Originally Posted by
Robert A. Zeichner
Measuring the comparative diameter of these two shutters is not as easy as you might think. The Compur shutter has many more blades in its diaphragm creating what appears to be almost a perfect circle. The Copal shutter's diaphragm contains fewer blades and the result is more of a polygon making measurement more of a challenge. By simply measuring the light passing through a known aperture, it was pretty easy to adjust the diaphragm on the Copal shutter to achieve the 1/3 stop increments.
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