Zoom Eyepieces: The Best and Their Issues

Zoom telescope eyepieces are often recommended for beginner astronomers as a tool to achieve any magnification within their specified focal length ranges.

By simply adjusting the zoom setting, we can alter the eyepiece’s focal length and go from a wider view of the sky to a close-up view of celestial bodies without the need to switch eyepieces. This kind of flexibility also proves beneficial when we’re tracking moving satellites or even terrestrial objects like birds and aircraft, as we can easily adjust our view.

The Good Side

Compared to purchasing multiple fixed focal length eyepieces of comparable quality, a single zoom eyepiece is often more cost-effective. It’s an attractive option for those keen on maximizing value while minimizing expenses. This becomes even more true if you are using a binoviewer and need 2 of each eyepiece—getting two zoom eyepieces is a lot cheaper than buying several pairs of high-quality eyepieces.

They are also useful if I’m trying to figure out the ideal magnification for an object. I also consider them ideal for my astronomy outreach sessions, where I want the ability to quickly switch magnifications without constantly swapping eyepieces. That way, I also don’t need to carry multiple eyepieces around, which is very convenient. Plus, I’m only putting a single ocular at risk of theft or damage at the outreach event, which also obviates the need for me to carry a bulky and easily misplaced eyepiece case.

The Limitations

While I agree that zoom eyepieces’ versatility is unparalleled, fundamental ways in which they operate introduce limitations. Zoom eyepieces usually work by moving internal magnifying lens groups to increase or decrease focal length. As a result, the apparent field of view can get narrow at longer focal lengths, refocusing is required during adjustments, and the field of view is rarely, if ever, constant. These are why I firmly believe that zoom eyepieces are a compromise.

• Optical Quality Compromises

I see optical quality compromises particularly in those zoom eyepieces targeted at beginners or available at a lower price point. Compared to their fixed focal length counterparts, I’ve noticed more off-axis aberrations. As we move away from the center of the eyepiece, we might encounter distorted images, reducing the clarity of peripheral celestial objects. Chromatic aberration, as well as glare and ghost images, also plagues cheaper units that I’ve tested.

Zoom eyepieces are also not particularly useful in telescopes with fast focal ratios. They are typically not designed for good performance with the steep angles of fast f/ratio telescopes’ light cones.

Additionally, the large number of lens elements in a zoom eyepiece can mean a dimmer, less crisp image than an eyepiece with a fixed focal length—especially if the eyepiece is cheaply made. They are never going to be the sharpest eyepieces you can get for your money.

• Inconsistent Field of View

While zoom eyepieces offer a wide apparent field of view at their shorter focal length range (high magnifications), this field often significantly narrows as I increase the focal length to lower magnifications. In contrast, most high-quality fixed focal length eyepieces maintain a wide apparent field of view.

• Inconsistent focus and eye relief

Most zooms I’ve used still require refocusing when I change focal lengths, whereas many fixed-length eyepiece sets are parfocal with each other even though I have to swap out oculars for different magnifications.

Many zooms also change in eye relief as I adjust focal length.

These factors are both just consequences of the optical design of most zooms.

You might benefit from getting a zoom eyepiece if you fit one or more of the following:

• Use a telescope with a focal ratio of f/6 or slower
• Want to buy a pair of zoom eyepieces for use in a binoviewer
• Do a lot of astronomy outreach

You probably won’t like a zoom eyepiece if you fit one or more of the following:

• 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 too stiff to easily adjust

All these said, not all zoom eyepieces are created equal, and one may or may not be what you need for your telescope.

Zoom Eyepieces I’d Avoid

• Cheap 7-21mm zoom eyepieces

I’ve found that cheap 7-21mm zoom eyepieces demand precise eye placement to avoid “blackout” when looking through them at low magnifications. At the 21mm setting, the apparent field of view (AFOV) is only 30°, feeling much like a drinking straw to look through. Even at the 7mm setting, it only expands to a still claustrophobic 43° AFOV. In telescopes faster than f/8 or so, I see these eyepieces performing poorly with numerous sharpness, chromatic aberration, and off-axis aberration issues—and there are quality control issues too with these eyepieces.

• Cheap 8-24mm goldline zoom eyepieces

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, minimal eye relief, and frequent quality control issues like dust particles and internal rattle. To me, they are pretty much only useful as kids’ toys.

Zoom Eyepieces I Can Recommend

Best Performance Zoom Eyepiece – Leica ASPH 8.9-17.8mm Zoom

The Leica ASPH 8.9–17.8mm Zoom is as good as they come.

I love this eyepiece’s wide apparent field of view, ranging from 60° at the 17.8mm setting to 80° at the 8.9mm setting. The eye relief is also excellent, making it easy to look through even if I have to wear glasses.

The correction is good in telescopes with focal ratios down to about f/4. Below that, I start seeing too much edge-of-field astigmatism and field curvature—even with a coma corrector. But very few people are likely to have such a fast telescope anyway.

Originally intended for spotting scopes, the ASPH eyepiece requires a specially made 1.25” or 2” adapter for use in telescopes.

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.

See Amazon Price

Best Value Zoom Eyepiece – Baader Hyperion Mark IV 8-24mm Zoom

I can use this as a 1.25” or 2” eyepiece, and it features an apparent field of 68° at 8mm to 50° at 24 mm (comparable to wide-angle and Plossl eyepieces). 50° is about as narrow as I’d generally consider comfortable or acceptable nowadays.

Correction towards the edges of the field of view is great in telescopes with focal ratios of f/8 and acceptable down to f/5, beyond which the sharpness towards the edge of the field of view drops dramatically.

Like the Leica ASPH, Baader offers a dedicated Barlow lens for this eyepiece.

$359 at HighPoint

Best Planetary Zoom Eyepiece – TeleVue Nagler 3-6mm Zoom

The Tele-Vue Nagler 3-6mm Zoom is a specialty eyepiece, and its short focal length means it is only good if your telescope and atmospheric conditions can handle the high magnifications it is likely to provide.

The Nagler Zoom is different from most zooms because it has a constant 50° apparent field and stays in focus even as I zoom. This eyepiece is razor-sharp in any telescope I’ve tried.

However, a mere 10mm of eye relief makes it a bit uncomfortable to look through and nearly impossible to use with glasses.

$452 at HighPoint

Best Cheap Zoom Eyepiece – Celestron 8-24mm Zoom

The Celestron 8-24mm Zoom has a narrow field of view of only 40° at its 24mm setting, increasing to 60° at 8mm.

This eyepiece is not exactly “premium,” and I’ve noticed 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.

$99.95 at HighPoint

Zoom or Barlow: What’s the Better Deal?

A zoom eyepiece should have several click-stop settings—usually around four—for different focal lengths.

Alternatively, a Barlow lens combined with a pair of eyepieces would give us the same number of magnification options, often with significantly sharper views as well as other benefits.

The trade-off is that a good Barlow and a pair of good eyepieces could end up costing a lot more than a single zoom eyepiece—and they’ll take up three times as much space as a zoom eyepiece.