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Message: <blockquote data-quote="henry link" data-source="post: 1650592" data-attributes="member: 6806">I need to correct the figures I presented yesterday for naked eye changes in IPD with distance. They were a little off, like by at least an order of magnitude ;-). I'm sure Ron's geometry for 8x and 10x roof prism binoculars is correct and those angles would need to be doubled for a typical Porro, so they are small but still  significant for pupil misalignment at close birding distances. For instance a 10x Porro with objective centers separated by 125mm would require each eye to tow in about 9 degrees off-axis to center an object at 4m. Even after setting the IPD with great care we're still left with bobbling hand held binoculars and constant rapid eye movements to throw everything off.</blockquote>