a) Field of the Invention
The present invention relates to a reading lens system which is used for forming images of bar codes and other coded information on light receiving devices such as CCD's.
b) Description of the Prior Art
Optical instruments which are used for reading coded information such as bar codes have been developed in various types such as stationary type and handy type, and diversified specifications are desired for a reading lens system. In recent days in particular, compactness, light weight, low manufacturing cost and easy handling are desired for a reading lens system, and importance is laid on its characteristics such as the lens system having a short total length (a distance as measured from an object surface to an image surface), the lens system being composed of a small number of lens elements, the lens system allowing little variation of the size of an image which caused by defocusing due to a variation of an object location and that the lens system produces aberrations such as astigmatism and distortion in small amounts for reserving a certain required field angle. Since reading errors are fatal for optical instruments of this kind, it is important to reduce variations of the size of an image caused when the image is defocused due to a variations of the object location.