The Canon EF 200-400mm f/4L IS USM Extender 1.4x is a new lens yet not been officially announced.
Recent sources told this lens will finally to be launched on May 14, 2013. And the release date will be in September or October 2013.
via: CR
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So the 1.4 extender is built-in … like ENG lenses have had for ever? I know my 1.4 extender with my 300/4 is excellent!