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Message: <blockquote data-quote="BruceH" data-source="post: 3270854" data-attributes="member: 106398">You had it right the first time.  With glasses, the eye cups are twisted in, without eye glasses, the eye cups are turned/extended out to allow the binocular to be braced against the face and provide (hopefully) the proper spacing between the eye and the eyepiece lens surface.  Here is an educational video from Eagle Optics that may make it easier to visualize what is going on.  <a href="https://www.youtube.com/watch?v=J0RCqpajMwA" target="_blank">https://www.youtube.com/watch?v=J0RCqpajMwA</a>There are two variables that come into play for proper fit.  One is the actual eye relieve and the other is the length of the eye cup in relationship to the eye relief.  A mis-match between the actual eye relief and the eye cup length is especially noticeable when the eye cups are extended.  If the eye cup length is to short in relation to the eye relief, then the lens surface is to close to the eye and black outs occur.  If the eye cup length is to long in relationship to the eye relief, then the eye is to far away from the surface of the lens and less then the full FOV is shown.</blockquote>