I didn't know about those military items. What form of killing needs such precision? 1/6400 seems a very curious unit, partly derived from halving successive units, in the way inches were used and partly from decimals. It corresponds to projecting a patch of light about just over 1Km wide on the Moon, more than adequate for a nuclear strike. I'm very curious, although this has nothing to do with Arca-Swiss cameras. In the context of the cameras, Metric seems to be a trademark, rather than a specification. There are scales on the camera. "Metric", meaning measure, occurs in other contexts, such as biometric and barometric. I wonder if the origin of this conversation lies in a conflation of the metric system and the decimal system? The original revolutionary metric system included decimal years of ten months with secular names for the months like Hot or Foggy and a decimal day, hour, minute and second. They did propose a system of angular measurement with 100° to the right angle. These proved either unpopular or unworkable. The Unites States has retained many units imposed on it by the Tyrant George III. I cannot explain cups for cooking. I do agree about setting apertures, particularly in recessed boards. Does anybody know of a solution to that little problem?