Originally Posted by
Dan Fromm
It seems that your lens could be a Heligon or a Xenotar. Both are fast double Gauss types, the Heligon has six elements in four groups, the Xenotar five elements in four groups. To tell them apart, count reflections in the rear cell. If a Heligon, you should see four strong reflections and one weak reflection, if a Xenotar four strong reflections and no weak reflection. The weak reflection, if present, may be hard to see. Count reflections with the diaphragm closed or, better still, the cell screwed out of the lens.
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