter. 
     1. The positive refractive power of the first lens element  3  may enhance converging capability of the imaging lens  10 . By arranging the aperture stop  2  on the object-side of the first lens element  3 , the system length may be further reduced. 
     2. By virtue of the convex portions  421 ,  621  and the concave portions  711 ,  821 , optical aberration of the image may be corrected. Cooperation with the concave portion  811  or the negative refractive power of the sixth lens element  8  may further enhance correction effect on optical aberration of the image. 
     3. Since the sixth lens element  8  is made of a plastic material, weight and cost of the imaging lens  100  may be reduced. 
     4. Through design of the relevant optical parameters, such as ALT/T2, ALT/AGmax, ALT/T4, EFL/T1, T5/T2, EFL/T2, EFL/AGmax, EFL/T5, EFL/T4, Gaa/AGmax and CTmax/T4, optical aberrations, such as spherical aberration, may be reduced or even eliminated. Further, through surface design and arrangement of the lens elements  3 - 8 , 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 seven preferred embodiments, it is known that the system length of this invention may be reduced down to below 5.5 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  100 , in which the imaging lens  100  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  100  is disposed, a holder unit  120  on which the barrel  21  is disposed, and an image sensor  130  disposed at the image plane  10  (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  100 . 
     Shown in  FIG. 32  is a second exemplary application of the imaging lens  100 . 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  100  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  100 . The optical filter  9  of the imaging lens  100  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  100  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.