Originally Posted by
Jim Galli
A picture might help to identify. If it's brass it's probably a petzval. Triplets are from the same era that Model T's turned black, and any of them are fairly easy to identify.
Here's how I measure. No black magic at all. Focus on something far enough away to be at infinity focus. At the midpoint between the front group and rear group measure to the ground glass. That's the focal length. Measure the width of the aperture. 40mm is probably a good number. Divide that width into the focal length and you're there. For studio focus just do the same thing over. Focus on an apple. Measure from GG to mid lens. Lets say for illustration you're now at 640mm focus. 40 into 640 is 16, you're at f16. No bellows factors needed.
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