Disclosed are stereo endoscopes where the optical train of one optical system is separated in a left and right optical train. The separation in a left and right optical train is achieved by splitting the entrance pupil in a left and right pupil half. To additionally achieve a significant stereoscopic disparity the distance between the left and right pupil half is expanded. The separation of the entrance pupil and the expansion of the left and right pupil half is achieved by a distally located prism block. This prism block consists of a pair of rhomboidal prisms which are located exactly at the entrance pupil inside the endoscope objective. The front lens group is replaced by an identical pair of front lens groups each assembled in front of one of the two rhomboidal prisms. Such stereo endoscopes with expanded pupil halves enable to build stereo endoscopes with very small diameter or stereo endoscopes for special surgical applications. The described stereo endoscopes also include video endoscopes where the aperture functioning as the entrance pupil is separated in two halves and expanded for better stereoscopic effect. The prism block of the disclosed stereo endoscopes can also be used to separate the exit pupil of the disclosed stereo endoscopes and expand the optical axes on the proximal side to adapt to different stereo endoscopic cameras.
