an embodiment of an image reader according to the present invention will be described below with reference to fig4 and 5 . in fig4 a symbol 1 denotes a light source . 2 denotes a reading device . 10 denotes an optical lens . s denotes a reading target in which a character and a picture are drawn on a front surface . the light source 1 is arranged on a rear side ( a surface where the character and the picture are not drawn ) of the reading target s . lights from the light source 1 are emitted onto the rear of the reading target s . the reading device 2 is arranged on a front side ( a surface where the character and the picture are drawn ) of the reading target s . shadow of the character and the picture ( an image ) based on the light from the light source 1 is read by scanning . the optical lens 10 is arranged between the reading target s and the reading device 2 . the optical lens 10 forms the image on an image forming surface 2 a of the reading device 2 . an image information inputted to the reading device 2 is converted into an electric signal . the reading device 2 is provided with a line memory ( not shown ) and an image processor ( not shown ). the reading device is , for example a ccd sensor and has a resolution characteristic of 1200 dpi . the electric signal is stored in the line memory . the image processor re - constitutes the image , in which distortion is small , in accordance with the stored electric signal . the optical lens 10 has a distortion characteristic that a distortion rate of a central part of a lens positioned at a center in a scan direction of the reading device 2 is larger than a distortion rate of an end part of the optical lens 10 . the end part of the optical lens 10 implies the vicinities of both ends in the scan direction . a lens having this characteristic is typically referred to as a wide - angle lens or a fish - eye lens . in the optical lens 10 , a resolution of the central part is set to be higher than that of the end part . for example , if the resolution of the end part is 600 dpi , that of the central part is set to 1200 dpi . in the optical lens 10 , the distortion rate of the end part is set to be larger , as shown in fig2 . this distortion characteristic enables the reading operation to an effective reading width to be carried out in an optical path ( focal length ) of a short length the focal length l 3 is shorter than the focal length ( l 1 : fig1 ) and the focal length ( l 2 : fig2 ). the usage of the optical lens 10 enables the effective reading width having the same value as the conventional lens to be reserved even if the length of the optical path is made shorter . if the length of the optical path is made shorter , the device can be miniaturized . the resolution of the central part in which the optical distortion is small is set to be higher than that of the end part . due to this characteristic , the resolution of the end part in which the distortion rate is large is maintained at a desirable value , which avoids a drop of information . this characteristic enables the reading performance to be kept equal to or higher than a predetermined standard at any position with regard to the effective reading width . this characteristic cancels out the necessity of the process for compensating for the drop of the information . as the distortion rate of the end part is larger , the formed image is gradually smaller from the central part to the end part . a consumption amount of memories per unit length of the effective reading width is reduced with approach to the end part . that is , the number of line memories used for storing the image can be reduced as compared with the case that a lens having a constant distortion rate is used . a capacity of the reduced line memories is shown by an equation ( a desirable resolution of a device / a resolution of a central part of a lens ) fig5 shows a resolution curve and a distortion curve in relation to the present invention . the resolution curve and the distortion curve in relation to the present invention are both represented by straight lines indicative of mere increase and decrease . other than the optical lens 10 having this characteristic , an optical lens having a characteristic represented by a non - linear curve may be employed . if an optical path is turned by a reflection mirror , the arrangement of the reading device can be changed . due to this change , the whole length of the optical path is fixed to thereby enable a distance ( focal length ) between the reading target and the reading device to be shorter . in this case , a conventional hand scanner ( or a handy scanner ) can be miniaturized without using a contact type image sensor . also , a manufacturing cost can be kept cheap . in the image reader described in this embodiment , the light source 1 is mounted on the rear side of the reading target s . the present invention can be applied to the configuration in which the light source is mounted on the front side of the reading target s . the image reader according to the present invention can correspond to any input of a color image input and a monochrome image input . in the image reader of the present invention as mentioned above , the distortion rate of the end part is set to be larger than that of the central part in the scan direction in the optical lens . as for the resolution of the optical lens , the resolution of the central part is set such that a predetermined resolution is obtained in the end part . this setting enables the resolution of the end part to be kept and also enables the effective reading width of the image to be expanded . if the optical lens having this characteristic is used , it is possible to reserve the effective reading width equal to that of the conventional image reader , even if the length of the optical path is made shorter . the reduction of the length of the optical path enables the image reader to be miniaturized .