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Message: <blockquote data-quote="henry link" data-source="post: 3440254" data-attributes="member: 6806">David,Looks like some genius at the ISO finally realized that expressing collimation errors in minutes of apparent angle rather than real angle greatly simplifies the standard since it doesn't vary with magnification.The old Japanese Telescope Inspection Institute (JTII) had what must be the strangest system of all (as represented in J.W. Seyfried "Choosing, Using, & Repairing Binoculars", pages 149-150). Collimation limits were "...expressed by the value of real angles in minutes...", but magnification was represented by broad ranges: Under 4.5X, 4.5X to 10X, 11X to 15X, 16X to 20X. The limits for 4.5X to 10X were: Vertical 4', Divergent 10', Convergent 6'.Under this system 10X binoculars are held to a very lax standard when expressed in apparent angle: Vertical 40', Divergent 100', Convergent 60'.Did you notice footnote 3 in the first link of your post #67 Apparently we can't even count on consistent definitions of convergence and divergence!Henry</blockquote>