Hi Marty:

In fact when you measure at R.O.C. (2xFl) you are measuring the Actual Focal Length but what you want is the Effective Focal Length (E.F.L.) which is what the optic will have when focused on an object at infinity. Many optics are never used at their E.F.Ls.. A camera or copy machine's A.F.L. is 2xE.F.l. for 1:1 copy scale. Such a machine will never be used at its E.F.L. or infinity but its lens will still be rated by such because it is a standard of reference.

Interestingly enough, the copy lens will not even perform well at its E.F.L. because it is designed for close work.

Cheers,
Rick