I'm stumped. I'm trying to compute the bellows factor for a 12" Dagor shooting close-ups and I'm using one of the usual formulas: x = bellows extension/focal length. I know I measure from the lens to the film plane to get the bellows ext ension. But from what point on the lens do I measure to get the bellows extensi on? The back of the lens? The middle? The front?
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