I will be traveling in the summer and have bought a Sigma 18-250 which I use on my 7D, I know its far from the best lens but it suits my needs well. Traveling light is a priority and the 18-250 gives me a good compromise between traveling light and image quality v affordability.

Im thinking about a 2x converter as I may need to film video from a distance. I have looked at this one.

Sigma DG tele-converters: Digital Photography Review (http://www.dpreview.com/news/0510/05100501sigma_teleconverters.asp)

At 500mm will my IS and AF still work and do you guys think it will provide decent results?

Many thanks

With that setup and a 2x converter, AF and IS won't work, the image will be very soft, low in contrast and chromatic aberrations may be quite bad.

A T3i would be a much better solution with it's 3x video crop mode. That's why I picked one up along with my 7D.

Kenko is what you want - its the factory version of the Tamron and maybe one or two others. my review of the 1.4X is here Kenko 1.4X Teleconverter Review (http://www.steveoakley.net/template_permalink.asp?id=263) . if the lens is a 4.5, then at about F8 1/2 ( 9.5) thats well past what the AF system will do.

if you want a LONG lens, try a Vivitar ( made by tokina ) 400mm 5.6 : review here Vivitar 400mm 5.6 Lens Review & Video : UPDATED (http://www.steveoakley.net/template_permalink.asp?id=138) which you can pick up on ebay for around $100 or so. it is heavy, but its a nice piece of glass, especially for the price. while you won't get AF, you may get AF confirm with a programmable adapter.