My Equipment - kenconger
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Canon's 1.4x tele-extender III multiplies the telephotos focal length 1.4x.  This particular extender is optically superb and preserves the image quality.  Autofocus is also maintained when combined with a lens having an f4 or faster maximum aperture.  The only con about using and extender is that it reduces the aperture by one f-stop.

Update: In 9/2011 I purchased the new version III converter.  The advanced features seem to make it a worthwhile purchase marrying up with the version II, 400/2.8 and certainly produces sharp images.