A system for monitoring the position of a blocking device on an ophthalmic lens (20) having at least one marking comprises: —a mechanical structure adapted to cooperate with the blocking device; —an image sensor (4) observing the ophthalmic lens (20); —a control unit (10) connected to the image sensor (4) and configured to produce an image having a point of reference with a determined position with respect to the mechanical structure and at least part of the ophthalmic lens (20) including said marking; and—a user interface (12) adapted to display a blocking device positional compensation proposal for an automatic positional compensation or for a manual positional compensation based on the comparison to a predetermined threshold of a distance between the point of reference and the marking on the image. A corresponding method and a method for edging an ophthalmic lens are also proposed.