Well, I put a magnifying glass lens onto a piece of black mat board and stuck it on my speed graphic. I like the DOF it gives, as well as the edge distortions. However I want to determine the F-stop. I know that F-stop is the focal length divided by the diameter of the lens. However, I am having some terminology difficulties. I know this is probably a dumb question, but here it goes... Is the focal length the distance from the front standard to the film plane when focused at infinity?