Abstract: A method, device, and system for optical near-field scanning microscopy on a test specimen in a liquid uses a light source that guides light focused on an optical fiber tip disposed near the test specimen. With the aid of the light beam extending largely parallel to the specimen surface, a diffraction pattern is formed downstream of the light source and the optical fiber tip. The diffraction pattern is detected and a resulting detection signal, which is indicative of a vibrational amplitude of the optical fiber tip, is used for controlling the distance between the optical fiber tip and the specimen, where the distance is related to the vibrational amplitude. Accordingly, optical near-field scanning microscopy may be performed in virtually any liquid, on non-smooth specimen surfaces, and on specimen surfaces whose features change with time.