Supplementary Lenses
A positive supplementary lens— commonly referred to as a wide-angle converter—will increase the angle of view (decrease the effective focal length range) of a zoom lens.
Conversely, a negative supplementary lens—also called a range extender or a lens extender—will increase focal length and narrow the angle of acceptance. The wide-angle, normal (middle rectangle), and 2X extender (smallest rectangle) effects are illustrated above.
A 2X negative supplementary lens can change a 100mm fixed focal length lens into a 200mm lens, or it can change a 12-120mm zoom lens into a 24-240mm zoom lens.
With some zoom lenses 2X range extenders are not exactly "supplementary"; they are built into the lens. Some automatically flip into place as needed; some have to be manually engaged.
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