Hmmm -- isn't it possible there could be mirror interference using the smaller format on a full frame? I'm not familiar with the Nikon stuff, but I'm thinking at least some Canon EFS lenses can be a problem and have a nub somewhere that prevents mounting them on a full frame body. Because the mirrors in a "crop frame" body are smaller, the rear lens elements can be allowed to stick out closer to the film plane. No 1st hand experience -- thus far.