An apparatus for determining the diameter of an ophthalmic compensation lens to be mounted onto a spectacle frame, or for controlling the mounting of an ophthalmic compensation lens onto the frame includes: an electronic tablet having a screen; a spectacle frame support having an opening and being suitable for holding the glasses frame in a predetermined position, one surface of the spectacle frame being placed near the surface of the screen; elements for storing at least one graphic image including 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 the frame; elements intended for processing the image and capable of generating a projected image; elements for displaying the projected image onto the screen; and elements for aligning the frame with regards to the projected image displayed on the screen.