Abstract:
Provided is a fast image reading lens system capable of achieving compactness and satisfactorily correcting various aberrations. The image reading lens system includes, in order from an object side: a positive first lens of which a convex surface faces toward the object side; a second lens formed as a biconcave lens; a third lens formed as a biconvex lens; a positive fourth lens of which a convex surface faces toward an image side; and a negative fifth lens of which a convex surface faces toward the image side. Assuming that a focal length of the whole lens system is f, a focal length of the fourth lens is f 4 , and a space between an image side surface of the fourth lens and an object side surface of the fifth lens on an optical axis is D 8 , the lens system satisfies the following conditional expressions: 0.06&lt;D 8 /f&lt;0.16, and 0.77&lt;f 4 /f&lt;1.81.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the Japanese Patent Application No. 2009-151148 filed on Jun. 25, 2009; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image reading apparatus such as an image scanner and an image reading lens system mounted on the image reading apparatus. 
     2. Description of the Related Art 
     Generally, image reading apparatuses such as copiers, facsimiles, and image scanners for reading an original image such as a document or an image and converting the image into digital image data have come into widespread use. In such image reading apparatuses, by using an imaging lens system (hereinafter referred to as an image reading lens system), the original image is formed on, for example, a CCD image sensor. In order to acquire image data with high fidelity to the original image, in the image reading lens system, various aberrations are suppressed with good balance, and thus a high resolving power is demanded. Furthermore, high aperture efficiency is demanded even in the corner of the screen. 
     Further, recently, in accordance with reduction in the size and cost of the image reading apparatuses, the image reading lens system has been further demanded to be reduced in size and cost. Further, in accordance with an increase in the image reading speed of the image reading apparatus, a faster (smaller F number) image reading lens system is demanded. 
     As the compact image reading lens system, there has been known a telephoto type lens system having four groups and four elements (for example, refer to JP-A-9-101452). Further, as the faster image reading lens system, there has been known a Gauss type lens system having four groups and six elements (for example, refer to JP-A-9-113802). Furthermore, as disclosed in JP-A-2000-241701, there has been an image reading lens system capable of achieving both of an increase in speed and reduction in size by adopting a configuration having five groups and five elements. 
     SUMMARY OF THE INVENTION 
     However, the image reading lens system which has four groups and four elements in JP-A-9-101452 is not able to correct chromatic aberrations sufficiently, and is inappropriate for reading of a color original image. Further, the image reading lens system having this configuration has a problem in that it is difficult to obtain a sufficient resolving power when the lens system is formed as a faster lens system. Further, the image reading lens system which has six groups and four elements in JP-A-9-113802 is formed as a fast lens system, in which it is difficult to make the diameter of the lens small, and thus the system is disadvantageous for compactness. For this reason, configurations for reducing the height of the lens by cutting the upper and lower ends of the lens respectively have also been contrived. However, there is still a problem in that it is difficult to rotate the lens in order to adjust the eccentricity thereof. In addition, the configuration having five groups and five elements in JP-A-2000-241701 has a problem in that correction of coma aberration is insufficient. 
     The invention has been made in view of the above-mentioned problems, and its object is to provide a low-cost, compact, and fast image reading lens system capable of maintaining high aperture efficiency up to the corner of a screen, correcting various aberrations with excellent balance, and having a high resolving power, and to provide an image reading apparatus having the image reading lens system. 
     In order to achieve the object, according to an aspect of the invention, an image reading lens system includes, in order from an object side: a positive first lens of which a convex surface faces toward the object side; a second lens formed as a biconcave lens; a third lens formed as a biconvex lens; a positive fourth lens of which a convex surface faces toward an image side; and a negative fifth lens of which a convex surface faces toward the image side. Assuming that a focal length of the whole lens system is f, a focal length of the fourth lens is f 4 , and a space between an image side surface of the fourth lens and an object side surface of the fifth lens on an optical axis is D 8 , the lens system satisfies the following conditional expressions (1) and (2):
 
0.06&lt; D 8/ f&lt; 0.16  (1), and
 
0.77&lt; f 4/ f&lt; 1.81  (2).
 
