With a 21mm focal length, the field of this 100° apparent field Ethos eyepiece is nearly 64% larger in area than the 82° field of a "space-walking" 20mm Nagler. The TeleVue Ethos delivers the true field size of a longer focal length (but narrower apparent field) eyepiece, such as a 26mm Nagler, but has the benefits of the higher power and darker sky background that are inherent in a shorter focal length.
There is a soft rolldown eyecup to shield the eye from ambient light (from a neighbor's security light, for example) and improve the image contrast. At a usable eye relief of 15mm, the 21mm Ethos would normally somewhat vignette the field of view for eyeglass wearers. However, since the 100° field is so wide that you can't see the full field without having to move your head from side to side to take it all in anyway, vignetting in the conventional sense is not a problem.
The 21mm Ethos has all the virtues that professional and amateur astronomers have come to expect from a TeleVue eyepiece: high contrast, comfortable eye relief, and full field sharpness - with well-controlled astigmatism, field curvature, lateral color, angular magnification distortion correction, and low pupil sensitivity (kidney bean effect) for daytime use. While sharpness is inherent to the optical design itself, the contrast is maximized through the intelligent use of flat finished baffles and ultra low reflectance, high efficiency coatings that are tuned to the glass composition of each lens element.
The Ethos is designed for 2" focusers and star diagonals only. A safety groove is machined into the 2" chrome barrel to engage your focuser thumbscrew. This will help prevent the eyepiece from falling should the thumbscrew accidentally loosen while observing.
The 21mm TeleVue Ethos eyepiece will accept TeleVue Dioptrx eyesight astigmatism correcting lenses.
Please note the 36 ounce (2.25 lb.) weight of this eyepiece if you have a Dobsonian telescope. Its use may require some temporary rebalancing of your scope, or the use of a TeleVue #TVEQC equalizing adapter with lighter 1.25" eyepieces to avoid balance problems.
