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The 12mm Pentax XF is not quite as good as the 8.5mm Pentax XF in a fast newtonian. It exhibits some field curvature, whereas the Pentax XF 8.5mm is perfectly flat. The 12mm Pentax XF is still a very good eyepiece, with excellent contrast and light throughput. It is probably still the best value for money at this price level. It's just not quite to the same high standard as the 8.5mm Pentax XF.
Cheers,
John B
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