PURPOSE:To obtain the long-focus objective which is bright, good in machinability, and low in cost by composing the objective of a negative meniscus lens having a convex surface on the object side and a positive biconvex lens, and satisfying specific conditions. CONSTITUTION:The objective consists of the negative meniscus lens which has the convex surface on the object side and the positive biconvex lens and the conditions shown by inequalities are satisfied. In the inequalities, nu+ and nu- are the Abbe numbers of the positive and negative lenses, n+ and n- are the (d)-line refractive indexes of the positive and negative lenses, theta+ and theta- are the theta of the partial fractional expression II of the positive and negative lenses, and nlambda is the refractive index of each spectrum line in lambda = g, G, and C. Consequently, the chromatic aberration and spherical aberration are compensated with good balance, the spherical aberration and comatic aberration are compensated at the same time, and after the chromatic aberration is corrected, the remaining chromatic aberration of high order can be suppressed low. Therefore, the lens which is bright and good in machinability is obtained at low cost without using fluorite.
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