Why? If you have Nikon glass that you want to use on your full frame Canon 5D, like I did on a shoot yesterday with a Nikon 58mm f1.2. It was just the thing. I don't know about this specific adapter, but there are many out there and some have a chip on the back that gives you focus verification in the view finder. You still have to stop down manually. Oh, and you can focus to infinity. It is a pretty inexpensive way to share lenses if you have mulitple systems. OM to EOS adapter works as well.