Patent ID: 6876390

Claim:
An image-forming optical system for forming an image of an object, said image-forming optical system comprising: a stop; a first prism; and a second prism; said first prism and said second prism being provided between said stop and an image plane; said first prism including, in order from an object side thereof: a first prism entrance surface through which a light beam passing through said stop enters said first prism; a first prism first reflecting surface that reflects the light beam entering said first prism; a first prism second reflecting surface that reflects the light beam reflected by said first prism first reflecting surface; and a first prism exit surface through which the light beam reflected by said first prism second reflecting surface exits from said first prism; wherein said first prism first reflecting surface and said first prism exit surface are an identical surface, and the light beam entering through said first prism entrance surface is incident on said first prism exit surface at an angle exceeding a total reflection critical angle, thereby allowing said first prism exit surface to produce a reflecting action so as to function as said first prism first reflecting surface; said second prism including: a second prism entrance surface through which the light beam exiting from said first prism exit surface enters said second prism; a second prism first reflecting surface that reflects the light beam entering said second prism; a second prism second reflecting surface that reflects the light beam reflected by said second prism first reflecting surface; and a second prism exit surface through which the light beam reflected by said second prism second reflecting surface exits from said second prism; wherein said second prism second reflecting surface and said second prism entrance surface are an identical surface, and the light beam reflected from said second prism first reflecting surface is incident on said second prism entrance surface at an angle exceeding a total reflection critical angle, thereby allowing said second prism entrance surface to produce a reflecting action so as to function as said second prism second reflecting surface, and wherein said first prism and said second prism satisfy the following conditions (1) and (2): P 1 x<P 2 x (1) P 1 y<P 2 y (2) where when a decentration direction of said imageforming optical system is a Y-axis direction, and a plane parallel to a light ray connecting a center of said stop and a center of said image plane is a YZ-plane, and further a direction perpendicularly intersecting the YZ-plane is an X-axis direction, P 1 x and P 1 y are powers in the X-axis direction and Y-axis direction, respectively, of said first prism, and P 2 x and P 2 y are powers in the X-axis direction and Y-axis direction, respectively, of said second prism, and further Px and Py are powers in the X-axis direction and Y-axis direction, respectively, of said image-forming optical system.