     The positive fourth lens of which a convex surface faces toward the image side may be a biconvex lens of which a convex surface faces toward the object side, or may be a lens of which the concave surface or the flat surface faces toward the object side. When the fourth lens is formed as a biconvex lens, it is more preferable to satisfy the following conditional expressions: “0.06&lt;D 8 /f&lt;0.14”, and “0.77&lt;f 4 /f&lt;1.59”. When the fourth lens is formed as a lens of a concave or flat surface and faces toward the object side, it is further more preferable to satisfy the following conditional expressions: “0.10&lt;D 8 /f&lt;0.16”, and “1.06&lt;f 4 /f&lt;1.81”. 
     Further, assuming that a refractive index of a j-th lens at the e-line is Nj, it is preferable that the image reading lens system according to the aspect should satisfy the following conditional expression (3):
 
1.55&lt;( N 1+ N 3+ N 4)/3&lt;1.74  (3).
 
     In other words, it is preferable that the average of refractive indices of the first, third, and fourth lenses which are positive lenses, that is, “(N 1 +N 3 +N 4 )/3” should satisfy the condition of the expression (3). 
     Furthermore, assuming that an Abbe number of the j-th lens at the d-line is νj, the image reading lens system according to the aspect should satisfy the following conditional expression (4):
 
13.5&lt;(ν1+ν3+ν4)/3−(ν2+ν5)/2  (4).
 
     Specifically, it is preferable that the difference between the average of the Abbe numbers of the positive lenses and the average of the Abbe numbers of the negative lenses, that is, “(ν 1 +ν 3 +ν 4 )/3−(ν 2 +ν 5 )/2” should satisfy the condition of the expression (4). 
     Any one of the conditional expressions (3) and (4) may be satisfied, but it is more preferable that both of them should be satisfied. 
     According to another aspect of the invention, an image reading apparatus includes the above-mentioned image reading lens system. 
     According to the aspects of the invention, the image reading lens system is able to maintain high aperture efficiency up to the corner of screen, sufficiently correct various aberrations with excellent balance, and thereby obtain a high resolving power while having a low-cost and compact configuration. Further, by using the image reading lens system, it is possible to embody an image reading apparatus capable of increasing the reading speed thereof without lowering the optical performance thereof while achieving compactness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view illustrating a configuration of an image reading lens system according to Example 1; 
         FIGS. 2A to 2D  are aberration diagrams illustrating spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the image reading lens system according to Example 1; 
         FIGS. 3A to 3D  are aberration diagrams illustrating lateral aberrations of the image reading lens system according to Example 1; 
         FIG. 4  is a sectional view illustrating a configuration of an image reading lens system according to Example 2; 
         FIGS. 5A to 5D  are aberration diagrams illustrating spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the image reading lens system according to Example 2; 
         FIGS. 6A to 6D  are aberration diagrams illustrating lateral aberrations of the image reading lens system according to Example 2; 
         FIG. 7  is a sectional view illustrating a configuration of an image reading lens system according to Example 3; 
         FIGS. 8A to 8D  are aberration diagrams illustrating spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the image reading lens system according to Example 3; 
         FIGS. 9A to 9D  are aberration diagrams illustrating lateral aberrations of the image reading lens system according to Example 3; 
         FIG. 10  is a sectional view illustrating a configuration of an image reading lens system according to Example 4; 
         FIGS. 11A to 11D  are aberration diagrams illustrating spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the image reading lens system according to Example 4; 
         FIGS. 12A to 12D  are aberration diagrams illustrating lateral aberrations of the image reading lens system according to Example 4; 
         FIG. 13  is a sectional view illustrating a configuration of an image reading lens system according to Example 5; 
         FIGS. 14A to 14D  are aberration diagrams illustrating spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the image reading lens system according to Example 5; 
         FIGS. 15A to 15D  are aberration diagrams illustrating lateral aberrations of the image reading lens system according to Example 5; 
         FIG. 16  is a sectional view illustrating a configuration of an image reading lens system according to Example 6; 
         FIGS. 17A to 17D  are aberration diagrams illustrating spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the image reading lens system according to Example 6; and 
         FIGS. 18A to 18D  are aberration diagrams illustrating lateral aberrations of the image reading lens system according to Example 6. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows an image reading lens system  11  used in an image reading apparatuses. The image reading lens system  11  forms images of original documents, which are placed at a predetermined interval so as to be close to the object side, on image sensors which are arranged at a predetermined interval so as to be close to the image side. 
     The image reading lens system  11  has a five-group five-element configuration in which first lens L 1  to fifth lens L 5  are arranged in order from the object side. The first lens L 1  is a lens which has a positive refractive power and of which a convex surface faces toward the object side. The second lens L 2  is formed as a biconcave lens, and the third lens L 3  is formed as a biconvex lens. The fourth lens L 4  is formed as a lens which has a positive refractive power and of which a convex surface faces toward the image side. The fifth lens L 5  is a lens which has a negative refractive power and of which a convex surface faces toward the image side. 
     When the fourth lens L 4  has a positive refractive power and of which a convex surface faces toward the image side, it may be a biconvex lens of which a convex surface faces toward the object side, may be a plano-convex lens of which the flat surface faces toward the object side, or may be a meniscus lens of which a concave surface faces toward the object side. Furthermore, in the lens configuration of  FIG. 1 , the fourth lens L 4  is formed as a biconvex lens. 
     Assuming that a focal length of the whole lens system is f, a focal length of the fourth lens L 4  is f 4 , and a space between the image side surface of the fourth lens L 4  and the object side surface of the fifth lens L 5  on an optical axis is D 8 , the image reading lens system  11  is configured to satisfy the following conditional expressions.
 
