a 2x example

[ScruffyJohn]

would/could someone post an example of what the 2x extender would produce? and maybe a 1.4x ? a friend and I are looking at both to use with our canon 100-300mm lens. Would we loose the AF feature ?

[Jon]

Yes, you would. The 300D won't autofocus with lenses slower than f/5.6. With a 1.4x TC you'd have an f/6.3-f/8, with the 2x you'd be at f/9-f/11.

[formula4speed]

I don't believe the Canon teleconverters would physically fit on your lens, 3rd party options might work like Sigma, Tamron or Kenko. If you get one that doesn't report the aperature it might autofocus, but probably not very well. Image quality will also degrade, more so with the 2x TC.

[Cadwell]

Correct, the Canon extenders will not fit that lens. Neither will the Sigma EX series TCs.