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Rob,
As you have noted the 14mm Pentax XW has a little bit of field curvature in faster scopes. This is evident when comparing to the 13mm T6 Nagler, the upside is the Pentax is sharper on axis and has higher light transmission. The 20mm Pentax XW has slightly worse field curvature than the 14mm Pentax XW and realistically I think if you don't need the 20mm of eye-relief for use with glasses, the 22mm and 24mm Panoptics are better eyepieces in the 1.25" versions and the 20mm Nagler T5 is the best choice in 2" version. The 20mm Nagler T5 is an absolutley outstanding eyepiece, unfortunately the eye-relief is a fraction short for me to use with glasses on. As you correctly point out the shorter focal length Pentax XW's (5mm,7mm and 10mm) exhibit a perfectly flat field. This is a common occurence that not all focal lengths in an eyepiece series perform at the same level. For instance in the Series 4000 Meade Ultrawides, the 8.8mm and the 14mm are the shining lights and clearly better performers than some other focal lengths in the series. Likewise with the TV Panoptics, the focal lengths from 22mm and above seem to perform slightly better than the 15mm and the 19mm.
Of the Pentax XW's I own and have used I would rate them as follows:-
5mm = 9.7/10
7mm = 9.7/10
10mm = 9.7/10
14mm = 9.2/10
20mm = 9.0/10
It was for this reason that I gave the line an overall rating of 9.5/10 when I posted that comparison in May.
Mike, Louie and myself are currently doing some evaluation comparing the 13mm NT6, 14mm Pentax XW and the 14mm Meade Series 4000 UWA. Mike will post a detailed review when its all finished. In short all the eyepieces are very very good and you won't go wrong with whichever of the 3 you chose. My initial thoughts are that if I didn't need glasses I would buy the Nagler due to its sharper stars at the EOF, needing glasses I am very happy to stick with the Pentax, which IMO outperformed the other 2 in terms of on axis sharpness and light transmission.
I am keen to compare the 10mm Pentax XW against either the 9mm or 11mm Nagler T6 for the simple reason I think the Pentax will beat the Nagler at this focal length, due to the fact the 10mm Pentax XW has a flatter field than the 14mm Pentax XW.
Clear Skies
John B
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