0.06&lt; D 8/ f&lt; 0.16  (1)
 
0.77&lt; f 4/ f&lt; 1.81  (2)
 
     The Conditional Expression (1) defines a ratio “D 8 /f” of the space D 8  between the fourth lens L 4  and the fifth lens L 5  to the focal length f of the whole system of the image reading lens system  11 . When the value of “D 8 /f” is set to be large, this is advantageous for correction of image field curvature and coma aberration. However, when the value is more than the upper limit of the Conditional Expression (1), the total length of the image reading lens system  11  increases, and thus it is difficult to achieve compactness. Further, when the value of “D 8 /f” is set to be small, this is advantageous for compactness of the image reading lens system  11 . However, when the value is less than the lower limit of the Conditional Expression (1), it is difficult to correct image field curvature and coma aberration satisfactorily. 
     The Conditional Expression (2) is a condition relating to distortion, and defines a ratio “f 4 /f” of the focal length f 4  of the fourth lens L 4  to the focal length f of the whole system of the image reading lens system  11 . When the value of “f 4 /f” is more than the upper limit of the Conditional Expression (2), distortion increases in the direction of “+”. In contrast, when the value is less than the lower limit, the distortion increases in the direction of “−”. Thus, these are not preferable. 
     Similarly to the image reading lens system  11  shown in  FIG. 1 , the fourth lens L 4  may be formed as a biconvex lens. In this case, it is more preferable to satisfy the following Conditional Expressions (1a) and (2a).
 
0.06&lt; D 8/ f&lt; 0.14  (1a)
 
0.77&lt; f 4/ f&lt; 1.59  (2a)
 
     Further, the fourth lens L 4  may be formed as a piano-convex lens or a meniscus lens of which a flat surface or a concave surface faces toward the object side. In this case, it is more preferable to satisfy the following Conditional Expressions (1b) and (2b).
 
0.10&lt; D 8/ f&lt; 0.16  (1b)
 
1.06&lt; f 4/ f&lt; 1.81  (2b)
 
     Further, in the image reading lens system  11 , when j=1, 2, . . . 5, a refractive index of a j-th lens at the e-line (a wavelength of 546.07 nm) is defined as Nj, and an Abbe number of the j-th lens at the d-line (a wavelength of 587.56 nm) is defined as νj. In this case, it is preferable to satisfy the following Conditional Expressions (3) and (4). The system may be configured so as to satisfy any one of the Conditional Expressions (3) and (4). However, it is more preferable that the system should be configured to satisfy both of them simultaneously.
 
1.55&lt;( N 1+ N 3+ N 4)/3&lt;1.74  (3)
 
13.5&lt;(ν1+ν3+ν4)/3−(ν2+ν5)/2  (4)
 
