With 110 deg., the Ethos-SX has 21% more AFOV area than the 100 deg. Ethos design. This permits 62% power increase from a 6mm Ethos, while retaining 68% of its TFOV. The 3.7mm actually has more TFOV than Tele Vue's 8mm Plossl, 6mm Radian and 5mm Nagler Type-6! In 1-1/4" mode, it is parfocal with the wide range of 1-1/4" Tele Vue Plossls, Panoptics, Radians, Naglers, Nagler Zooms and 6mm and 8mm Ethos eyepieces. The screw-on 2" adapter (included) is designed to make it parfocal with 13mm and 10mm Ethos when they are used in their 2" modes.