PURPOSE:To provide an objective lens which can be a reference for objective lenses for astronomical telescopes and optical measurements of ultrahigh precision and can be regarded virtually aberration-free up to large aperture ratios, by regulating the focal lengths and air spaces of 3 kinds of lenses to specific conditions. CONSTITUTION:The positive 1st lens L1, the negative 2nd lens L2, and the 3rd lens L3 which is a positive meniscus lens directing its convex face to the image side which are mutually separated are provided successively from the incident light side (left side in the figure). When the focal length of the entire system is defined as (f), the focal lengths of the respective lenses are defined as f1, f2, f3 respectively, the air space between the 1st lens L1 and the 2nd lens L2 is defined as d2, the air space between the 2nd lens L2 and the 3rd lens L3 as d4 and the Abbe number of the 1st lens L1 as nud1, the respective conditions of the equations 1-5 are satisfied.
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