I have just bought an old vintage brass lens that I intend to use for landscape photography on my 8x10in camera. It is the Darlot Hemispherique 130mm built for Marion & Cie. I believe it is a homocentric design. My question is it has three lever stops marked 3, 5 and 7. Are these actual diameters in mm (3 is the smallest opening) or are they an unusual aperture scale?
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