N4646W wrote:
Well, yes, "if" you are in a lab, tightly controlled conditions, engineering the design on paper, or you have some magical lens that can compensate for humidity changes, wind current, temp differences between subject and capture element, take into consideration light diffraction from the surrounding, lens and camera are within ideal tolerances, ect.
In the real world not so much. Rule of thumb is about the best guess.
Ron
Then as a rule of thumb it’s pretty useless.