Excerpts from The Backyard Astronomer's Guide review of the 3-6mm Nagler Zoom.

• An exceptional zoom eyepiece is the 3mm-to-6mm Nagler Zoom, a specialized high-power eyepiece for dialing in the optimum power for planetary viewing with short-focal-length telescopes, such as fast apo refractors.
• Imaging is superb, and - a first - the apparent field stays constant throughout the zoom. Click stops define the 6mm, 5mm, 4mm and 3mm positions, through the observer can select any value in between.

From Sky & Telescope's Hot Products review of the 3-6mm Nagler Zoom.

THERE HAVE BEEN ZOOM EYEPIECES IN THE PAST, BUT NONE QUITE LIKE THE Tele Vue 3mm-6mm Nagler Zoom ... . Since the time of Galileo, short-focal-length (hence high-magnification) eyepieces have been notoriously difficult to look through. That has changed in recent years, especially with the introduction of Tele Vue's line of Radian eyepieces (a 1999 Hot Product). The new Tele Vue zoom is small, lightweight, and offers the same magnifications (with click-stop settings) as four Radians while maintaining a constant focus and 50 degree apparent field. It also lets observers choose in-between focal lengths, thus giving them the ability to tune the magnification to atmospheric conditions, which is very critical for high-power viewing. Although the zoom's 10-millimeter eye relief isn't long enough for eyeglass wearers, it is enough to make this a very comfortable eyepiece to look through.