Zoom telescope eyepieces are often recommended for beginner astronomers as a tool to achieve varying magnifications. However, not all zoom eyepieces are created equal, and one may or may not be what you need for your telescope.
Advantages & Disadvantages of Zoom Eyepieces
Zoom eyepieces, of course, allow you to achieve any magnification within their range. However, zoom eyepieces are limited by the fundamental ways in which they work. Zoom eyepieces usually work like a telephoto lens, moving internal magnifying lens groups to increase or decrease focal length. At the long end of a zoom eyepiece’s focal lengths, the apparent field of view can get narrow, and refocusing is required when making adjustments. The field of view is rarely, if ever, constant, either. The large number of lenses usually used in a zoom eyepiece can often mean they provide a dimmer image than an eyepiece with a fixed focal length, especially if they are cheaply made.
Zoom eyepieces are also not particularly useful in telescopes with fast focal ratios. They are typically not designed for good performance with the shallow angles of fast f/ratio telescopes’ light cones, and the varying spacing of the internal lenses means it is basically impossible for them to achieve good correction across the entire range of focal lengths.
Zoom eyepieces are definitely a compromise. They are never going to be the sharpest eyepieces you can get for your money. They are, however, useful if you are trying to figure out an ideal magnification for an object. They are also ideal for astronomy outreach with other people, where you may want the ability to quickly switch magnifications and/or minimize the risk of damage to multiple eyepieces from accidents.
You might benefit from getting a zoom eyepiece if you:
- Use a telescope with a focal ratio of f/6 or slower
- Want to buy a pair of eyepieces for use in a binoviewer
- Do a lot of astronomy outreach
You probably won’t like a zoom eyepiece if you:
- Want wide-angle views at low magnifications
- Use a telescope with a focal ratio faster than f/6
- Want the sharpest possible views of planets and other high-resolution targets
- Observe in cold weather which can make the zoom mechanism get too stiff to easily adjust
Best Zoom Eyepieces – Our Recommendations
1. Best Performance Zoom Eyepiece – Leica ASPH 8.9-17.8mm Zoom
This eyepiece has a wide apparent field of view, ranging from 60 degrees at the 17.8mm setting to 80 degrees at the 8.9mm setting. The eye relief is also excellent, making it easy to look through even if you have to wear glasses. The correction is good down to f/4. Below f/4, this eyepiece has too much edge-of-field astigmatism and curvature to be used even with a coma corrector, but few are likely to have such a fast instrument. Originally intended as a spotting scope eyepiece, the ASPH will require a specially made 1.25” or 2” adapter for use in a telescope. Of course, the price is quite high, to the point where buying a set of well-corrected premium ultra-wide angle eyepieces is actually cheaper.
2. Best Value Zoom Eyepiece – Baader Hyperion Mark IV 8-24mm Zoom
The Baader Hyperion Mark IV 8-24mm Zoom can be used as a 1.25” or 2” eyepiece and features an apparent field of 68 degrees at 8mm to 50 degrees at 24mm, or comparable to wide-angle and Plossl eyepieces. 50 degrees is about as narrow as is generally considered comfortable or acceptable nowadays. Correction towards the edges of the field of view is great at f/8 and acceptable down to f/5, at which point sharpness towards the edge of the field of view drops dramatically. Like the Leica ASPH, Baader offers a dedicated Barlow lens for this eyepiece.
3. Best Planetary Zoom Eyepiece – TeleVue Nagler 3-6mm Zoom
The Nagler Zoom is different from most zooms because it has a constant 50-degree apparent field and stays in focus even as the barrel extends. This eyepiece will be razor-sharp in any telescope. However, a mere 10mm of eye relief makes it a bit uncomfortable to look through and nearly impossible to use with glasses.
4. Best Cheap Zoom Eyepiece – Celestron 8-24mm Zoom
The Celestron 8-24mm Zoom has a narrow field of only 40 degrees at its 24mm setting, increasing to 60 degrees at 8mm. This eyepiece is not exactly “premium” and has some chromatic aberration as well as issues with sharpness in telescopes below f/6 to f/7. However, it’s great for astronomy outreach or binoviewing as it’s comfortable to look through and a lot less expensive than other offerings.
Other Zoom Eyepieces to Consider
- The Pentax XL 8-24mm SMC Waterproof Zoom is gigantic, dwarfing many eyepieces though still working in a 1.25” barrel. Using the Pentax XL may be a problem if you have a small telescope due to the sheer weight of this eyepiece. It has an apparent field of view ranging from 60 degrees at 8mm to 38 degrees at 24mm. In addition to being waterproof, the Pentax XL 8-24mm is extremely sharp and works well down to f/5 or so.
- The Pentax XF 6.5mm-19.5mm XF Zoom has an apparent field varying from 42 degrees at 19.5mm to 60 degrees at 6.5mm. It is sharp in telescopes slower than f/7 but has severe issues with field curvature and chromatic aberration in faster instruments. It’s also rather short on eye relief. For the price, it probably isn’t worth purchasing if you can get your hands on the Baader Hyperion Zoom instead.
Zoom Eyepieces to Avoid
Cheap 7-21mm zooms have issues with requiring precise eye placement to avoid “blackout” when looking through them at low power, and at the 21mm setting, the apparent field of view is only 30 degrees, feeling much like a drinking straw to look through, and expanding to a claustrophobic 43 degrees at the 7mm setting. These eyepieces also perform poorly in telescopes faster than f/8 or so, with numerous sharpness, chromatic aberration, and off-axis aberration issues. There are quality control issues with these eyepieces too.
The cheap 8-24mm “goldline” zoom designs often sold for under $50 USD have extremely narrow fields of view, loads of chromatic aberration and field curvature, little eye relief, and often quality control issues like dust particles and internal rattle. They are pretty much only useful as kids’ toys.