     The Conditional Expression (3) is a condition relating to lateral chromatic aberration and aperture efficiency, and defines an average value “(N 1 +N 3 +N 4 )/3” of the refractive index N 1 , the refractive index N 3 , and the refractive index N 4  of the first lens L 1 , the third lens L 3 , and the fourth lens L 4  which are positive lenses. When the value of “(N 1 +N 3 +N 4 )/3” is more than the upper limit of the Conditional Expression (3), the aperture efficiency decreases, and this causes deterioration in relative illumination. In contrast, when the value is less than the lower limit thereof, it is difficult to correct lateral chromatic aberration satisfactorily. 
     The Conditional Expression (4) is a condition relating to longitudinal chromatic aberration, and defines a difference “(ν 1 +ν 3 +ν 4 )/3−(ν 2 +ν 5 )/2” between the average value of the Abbe numbers ν 1 , ν 3 , and ν 4  of the first lens L 1 , the third lens L 3 , and the fourth lens L 4  which are positive lenses and the average value of the Abbe numbers ν 2  and ν 5  of the second lens L 2  and the fifth lens L 5  which are negative lenses. When the value of “(ν 1 +ν 3 +ν 4 )/3−(ν 2 +ν 5 )/2” is less than the lower limit of the Conditional Expression (4), it is difficult to correct longitudinal chromatic aberration satisfactorily. 
     As described above the image reading lens system  11  is configured to have five groups and five elements, and to satisfy the conditional expressions. With such a configuration, in the image reading lens system, various aberrations, particularly, image field curvature is appropriately corrected. Thus, it is possible to obtain a high resolving power of, for example, about 600 dpi and a speed of F 5.0 or less, and maintain an aperture efficiency of nearly 100% even in the most-peripheral portion, and it is also possible to achieve compactness by adopting the configuration having five groups and five elements. 
     Furthermore, the image side and the object side of the image reading lens system  11  may be reversed. In other words, in order from the image side, the first lens L 1 , the second lens L 2 , the third lens L 3 , the fourth lens L 4 , and the fifth lens L 5  may be arranged. In this case, the lens system can be used as an image magnification lens. Further, the lens surfaces of the lenses L 1  to L 5  may be formed to be aspheric. In this case, by employing the aspheric surface, it is possible to perform more favorable correction. 
     EXAMPLES 
     Examples 1 to 6 of the image reading lens system according to the embodiment of the invention will be described in detail. Hereinafter, in Example 1 to 6, the elements and spaces will be referenced by common reference numerals and signs. Further, in Examples 1 to 6, when the surface number i (i=1 to 12) is given in increasing order from the object side lens surface of the first lens L 1 , each surface of the lenses is represented by Si, and the radius of curvature of the lens surface Si is represented by Ri (mm). Further, the space between the lens surface Si and the lens surface S(i+1), which are adjacent to each other, on the optical axis, that is, the on-axis surface spacing is represented by Di (mm). In addition, the refractive index of each lens at the e-line is represented by N, and the Abbe number at the d-line is represented by ν. 
     Example 1 
     Example 1 is the image reading lens system  11  shown in  FIG. 1 . Specifically, the lens system has a five-group five-element configuration in which the first to fifth lenses L 1  to L 5  are arranged. The first lens L 1  is formed as a lens which has a positive refractive power and of which a convex surface faces toward the object side. The second lens L 2  is formed as a biconcave lens, the third lens L 3  and the fourth lens L 4  are formed as biconvex lenses, and the fifth lens L 5  is formed as a negative lens of which a convex surface faces toward the image side. Table 1 shows lens data in Example 1. Further, Table 2 shows the focal length f (mm) of the whole system, the F number, the angle of view  2 ω (°), and the imaging magnification β of the image reading lens system  11 . 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 RADIUS OF 
                 ON-AXIS 
                 REFRACTIVE 
                 ABBE 
               
               
                 SURFACE 
                 CURVATURE 
                 SURFACE 
                 INDEX 
                 NUMBER 
               
               
                 NUMBER 
                 (mm) 
                 SPACING  
                 (e-LINE) 
                 (d-LINE) 
               
               
                 (Si) 
                 (Ri) 
                 (mm) (Di) 
                 (N) 
                 (ν) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 13.371 
                 4.22 
                 1.71615 
                 63.4 
               
               
                 2 
                 43.112 
                 0.49 
                   
                   
               
               
                 3 
                 −44.124 
                 1.34 
                 1.65222 
                 33.8 
               
               
                 4 
                 13.977 
                 1.70 
                   
                   
               
               
                 5 
                 20.218 
                 9.00 
                 1.71615 
                 63.4 
               
               
                 6 
                 −36.988 
                 0.20 
                   
                   
               
               
                 7 
                 236.730 
                 6.97 
                 1.71615 
                 63.4 
               
               
                 8 
                 −32.538 
                 3.76 
                   
                   
               
               
                 9 
                 −12.377 
                 3.98 
                 1.61669 
                 44.3 
               
               
                 10 
                 −117.370 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 f (mm) 
                 F NUMBER 
                 2ω (°) 
                 IMAGING MAGNIFICATION β 
               
               
                   
               
             
             
               
                 34.58 
                 4.5 
                 47.5 
                 0.11 
               
               
                   
               
             
          
         
       
     
     The focal length f 4  of the fourth lens L 4  according to Example 1 is “40.38 (mm)”. The numerical values corresponding to the Conditional Expressions (1) to (4) in Example 1 are set to satisfy the Conditional Expressions (1) to (4) and the Conditional Expressions (1a) and (2a) as shown in Table 3. 
     
