Tele Vue's 3mm-to-6mm Nagler Zoom is an exceptional zoom eyepiece , a specialized high-power eyepiece for dialing in the optimum power for planetary viewing with short-focal-length telescopes, such as fast apo refractors. It features click stops at half-millimeter intervals of focal length, a constant 50° apparent field of view, and an eye relief of 10 mm -- not sufficient for eyeglass wearers, but remarkably comfortable for an eyepiece of such short focal length
