A review of the 20mm ES68, 20mm Pentax XW, and 20mm Waterproof Meade UWA
The eyepiece market provides numerous options with a 20mm focal length ranging from inexpensive 20mm plossls to expensive “premium” options offered by Pentax and Televue. Eyepieces with a 20mm FL are very versatile giving exit pupils ranging from 4.4mm to 2.0mm with scopes having focal ratios from F/4.5 to F/10...
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