Foward-oblique viewing optical system

Claim 1

A forward-oblique viewing optical system for endoscopes comprising a correcting prism, a concave lens arranged in front of said correcting prism, an observing-direction changing prism, an objective and an image-transmitting optical element, said correcting prism having an entrance surface arranged perpendicular to the oblique-viewing direction of the endoscope which is inclined at an angle substantially less than 75° in respect to the forward-viewing direction of the endoscope and an exit surface inclined in respect to the forward-viewing direction with an angle of inclination smaller than that of said entrance surface, said observing-direction changing prism having an entrance surface opposed to said exit surface of said correcting prism and separated therefrom, a reflecting surface inclined in respect to the forward-viewing direction by an angle of inclination smaller than that of said entrance surface of said observing-direction changing prism and an exit surface arranged perpendicular to the optical axis of said objective system rays entering said observing-direction changing prism being reflected by said reflecting surface and directed toward said entrance surface of said observing-direction changing prism, being further reflected by said entrance surface of said observing-direction changing prism and then going out of said observing-direction changing prism through said exit surface, said objective being arranged in rear of said observing-direction changing prism, said image-transmitting optical element being arranged in rear of said objective, the entrance surface of said correcting prism and the axis of rays entering said correcting prism approximately coinciding with the intersecting point of said entrance surface of said correcting prism and the optical axis of said objective.