There are a few question I have to get started on this. I have been looking between the 400mm canon f2.8 and the 600mm canon f4. Does anyone have any advice as to which I should purchase? And a question about teleconverters. I was browsing ebay today and I noticed the Canon 2x teleconverter. Would this work with either of the lenses and how does this 2x teleconverter compare to the 1.4x teleconverter.