Abstract: According to an exemplary embodiment, a process for coupling a mechanical interface unit to a suction-ring unit retained on an eye by suction force includes a step of relative approximating of the interface unit to the suction-ring unit in an axial direction as far as a first, predetermined relative position of the two components, which is preferentially detected by suitable sensorics. When the first relative position is attained, the evacuation is begun of a suction chamber formed between the interface unit, the suction-ring unit and the surface of the eye, whereby the partial vacuum generated establishes in the suction chamber a contact between the surface of the eye and an applanation plate retained on the interface unit, or enlarges a region of existing contact. The aspirating of the eye onto the applanation plate prevents compression loads and shear loads on the eye such as normally cannot be avoided when impressing the plate onto the eye.