| 摘要 |
PURPOSE:To provide an objective lens for video discs of a weight about 1/5 that of conventional lenses with an objective lens of 0.45 to 0.05 in numerical aperture (N.A) and 2 mm in working distance by constituting the lens with two groups of lenses which satisfy the specific conditions. CONSTITUTION:The 1st group lens comprises a cemented positive lens of a positive lens and negative lens and the 2nd group lens compreses a positive meniscus lens whose concave face is faced to the image side. Where the radii of curvature of the cemented face of the 1st group lens and the face of the image side of the 2nd group lens are let to be r2, r5, the refractive indices of the respective lenses for a wavelength 632.8 mm to be n1, n2, n3 and the focal length of the entire system to be f, the conditons of formulas (1), (2) are satisfied. The conditions of formula (1) are for correcting the spherical aberrations which produce in a large amount on the 3rd and 5th faces of the objective lens at the cemented face of the 1st group lens. The conditions of formula (2) are provided in order to maintain sine conditions well and are for correcting the coma aberrations produced on the cemented faces of the 1st group lens and other faces when said cemented face is selected within the range of the conditions (1). |
