Slight error here. I am not trying to find what is "right." I was simply experimenting to find the minimum f-stop that would tend to keep that rear eye in focus when using classic portrait lenses in the 100mm range at a classic head and shoulders portrait distance. The rule of thumb seems to be f4 will do it on an FX body and f2.8 will do it on a DX body.
We could say DX gives you a one stop DOF advantage in low light when shooting portraits and trying to keep the full face in reasonable focus. This is not generally commented upon as an advantage to shooting DX. It may be a factor when the new D400 and D72000 come out if they have high ISO equivalent to the new FX bodies. Using f1.8 in DX(and equivalent lenses) will give you about the same DOF as f2.8 in FX and shooting at f2.8 in low light will give you about the same DOF as using f4 on an FX body. Hence, if you are in low light and want to keep the full face reasonably sharp grab your D400 DX body because that allows you to do at f2.8 what you will need f4 to do if you are using your D4 body? Is this true? Conversely, if you are working in f2.8 available light and want to blur the background more grab the FX body and shoot at f2.8 in FX rather than in DX because it will give you shallower DOF in the same light? Just have the model turn her head more directly to you to keep both eyes sharp (or do a profile where you won't see that rear eye or just let it blur).
