| 摘要 |
PURPOSE:To make a telephoto ratio small, to make both a length and a diameter compact, and to correct satisfactorily various aberrations by setting a specified condition between Abbe numbers of a negative meniscus lens and a biconvex lens of the fifth group, in a lens group of a five group constitution consisting of a variable power system and an image forming system. CONSTITUTION:The titled lens is constituted of a variable power system consisting of the first group for focusing, having a positive refractive power, the second group having a negative refractive power and moving along the optical axis for a variable power, and the third group having a positive refractive power and moving by drawing a locus of a concave shape to an object side along the optical axis in order to correct of a variation of an image face caused by a movement of the second group, and an image forming system having a positive refractive power, in order from the object side. The image forming system consists of the fourth group which is a cemented lens of a biconvex lens and a negative lens, and whose composite refractive power is positive, and the fifth group consisting of a negative meniscus lens being convex to an image side and a biconvex lens, and a condition of the expression is satisfied. In this way, the diameter direction is also compact in the same way as the length direction, and also a well-balanced aberration correction is executed extending over the whole focal distance range.
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