Patent ID: 8284494

Claim:
An optical system for observation of a front-direction object and a substantially-lateral-direction object, the optical system comprising, in order from a front-direction-object side: a front group with a negative refracting power having a reflecting/refracting optical element; an aperture stop; and a rear group with a positive refracting power having a moving lens component that is movable along an optical axis; wherein the reflecting/refracting optical element has: a first face that is formed on the front-direction-object side and that has a first transmitting surface formed with the optical axis being at a center thereof and a first reflecting surface annularly formed around the first transmitting surface and directed toward an image side, a second face that is formed on the image side and that has a second transmitting surface formed with the optical axis being at a center thereof and a second reflecting surface annularly formed around the second transmitting surface and directed toward the front-direction-object side, and a third face formed as a transmitting surface between the first face and the second face, and wherein a movement of the moving lens component changes a refracting power ratio of the front group to the rear group, to enlarge or shrink, in an observation region, a region in which an image of the front-direction object is formed.