Abstract: The apparatus has: a rocking support and an upper clamping member for blocking the lens disc; a first transparent screen provided with reference lines; an image pick-up which obtains images of the lines and images of the disc; an optical system located between the means and the pick-up; an electronic circuitry for processing the images and the modifications in the reference lines caused by inserting the disc between the lines and the pick-up, building a synthetic image; a digital reader for frame forms, in communication with the circuitry; and a second screen for displaying a synthetic image superimposed on the frame data.