Equivalence refers to taking the "same" picture with FX and DX. By the same we mean from the same position (perspective) and with the same FOV (framing). Okay, let's say we're taking a portrait. We shoot with the D800E at 105mm f4 and our subject is 10' away. We get a total DOF of .64 feet. Now we shoot the D7100 at 70mm at f2.8. Our total DOF is now .69 feet - close enough for rock. Note that assuming the same shutter speed, we'd shoot the DX camera at one stop lower ISO. Diffraction, it turns out, is precisely correlated with DOF and will also be the same here.