       
         
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                   
                 D8/f 
                 0.109 
               
               
                   
                 f4/f 
                 1.168 
               
               
                   
                 (N1 + N3 + N4)/3 
                 1.716 
               
               
                   
                 (ν1 + ν3 + ν4)/3 − 
                 24.4 
               
               
                   
                 (ν2 + ν5)/2 
               
               
                   
               
             
          
         
       
     
       FIGS. 2A to 2D  and  3 A to  3 D show various aberrations of the image reading lens system  11 .  FIG. 2A  shows spherical aberration,  FIG. 2B  shows astigmatism,  FIG. 2C  shows distortion (distortion aberration), and  FIG. 2D  shows lateral chromatic aberration. In the spherical aberration diagram of  FIG. 2A , the spherical aberration at the e-line is indicated by the solid line, the aberration at the g-line (a wavelength of 435.84 nm) is indicated by the chain line, and the aberration at the C-line (a wavelength of 656.28 nm) is indicated by the chain double-dashed line. In the astigmatism diagram of  FIG. 2B , the aberration on the sagittal image plane is indicated by the solid line, and the aberration on the tangential image plane is indicated by the dashed line. In the lateral chromatic aberration diagram of  FIG. 2D , when the e-line is set as a reference, the aberration at the g-line is indicated by the chain line, and the aberration at the C-line is indicated by the chain double-dashed line. 
     Further,  FIGS. 3A to 3D  show lateral aberrations. Specifically,  FIGS. 3A to 3D  show lateral aberrations at the e-line when the angle of view is 23.8°, 17.1°, 12.4°, and 0° (on the axis). 
     Example 2 
     The image reading lens system  12  according to Example 2 shown in  FIG. 4  has a five-group five-element configuration in which first lens L 1  to fifth lens L 5  are arranged in order from the object side. The first lens L 1  is a lens which has a positive refractive power and of which a convex surface faces toward the object side. The second lens L 2  is formed as a biconcave lens, and the third lens L 3  is formed as a biconvex lens. The fourth lens L 4  is formed as a positive meniscus lens of which a concave surface faces toward the object side and of which a convex surface faces toward the image side. The fifth lens L 5  is a negative lens of which a convex surface faces toward the image side. 
     Table 4 shows lens data of the image reading lens system  12 . Further, Table 5 shows the focal length f (mm) of the whole system, the F number, the angle of view  2 ω (°), and the imaging magnification β of the image reading lens system  12 . The focal length f 4  of the fourth lens L 4  according to Example 2 is “52.70 (mm)”. The numerical values corresponding to the Conditional Expressions (1) to (4) are set to satisfy the Conditional Expressions (1) to (4) as shown in Table 6. Further, the values are also set to satisfy the Conditional Expressions (1b) and (2b). 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                 RADIUS OF 
                 ON-AXIS 
                 REFRACTIVE 
                 ABBE 
               
               
                 SURFACE 
                 CURVATURE 
                 SURFACE 
                 INDEX 
                 NUMBER 
               
               
                 NUMBER 
                 (mm) 
                 SPACING 
                 (e-LINE) 
                 (d-LINE) 
               
               
                 (Si) 
                 (Ri) 
                 (mm) (Di) 
                 (N) 
                 (ν) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 15.179 
                 3.00 
                 1.71615 
                 53.9 
               
               
                 2 
                 66.033 
                 0.45 
               
               
                 3 
                 −42.266 
                 1.64 
                 1.62409 
                 36.3 
               
               
                 4 
                 16.111 
                 2.87 
               
               
                 5 
                 21.030 
                 7.95 
                 1.62287 
                 60.3 
               
               
                 6 
                 −30.227 
                 0.22 
               
               
                 7 
                 −260.389 
                 7.67 
                 1.75844 
                 52.3 
               
               
                 8 
                 −35.089 
                 4.89 
               
               
                 9 
                 −12.146 
                 4.99 
                 1.61669 
                 44.3 
               
               
                 10 
                 −110.120 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 f (mm) 
                 F NUMBER 
                 2ω (°) 
                 IMAGING MAGNIFICATION β 
               
               
                   
               
             
             
               
                 39.75 
                 5 
                 41.9 
                 0.11 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
             
               
                   
                 D8/f 
                 0.123 
               
               
                   
                 f4/f 
                 1.326 
               
               
                   
