The Canon EF 2x II Extender is an easy and very affordable way to dramatically extend the reach (focal length) of compatible Canon L Series lenses, and a few additional lenses.
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The Canon EF 2x II Extender mounts between the compatible lens and body, and multiplies the focal length settings of your lens by 2. For example, the Canon 2x makes a Canon EF 70-200mm f/2.8L IS Lens into a 140-400mm f/5.6 IS lens - and yes, IS still functions normally. A Canon EF 70-200mm f/4.0L Lens mounted over a 2x becomes a 140-400mm f/8 lens. A Canon EF 300mm f/2.8L IS Lens becomes a 600mm f/5.6 IS lens.