- A 10mm eyepiece delivers high magnification for planets, the Moon's craters, and tight double stars.
- A 20mm eyepiece gives moderate power with a wider field of view, making it more versatile for general observing.
- Most observers use a 20mm more often, but a 10mm is essential for planetary detail.
- Eye relief is a real issue at 10mm in cheap Plossl designs. Wide-angle eyepieces solve it.
- If you can only buy one, a zoom eyepiece (7-21mm) covers both focal lengths in a single barrel.
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You just bought a telescope, and now you're staring at eyepiece listings wondering: do I need a 10mm or a 20mm?
The short answer: they do completely different jobs. One zooms in tight on planets. The other gives you a wide, bright view for sweeping the sky. Understanding the difference will save you from buying the wrong one first.
Let's break it down with real numbers, real targets, and real product recommendations at every price point.
The Quick Answer
- 10mm = high power. Planets, lunar craters, double stars, planetary nebulae. Needs steady skies.
- 20mm = medium power. Star clusters, nebulae, galaxies, the full Moon, general scanning. Works in any conditions.
- Own both? Start with the 20mm to find your target, swap to 10mm for detail. That's what experienced observers do.
How Magnification Actually Works
Magnification = your telescope's focal length ÷ the eyepiece focal length. That's the only formula you need.
Here's what that looks like in practice with common telescopes:
| Telescope | Focal Length | With 20mm | With 10mm |
|---|---|---|---|
| Short refractor (70mm f/5.7) | 400mm | 20x | 40x |
| 6" Dobsonian (f/8) | 1200mm | 60x | 120x |
| 8" Dobsonian (f/6) | 1200mm | 60x | 120x |
| Celestron NexStar 4SE (f/13) | 1325mm | 66x | 133x |
| Celestron NexStar 6SE (f/10) | 1500mm | 75x | 150x |
Notice the pattern: a 10mm always gives exactly double the magnification of a 20mm on the same scope. That's the core tradeoff. Double the zoom, but half the field of view and a dimmer image.
Related: Telescope Magnification: Beginner's Guide With Examples
Side-by-Side Comparison
| Feature | 10mm Eyepiece | 20mm Eyepiece |
|---|---|---|
| Magnification | High (2x the 20mm) | Moderate |
| Field of view | Narrow | Wide |
| Image brightness | Dimmer | Brighter |
| Eye relief (Plossl) | ~6-7mm (tight) | ~12-15mm (comfortable) |
| Eye relief (wide-angle) | ~13-20mm | ~15-20mm |
| Best for | Planets, Moon detail, double stars | Star clusters, nebulae, general use |
| Seeing sensitivity | High (blurs in turbulence) | Low (forgiving) |
| How often you'll use it | ~30% of sessions | ~70% of sessions |
The 20mm is the workhorse. The 10mm is the specialist. You want both in your case eventually, but they serve very different roles.
What a 10mm Eyepiece Is Best For
Planets
This is where a 10mm earns its place. On a steady night with a 6" or larger scope, you'll see:
- Jupiter: Cloud bands, the Great Red Spot, and all four Galilean moons as distinct points
- Saturn: The Cassini Division in the rings, the shadow of the planet on the rings, and Titan
- Mars: Dark surface features (Syrtis Major) and polar ice caps during opposition
- Venus: Crescent phases clearly defined
A 20mm will show you these planets too, but they'll be small disks. The 10mm is what makes them look like actual worlds.
Related: 5 Best Telescope Eyepieces for Viewing Planets
The Moon (Close-Up)
With a 10mm, you're not looking at the Moon. You're exploring it. Individual craters jump out with sharp shadow detail along the terminator line. Mountain ranges look three-dimensional. Rilles (lava channels) become visible in the maria. The experience is completely different from a wider eyepiece.
Related: What Telescope Do I Need to See Saturn?
Double Stars & Planetary Nebulae
Splitting tight double stars (like Albireo or the Double Double in Lyra) requires magnification. A 10mm on a 1200mm scope gives 120x, enough to cleanly split many pairs. Small planetary nebulae like the Ring Nebula (M57) also benefit from the higher power to reveal their structure.
What a 20mm Eyepiece Is Best For
Deep Sky Objects
Galaxies, nebulae, and star clusters are big, faint targets. They need the wider field of view and brighter image that a 20mm provides:
- Orion Nebula (M42): Fills the field with glowing gas and embedded stars
- Andromeda Galaxy (M31): The bright core and dust lanes visible in a single view
- Pleiades (M45): The full cluster with nebulosity around the brightest stars
- Hercules Cluster (M13): A dense ball of stars starting to resolve at the edges
Try viewing M42 at 10mm and then at 20mm. At 10mm you see the Trapezium cluster in the center. At 20mm you see the entire nebula with its wings. Both views are worth having.
