I know that Teleconverters increase reach on a lens but I have a few questions about them. I would look to get the Canon EF 1.4x in the future. 1. How does it go onto the camera? 2. Can I mount a EF-S lens on a EF mount on a XSi? 3. Why does the 2x teleconverter reduce autofocus capabilities? 4. why do you give up 1 f-stop by using one? 5. How much would a 55-250mm lens be turned into with a 1.4x?