An apparatus (100, 200, 300, 400) for determining the diameter of an ophthalmic compensation lens intended to be mounted in a spectacle frame or for checking the mounting of an ophthalmic compensation lens in a spectacle frame, characterized in that the apparatus comprises: - an electronic tablet having a screen (111, 113, 114), - a spectacle frame support, the spectacle frame support exhibiting an opening and being suitable for holding said spectacle frame in a determined position, a face of the spectacle frame being placed in the vicinity of the surface of the screen (111, 113, 114), the spectacle frame support being adapted to allow tilting of the spectacle frame about an axis Y passing through the sagittal plane of the spectacle frame, for pivoting to the right and to the left, so as to bring the right, respectively left rim of the spectacle frame, close to the surface of the flat screen and - means for storing at least one graphical image comprising at least one graphical reference marker associated with the ophthalmic compensation lens and/or the patient intended to wear the ophthalmic compensation lens mounted in said frame; - means for processing said image which are suitable for deforming said image by half-image depending on a dihedral angle of the spectacle frame and depending on the pivoting direction of the frame support so as to generate a projected image; - means for displaying said projected image (160, 170, 180, 190) on the screen, and - means for aligning said frame in relation to the projected image displayed on the screen (111, 113, 114), - said electronic display means being configured to allow an operator to visually determine a diameter of lens intended to be mounted in said spectacle frame by projecting at least one rim of the frame via said opening in relation to the position of the at least one corresponding graphical reference marker in the projected image displayed on the screen or to visually check the mounting of an ophthalmic compensation lens in said spectacle frame by projecting a physical reference marker associated with the lens mounted in said frame in relation to the position of the at least one corresponding graphical reference marker in the projected image displayed on the screen. - an electronic tablet having a screen (111, 113, 114), - a spectacle frame support, the spectacle frame support exhibiting an opening and being suitable for holding said spectacle frame in a determined position, a face of the spectacle frame being placed in the vicinity of the surface of the screen (111, 113, 114), the spectacle frame support being adapted to allow tilting of the spectacle frame about an axis Y passing through the sagittal plane of the spectacle frame, for pivoting to the right and to the left, so as to bring the right, respectively left rim of the spectacle frame, close to the surface of the flat screen and - means for storing at least one graphical image comprising at least one graphical reference marker associated with the ophthalmic compensation lens and/or the patient intended to wear the ophthalmic compensation lens mounted in said frame; - means for processing said image which are suitable for deforming said image by half-image depending on a dihedral angle of the spectacle frame and depending on the pivoting direction of the frame support so as to generate a projected image; - means for displaying said projected image (160, 170, 180, 190) on the screen, and - means for aligning said frame in relation to the projected image displayed on the screen (111, 113, 114), - said electronic display means being configured to allow an operator to visually determine a diameter of lens intended to be mounted in said spectacle frame by projecting at least one rim of the frame via said opening in relation to the position of the at least one corresponding graphical reference marker in the projected image displayed on the screen or to visually check the mounting of an ophthalmic compensation lens in said spectacle frame by projecting a physical reference marker associated with the lens mounted in said frame in relation to the position of the at least one corresponding graphical reference marker in the projected image displayed on the screen.