Optical system for a side-viewing endoscope, and side-viewing endoscope

An optical system having a viewing angle θ relative to a longitudinal axis including: a prism group to deflect incident light by reflection at first and second surfaces including prisms with mutually adjacent boundary surfaces arranged in pairs and separated by a gap, a total reflection of incident light from outside a field of view takes place at a boundary surface between a prism and a corresponding gap, the prism group has a cylindrical envelope (D), and an input-side prism is configured to have a wedge-shape with angle β and an optical path length α; an entry surface of the prism has a length L, which is a length of a line of intersection of the entry surface with a plane that is spanned by a central beam path; and the first prism meets the conditions: α