When light hits an object, diffraction causes the beam to spread over an area that depends on the aperture of the lens. The phase filters make this area smaller but also cause a reduction in the beam brightness (see image). Sales wondered what could be the smallest possible focal spot a beam could be subjected to before quantum mechanical effects interfered with the beam. Using standard Fresnel approximations to describe diffraction, and Bessel functions to model the filters, he showed that the theoretical minimum spot size of a circular beam through a filter is 0.94 that of the smallest unfiltered spot size, but approximately 0.5 of the spot size if the beam narrows along one of its axis.