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Message: <blockquote data-quote="elkcub" data-source="post: 3194086" data-attributes="member: 14473">I'm afraid there is a semantic problem.  It's generally assumed in visual instrument design that the optical axes will correspond with the user's visual axes.  Since the eye seeks to obtain the lowest image blur, it will instinctively move to center a telescope to make that happen.  When confronted with divergent telescopes this is not possible, because to do so would require rotating the eyes outward where nature did not provide an appropriate motor mechanism.  Moreover, if the eyes could diverge there would be no corresponding points on the retinae and we would have vision systems that operate like birds with eyes on either side of their heads.  Convergence is what allows for a geometric horopter, — which is the basis for stereopsis.  Regarding Henry's related point it is also true that perfectly parallel tubes are only ideal at optical infinity.  At closer working distances some increasing amount of convergence is necessary to keep a target simultaneously focused on both retinae (which is the purpose of convergence).  The brain can deal with the problem to some extent (I don't know the tolerances) but at some point either: (a) one has to move the instrument's IPD closer, (b) close one eye and view monocularly, or (c) buy something like a Papilio that converges the optical axes to correspond with the visual axes.  Other than (c), any binocular built with with fixed divergent telescopes would exceed the brain's tolerance at a greater distance, and make that problem worse. EdPS.  It just occurred to me that if making matters a little worse at infinity doesn't matter, why not make the tubes slightly convergent?  Extra FOV is gained since the left side of the right field extends over the left field, and visa-versa.  Also, being pre-converged, there is some built-in compensation for closer working distances.  And, unlike a divergent tube design there actually would be an ideal distance at which everything worked perfectly.  :smoke:</blockquote>

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