Do Zoom Eyepieces Work?
Zoom eyepieces provide the ability to obtain a range of focal lengths from a single eyepiece. It's basically a regular eyepiece with a moveable Barlow lens, though some of the eyepiece elements may also move. By varying the distance between the eyepiece and Barlow, you change the magnification. There have been good reports about the Tele Vue version, though most experienced observers tend to avoid zoom eyepieces. The problem is that for any given Barlow lens, there is only one position at which it performs optimally. A Barlow lens used outside its designed amplification factor induces spherical aberration. So while zoom eyepieces can be very convenient to use at times, the image is seldom as sharp as that provided in a single focal length eyepiece. Moreover, due to the torque required to rotate the barrel, you often end up having to remove the zoom eyepiece from the focuser in order to adjust the magnification. This is especially true in cold weather when it seems to tighten up quite a bit. I owned one of the less expensive zoom eyepieces a number of years ago, but before long I junked it!