I have a magic lantern Petzval I want to use with slide film, and to do so, i'm going to need a more accurate aperture measurement than what i've been working with. With a multi-element lens with the elements having plenty of airspace between them, where do I measure from? Here is what I did, I racked the lens back so it was sitting about half and half in the lensboard. Then, I measured from the ground glass for the lensboard (middle of the lens-ish) and took the measurement (5") then I took a measurment of the front diameter of the lens (1.6"). 5" = 127mm 1.6" = 40.64mm 127 / 40.64 = f 3.12 Did I do this correctly? Are there better methods to calculate the aperature of multi-element lenses such as this? Can anyone offer improvments to what I did?