ok, I'm opening myself up for total ridicule here.. LOL... But I am going to ask a very basic question about FX vs. DX.. but not about the camera... I understand the difference there....but my question has to do with FX vs. DX lenses. I think I have 3 lenses that can go either way. Nikon 80-200mm f2.8, Nikon 50mm f1.4, Tamron 28-75 F2.8 But I have 2 that are only DX: Nikon 18-200mm f3.5-5.6, Tamron 10-20mm f3.5-5.6 My question is this: What physical property in a lens makes a DX lens not able to be used on a FX insofar as filling the image sensor? 1) Is it that the max aperture has to be 2.8 or better for it to be a FX compatible lens? OR 2) Is it that the lens curvature has to bend the light less so that it goes to the FX sensor width? Just curious. Thanks in advance.