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A teleconverter (sometimes called tele extender) is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens. For example, a 2× teleconverter for a 35 mm camera enlarges the central 12×18 mm part of an image to the size of 24×36 mm in the standard 35 mm film format. Teleconverters are typically made in 1.4×, 1.7×, 2× and 3× models, of which 1.4× and 2× are most common. The use of a 2× teleconverter gives the effect of using a lens with twice the focal length. It also decreases the intensity of the light reaching the film by a factor of 4 (an equivalent of doubling the focal ratio) as well as the resolution (by a factor of 2). ==Function== A teleconverter works similarly to a ''telephoto group'' of a proper telephoto lens. It consists of a group of lenses which together act as a single diverging lens. The location of a teleconverter is such that the image produced by the objective is located behind the teleconverter in a distance smaller than its focal length. This image is a ''virtual object'' of the teleconverter which is then focused further away and thus enlarged. For example when a single negative lens is placed so that the image formed by the objective is located in the midpoint between the lens and its focal point the lens produces the image in its focal point enlarging it two times thus acting as a 2× teleconverter. When used with somewhat slow lenses they may reduce the effective aperture enough that the camera's autofocus system will no longer work; depending on the camera system, this may range from f/5.6 to f/8. Dedicated teleconverters only work with a limited number of lenses, usually telephoto lenses made by the same manufacturer, or by a third-party manufacturer to a matching standard. Using a teleconverter with an existing lens is usually less expensive than acquiring a separate, longer telephoto lens, but as the teleconverter is magnifying the existing image circle, it also magnifies any aberrations. Image:Viewfinder-SLR-300mm.swn.jpg|Camera viewfinder with 300mm telephoto lens. Image:Viewfinder-SLR-300mm-2xTeleconv.swn.jpg|Camera viewfinder with 300mm telephoto lens and 2x teleconverter. Image:Leica-doubleur-p1020785.jpg|A Leica R series doubler, with the female part of in bayonet mount... Image:Leica-doubleur-p1020786.jpg|...and the male part. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「teleconverter」の詳細全文を読む スポンサード リンク
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