"It has now been 20 years since Tele Vue designed the original 13mm Nagler eyepiece. From then on Tele Vue has always sought ways to obtain the largest possible true fields (i.e. longer focal length) while maintaining the basic Nagler concept; 82 degree apparent field with good correction to the edge. The previous Nagler versions were not conducive to scaling to longer focal lengths. A totally innovative design using four different exotic glass types (some larger than the 70mm Pronto objective!) in a 6 element configuration, opened the door to the largest true field possible in the Nagler eyepiece family. With fewer elements, Tele Vue also retained the high contrast achieved on the Radians and Nagler Type 4s. Although the sheer size and element shapes preclude using an ""instadjust"" eyeguard, the eye- relief is quite comfortable and the weight was actually kept below that of the 20mm Nagler Type 2! The true field is, however, more than 50% wider (or 2.35x the area). Tele Vue hopes the 31mm Nagler Type 5 gives you as much of a ""spacewalk"" jolt as the original 13mm Nagler did 20 years ago."