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Message: <blockquote data-quote="elkcub" data-source="post: 3440421" data-attributes="member: 14473">In the midst of ever-growing opto-mechanical confusion/consternation, I would suggest that the root demand is to stay within human observer tolerances.  If an observer attempts to align his/her eyes simultaneously with each of the optical axes of a binocular, then convergent axes force the eyes to diverge.  Here, there is no acceptable tolerance, because human eyes are simply not designed to do so.  If the axes diverge, then the eyes must converge.  That's ok.  Note, however, that the SPIE specs indicate a minimum of 5 to a maximum of 30 arcmin.   That's interesting, why a minimum?At optical infinity aren't the optical axes of the eyes perfectly parallel?  Actually, not.  Convergence and accommodation are physiologically connected through an adaptive feedback control system.  Resting eyes tend to accommodate by about one diopter, so some amount of convergence also occurs sympathetically.  My guess is that the binocular specs cater to this property of the visual system, recognizing that individuals differ and can also adapt the tonic state of their feedback controller.  Binoculars are simple; humans are not.  Anyway, as stated, the SPIE specs are independent of magnification.  Something's got to be simple.  Ed</blockquote>