I too wondered how how much of the variation in times was from the shutter tester and how much from the shutters. When I tested my Mamiya 7 lenses, which have electronically controlled leaf shutters, I had my answer. I remember being stunned that the 1/2 came out to .50000 and the 1 was 1.00000 and the others were either dead on or off by a minuscule amount. At that point I determined for practical purposes all the error and variation was due to the shutters, or testing error.
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