The invention relates to an apparatus (100) for determining the diameter of an ophthalmic compensation lens to be mounted onto a glasses frame, or for controlling the mounting of an ophthalmic compensation lens onto a glasses frame. According to the invention, the apparatus comprises: an electronic tablet having a screen (111); a glasses frame holder (120, 121, 122) having an opening and being suitable for holding said glasses frame in a predetermined position, one surface of the glasses frame being placed near the surface of the screen (111); a means for storing at least one graphic image comprising at least one graphic reference mark associated with the ophthalmic compensation lens and/or with the patient who is to wear the ophthalmic compensation lens mounted onto said frame; a means intended for processing said image and capable of generating a projected image; a means for displaying said projected image onto the screen; and a means for aligning said frame with regards to the projected image displayed on the screen.