Patent ID: 6134052
Filing Date: 2000-10-17
Classification: G02B

Abstract:
A variable-power type observational optical system having an objective lens system and a variable-power type eyepiece lens system, in this order from the object, said variable-power type eyepiece lens system comprising:a positive first lens group including a negative meniscus lens element and a bi-convex positive lens element which are cemented together;a second lens group including a single plastic-made bi-convex lens element, at least one surface of which is formed as an aspherical surface;a third lens group including a single plastic-made bi-concave lens element, at least one surface of which is formed as an aspherical surface;wherein an object image by said objective lens system is formed between said second and third lens groups, and when variable power is performed from the long focal length extremity towards the short focal length extremity, said second lens group is moved closer to said first lens group, and said third lens group is moved away from said first lens group, and said variable-power type observational optical system satisfies the following conditions:1.5<.DELTA.LD/M<3.01.8<f2/fS<2.6whereinM designates the magnification ratio of said eyepiece lens system;.DELTA.LD designates the change of the overall length of said eyepiece lens system upon variable power being performed;f2 designates the focal length of said second lens group; andfS designates the focal length of the overall system of said eyepiece lens system at the short focal length extremity.