Canon Tip on Using Their Extenders 1.4x or 2x
A friend of mine shared this tip and I thought BPN folks might be interested in it too.
Using EF Extenders
If you decide that you need to use one of Canon’s EF Extenders (they come in 1.4x or 2x magnifications) with your lens to extend the focal length order to achieve the shot you’re thinking of, there is a simple process you should always follow to ensure the best results. Instead of attaching the extender to the camera and then the lens on the front of the extender, you should always remember to first attach the extender to the lens, and only then attach the extender/lens combination to the camera. This ensures that the combination of lens and extender information is correctly reported to the camera and you get the full benefits of information being delivered, and hence optimal performance from your EOS system.
It's from "Tips & Tricks from Canon"
http://cpn.canon-europe.com/content/...psandtricks.do
Worth seeing their other tips too.