In conventional zooms, the apparent field of view and the usable eye relief change as the focal length changes. Likewise, because conventional zooms are not parfocal (they don’t focus at the same point at all focal lengths), objects go out of focus as the magnification is changed with an ordinary zoom. These anomalies don’t happen with this very high power Nagler zoom. It physically lengthens and contracts as the focal length is changed, as can be seen in the feature images you can click on above.
This unique approach (conventional zooms all move lenses within their body to change their focal length, while the Nagler physically changes its 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.
This high-power 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 to other zooms with an additional two or three light-scattering lenses in their light path. The eyepiece has click stops at 2mm, 2.5mm, 3mm, 3.5mm, and 4mm 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 under 800mm in focal length, like the TV-60. 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. This is the ultimate tool for tweaking out the last bit of observing power that the atmosphere will permit.
If you own an ultra-premium short focal length refractor, or any other top quality short focal length scope, there’s no longer a reason to have to accept second best when it comes to high power lunar and planetary observing. This high-power Nagler zoom will match the quality of your scope optics. It will put to rest forever the idea that it is impossible to produce a zoom eyepiece that does not degrade the image.