Anyone used a Canon 1.4 or 2x extender? Any good? Do they affect quality much? I have a 100-400 Canon zoom. The extender should be compatable (camera a bit older though - 40D). Going on hols to Canada and I want to get decent bear pics without getting too close! I know I'll lose an f-stop or 2 which I'm not bothered about, but looking for feedback from anyone who has used one. Ta. I'd probably go for a mk11, as the price is a bit more reasonable than the newer mk111.