Fisheye for D800?
The 8mm Rokinon/Samyang is great on FX if you want the actual circular image. Shaving the hood is very easy, and the flare is still very well controlled despite it being cut off. The actual lens comes apart quite easily, and when shaved offers 180 degrees horizontally, and nearly 180 (maybe 160-070 vertically); on a full frame sensor. You'll have to fiddle a bit with exposure compensation since the matrix metering sees all the black and tries to overcompensate, but it works VERY well and I certainly won't be selling mine. Just make sure to adjust the focus range; mine was way out from the factory. Adjusting is easy by pulling back the rubber ring, loosening the 1mm screws, and adjusting so that it is accurate. After that you will have a fisheye that focuses very close, and will give infinite DOF after 3m even at 3.5. At f8 and above everything is in sharp focus.