The 12mm Vernonscope Brandon eyepiece is designed for high power use with any telescope type, at focal ratios f/7 and above. It provides highly detailed/high contrast close-up lunar, planetary, star cluster, and binary star observing. With shorter focal length scopes, down to f/4, it is still exceptionally sharp at the center of the field (although somewhat less so at the edges).
While the designed and technically correct eye relief is considerably longer at 9.5mm, because of the recessed location of the eye lens, the measured usable eye relief of 4mm is too short for unvignetted use with eyeglasses. However, most of the objects being observed at the magnification this eyepiece provides are small. They are typically placed at the center of the field, where vision is sharpest and vignetting of the edge of the field due to eyeglasses should not be an issue. As a personal aside, your author uses this eyepiece in both his 3.5” and 7” Questars and has never had a problem with the eye relief, even when wearing glasses.
For a brief discussion of how we define and measure usable eye relief and its importance to you, an explanatory text box will pop up when you move your cursor over the term “Eye Relief” in the "Tech Details" pull-down tab above. Removing the eyepiece’s rubber eyecup will provide several mm of additional usable eye relief, but at the expense of exposing your eyeglass lenses to the metal top of the eyepiece.
The Brandon is the sharpest design currently available for high magnification eyepiece projection photography. The body of the 12mm is considerably narrower than most similar focal length Plössl eyepieces, allowing its use in an eyepiece projection adapter much more readily than a wider Plössl. Its rolldown rubber eyecup can be removed, should the eyepiece/eyecup combination still be too wide to fit easily into your eyepiece projection adapter.
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