Nagler type 4 
82° apparent field, 17mm or more eye relief in all models. Focal lengths: 22mm (2") and 17mm (2") have 7 elements in 5 groups. 12mm has 6 elements in 4 lens groups. 

Nagler type 5 
82° apparent field, focal lengths available: 31mm (2"), 26mm (2"), 20mm (2"), and 16mm. Eye relief is approximately 62% of focal length. 6 elements in 4 lens groups. 

Nagler type 6 
82° apparent field, 12mm eye relief in all models. Focal lengths available: 13mm, 11mm, 9mm, 7mm, 5mm, 3.5mm, 2.5mm. 7 elements in 4 groups.