A method implemented by computer device of determining a correcting optical function to be applied on a virtual image to be displayed to a user of an optical system including an optical lens to be placed in front of a user eye and including a back face having an optical design; a see-through displaying device configured to display the virtual image towards the eye, the virtual image being transmitted at least through the back face of the optical lens defining a display area on the back face; the method includes a step, during which optical design data relating to the optical design of the back face of the optical lens are provided; a display area data providing step, during which display area data relative to at least one parameter of the display area are provided; and a correcting optical function determining step, during which a correcting optical function is determined based at least on the optical design data and on the display area data.