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Message: <blockquote data-quote="denco@comcast.n" data-source="post: 3371072" data-attributes="member: 25300">It seems binoculars are considered an Afocal system. Here are some mathematics that helps explain it in a more easily to understand format. <a href="http://api.viglink.com/api/click?format=go&jsonp=vglnk_145825164887411&key=7cbadd430ab8818512444a794700951f&libId=ilwtalfq01010qwu000DAh2utqzqc&loc=http%3A%2F%2Fwww.birdforum.net%2Fshowthread.php%3Ft%3D321037%26page%3D3&v=1&out=http%3A%2F%2Fspie.org%2Fpublications%2Foptipedia-pages%2Fpress-content%2Ffg01%2Ffg01_p18_afocal_systems&ref=http%3A%2F%2Fwww.birdforum.net%2Fforumdisplay.php%3Ff%3D112&title=Optimize%20for%20size%20and%20weight%20at%20expense%20of%20optical%20performance%3F%20-%20Page%203%20-%20BirdForum&txt=http%3A%2F%2Fspie.org%2Fpublications%2Foptiped...afocal_systems" target="_blank">http://api.viglink.com/api/click?format=go&jsonp=vglnk_145825164887411&key=7cbadd430ab8818512444a794700951f&libId=ilwtalfq01010qwu000DAh2utqzqc&loc=http://www.birdforum.net/showthread.php?t=321037&page=3&v=1&out=http://spie.org/publications/optipedia-pages/press-content/fg01/fg01_p18_afocal_systems&ref=http://www.birdforum.net/forumdisplay.php?f=112&title=Optimize for size and weight at expense of optical performance? - Page 3 - BirdForum&txt=http://spie.org/publications/optiped...afocal_systems</a>"An afocal system is formed by the combination of two focal systems. The rear focal point of the first system is coincident with the front focal point of the second system. Rays parallel to the axis in object space are conjugate to rays parallel to the axis in image space. The transverse and longitudinal magnifications are constant. Equispaced planes map into equispaced planes. The relative axial spacing changes by the longitudinal magnification m. Because the magnification is constant, the cardinal points are not defined for an afocal system, and the Gaussian and Newtonian equations cannot be used to determine conjugate planes. However, any pair of conjugate planes coupled with m can be used. A convenient pair is the front focal point of the first system FF1 and the rear focal point of the second system F'R2."</blockquote>

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