Patent ID: 7133210

Claim:
An eyepiece lens comprising: a first lens group having a negative refracting power including a negative meniscus lens with a concave surface facing an eyepoint side; a second lens group having a positive refracting power including a lens with convex surfaces on both ends; and a third lens group having a negative refracting power including a negative lens; said first - third lens groups being arranged in an order from the eyepoint side, said eyepiece lens being capable of varying a diopter by moving the second lens group along an optical axis, wherein the eyepiece lens is configured by aspherical surfaces to make the positive refracting power weak while at least one surface of the convex surfaces of the lens deviates from the optical axis, and in such a manner that conditions of a following formula are satisfied: 1.6 <f 1 /f 3<2.5 −0.55<S3≦0 where f1 indicates a focal length of the first lens group, f3 indicates a focal length of the third lens group, and S3 indicates a shape factor of the third lens group, and further wherein said shape factor S3 is defined by a conditional formula S 3=( re 3 +rs 3 )/( re 3 −rs 3 ) where re 3 indicates a radius of curvature of the third lens group on the eyepoint side and rs 3 indicates a radius of curvature thereof on the object side when the lens surfaces are spherical, while re 3 indicates a paraxial radius of curvature of the third lens group on the eyepoint side and rs 3 indicates a paraxial radius of curvature thereof on the object side when the lens surfaces are aspherical.