| 摘要 |
<p>An imaging optical system comprises: a first lens having a positive refractive power on an optical axis; a second lens having a negative refractive power on the optical axis; a third lens having a meniscus shape protruding toward the imaging plane side and a positive refractive power on the optical axis, the positive refractive power decreasing with distance from the optical axis and turning negative in the vicinity of a main light beam of a light flux reaching the maximum image height; a fourth lens having a positive refractive power on the optical axis; and a fifth lens having a negative refractive power on the optical axis, the negative refractive power decreasing with distance from the optical axis. The first to fifth lenses are arranged in this order from the object side to the imaging plane side. The imaging optical system satisfies the formulae: ?1/?2 > 1.3 (1); ?1/?3 > 1.3 (2); and -2 < f1/f5 = -1.2 (4), where ?i (i being an integer of 1 to 3) represents any of the Abbe's numbers of the first to third lenses, and fi (i being an integer of 1 to 5) represents any of the focal lengths of the first to fifth lenses.</p> |
