Originally Posted by alanrockwood
... (I'm overlooking some subtleties here, such as telephoto or retro-focus designs, in which the physical lens diameter does not equal the effective optical diameter.)
Correction: I'm not sure the statement is correct about the physical lens diameter not equaling the effective diameter for certain types of lenses. This will take a bit more thought.
Regardless, it is a fine point that doesn't significantly affect the rest of what I said.