A preparation device (1) for preparing an ophthalmic lens to be machined, includes a support (100) suitable for holding the ophthalmic lens, blocking elements (400) including at least one blocking accessory (450) and a manipulator arm (401) for manipulating the blocking accessory, an acquisition device (200) suitable for acquiring an image of the ophthalmic lens, electronic and/or computer processor elements (700) suitable firstly for deducing an optical frame of reference of the ophthalmic lens, and secondly for controlling the position of the manipulator arm and/or of the support to apply the blocking accessory against the ophthalmic lens in a given blocking position. The blocking elements and/or the support include at least one marker element and the electronic and/or computer processor elements are adapted to identify the position of the marker element in an image acquired by the acquisition device, in which image there appears an image of the marker element.