Patent ID: 7184208

Claim:
A variable power finder comprising: an objective optical system having a positive refractive force; an inverting optical system for converting an inverted image taken by said objective optical system into an erecting image; and an ocular optical system having a positive refractive force to observe said erecting image obtained by said inverting optical system, said objective optical system, said inverting optical system, and said ocular optical system being arranged in order from an object side, wherein said objective optical system including a negative first lens group, a positive second lens group, and a positive third lens group, an image side lens surface and an object side lens surface of each of the first and second lens group including a respective aspherical surface, said second lens group configured to be displaced along an optical axis to provide the variable power and to compensate a visibility change caused by said variable power through said displacement of said first lens group, and said third lens group includes a lens prism having two reflection surfaces on separate sides of the lens prism and two convex refraction surfaces adjacent each other on a common side of the lens prism, said two convex refraction surfaces forming an entrance face and an exit face of the lens prism on said common side, and expressions (1) to (4) below are satisfied: f 3 /fw ≧2.5 (1) −1.0<( R 32 +R 31)/( R 32 −R 31)<1.0 (2) L 1 /fw 2 ≦0.45 1/mm (3) L 2 /fw 2 ≦0.03 1/mm (4) where f 3 is a focal distance in millimeters (mm) of said third lens group, fw is a focal distance in mm of said objective optical system measured when said second lens group is displaced along the optical axis at a wide angle end of said variable power, R 31 is a curvature radius of an object side lens surface of said lens prism, R 32 is a curvature radius of an image side lens surface of said lens prism, L 1 is an air equivalent distance in mm from a middle imaging surface to the object side lens surface of said lens prism, and L 2 is a distance in mm from the middle imaging surface to said image side lens surface of the lens prism.