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In optics an afocal system (a system without focus) is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length.〔(Daniel Malacara, Zacarias Malacara, Handbook of optical design. Page 379 )〕 This type of system can be created with a pair of optical elements where the distance between the elements is equal to the sum of each element's focal length (''d'' = ''f''1+''f''2). A simple example of an afocal optical system is an optical telescope imaging a star, the light entering the system is at infinity and the image it forms is at infinity (the light is collimated).〔(Virendra N. Mahajan, Ray geometrical optics, page 38 )〕 Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam, increasing magnification. The magnification of such a telescope is given by : Afocal systems are used in laser optics, Infrared and forward looking infrared systems, camera zoom lenses and telescopic lens attachments such as teleside converters,〔(Rudolf Kingslake, Optics in photography, page 187 )〕 and photography setups combining cameras and telescopes (Afocal photography). ==See also== * Lens (optics) * Teleside converter 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「Afocal system」の詳細全文を読む スポンサード リンク
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