A method for customizing a head mounted device adapted to generate a virtual image for a wearer, the method comprises: obtaining a 3D eye pupil position of the wearer in a reference frame and corresponding to a predetermined gaze direction, and customizing the head-mounted device based on the 3D eye pupil position so that the position of a focus area (EMB) at least partly coincides with the position of the eye pupil in the predetermined gaze direction, wherein customizing the head-mounted device comprises controlling a recording of a holographic mirror based on the 3D eye pupil position in the reference frame.