I *thought* bellows factor refered to the fact that effective f number is f number times (1 + magnification). (Ie, if you're shooting at f/10 and magnification 1:1 then you must expose as if you were shooting f/20). With ttl metering, we don't have to worry about that much, except to know we'll need more light for macros.


Effective f number also gives rise to more diffraction. I don't think you use this "effective f number" in dof calculations. dof is proportional to f number (the real one, not effective f) and inversely proportional to magnification.


But I don't see how this changes the fovcf rule. That is, it seems to me that a 100mm lens on a 1.6fovcf camera shooting at f/10 and 1:1 magnification still acts like 160mm f/16 at 1:1 would on ff (in terms of exposure and dof).


I'm probably wrong, though [^o)]