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Message: <blockquote data-quote="elkcub" data-source="post: 1265177" data-attributes="member: 14473">The "emmetropic eye" is one in which distant objects are sharply focused on the retina with fully relaxed accommodation.  It is an ideal state.  "Far-sighted" (hypermetropic) young eyes can often compensate for lack of emmetropia via accommodation, but old ones need glasses.  "Near-sighted" (myopic) eyes always need glasses, whether they are young or old.  Incidentally, this also relates to another human factor — how instrument focusing is accomplished.  To assure the most relaxed accommodation possible, the image should be brought into focus from far to near.  The Navy used to go to some lengths training sailors this way, to minimize eye fatigue over long periods on watch.  I wouldn't be surprised if astronomers use a similar rule of thumb.The relevance here, of course, is that eye strain contributes to measurement variability.  For vision studies, this technique would qualify a "trained observer."You asked.Ed</blockquote>