The DX Digital System isn't full frame, or full 35mm. So, if you put a Full Frame, 50mm lens on the camera, it will only see about 2/3rds of the lens. The full frame cameras like the D3, D700 or Canon 5d will use the full lens area and you will get a true 50mm lens. When you put that same lens on the DX cameras, it will be more like a 72mm lens. Check out info on google about the Nikon DX system vs FX Full Frame.
Hope that helps!