Eyepieces of course

The original version of the previous post refered to 'objectives' instead of 'eyepieces' Thanks to a contributor to Birdforum for pointing out the obvious error. I have only two excuses: 1. I had been working on a consultancy project involving different microscope objectives and must have had objectives on the brain when I was writing about eyepieces for telescopes; 2. Is age catching up with me?  The post, including the diagram, has now been corrected.