Abstract: There is disclosed an optical unit which is primarily intended to retrofit existing work piece processing machines such as coordinate measuring machines, milling machines or EDM machines. The optical unit includes a zoom lens with a lens housing, and an optical coupling for a video camera. The lens housing isolates the optical elements against vibration during adjustment of the zoom lens, and for this purpose a flexible belt link is provided between a lens focusing knob and the zoom lens. An optical coupling having a relay or copying lens and an extender lens is included between the video camera and the zoom lens. This coupling effectively decrease the back focal length of the high magnification zoom lens and greatly compress the overall assembly. The optical coupling includes a coupling housing having a focusing cylinder that rotatably adjusts the position of the relay lens.