Placed on the rear of the lens, extenders increase the focal length at the loss of exposure. Optical quality of the current extenders is very good. Canon manufactures two extenders: the EF 1.4x II Extender and the 2x II Extender. As the name implies, the 1.4x Extender magnifies the focal length 1.4 times at the expense of one stop exposure. Thus, the 200mm f/2.0 lens becomes a 280mm f/2.8 lens. The EF 2x II Extender doubles the focal length, but at the loss of two stops.
The extenders are designed to work with a specific group of lenses only. These include any fixed focal length lens over 135mm (other than the 135mm soft focus lens), as well as the 70-200mm L series of lenses, and the 100-400mm IS zoom lens.
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