The Moon (Full Disk)
A 20mm at moderate power shows you the entire lunar disk with enough detail to identify major features: craters Tycho, Copernicus, and Plato; the maria; the mountain ranges at the terminator. It's the "big picture" view that orients you before zooming in.
Finding and Framing Targets
Even experienced observers start every session with a medium-power eyepiece. The wider field makes it far easier to locate objects, center them, and then swap to higher power. If you jump straight to 10mm, you'll spend 10 minutes trying to find what the 20mm would've shown you in seconds.
Poor Seeing Nights
On nights when the atmosphere is turbulent (stars twinkling hard, images boiling at high power), a 20mm saves the session. Lower magnification is far more forgiving of bad seeing. You can still enjoy great views of clusters, wide double stars, and nebulae when the 10mm would show nothing but mush.
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Best 10mm Eyepieces (Every Budget)
Not all 10mm eyepieces are created equal. The difference between a $15 Plossl and a $150 premium eyepiece is mostly eye relief and apparent field of view. Here are the best options from budget to premium.
Budget: Astromania 10mm Plossl (~$18)
The Astromania Plossl does one thing well: it delivers sharp, high-contrast views for very little money. If you're upgrading from a kit eyepiece and don't wear glasses, this is the no-brainer starting point. The 52-degree field of view is standard for Plossls, and the metal construction feels solid.
The downside is the eye relief. At 10mm focal length, Plossls give you roughly 7mm of working distance. You have to press your eye very close to the lens. If you wear glasses, skip this and go to the next tier.
Best Value: SVBONY 10mm 62° Wide Angle (~$28)
This is the eyepiece most experienced observers recommend to beginners. The 62-degree field of view is a noticeable jump from the 52 degrees of a Plossl. The wider field makes it easier to keep objects centered and gives a more immersive viewing experience. The aspheric element helps control edge distortion.
For roughly $10 more than the Astromania Plossl, you get a meaningfully better experience. If you're buying one 10mm eyepiece and want the best bang for your dollar, this is it.
Mid-Range: Celestron Ultima Edge 10mm (~$70)
The Ultima Edge line is Celestron's answer to the question "what comes after Plossls?" The flat field design means stars are sharp across the entire view, not just the center. This matters most with faster telescopes (f/5 or lower) where Plossls show noticeable edge distortion.
If you own a Dobsonian or a fast Newtonian reflector, the Ultima Edge is worth the upgrade. The parfocal design also means you can swap between Ultima Edge eyepieces without refocusing, which is genuinely convenient.
Premium: Celestron 10mm Luminos (~$130)
The Luminos is the eyepiece that makes you understand why people spend money on glass. At 82 degrees apparent field of view, you stop feeling like you're looking through a tube and start feeling like you're floating in space. Jupiter with its moons, Saturn with its rings, the Moon's craters: everything becomes an experience rather than an observation.
The 15mm eye relief is comfortable even with glasses, and the 7-element design keeps the image sharp to the edges. This is a 2-inch barrel eyepiece, so make sure your focuser accepts 2-inch eyepieces.
Premium (Modular): Baader Hyperion 10mm (~$150)
The Hyperion's killer feature is the 20mm eye relief at a 10mm focal length. That's nearly unheard of. Eyeglass wearers get the same comfortable experience as everyone else. The modular system lets you buy optional fine-tuning rings to shift the focal length (e.g., turning a 10mm into an 8mm or 13mm), so one eyepiece covers multiple magnifications.
It works in both 1.25" and 2" focusers out of the box. If you're building a serious eyepiece collection and want flexibility, the Hyperion system is hard to beat.
Best 20mm Eyepieces (Every Budget)
A 20mm eyepiece is your do-everything workhorse. It's the one that stays in the focuser 70% of the time. Here are the best picks.
Budget: WEOOEN 20mm Plossl (~$12)
At 20mm, the Plossl design actually works well. Unlike the 10mm where eye relief is painfully tight, a 20mm Plossl gives you about 12mm of working distance, which is fine for most people. The WEOOEN delivers clean views with good contrast and a solid metal barrel.
If your telescope came with a plastic 20mm kit eyepiece, swapping it for this metal Plossl is a noticeable improvement in both image quality and build quality.
Best Value: Explore Scientific 62° 20mm Waterproof (~$85)
Explore Scientific's 62-degree series punches well above its price class. The waterproof construction means you don't have to worry about dew or accidentally setting the eyepiece down on wet grass. The 62-degree field is wide enough to feel immersive without breaking the bank, and the near-parfocal design across the line means switching eyepieces is smooth.
This is the 20mm eyepiece that will stay in your collection for years, even as you upgrade the rest of your gear.
