There are several web sites which provide calculators for determining diffraction. Here is one which also explains the whole concept.
http://www.cambridgeincolour.com/tutorials...photography.htmWith a MFDB, printing to moderate sizes, the lenses are diffraction limited at around F11.
If you print smaller, you can stop down more (because the diffraction will be less apparent).
If you use larger film, the same holds true.
The lenses designed for 645 are optimized at F11 or F16 because, beyond that, they are diffraction limited.
The lenses designed for 4x5 can be optimized for F22 or F32 because, beyond that, they are diffraction limited.
The real upside to this, is that the strobe equipment weighs half as much to get the same focus.