I am tempted to believe that any lens is going to display this artifact of spots showing up on the negative under such conditions as are being discussed. If the diaphragm is made more circular then I would expect the spots to be more circular. I am not saying that proper lens formultation and construction is unable to help but I would expect spots to appear when conditions of the type being discussed are severe enough. If a person prefers circular spots to hex shaped then will Zeiss actions to their liking. Having a morecircular diaphragm is helpful in other areas also.