An instrument has a light-emitting and light-receiving assembly including an image capture unit and image processing unit; a backscatterer and an opening provided therein; a support for receiving an ophthalmic lens between the assembly and backscatterer, the assembly, support and backscatterer placed so that an incident light beam traverses the lens, strikes the backscatterer, returns and re-traverses the lens to arrive at the capture unit; the light-receiving assembly, the support, backscatterer and opening configured so that the assembly receives light from the beam; and the opening and a drive device for cyclically driving and making the backscatterer perform an identical movement in each cycle, configured so that a fixed zone opposite the backscatterer includes at least one part of which, over the course of a cycle, every point is at times perpendicular to the opening and at times perpendicular to a solid portion part of the backscatterer.