| 摘要 |
PROBLEM TO BE SOLVED: To provide a low cost optical system for reading in which an F number is made brighter, a half view angleωis made wider, various aberrations are excellently compensated for over the wide temperature range of 0 deg.C to 65 deg.C and high contrast is obtained in a high spatial frequency region. SOLUTION: In order to reduce the cost, plastic lenses are used for a first lens L1 and a second lens L2 of an optical system constituted by three groups of six lenses. The fluctuation of the focal distance of the plastic lenses caused by the variation of the environmental temperature is excellently compensated for by the appropriate combination of positive and negative lenses. Moreover, by selecting and setting appropriate values for the refractive indexes of the shapes and materials used for six single lenses L1 to L2 and the Abbe constant, sphericality, astigmatism, distortion, coma and sine conditions are excellently compensated for while keeping a near 100% aperture efficiency to the vicinity of the peripheral section of the view angle and the balance between on and off axis aberrations in properly kept.
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