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Message: <blockquote data-quote="henry link" data-source="post: 1961252" data-attributes="member: 6806">Paul,The link that Rick supplied has some simple cutaway diagrams here:<a href="http://www.kowa-prominar.ne.jp/pro/lens/feature.htm" target="_blank">http://www.kowa-prominar.ne.jp/pro/lens/feature.htm</a>They look a lot like the scope optics until you get to the extra doublet behind the focusing group. Maybe that doublet changes the position of the focal plane to make the lens more compact or possibly massages the field curvature to match a flat sensor for photographic use. The visual back then has another lens or group in front of the prism. Maybe that lens returns the field curvature to its original shape to match the eyepiece field curvature.  Henry</blockquote>