Tele Vue 3.5mm Nagler
Designed by Al Nagler and sporting his name, the Nagler eyepiece design came about as an effort to produce a sharp, high contrast wide angle "spacewalk" 82deg AFOV(apparent Field Of View) viewing experience through a telescope.
To achieve the full magnification range over 82deg apparent field of view requires two types: Nagler Type-5 with focal lengths of 31, 16mm and Nagler Type-6 with focal lengths of 13, 11, 9, 7, 5, 3.5, 2.5mm.
The Type-5 permits the largest true field possible (31mm has 42mm field stop).
Type-6 models have shorter focal lengths, are parfocal, with similiar size and weight, and all with a comfortable 12mm eye relief.
The Type-4 models feature about 18mm of eye relief and with the "instajust" eyeguard are best for eyeglass wearers or Dioptrx users in their 22, 17, 12mm focal length range.
Note: some Nagler models have been retired.