Quote Originally Posted by Robert A. Zeichner View Post
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.
If you want to be picky measure it at the smallest value for both of them - the shape difference is negligible - the highest value are compensated by the lowest ones to result in the same average value. Still it'll be more precise than introducing all the errors you get with your exposure meter method. Also it is not difficult to measure the average value with a calliper if you want to open it in between the smallest and the highest value.