Jon - You said: "One divided by twice the focal length."
Bless you for putting this equation in English. I've tried in the past to explain this to several people. I know what it means and make use of it. But in the end, I've had to look at and accept a quote on a tee shirt my wife bought for me, which says: "I can explain it to you... but I can't understand it for you." Most of us just don't speak "equation."
You have clarified this principle for the ages! Now I know how to explain it without using terms like reciprocal or writing the equation (which I have been known to do incorrectly.)