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Old January 16th, 2013, 06:15 PM
Doug Kerr Doug Kerr is offline
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Join Date: May 2006
Location: Alamogordo, New Mexico, USA
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The principle here is just that of any inboard focal length converter, in this case with a ratio less than 1.

All the basic math works the same way. The f-number of the "new composite" lens is less (larger relative aperture) than that of the base lens itself (its true f-number, not an "effective f-number", as is often said).

The focal length converters we most often use were originally introduced to "convert" a "normal" lens into a "telephoto" lens (using that term in the sense of a lens with a focal length significantly greater than that of a "normal" lens), and thus were often called "teleconverters". That name stuck, even when we apply them to a lens that is already a "telephoto" lens.

The device discussed here could be called, in that same vein, a "wideconverter".

Best regards,

Doug
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