I've been checking B&H for the Canon 100mm f2.8 which I thought has been out of stock for the past few weeks but then just now I realize there is a "Imported" version and a "USA" version. Is there any difference in these lenses? One is priced at $490(usa) and the other $509(imported) The imported version is out of stock but not the USA version. When I had called B&H the other day they just said the lens was out of stock but didnt mention anything about 2 different versions.. Can anyone shed some light on this? Thanks!