hundreds of gravitational lenses are currently known. about half a dozen of them are partial einstein rings with diameters up to an arcsecond, although as either the mass distribution of the lenses is not perfectly axially symmetrical, or the source, lens, and observer are not perfectly aligned, we have yet to see a perfect einstein ring. most rings have been discovered in the radio range. the degree of completeness needed for an image seen through a gravitational lens to qualify as an einstein ring is yet to be defined.