If you’re new to telescopes, you might wonder which telescope lens is stronger 10mm or 20mm. It’s a common question, and the answer is key to getting great views of the night sky. The strength of a telescope eyepiece, which is the lens you look through, determines how much you can magnify an image. Understanding this basic idea will help you choose the right eyepiece for any target.
Let’s clear up the main point right away. In direct comparison, the 10mm eyepiece is the stronger lens. It provides higher magnification than a 20mm eyepiece when used in the same telescope. But “stronger” doesn’t always mean “better.” The best choice depends entirely on what you want to look at. Sometimes, a lower magnification with a wider view is much more useful.
Which Telescope Lens Is Stronger 10mm Or 20mm
To understand why the 10mm is stronger, you need to know about focal length. The focal length of an eyepiece is the number in millimeters printed on its side, like 10mm or 20mm. This number works with your telescope’s own focal length to create magnification.
Here’s the simple rule: a smaller eyepiece number means higher magnification. A 10mm eyepiece will always magnify more than a 20mm eyepiece on the same telescope. It’s like using a zoom lens on a camera; the 10mm lets you zoom in closer to the object.
How Magnification Actually Works
You calculate magnification using a straightforward formula. You divide your telescope’s focal length by the eyepiece’s focal length. The result is the magnification power.
Magnification = Telescope Focal Length / Eyepiece Focal Length
Let’s see an example with a common telescope that has a 1000mm focal length:
- With a 20mm eyepiece: 1000mm / 20mm = 50x magnification.
- With a 10mm eyepiece: 1000mm / 10mm = 100x magnification.
As you can see, the 10mm eyepiece doubles the magnification power. It makes objects appear twice as large as the 20mm does. This is the sence in which it is “stronger.”
Why More Power Isn’t Always the Answer
While the 10mm is stronger, higher magnification has trade-offs. It’s not the best tool for every job. Increasing magnification also amplifies problems.
- Dimmer Image: The light gathered by your telescope is spread over a larger area, making the view darker.
- Shakier View: Any vibration in your telescope mount or tripod is also magnified, making the image wobble more.
- Narrower Field of View: You see a much smaller patch of sky, making it harder to find and track objects.
- Atmospheric Distortion: On most nights, Earth’s atmosphere blurs details at very high powers. The view can become mushy.
The 20mm eyepiece, with its lower power, gives you a brighter, wider, and more stable view. This is often more pleasing and practical.
Choosing the Right Eyepiece for the Target
Now that you know the 10mm is stronger, let’s see when to use each one. Smart astronomers switch eyepieces based on their target.
When to Use the 20mm Eyepiece (Lower Power)
The 20mm is your go-to eyepiece for large, faint objects and for finding your way around. It’s perfect for:
- Viewing large star clusters and nebulae like the Pleiades or the Orion Nebula. You need the wide view to see the whole object.
- Finding objects initially. It’s much easier to navigate the sky with a wider view.
- Observing the full Moon. You can see the entire lunar disc comfortably.
- Nights with poor “seeing” (when the air is turbulent).
When to Use the 10mm Eyepiece (Higher Power)
Use the stronger 10mm lens when you want to see fine details on bright, compact objects. It’s ideal for:
- Studying the Moon’s surface: Craters, mountains, and rilles come into sharp detail.
- Viewing planets: You can see Jupiter’s cloud bands, Saturn’s rings, and the polar caps on Mars.
- Splitting double stars: Higher power helps separate close pairs of stars.
- Looking at the cores of bright star clusters.
Beyond 10mm and 20mm: Building Your Eyepiece Collection
Most telescopes come with one or two basic eyepieces. To get the most from your scope, you’ll want a small set. A good starter kit covers low, medium, and high magnification.
Here is a simple three-eyepiece plan for a telescope with a 1000mm focal length:
- Low Power (Wide View): A 25mm or 32mm eyepiece. Gives about 30x-40x magnification. Perfect for big nebulae and star-hopping.
- Medium Power (All-Purpose): A 15mm or 18mm eyepiece. Gives about 55x-65x magnification. Great for many clusters and larger galaxies.
- High Power (Detail): Your 10mm eyepiece. Gives 100x magnification. Used for planets, the Moon, and double stars.
