A method is described for fixing a lens of an optical group with respect to an optical sensor in an image acquisition device comprising the steps of housing the optical sensor in a housing, fixing a lower holder of the optical group to the housing, aligning an upper holder of the optical group, wherein the lens is placed, with the sensor so as to align a focusing point of the lens with respect to the sensor, welding the upper holder to the lower holder. The welding step may be performed by means of ultrasounds.