I have picked up a Hugo Meyer Convertible Plasmat 5X7 lens. It comes with 3 elements, which are 9 1/8th", 12" and 13 1/4". This means using it, combined, it would be 9+12, 12+13, 9+13. What is the formula for coming up with the focal length of those combinations, should I choose to do this mathematically instead of putting them together and measure from the lens board to the ground glass? 12+13 appears to be around 190mm just playing with it here in the office. The set came with a nice old box but, unfortunately, no paper work so I need to figure out the f stop table for the mm scale. Many thanks for your help.
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