Patent ID: 8331039

Claim:
An optical system for observing a front object and an approximately lateral object, comprising, in order from a front-object side: a front group having a reflective refractive optical element, a refractive power of the front group being negative; an aperture stop; and a rear group having a moving lens unit, a refractive power of the rear group being positive, wherein the moving lens unit is configured to be movable in a direction along an optical axis, wherein the reflective refractive optical element includes: a first face which has a first transmitting surface and a first reflective surface and is formed on the front-object side, where the first transmitting surface is formed with the optical axis being at a center thereof and the first reflective surface is formed in a shape of a ring around the first transmitting surface and faces toward the image side, a second face which has a second transmitting surface and a second reflective surface and is formed on an image side, where the second transmitting surface is formed with the optical axis being at a center thereof and the second reflective surface is formed in a shape of a ring around the second transmitting surface and faces toward the front-object side, and a third face which is formed as a transmitting surface between the first face and the second face, and wherein the optical system makes focus on an object while hardly changing, in an entire observation area, an area in which an image of the front object is formed, by moving the moving lens unit to the extent that the refractive power of the front group and the refractive power of the rear group hardly change relative to each other.