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Message: <blockquote data-quote="elkcub" data-source="post: 1650731" data-attributes="member: 14473">Thanks, Ron.I've found very few books that discuss the issue of binocular tube convergence.  For those interested, however, Chapter 37 of "The Eye and Visual Optical Instruments" by Geo. Smith and Davis Atchison, has an interesting discussion of several issues mentioned in this thread.  The math is fully worked out, but, for those who would like to ignore the human oculomotor system (in deference to just the properties of the instrument, for example <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />), I'm afraid it might be somewhat frustrating.  The visual accommodation and convergence responses are inherently coupled, and both change with age.  The coupling can also be modified by learning.  "The ability to converge decreases with age (Pickwell, 1985), and this is accompanied by an increase in exophoria [tendency of eye to turn outwards relative to its fellow eye] with age.  Thus, according to MacLeod and Bannon (1973), older users may be better off with parallel binocular tubes."  pg. 735.  Good thing too, since I've gotten older and older over the years.  But, I didn't know that I was exophoric in addition to lacking accommodation.  Learn something every day, one does.    Someone above alluded to the the question of optical aberrations when viewing off-axis or cross axis.  I've been puzzled about the same thing for years, since it must be true that this circumstance occurs with close viewing, regardless of changing the binoculars' IPD.  All this is mainly academic, of course, and has little practical value.  I set my binoculars' IPD with a scale, as I think you described earlier, Ron.      Ed</blockquote>

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