                 (N1 + N3 + N4)/3 
                 1.699 
               
               
                   
                 (ν1 + ν3 + ν4)/3 − 
                 15.2 
               
               
                   
                 (ν2 + ν5)/2 
               
               
                   
                   
               
             
          
         
       
     
     Further,  FIG. 5A  shows spherical aberration of the image reading lens system  12 ,  FIG. 5B  shows astigmatism thereof,  FIG. 5C  shows distortion (distortion aberration) thereof, and  FIG. 5D  shows lateral chromatic aberration thereof.  FIGS. 6A to 6D  show lateral aberrations at the e-line when the angle of view is 21.0°, 15.0°, 10.8°, and 0° (on the axis). 
     Example 3 
     The image reading lens system  13  according to Example 3 shown in  FIG. 7  has a five-group five element configuration in which the first lens L 1  to fifth lens L 5  are arranged similarly to Example 2. However, the lens system is different in that the fourth lens L 4  is formed as a positive piano-convex lens of which a flat surface faces toward the object side and of which a convex surface faces toward the image side. 
     Table 7 shows lens data of the image reading lens system  13 . Further, Table 8 shows the focal length f (mm) of the whole system, the F number, the angle of view  2 ω (°), and the imaging magnification β of the image reading lens system  13 . The focal length f 4  of the fourth lens L 4  according to Example 3 is “70.42 (mm)”. 
     The numerical values corresponding to the Conditional Expressions (1) to (4) are set to satisfy the Conditional Expressions (1) to (4) as shown in Table 9. Further, the values are also set to satisfy the Conditional Expressions (1b) and (2b). 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                   
                 RADIUS OF 
                 ON-AXIS 
                 REFRACTIVE 
                 ABBE 
               
               
                 SURFACE 
                 CURVATURE 
                 SURFACE 
                 INDEX 
                 NUMBER 
               
               
                 NUMBER 
                 (mm) 
                 SPACING 
                 (e-LINE) 
                 (d-LINE) 
               
               
                 (Si) 
                 (Ri) 
                 (mm) (Di) 
                 (N) 
                 (ν) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 18.699 
                 4.74 
                 1.71615 
                 53.9 
               
               
                 2 
                 74.904 
                 0.79 
               
               
                 3 
                 −46.239 
                 3.19 
                 1.65222 
                 33.8 
               
               
                 4 
                 19.567 
                 2.76 
               
               
                 5 
                 29.516 
                 8.92 
                 1.71615 
                 53.9 
               
               
                 6 
                 −38.039 
                 1.45 
               
               
                 7 
                 0.000 
                 8.43 
                 1.71615 
                 53.9 
               
               
                 8 
                 −50.429 
                 6.58 
               
               
                 9 
                 −16.729 
                 4.53 
                 1.61669 
                 44.3 
               
               
                 10 
                 −126.000 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 f (mm) 
                 F NUMBER 
                 2ω (°) 
                 IMAGING MAGNIFICATION β 
               
               
                   
               
             
             
               
                 46.62 
                 4.5 
                 36.5 
                 0.11 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE 9 
               
               
                   
                   
               
             
             
               
                   
                 D8/f 
                 0.141 
               
               
                   
                 f4/f 
                 1.511 
               
               
                   
                 (N1 + N3 + N4)/3 
                 1.716 
               
               
                   
                 (ν1 + ν3 + ν4)/3 − 
                 14.9 
               
               
                   
                 (ν2 + ν5)/2 
               
               
                   
                   
               
             
          
         
       
     
     Further,  FIG. 8A  shows spherical aberration of the image reading lens system  13 ,  FIG. 8B  shows astigmatism thereof,  FIG. 8C  shows distortion (distortion aberration) thereof, and  FIG. 8D  shows lateral chromatic aberration thereof.  FIGS. 9A to 9D  show lateral aberrations at the e-line when the angle of view is 18.3°, 13.0°, 9.4°, and 0° (on the axis). 
     Example 4 
     The image reading lens system  14  according to Example 4 shown in  FIG. 10  has a five-group five element configuration in which the first lens L 1  to fifth lens L 5  are arranged similarly to Example 1. In the configuration, the fourth lens L 4  is formed as a biconvex lens. 
     Table 10 shows lens data of the image reading lens system  14 . Further, Table 11 shows the focal length f (mm) of the whole system, the F number, the angle of view  2 ω (°), and the imaging magnification β of the image reading lens system  14 . The focal length f 4  of the fourth lens L 4  according to Example 4 is “45.15 (mm)”. 
     The numerical values corresponding to the Conditional Expressions (1) to (4) are set to satisfy the Conditional Expressions (1) to (4) and the Conditional Expressions (1a) and (2a) as shown in Table 12. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                   
                 RADIUS OF 
                 ON-AXIS 
                 REFRACTIVE 
                 ABBE 
               
