This is interesting... just discovered it on the zeiss.com website.
Originally Posted by bjorke
...In addition to this, ZF lenses come with a completely newly-designed iris featuring a circular shape, not those hexagons, octagons and similar geometries that give out-of-focus highlights an artificial, unnatural look. While the Japanese have appreciated this natural look for a decade or more, Hollywood has only recently rediscovered it. Curiously, it was the Japanese photo equipment makers who were mainly responsible for introducing the hexagon and octogon iris; European makers favored the circular iris until the 1960's...