Patent ID: 8194318

Claim:
A telescope optical system comprising, in order from an object side: an objective lens; an erecting prism; and an eyepiece; the objective lens comprising, in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, and a third lens group having positive refractive power, the first lens group being disposed, in order from the object side, a negative lens, a positive lens, and a positive lens, the objective lens of the telescope optical system forming an image of an object between the erecting prism and the eyepiece, focusing being carried out by moving the second lens group along an optical axis, an image position being movable by moving the third lens group in a direction perpendicular to the optical axis, and the following conditional expression being satisfied: 1.5<f1/(−f2)<4 where f 1 denotes a focal length of the first lens group, and f 2 denotes a focal length of the second lens group.