Optical instrument for locating at least one characteristic point of an ophthalmic lens, comprising: - a light emission and reception assembly, comprising an image capture unit and an image utilization unit linked to the image capture unit; - a light return unit; - a support configured to receive said ophthalmic lens between said light emission and reception assembly and said light return unit, with said light emission and reception assembly, said support and said light return unit which are disposed so that an incident light beam issuing from said light emission and reception assembly passes through said ophthalmic lens, encounters said light return unit, returns towards said ophthalmic lens, passes once again through said ophthalmic lens and returns to said light emission and reception assembly so as to reach the image capture unit, which is configured to provide the image utilization unit with images so as to identify and locate predetermined indications liable to be present on the ophthalmic lens so as to give the location of said at least one characteristic point; - an aperture made in said light return unit; and - a light reception and analysis assembly, situated on the side of the light return unit opposite to said support, with said support, said light return unit and said aperture which are configured so that said light reception and analysis assembly receives light, from said incident beam, that has passed through said ophthalmic lens and through said aperture, said light reception and analysis assembly being configured to locate said at least one characteristic point of the ophthalmic lens on the basis of the light received; characterized in that said aperture (41; 141; 241; 341; 441; 541) and a device (50; 71) for cyclic driving of the light return unit, making the light return unit make one and the same motion at each cycle, are configured so that a fixed zone facing the light return unit comprises at least one part (60; 63; 75), each site of which, in the course of a cycle, is at times in line with said aperture (41; 141; 241; 341; 441; 541) and at times in line with a solid part of the light return unit. - a light emission and reception assembly, comprising an image capture unit and an image utilization unit linked to the image capture unit; - a light return unit; - a support configured to receive said ophthalmic lens between said light emission and reception assembly and said light return unit, with said light emission and reception assembly, said support and said light return unit which are disposed so that an incident light beam issuing from said light emission and reception assembly passes through said ophthalmic lens, encounters said light return unit, returns towards said ophthalmic lens, passes once again through said ophthalmic lens and returns to said light emission and reception assembly so as to reach the image capture unit, which is configured to provide the image utilization unit with images so as to identify and locate predetermined indications liable to be present on the ophthalmic lens so as to give the location of said at least one characteristic point; - an aperture made in said light return unit; and - a light reception and analysis assembly, situated on the side of the light return unit opposite to said support, with said support, said light return unit and said aperture which are configured so that said light reception and analysis assembly receives light, from said incident beam, that has passed through said ophthalmic lens and through said aperture, said light reception and analysis assembly being configured to locate said at least one characteristic point of the ophthalmic lens on the basis of the light received;