Patent ID: 8310769
Filing Date: 2012-11-13
Classification: G02B

Abstract:
1. An optical system for observing 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; wherein the reflecting/refracting optical element is constructed of a first lens and a second lens arranged on an image side of the first lens, which are cemented together; has a first face that is formed on the front-direction-object side of the first lens and that has a first transmitting surface formed with an optical axis being at a center thereof and a first reflecting surface annularly formed around the first transmitting surface and directed toward the image side, a second face that is formed at a cemented surface between the first lens and the second lens 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, a third face formed as a transmitting surface on the image side of the second lens, and a fourth face formed as a transmitting surface on a circumferential face of the first lens; and satisfies the following conditions: where n d RL1 is a refractive index of the first lens for d-line rays, n d RL2 is a refractive index of the second lens for d-line rays, v d RL1 is an Abbe's number of the first lens for d-line rays, and v d RL2 is an Abbe's number of the second lens for d-line rays, and wherein the cemented surface is formed to be concave toward the image side.