A variable-focus variable-magnification optical coupler is provided, for coupling an endoscope to a variety of different cameras having image planes at different positions while maintaining "true-zoom" functioning of the optical coupler. By "true-zoom", it is meant that the focal length of the optical coupler's lens system can be varied without having the image out of focus. That is, once an image is focussed with the optical coupler, the image remains in focus even when the size of the image is changed by a zoom lens of the coupler. In one embodiment, the optical coupler includes an adjustment mechanism which is actuated to move a focus lens and a zoom lens assembly together along the length of the coupler relative to a camera mount. In another embodiment, an adjustment mechanism moves a back focal lens is moved relative to the camera mount to account for the different image plane of the camera.