| 摘要 |
PURPOSE:To suppress the change of aberration due to the change of the wavelength of a light source as small as possible by constituting the titled lens of five lenses in four groups, i.e. the 1st group consisting of a positive lens, the 2nd group consisting of a negative meniscus lens, the 3rd group having a junction lens consisting of a positive lens and a negative lens, and the 4th group consisting of a positive meniscus lens, and satisfying these lens groups with specific conditions. CONSTITUTION:The lens for an optical disk is constituted of five lenses in the four groups, i.e. the 1st group consisting of a positive lens, the 2nd group consisting a negative meniscus lens turning its concave surface to the light source side, the 3rd group constituted of a junction lens consisting of a positive lens and a negative lens, and the 4th group consisting of a positive meniscus lens turning its convex surface to the light source side, and these lenses are satisfied with the conditions shown by the inequalities. Provided that (f) is the focal distance of the whole system, f1,2 is the composite focal distance of the 1st and 2nd group lenses, r3 is the radius of curvature of the light source side surface of the 2nd group lens, r6 is the radius of curvature of the adhering surface of the 3rd group lens, and upsilon3-upsilon4 is the difference of Abbe's numbers of respective lenses constituting the 3rd group. Consequently, various aberrations, especially chromatic aberration, can be efficiently corrected.
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