Related: Telescope Eyepiece Sizes: Full Guide
The Zoom Eyepiece Alternative: Why Not Both?
If the 10mm-vs-20mm question is causing decision paralysis, here's the cheat code: a zoom eyepiece that covers both.
The SVBONY SV135 zooms from 7mm (even higher power than 10mm) to 21mm (slightly wider than 20mm). You twist the barrel to change magnification on the fly. Found a planet at 21mm? Zoom in to 7mm without pulling the eyepiece out. It's the Swiss Army knife approach.
The tradeoffs:
- The apparent field of view narrows as you zoom in (from about 40° at 21mm to 30° at 7mm)
- A dedicated wide-angle 10mm like the SVBONY 62° will give you a wider, more immersive view than the zoom at 10mm
- Image quality is slightly below a premium fixed eyepiece at any given focal length
The advantages:
- One eyepiece covers the entire 7-21mm range, no swapping in the dark
- Great for learning what magnification works best for different targets
- Perfect travel eyepiece when you don't want to pack a full case
- Costs less than buying separate 10mm and 20mm eyepieces
For beginners who want to explore, this is genuinely the smartest first purchase. Once you know your preferred magnifications, you can buy dedicated eyepieces to improve on the zoom's performance at those specific focal lengths.
Related: Zoom Telescope Eyepieces: A Guide to Types & Models
Which Should You Buy First?
Here's the decision tree:
- You only have the kit 25mm eyepiece → Buy a 10mm. It fills the biggest gap and gives you a dedicated planetary eyepiece. The kit 25mm handles the wide-field role well enough for now.
- You have a 25mm and want better wide views → Buy a quality 20mm (Explore Scientific 62°). It's a real upgrade over the kit piece.
- You can't decide → Buy the SVBONY SV135 zoom (7-21mm). Covers everything. Upgrade to dedicated eyepieces later.
- You wear glasses → Avoid Plossl at 10mm. Go SVBONY 62° or Baader Hyperion for the eye relief.
- Budget is $30 total → SVBONY 10mm 62° wide angle. Best single upgrade for the money.
The honest truth: most observers eventually own both. A 20mm and 10mm together cover 80% of what you'll ever want to look at. Add a 2x Barlow lens, and those two eyepieces become four (effectively 20mm, 10mm, 10mm Barlowed = 5mm, and 20mm Barlowed = 10mm overlap).
Frequently Asked Questions
Is a 10mm or 20mm eyepiece better for planets?
A 10mm is better for planets when the seeing is steady. It delivers roughly double the magnification, revealing cloud bands on Jupiter and the Cassini Division in Saturn's rings. On turbulent nights, the 20mm produces steadier views.
What magnification does a 10mm eyepiece give?
Divide your telescope's focal length by 10. A 1000mm telescope gives 100x. A 650mm scope gives 65x. A 1500mm scope gives 150x.
Can I use a 10mm eyepiece on a beginner telescope?
Yes, but results depend on the scope. On a short 400mm refractor, a 10mm only gives 40x, which is fine. On a long 1500mm scope, the 150x may exceed what the optics and atmosphere support. Start with 20mm, then try 10mm when the image is steady.
Should I buy a 10mm or 20mm eyepiece first?
If you only have the kit 25mm, buy a 10mm first. It fills the biggest gap. If you already have a short focal length eyepiece, a quality 20mm upgrade will serve you more often. If truly undecided, the SVBONY SV135 zoom (7-21mm) covers both.
Is eye relief a problem with 10mm eyepieces?
In Plossl designs, yes. A 10mm Plossl gives about 6-7mm of eye relief, which is tight. Wide-angle designs like the SVBONY 62° (15mm) or the Baader Hyperion (20mm eye relief) solve this completely.
Do I need both a 10mm and 20mm eyepiece?
Ideally, yes. They complement each other perfectly. Use the 20mm to find and frame targets, swap to 10mm for detail. If budget is tight, a zoom eyepiece like the SVBONY SV135 (7-21mm) covers both focal lengths.
What's the best budget 10mm eyepiece?
The SVBONY 10mm 62° wide angle (~$28) offers the best value. Wider field than a Plossl, decent eye relief, and sharp optics. The Astromania 10mm Plossl (~$18) is the absolute cheapest option that still performs well.
Related Guides
- 10mm vs 25mm Eyepiece Comparison
- 9mm vs 25mm Eyepiece Comparison
- 25mm vs 32mm Eyepiece Comparison
- Best Eyepieces for Viewing Planets
- Telescope Eyepiece Sizes (Full Guide)
- Zoom Telescope Eyepieces Guide
- Telescope Magnification Beginner's Guide
- Barlow Lenses Explained
- Barlow Lens vs Eyepiece
- 2-Inch Telescope Eyepieces Guide
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