| 摘要 |
PURPOSE:To obtain an achromat-class objective of about x20 power which has large N, A and operation distance and superior contrast and resolving power and provides a flat image surface over a wide visual field by forming the objective of three lens groups and satisfying specific conditions. CONSTITUTION:This objective consists of the 1st lens group G1 which has a meniscus lens element having a large-curvature concave surface on an object side and a positive meniscus lens element having a concave surface on the object side, the 2nd lens group G2 which has a single positive lens element and a cemented lens element of a negative, a positive, a negative lens, and the 3rd lens group G3 having a cemented lens element which is formed by cementing a positive and a negative lens together and has small refracting power. Conditions shown by inequalities are satisfied, where (f) is the composite focal length of the whole system, f1 the focal length of the 1st lens group G1, n1 the refractive index of the meniscus lens element closest to the object side in the 1st lens group G1, f21 the focal length of the single positive lens element in the 2nd lens group G2, nu21 the Abbe number of the single positive lens component, and nu2P the Abbe number of the positive lens forming the cemented lens component in the 2nd lens group G2.
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