Michael Mutmansky asked a question that I thought I knew the answer to and found out I didn't.(ie. which ap scale is right etc?)
I thought apertures were simply the mathematical inverse relation of the focal l ength. Simply F10 for a 210mm lens would be 1/10th of 210 or 21mm.
I started checking "knowns" on lenses that I knew were in their manufacturers or iginal configuration and found that it "just ain't so." I did find that a ver y general rule of thumb that would probably get you into the ballpark is Focal L ength minus 25% and then do the inverses. For example the inverse relation work s well on a 120mm Symmar if I reduce that 120 by 25% to 90 and then calculate 1/ 5.6, 1/8, 1/11 of the 90, not 120.
I surmise that light transmission, number of air surfaces, modern coatings vss. single coatings vss. no coatings probably all play into the formula.
Someone who knows, help me out here. If I mount a barrell lense in a shutter is it impossible for me to correctly calculate an aperture scale myself? Is it bl ack magic that only the original designers can do correctly? How does Wisner do it with say a casket of plasmats that are mix and match in a hundred different ways?
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