Choosing the right telescope lens can feel like a big decision, especially when you’re just starting out. You’ve probably seen the terms 10mm and 20mm and wondered what the difference really means for what you can see. It’s totally normal to find the 10mm vs 20mm telescope lens question a little confusing at first.
But don’t worry, it’s not as complicated as it seems! We’ll break it down simply so you can make a clear choice. Let’s get started on figuring out what each one does and how it helps you explore the night sky.
Magnification and Field of View Explained
When you look through a telescope, the lens, often called an eyepiece, is what you hold up to your eye. This eyepiece is a key part of your telescope, and it determines two main things about what you see: magnification and field of view. Think of magnification as how much closer something appears, and field of view as how much sky you can see at once. These two elements work together to create your observing experience.
Understanding Magnification
Magnification tells you how much bigger an object will look through your telescope. It’s calculated by dividing the telescope’s focal length by the eyepiece’s focal length. For example, if your telescope has a focal length of 1000mm and you use a 10mm eyepiece, your magnification is 1000mm / 10mm = 100x. If you use a 20mm eyepiece with the same telescope, your magnification is 1000mm / 20mm = 50x.
Higher magnification makes objects appear larger, which is great for seeing details on planets like Jupiter’s moons or the rings of Saturn. However, very high magnification can also make the image dimmer and shakier, especially if the air isn’t steady. It can also narrow your view, making it harder to find faint objects.
What is Field of View
The field of view (FOV) is the area of the sky you can see through your eyepiece. It’s usually measured in degrees. A wider field of view lets you see more of the sky at one time. This is like looking through a wide-angle camera lens versus a telephoto lens.
Eyepieces with longer focal lengths, like a 20mm, generally offer a wider field of view. This makes them excellent for scanning the sky, finding objects, and viewing large celestial objects like star clusters or nebulae. A wider FOV can also make it easier to track moving objects like the Moon as it drifts across the sky.
10mm vs 20mm Telescope Lens Key Differences
The main difference between a 10mm and a 20mm telescope lens, or eyepiece, lies in their focal length. This difference in focal length directly impacts both magnification and field of view. Understanding these impacts is essential for choosing the right eyepiece for different observing situations.
10mm Eyepiece Characteristics
A 10mm eyepiece has a shorter focal length. As we saw, this means it will provide higher magnification when used with your telescope.
* Higher Magnification: This makes distant objects appear larger and closer.
* Narrower Field of View: You’ll see a smaller patch of the sky.
* Best For: Detailed views of planets, the Moon, and binary star systems. When you want to see the fine details, a 10mm is often the go-to.
* Potential Drawbacks: Can make images appear dimmer. More susceptible to atmospheric turbulence, causing a “wobbly” image. Finding faint, widespread objects can be challenging.
Imagine looking at the Moon. With a 10mm eyepiece, you might see craters and surface features in great detail, but you’ll only see a small section of the Moon at a time.
20mm Eyepiece Characteristics
A 20mm eyepiece has a longer focal length. This leads to lower magnification and a wider field of view.
* Lower Magnification: Objects will appear smaller but more in context.
* Wider Field of View: You’ll see a larger area of the sky.
* Best For: General stargazing, finding objects, viewing large deep-sky objects like the Andromeda Galaxy or the Pleiades star cluster. Great for casual viewing and when starting out.
* Potential Advantages: Images tend to be brighter. Easier to locate and track celestial objects. More steady image, especially in less-than-perfect atmospheric conditions.
Using the Moon example again, a 20mm eyepiece would show you a larger portion of the Moon, allowing you to see its context within a larger expanse of space. It’s like getting a wider shot in a movie.
When to Use Each Eyepiece
The choice between a 10mm and a 20mm eyepiece isn’t about which one is “better,” but rather which one is better for what you want to see right now. Different celestial objects and observing conditions call for different tools.
Using Your 10mm Eyepiece
Your 10mm eyepiece is your high-detail specialist. It’s your best friend when you want to zoom in.
