In conventional zooms, the apparent field of view and the usable eye relief change as the focal length changes. Likewise, objects go markedly out of focus as the magnification is changed, as conventional zooms are not parfocal (they don't focus at the same point at all focal lengths). These anomalies don't happen with the Nagler zoom. The Nagler zoom physically lengthens and contracts as the focal length is changed.
This unique approach (all other zooms move lenses within their body to change their focal length) allows the apparent field, eye relief, and focus to stay the same, no matter what the focal length. You don't have to constantly adjust the focus as you change powers, as you do with other zooms (minor tweaking may be necessary, however). And you don't have to adjust your eye position to accommodate a changing eye relief and a changing field stop diameter.
The Nagler zoom has five fully-multicoated elements in three groups, similar to a fine camera lens. By keeping the number of optical elements low, the contrast stays high when compared with other zooms that have an additional two or three light-scattering lenses in their light path. The eyepiece has click stops at 3mm, 4mm, 5mm, and 6mm for quick changes to precise preset magnifications. Naturally, you can use any intermediate focal length between the click-stops, as well. There's a fold-down rubber eyeguard and a rubber grip ring.
The Nagler zoom is designed to provide the best balance of contrast, comfortable eye relief, and resolution for the critical planetary observer, particularly those with ultra-premium short focal length refractors. It allows the observer to dial in the right balance between magnification, exit pupil, and atmospheric turbulence to achieve the best possible image for the observing conditions at any given moment.
If you own an ultra-premium short focal length refractor, or any other top quality scope, there's no longer a reason to have to accept second best when it comes to high power lunar and planetary observing. There's finally a zoom eyepiece that matches the quality of your scope optics. This Nagler zoom will put to rest forever the idea that it is impossible to produce a zoom eyepiece that does not degrade the image.
