I'm thinking of purchasing a 2x AutoFocus TelePhoto Converter for NIKON D5100 or something similar to use with my 55-300 lens since I can't afford a 600 mm lens. Is there a difference on the ones with auto focus. What brands work best and how do I make sure it's compatible? I would like for it to extend to 600 mm for bird photography. I have heard pros and cons. Any suggestions and advice would be appreciated.