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Message: <blockquote data-quote="elkcub" data-source="post: 1652559" data-attributes="member: 14473">Verily.  Taking this discussion to a practical conclusion, you are correct.  Once a human puts eyes to an instrument, all bets are off with regard to knowing the specifics.  There is a large literature, for example, having to do with instrument myopia (and related topics like "open field myopia" and "dark focus") that hasn't even been given considered.  I won't go into that, in deference to those who don't venture much beyond the "exit pupil," but, at best we have a kind of refrigerator light dilemma: i.e., does the darn thing really go off when the door is closed, ... all the time?  In the case of optical instruments, including binoculars, accommodation is usually not as idealized, even for emmetropes (perfectly sighted folks) viewing images at infinity.  They accommodate closer by varying amounts — as much as 4 diopters.  Accommodation is physiologically connected to convergence (and inter-pupil distance), as well as pupil diameter, incidentally, which decreases with accommodation and convergence.  Add individual differences to the soup, and we have a devil's brew.  In practice it is best to clear ones mind while using a binocular and "go with the flow."  :t:Ed</blockquote>