Best longer zoom for Canon XSi?

[rapsac]

I have recently bought a Canon XSi. I have the 18-55 and 55-250 lenses, and am wondering what is the best longer - say up to 400mm - zoom, for birds and other wildlife?

[McQ]

If you already have a zoom lens that goes out to 250 mm, I think you would likely be much better off with a fixed focal length lens at 300 or 400mm. You can always add an extender if you need a litter more reach, and still not be much worse off (if at all) than the quality you would get with a zoom. If you must go for a zoom though, Canon's 100-400mm f/5.6 lens is really the only option, but it is notoriously soft at the long end of that range. If you do not need image stabilization, then Canon's 400mm f/5.6L prime lens is a fantastic bang for the buck...

[rapsac]

Thank you, McQ. That makes a lot of sense.

What about extenders? Can I use an extender with the 55-250?

I seem to remember reading somewhere on the web that someone actually got better results with a Sigma than a Canon extender.

[McQ]

Extenders only work with a limited range of lenses. The vast majority of these are Canon's L-series models. Check out Canon's website for a listing of compatible models.

That may be true about the Sigma, although I think that comparison was with the earlier Mark I series of Canon extenders, not the newer Mark II series. The Canon model is definitely more expensive, but I do not think that anything matches them right now when it comes to edge sharpness and chromatic aberration.