A method and instrument to irradiate a light adjustable lens, for example, inside a human eye, with an appropriate amount of radiation in an appropriate pattern by measuring aberrations in the system containing the lens; aligning a source of the modifying radiation so as to impinge the radiation onto the lens in a pattern that corresponds to the aberration; and controlling the quantity of the impinging radiation whereby to decrease the aberration. The quantity of the impinging radiation is controlled by controlling the intensity and duration of the irradiation. The pattern is controlled and monitored while the lens is irradiated.