Assuming you want a fish eye (8-10mm), if you want to save money, get this: http://www.dpreview.com/news/2011/11/25/samyang8nikon
(make sure to get the new CPU version)
It comes rebranded in the USA because of a trademark dispute with Samsung; you can find it as Opteka, Rokinon, Bower, etc...
For video and stopped down this lens is sharp enough. Otherwise the Nikon 10.5mm is a far better choice.