I getting more and more fond of my old 6x9 folder, but because it was a consumer item, it does not have the standard f/stops increments, nor does it have any focal length information. I'm interested in measuring those mostly to get my light metering estimations right and for the entertainment. Now I know that f/stop is a relationship between aperture and focal length, and I'm sure I can find the formula somewhere on the net. But regarding focal length, what should I measure? The camera covers 6x9, and focuses on two fixed positions, "near" and "far". Should I just measure the distance between the lens and the film plane? It's not even a triplet, it's barely a single piece of converging glass.