Apparatus and methods are described including a corrective optical element having a thickness and/or a curvature that is different in different regions of the corrective optical element, such that the corrective optical element is configured, upon being adhered to any one of a plurality of differently-shaped optically-corrective single-vision lenses, to change a focal length of the optically-corrective single-vision lens differently in different regions of the optically-corrective single-vision lens. The corrective optical element is shapeable such that the corrective optical element can conform with a shape of any one of the plurality of differently-shaped optically-corrective single-vision lenses. Other applications are also described.