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If that 0.5mag claim were true, the transparency of "brand X" eyepieces would have to be less than 63%.
It could be true if "brand x" didn't have multicoated surfaces on all air-glass interfaces, a distinct possibility in the 1970's.
But virtually all decent modern eyepieces are fully multicoated apart from the crap Meade MA ones, in which case the transparency will be better than 95% and that claim is simply untrue.
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