You might later add a very high-power eyepiece, like a 6mm or 7mm, for nights of exceptional atmospheric stability.
Other Factors That Affect Your View
The focal length isn’t the only thing that matters on an eyepiece. Two other specs are crucial: the Apparent Field of View and the Eye Relief.
Apparent Field of View (AFOV)
This is how wide the view feels when you look into the eyepiece. It’s measured in degrees. A wider AFOV feels like looking out a picture window instead of a porthole. Even at high power with a 10mm, a wide AFOV eyepiece can show more of the sky around your target, which is very helpful.
Eye Relief
This is the distance your eye must be from the lens to see the whole view. Longer eye relief is more comfortable, especially for people who wear glasses. Some high-power eyepieces have very short eye relief, forcing you to press your eye right against the lens.
When shopping, you might see codes like “10mm 52°” or “20mm 68°.” The first number is focal length (strength), and the second is the Apparent Field of View (comfort and wideness).
Common Mistakes Beginners Make
Knowing which lens is stronger helps you avoid typical errors. Here are a few to watch out for:
- Using too high magnification too soon. Always find and center your target with a low-power (like the 20mm) eyepiece first. Then switch to the stronger 10mm.
- Expecting Hubble-like views. Even with a strong eyepiece, planets will appear small. Patience and practice reveal the details.
- Ignoring atmospheric conditions. If the stars are twinkling violently, it’s a bad night for high power. Stick with the 20mm.
- Forgetting to let your telescope cool. If you bring a warm telescope outside, the air inside it will swirl and ruin the view at any magnification. Let it adjust to the outdoor temperature for 30 minutes.
Practical Steps for Using Your Eyepieces
Let’s put this knowledge into a simple routine for a night of observing.
- Set up and align your finder scope in daylight. This is the most important step for success.
- Let your telescope cool down outside for at least 20-30 minutes.
- Start with your 20mm eyepiece (lowest power) to find your first target.
- Once the object is centered and in focus, take time to observe it. Note the context and overall shape.
- Carefully swap to the 10mm eyepiece. Do this gently to avoid bumping the telescope.
- Refocus. The stronger lens will always require a slight adjustment to the focus knob.
- Observe the new details. Wait for moments of steady air to see the sharpest views.
- If the image becomes too dim, fuzzy, or shaky, go back to the 20mm. The conditions might not support high power that night.
FAQ Section
Is a 10mm or 20mm eyepiece better for planets?
For planets, the stronger 10mm eyepiece is generally better because it provides the higher magnification needed to see details like Jupiter’s bands or Saturn’s rings. Start with the 20mm to find the planet, then switch to the 10mm.
What does the mm mean on a telescope lens?
The “mm” stands for millimeters and refers to the eyepiece’s focal length. A smaller number (like 10mm) means higher magnification. A larger number (like 20mm) means lower magnification but a wider field of view.
Can I use both a 10mm and 20mm eyepiece together?
No, you use only one eyepiece at a time in the telescope’s focuser. You switch between them to change the magnification power. Some accessories, like Barlow lenses, can effectively combine with an eyepiece to double or triple its power.
Which is stronger, 9mm or 12mm?
Following the same rule, the 9mm eyepiece is stronger than the 12mm. It will provide higher magnification on the same telescope because it has a smaller focal length number.
Why is everything blurry when I use my 10mm lens?
If your 10mm view is blurry, first try refocusing carefully. If it’s still blurry, the main causes are: 1) The telescope isn’t cooled down, 2) The atmosphere is too turbulent for high power, 3) The object isn’t perfectly centered, or 4) You may be exceeding your telescope’s useful magnification limit.
Final Thoughts on Lens Strength
So, which telescope lens is stronger 10mm or 20mm? You now know it’s the 10mm. Remember, strength (magnification) is just one tool in your toolbox. The 20mm’s wider, brighter view is equally valuable. The skill of a good astronomer lies in knowing which tool to use and when.
The best approach is to experiment. Look at the Moon with both eyepieces. Find a bright star cluster and switch between them. See how the view changes on a planet. Over time, you’ll develop an instinct for which eyepiece to grab first. This understanding, more than any piece of equipment, is what makes astronomy rewarding. Clear skies and happy viewing.