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Message: <blockquote data-quote="jring" data-source="post: 3590206" data-attributes="member: 133897">Hi,if we assume that the zoom on the 553/4 is actually the small body zoom (which the image showing it labeled 60x linked by me somewhere above in this thread implies), the calculation is as follows:The small body scopes (except for the odd 82SV) all have a focal length of 420mm and the small body zoom yields 20-60x on those - which makes it a 7-21mm zoom.So if this zoom is used in the 553/4 and gives 15-45, we get 7x45 or 21x15 = 315mm as the focal length of the 550 bodies. So the focal ratio is 315mm/55mm = 5.7...A 55m doublet with fluorite at that focal ratio should show very good CA control - maybe better than is needed for 45x...But why oh why did they use that zoom... sure, the magnification range of 15-37x with the big body zoom would look a bit strange, but then they could have increased the focal length of the body a bit to 380mm and it would have been roughly 19-45x... that would have made the objective f6.9 and any FPL-51 clone ED glass would have given great results too to bring down the price.I'm quite sure they could have kept this one below 1000g and a bit longer than it's now and it would have been quite a bit more attractive.Joachim</blockquote>