* Planetary Observation: When planets like Jupiter, Saturn, Mars, or Venus are well-positioned in the sky and the atmosphere is calm, a 10mm eyepiece can reveal astonishing detail. You might see Jupiter’s Great Red Spot, Saturn’s Cassini Division in its rings, or the polar ice caps on Mars.
* Lunar Details: For studying the Moon, the 10mm offers a close-up view of craters, mountains, and valleys. You can spend hours just exploring different lunar features.
* Double Stars: Some binary star systems are very close together. Higher magnification helps to separate these stars and appreciate their binary nature.
* When to Be Cautious: If the air is “shimmery” (poor seeing conditions), high magnification can make the view very unstable. You might find yourself better off with a lower power eyepiece.
Using Your 20mm Eyepiece
The 20mm eyepiece is your workhorse for general sky surveys and larger objects. It’s often the most used eyepiece for many beginners and experienced astronomers alike.
* Finding Objects: The wider field of view makes it much easier to locate faint objects like galaxies, nebulae, and star clusters. You can sweep the sky and spot your target before switching to higher magnification if needed.
* Deep-Sky Objects: Many large and beautiful objects, such as the Andromeda Galaxy, the Orion Nebula, or the Pleiades, are best viewed with a wider field of view to appreciate their full extent.
* General Stargazing: For simply enjoying the night sky, the 20mm provides a comfortable and expansive view. It’s great for looking at the Milky Way or large open star clusters.
* Low Power, High Comfort: Lower magnification is generally easier on the eyes and provides a more stable image, especially on nights with turbulent air. This makes it ideal for longer observing sessions.
Factors Affecting Your View
Beyond the eyepiece itself, several other things influence what you see. Knowing about these factors can help you get the best possible views with either your 10mm or 20mm eyepiece.
Telescope Aperture
The aperture of your telescope is the diameter of its main lens or mirror. A larger aperture gathers more light, which is crucial for seeing fainter objects and for higher magnification. While eyepiece focal length affects magnification, the telescope’s aperture sets the ultimate limit on how much detail you can resolve.
Atmospheric Seeing
The condition of the Earth’s atmosphere significantly impacts what you see. “Seeing” refers to the steadiness of the air. On nights with good seeing, the air is calm, and images are sharp. On nights with poor seeing, the air is turbulent, causing images to appear blurry and wavy, especially at high magnifications.
Light Pollution
Where you are observing from matters a lot. Light pollution from city lights can wash out fainter objects, making them difficult or impossible to see. A darker sky will reveal much more, regardless of your eyepiece choice.
Eyepiece Types and Their Impact
Not all eyepieces are created equal, even with the same focal length. Different designs affect image quality, field of view, and comfort.
Plössl Eyepieces
Plössl eyepieces are very common and offer a good balance of performance and price. They typically have a 50-degree field of view. A 10mm Plössl will give you high magnification with a moderate FOV, while a 20mm Plössl will offer lower magnification with a wider FOV.
Wide-Field Eyepieces
Some eyepieces, like those with 65-degree or even 80-degree fields of view, offer a much more immersive experience. If you have a 10mm or 20mm eyepiece with a wider FOV, it can make objects seem to pop out of the background more dramatically.
Comparing Magnification and Field of View Visually
Let’s visualize what you might see. Imagine you are looking at the Moon.
* With a 10mm eyepiece (high magnification, narrow FOV): You see a section of the Moon about the size of a large coin held at arm’s length. You can clearly make out individual craters, the texture of the lunar surface, and perhaps some mountain ranges. It feels like you’re very close.
* With a 20mm eyepiece (lower magnification, wide FOV): You see an area of the Moon about the size of a dinner plate held at arm’s length. You can see entire crater formations, a good portion of a lunar mare (the dark, smooth areas), and the overall landscape of that region. You get a sense of the larger features and their arrangement.
Now consider a large nebula like the Orion Nebula.
* With a 10mm eyepiece: You might see a bright, detailed core of the nebula but miss the fainter, more extensive surrounding gas clouds.
* With a 20mm eyepiece: You can encompass the entire nebula, including its fainter outer regions, allowing you to appreciate its full size and shape.
