An optical system for use with a stereo video endoscope with a fixed, lateral viewing direction, including: laterally-viewing distal and proximal optical assemblies, similarly configured left and right lens system channels; the distal optical assembly couples incident light from an object space into the left and right lens system channels; the distal optical assembly includes an entrance lens, a deflection prism group and an exit lens in a direction of incident light; the deflection prism group includes first and second prisms in the direction of incident light; the first prism includes first entrance and first exit sides at an angle relative thereto; the second prism includes a second entrance side, a reflection side and a second exit side; and the first entrance side of the first prism and the reflection side of the second prism enclose an angle that is greater than a total reflection angle of the second prism.