Lumping the Naglers together isn't appropriate. The type 4, 5, and 6 Naglers are three completely different designs, and none of them cover the full focal length range.
I disagree with that...and so does the TV literature. The T6 design optimizes the 82 AFOV in shorter FLs, and the T5 design optimizes it for the longer FLs. This was and is the strategy they used. Then the T4 line was done to address those who need longer ER. So the Nagler T5 and T6 combination of prescriptions is the TV "solution" to a full range of 82 degree AFOV EPs with mid-ranged ER.
From the TV Site:
[i]Why Are There 3 Types of Naglers?
To achieve the full magnification range over 82° apparent field of view requires two types: Nagler Type-5 with focal lengths of 31, 26, 20, 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), while the shorter focal length Type-6 models 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. i]