Which Eyepiece is Better for Beginners
For someone just starting out, a 20mm eyepiece is often recommended as a first purchase or as part of a starter set. Here’s why:
* Ease of Use: The wider field of view and lower magnification make it much easier to find and observe celestial objects. There’s less frustration in trying to locate targets.
* Versatility: A 20mm is useful for a wide range of objects, from the Moon and planets to brighter deep-sky objects. It’s a great all-around eyepiece.
* Comfort: The lower magnification is easier on the eyes and provides a more stable image, which is more enjoyable for beginners.
This doesn’t mean a 10mm eyepiece isn’t useful for beginners. If you are particularly interested in detailed views of the Moon and brighter planets, and you are patient with finding objects, a 10mm can also be a rewarding choice. However, the 20mm usually offers a gentler introduction to astronomy.
Building Your Eyepiece Collection
Most telescopes come with one or two eyepieces. As you become more interested, you’ll likely want to expand your collection.
Complementary Focal Lengths
It’s helpful to have a range of focal lengths. A common approach is to have:
* A low-power eyepiece (like 20mm or 25mm) for wide-field views and finding objects.
* A medium-power eyepiece (like 10mm or 12mm) for detailed views of planets and smaller objects.
* A high-power eyepiece (like 5mm or 6mm) for very detailed planetary or lunar viewing, but only on nights with excellent atmospheric conditions.
Magnification Limits
Remember that there’s a limit to how much magnification your telescope can effectively use. This is called the “useful magnification.” It’s roughly 50x per inch of aperture. So, a 4-inch telescope (102mm) has a useful magnification limit of around 200x. Pushing beyond this often results in a dim, blurry, and uninteresting view, even with a short focal length eyepiece.
Frequently Asked Questions
Question: Which eyepiece is best for viewing galaxies
Answer: For viewing galaxies, a 20mm eyepiece or even longer focal length eyepieces with a wide field of view are generally better. Galaxies are large, faint objects, and a wider view helps you see their full extent and context. High magnification often makes them appear dimmer and harder to find.
Question: Can I use a 10mm eyepiece on any telescope
Answer: Yes, you can physically insert a 10mm eyepiece into most standard telescope focusers. However, whether it provides a good view depends on your telescope’s focal length and aperture. Very small telescopes might not provide a stable or detailed image with high magnification from a 10mm eyepiece.
Question: Is a 20mm eyepiece always less powerful than a 10mm
Answer: Yes, for the same telescope, a 20mm eyepiece will always provide lower magnification than a 10mm eyepiece. This is because magnification is calculated by dividing the telescope’s focal length by the eyepiece’s focal length. A larger denominator (20mm) results in a smaller quotient (magnification).
Question: Should I buy a 10mm or 20mm eyepiece first
Answer: For most beginners, a 20mm eyepiece is a better first choice. It offers a wider field of view and lower magnification, making it easier to find and observe a wider variety of celestial objects. It’s more versatile for general stargazing.
Question: How do eyepiece focal lengths affect brightness
Answer: Shorter focal length eyepieces (like 10mm) result in higher magnification and typically produce a dimmer image because the light is spread over a larger apparent area. Longer focal length eyepieces (like 20mm) provide lower magnification and tend to produce brighter images.
Final Thoughts
When comparing a 10mm vs 20mm telescope lens, remember that each has its own strengths. The 10mm lens is your tool for getting up close and personal with planets and lunar details, offering a magnified view that reveals fine features. It’s excellent for when you want to explore the intricate surfaces of celestial bodies. On the other hand, the 20mm lens excels at providing a broader perspective. It’s perfect for sweeping the night sky, locating larger deep-sky objects like nebulae and galaxies, and enjoying a more expansive view of star fields. For newcomers, starting with a 20mm is often easier because of its wider field of view and lower magnification, which makes finding and observing objects more forgiving. However, both are valuable additions to any astronomer’s kit. As you gain experience, you’ll find yourself reaching for different eyepieces depending on what you’re observing and the current sky conditions. Don’t be afraid to experiment, and enjoy the process of discovering the universe, one eyepiece at a time.
