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Message: <blockquote data-quote="henry link" data-source="post: 1649529" data-attributes="member: 6806">Interesting discussion, gentlemen. I think it would a good idea for someone (Ron?) to start a separate IPD thread or this digression will be buried forever.To throw in another twist. I just made a crude sighting device, which I'll describe later for those who want to try it. It allowed me to establish my infinity IPD and also IPDs for closer distances when the eyes are towed in. I found there's roughly a 4mm decrease in my naked eye IPD when observing an object at 10m compared to infinity, so I assume the decrease would be 2mm at 20m and 1mm at 40m. These differences should apply to the IPD with roof prism binoculars, but would double when using Porros with objectives spaced twice as wide as the eyes. These are larger differences than I would have expected (I'm sure Ron could easily do the math) and suggest to me that a single hypercritical IPD setting will only work for long distances.HenryEdit: Sorry folks. The figures I gave above were seriously miscalculated to say the least. The real numbers are far less for naked eye IPD. I haven't recalculated them, but I trust Ron's numbers for 8x and 10x roof prism binoculars which are still significant (even more for Porros) at distances below 10m.</blockquote>