An optical system for use with a stereo video endoscope with a fixed lateral viewing direction. The optical system including: a laterally-viewing distal optical assembly; and a proximal optical assembly, the distal optical assembly and proximal optical assembly jointly establishing a beam path, the proximal optical assembly including: a left channel lens system; and a right channel lens system similarly configured to the left channel lens system; wherein the distal optical assembly establishes an optical axis and is configured to couple incident light along the beam path from an object space into the left channel lens system and into the right channel lens system of the proximal optical assembly; and the optical system comprises an angle-selective optical element with a surface oriented perpendicular to the optical axis of the distal optical assembly, the surface being located in the beam path and coated with an incidence-angle-selective dielectric coating.