Zoom eyepieces offer a convenient method of changing magnification without having to use multiple eyepieces. Unfortunately–there being no free lunch and all–there is a trade-off in optical quality when using a zoom eyepiece.
Above: A typical zoom eyepiece, TeleVue’s 8-24mm Click Stop Zoom
Advantages of Zoom Eyepieces
The main advantage of a zoom eyepiece is that it avoids the need to carry around, use, and own multiple eyepieces. A zoom covers a magnification range that normally requires at least 3 separate eyepieces. Most zooms are in the range of 8mm to 24mm. On most telescopes this range is from 40x to 100x at the low end of the magnification range, and from 125x to 300x on the high end. One good quality zoom eyepiece is usually less expensive than buying, say, a 25mm, a 15mm, and an 8mm eyepiece.
Instead of having to switch back and forth between different eyepieces, fishing around in the dark trying to find the right size, you can simply rotate a zoom eyepiece to change magnification and field of view. Zooms are also popular for star parties (especially events for kids) where one eyepiece can be used (i.e., sacrificed) instead of having to take out the better eyepieces from your collection.
Another big advantage of a zoom is that you can adjust it exactly to the appropriate viewing conditions. For example, on an 8″ SCT, a typical zoom eyepiece covers a magnification range of 80x to 250x. If you are observing the planets on a night of excellent seeing, 250x might work well. However, on a less-than-ideal night, maybe 200x would be preferable. And on a mediocre night, 150x might be all you could use. With fixed eyepieces, you would have to own an 8mm, a 10mm, and a 13mm eyepiece to cover all those possibilities, while a single zoom could be adjusted as needed. That being said, most observers still use fixed eyepieces, for the reasons below.
Disadvantages of Zoom Eyepieces
The primary drawback to zoom eyepieces is the loss of optical quality. Photographers know this effect from camera lenses: zooms are always more convenient but never as good optically as a good quality fixed lens. The same goes for telescope eyepieces.
Zoom eyepieces suffer from internal reflections because of the greater number of lenses and the fact that the lenses must move within the eyepiece. This means that the lens cannot be optimized for one focal length as a fixed eyepiece can. Instead, zooms are an optical compromise because of the moving lens internal elements. They have to perform well at a variety of focal lengths.
Zooms have more restricted field of view than fixed eyepieces. The apparent field of a zoom eyepiece usually changes throughout the zoom range but is normally a bit less (40-45°) than that of a typical standard eyepiece.
Since zoom eyepieces cover the short focal length (usually high magnification) range, they must be specially designed to have long eye relief if they are to be comfortable to use at the short end of the zoom range, especially for eyeglass-wearers. Some less-expensive zooms do not have long eye relief and will be harder to view through. Those with long eye relief are preferable but obviously more expensive.
Zoom Eyepiece Prices
Inexpensive zoom eyepieces cost around $100-150. These are usually not the best eyepieces in terms of overall sharpness, contrast, and eye relief. The most popular zooms are in the $200 price range. These are very good quality, although they do not perform quite as well as fixed eyepieces. The most expensive zooms are extremely high quality, such as the Pentax series, or special designs such as the TeleVue Nagler Zooms. These eyepieces cost $300-400.