               
                 SURFACE 
                 CURVATURE 
                 SURFACE 
                 INDEX 
                 NUMBER 
               
               
                 NUMBER 
                 (mm) 
                 SPACING 
                 (e-LINE) 
                 (d-LINE) 
               
               
                 (Si) 
                 (Ri) 
                 (mm) (Di) 
                 (N) 
                 (ν) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 16.458 
                 5.87 
                 1.62033 
                 63.4 
               
               
                 2 
                 48.007 
                 0.88 
               
               
                 3 
                 −39.847 
                 2.66 
                 1.63003 
                 35.7 
               
               
                 4 
                 18.292 
                 1.18 
               
               
                 5 
                 17.869 
                 8.28 
                 1.71615 
                 53.9 
               
               
                 6 
                 −82.942 
                 0.38 
               
               
                 7 
                 96.049 
                 7.21 
                 1.83932 
                 37.2 
               
               
                 8 
                 −60.441 
                 5.16 
               
               
                 9 
                 −10.923 
                 6.02 
                 1.65222 
                 33.8 
               
               
                 10 
                 −30.927 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                 TABLE 11 
               
               
                   
               
               
                 f (mm) 
                 F NUMBER 
                 2ω (°) 
                 IMAGING MAGNIFICATION β 
               
               
                   
               
             
             
               
                 43.40 
                 4.5 
                 39.5 
                 0.11 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE 12 
               
               
                   
                   
               
             
             
               
                   
                 D8/f 
                 0.119 
               
               
                   
                 f4/f 
                 1.040 
               
               
                   
                 (N1 + N3 + N4)/3 
                 1.725 
               
               
                   
                 (ν1 + ν3 + ν4)/3 − 
                 16.8 
               
               
                   
                 (ν2 + ν5)/2 
               
               
                   
                   
               
             
          
         
       
     
     Further,  FIG. 11A  shows spherical aberration of the image reading lens system  14 ,  FIG. 11B  shows astigmatism thereof,  FIG. 11C  shows distortion (distortion aberration) thereof, and  FIG. 11D  shows lateral chromatic aberration thereof.  FIGS. 12A to 12D  show lateral aberrations at the e-line when the angle of view is 19.8°, 14.1°, 10.2°, and 0° (on the axis). 
     Example 5 
     The image reading lens system  15  according to Example 5 shown in  FIG. 13  has a five-group five element configuration in which the first lens L 1  to fifth lens L 5  are arranged similarly to Example 1. In the configuration, the fourth lens L 4  is formed as a biconvex lens. 
     Table 13 shows lens data of the image reading lens system  15 . Further, Table 14 shows the focal length f (mm) of the whole system, the F number, the angle of view  2 ω (°), and the imaging magnification β of the image reading lens system  15 . The focal length f 4  of the fourth lens L 4  according to Example 5 is “52.15 (mm)”. The numerical values corresponding to the Conditional Expressions (1) to (4) are set to satisfy the Conditional Expressions (1) to (4) and the Conditional Expressions (1a) and (2a) as shown in Table 15. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 13 
               
               
                   
               
               
                   
                 RADIUS OF 
                 ON-AXIS 
                 REFRACTIVE 
                 ABBE 
               
               
                 SURFACE 
                 CURVATURE 
                 SURFACE 
                 INDEX 
                 NUMBER 
               
               
                 NUMBER 
                 (mm) 
                 SPACING 
                 (e-LINE) 
                 (d-LINE) 
               
               
                 (Si) 
                 (Ri) 
                 (mm) (Di) 
                 (N) 
                 (ν) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 17.585 
                 6.09 
                 1.62033 
                 63.4 
               
               
                 2 
                 51.185 
                 0.94 
               
               
                 3 
                 −41.950 
                 2.88 
                 1.63003 
                 35.7 
               
               
                 4 
                 19.498 
                 1.29 
               
               
                 5 
                 19.233 
                 8.55 
                 1.71615 
                 53.9 
               
               
                 6 
                 −71.443 
                 0.72 
               
               
                 7 
                 113.978 
                 7.80 
                 1.83932 
                 37.2 
               
