Ian, the Ross lens was made under licence from Zeiss. Ross made them f:16, but Zeiss were not willing to accept the quality loss caused by increasing the maximum aperture from f:18 to f:16. Besides, the 4,5 - 6,3 - 9 - 12 - 18 - 25 aperture scale was "standard" in Germany at the time (one of many "standards", but at least that's what Zeiss used).