| 摘要 |
PURPOSE:To correct the variance of the focal length of a plastic lens due to temperature to compensate the movement of the image surface, by using one glass lens at least in the first lens group and satisfying specific conditions in respect to the lens focal length. CONSTITUTION:The first lens group consists of a high-dispersion plastic lens L1, a low-dispersion plastic lens L2, and a glass lens L3; and when the focal length of the glass lens, the Abbe's number of the glass lens, the focal length of the high-dispersion plastic lens, and the focal length of the whole of the first lens group are denoted as fG, vG, fP, and fF, respectively, these lenses satisfy inequalities (1) and (2). The first lens group is constituted with lenses consisting of plural materials different in thermal expansion rate or temperature change rate of refractive index to correct the variance of the focal length, thus compensating the variance of the focus position. |