               
                 8 
                 −68.847 
                 5.38 
               
               
                 9 
                 −11.765 
                 6.64 
                 1.65222 
                 33.8 
               
               
                 10 
                 −33.713 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                 TABLE 14 
               
               
                   
               
               
                 f (mm) 
                 F NUMBER 
                 2ω (°) 
                 IMAGING MAGNIFICATION β 
               
               
                   
               
             
             
               
                 46.87 
                 4.5 
                 36.3 
                 0.11 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE 15 
               
               
                   
                   
               
             
             
               
                   
                 D8/f 
                 0.115 
               
               
                   
                 f4/f 
                 1.113 
               
               
                   
                 (N1 + N3 + N4)/3 
                 1.725 
               
               
                   
                 (ν1 + ν3 + ν4)/3 − 
                 16.8 
               
               
                   
                 (ν2 + ν5)/2 
               
               
                   
                   
               
             
          
         
       
     
     Further,  FIG. 14A  shows spherical aberration of the image reading lens system  15 ,  FIG. 14B  shows astigmatism thereof,  FIG. 14C  shows distortion (distortion aberration) thereof, and  FIG. 14D  shows lateral chromatic aberration thereof.  FIGS. 15A to 15D  show lateral aberrations at the e-line when the angle of view is 18.2°, 12.9°, 9.3°, and 0° (on the axis). 
     Example 6 
     The image reading lens system  16  according to Example 6 shown in  FIG. 16  has a five-group five element configuration in which the first lens L 1  to fifth lens L 5  are arranged similarly to Example 1. In the configuration, the fourth lens L 4  is formed as a biconvex lens. 
     Table 16 shows lens data of the image reading lens system  16 . Further, Table 17 shows the focal length f (mm) of the whole system, the F number, the angle of view  2 ω (°), and the imaging magnification β of the image reading lens system  16 . The focal length f 4  of the fourth lens L 4  according to Example 6 is “36.66 (mm)”. The numerical values corresponding to the Conditional Expressions (1) to (4) are set to satisfy the Conditional Expressions (1) to (4) and the Conditional Expressions (1a) and (2a) as shown in Table 18. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 16 
               
               
                   
               
               
                   
                 RADIUS OF 
                 ON-AXIS 
                 REFRACTIVE 
                 ABBE 
               
               
                 SURFACE 
                 CURVATURE 
                 SURFACE 
                 INDEX 
                 NUMBER 
               
               
                 NUMBER 
                 (mm) 
                 SPACING 
                 (e-LINE) 
                 (d-LINE) 
               
               
                 (Si) 
                 (Ri) 
                 (mm) (Di) 
                 (N) 
                 (ν) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 14.661 
                 3.34 
                 1.62033 
                 63.4 
               
               
                 2 
                 38.051 
                 0.80 
               
               
                 3 
                 −49.643 
                 3.00 
                 1.63003 
                 35.7 
               
               
                 4 
                 19.112 
                 1.78 
               
               
                 5 
                 15.517 
                 7.96 
                 1.73234 
                 54.7 
               
               
                 6 
                 −297.498 
                 0.97 
               
               
                 7 
                 112.626 
                 6.79 
                 1.80811 
                 46.6 
               
               
                 8 
                 −39.112 
                 2.81 
               
               
                 9 
                 −9.328 
                 4.96 
                 1.65803 
                 39.7 
               
               
                 10 
                 −32.184 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
           
               
                 TABLE 17 
               
               
                   
               
               
                 f (mm) 
                 F NUMBER 
                 2ω (°) 
                 IMAGING MAGNIFICATION β 
               
               
                   
               
             
             
               
                 39.89 
                 5 
                 41.8 
                 0.11 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                   
                 TABLE 18 
               
               
                   
                   
               
             
             
               
                   
                 D8/f 
                 0.070 
               
               
                   
                 f4/f 
                 0.919 
               
               
                   
                 (N1 + N3 + N4)/3 
                 1.720 
               
               
                   
                 (ν1 + ν3 + ν4)/3 − 
                 17.2 
               
               
                   
                 (ν2 + ν5)/2 
               
               
                   
                   
               
             
          
         
       
     
     Further,  FIG. 17A  shows spherical aberration of the image reading lens system  16 ,  FIG. 17B  shows astigmatism thereof,  FIG. 17C  shows distortion (distortion aberration) thereof, and  FIG. 17D  shows lateral chromatic aberration thereof.  FIGS. 18A to 18D  show lateral aberrations at the e-line when the angle of view is 20.9°, 15.0°, 10.8°, and 0° (on the axis).