4mm eyepiece vs 20mm eyepiece

Which is Stronger? 4mm Eyepiece vs 20mm Eyepiece!

Typical beginner telescopes come with 10mm and 25mm eyepieces. These are great for starters. But you want to purchase some smaller focal length eyepieces – maybe a 4mm and a 20mm. 

Now, as a beginner what’s the difference between a 4mm Eyepiece vs 20mm Eyepiece?

4mm eyepiece’s magnification will be much greater than the 20mm eyepiece. But you will never get a good image with a 4mm eyepiece with a beginner telescope. Yet with a 20mm eyepiece, you’ll get clearer images. It’s also quite difficult to use a 4mm eyepiece without good seeing conditions.

I have seen many beginner astrophotographers make mistakes with eyepieces. Thinking smaller focal lengths will improve their images. 

Hence in this article, I will discuss the two eyepieces and clear the confusion. 

Hop in if you are in the same boat as them. 

Key Differences Between a 4mm Eyepiece and a 20mm Eyepiece

Typically, one 25mm and one 10mm telescope eyepiece comes with your telescope purchase. But, 4mm and 20mm eyepieces are also great for stargazing.

I made a table of the key differences between the eyepieces respective to a beginner telescope. That’s a quick rundown of the things I will discuss in the subsequent section – 

Factor4mm Eyepiece20mm Eyepiece
Observation RangeDeep Sky Objects, Moon and PlanetsStar Clusters, Nebula and Galaxies
Field of View52 Degree66 Degree
SpecialtyFor smaller objects, objects closer For Larger objects, objects far away 
PriceCheck Price on AmazonCheck Price on Amazon

 As you see, each of the two eyepieces serves a different purpose. Let’s discuss that in detail. 

4mm Eyepiece
Source: Wikipedia

Detailed Comparison: 4mm vs 20mm Eyepiece

I’ll describe how different 4mm vs. 20mm eyepieces are in detail in this section. To assist you in deciding which one to utilize based on your goals.


The focal length of a healthy, relaxed eye is around 1.7 cm. It’s when viewing an object that is infinitely far away (17 mm). Our eyes shift the focal length to roughly 22mm when we see a closer object. 

When stable, the item appears closer to us optically when our field of view is reduced. With telescopes, the same thing takes place. Our field of vision hasn’t altered when we use naked eyes to observe the night sky.

Now, when we gaze through a telescope, we are both reducing our field of view. And it draws the thing closer. 

A smaller eyepiece number indicates a higher magnification. Five times more magnification would be available from a 4mm eyepiece than from a 20mm eyepiece. 

With a Pentax K-70 Astrophotography camera, a 20mm eyepiece will give you clear and pristine shots.

Additionally, it indicates that a single eyepiece provides various magnifications depending on the scope. On a short-focal-length scope, a 4mm eyepiece would have low power. But on a long-focal-length coverage, it would have tremendous power.

In a 1000mm telescope, a 4mm eyepiece will give you 250x magnification. And a 20mm eyepiece will give you 50x magnification.

The high magnification doesn’t mean better, rather it’s difficult for beginners. Take me for an example. I’ve never gotten a decent image with a 4mm eyepiece in my beginner days.

Whereas, a 20mm eyepiece gives you much better images. A true beginner-friendly eyepiece. The downside is less magnifying power. 

Observation Range:

So, what can you see with a 4mm eyepiece?

With a 4mm eyepiece, you can watch planets and the moon. However, due to strong magnification, the images will be blurred. You will hardly recognize the planets. Thus the 4mm eyepiece is difficult for beginners. Handling the 250x magnification of a 4mm eyepiece takes time to master. 

On the contrary, with a 20mm eyepiece, you’ll be able to watch faraway objects. Like star clusters, nebulae, galaxies, and deep space objects. 

And you’ll also be able to get decent images with a 20mm eyepiece. However, once you learn the proper usage of a 4mm eyepiece, you can take beautiful images. Like this person.

Field of View:

The field of view for a 4mm eyepiece is 52 degrees. And it’s 66 degrees for a 20mm eyepiece. 

The FOV of many deep-space objects is too big when eyepieces with focal lengths of 4mm. Sometimes it may obscure many stars, making it impossible for you to appreciate each one separately. 

Observing star clusters and galaxy groupings with an eyepiece 20 mm eyepiece is more comfortable and convenient. 

The FOV is sufficient to provide a perfect view of both upside-down. And right-side-up deep space objects with an eyepiece of 20mm. 

As a result, eyepieces with focal lengths greater than 20 mm offer the best of both worlds. The majority of telescope eyepiece magnification recommendations for focal lengths 20mm are at least 30x to 40x.

20mm eyepiece
Source: Astronomy Stack Exchange


Lenses can be very expensive. Choosing the wrong lens can will be a huge loss. Thus, let’s see the price of each lens.

The 4mm eyepiece costs around $13-$189. You don’t need to spend bucks to buy a 4mm eyepiece. Thus, some of the best 4mm eyepiece are recommended below.

For crystal clear images, you can go for Celestron Omni 4mm eyepiece. You can check the price on Amazon.

If you want more detailed surface imagery of planets, than SVBONY Eyepieces are good option. You can check price on Amazon.

You can get the Astromania 1.25″ 4mm Plossl Telescope Eyepiece for more durability. You can check for the price on Amazon.

On the other hand, the 20mm eyepiece is more expensive than 4mm eyepiece. It costs around $22-$399. 

So, Which Eyepiece Should You Go for?

After discussing the key differences, it’s time for a verdict. Which one is better?

Actually, the decision really depends on your intention of using this eyepiece. But Here is my 2cents. 

The eyepieces’ specialty is in the sense of what it works best for. In this case, 4mm eyepieces are actually the best for powerful telescopes, not beginner ones.

20mm eyepieces are actually one of the best types of eyepieces. Seeing the condition and still air is very important to get good Astro-images. 

With 20mm eyepieces, you will still be able to get good images. Without being too bothered about the seeing condition. 

With a 4mm eyepiece, you’ll have to be in good seeing condition. Only then you’ll get a good picture.

field of view for a 20mm eyepiece
Source: Cloudy Nights

Frequently Asked Questions (FAQs)

What Magnification Is A 4mm Lens?

You can get 50x, 120x, and 300x magnifications with a 4mm, 10mm, and 24mm eyepiece. Which is an excellent variety of magnifications for this instrument. With these eyepieces, you may reach 50x, 100x, 120x, 240x, and 300x magnifications. But you will have to pair a decent 2x Barlow with the 4mm.

Which Telescope Lens Is Stronger 10mm or 20mm?

Simply divide the focal length of the telescope by the focal length of the eyepiece. It gives you magnification. This implies that higher magnification is provided by a smaller number on an eyepiece. The magnification offered by a 10mm eyepiece would be twice that of a 20mm eyepiece.

Is A 20mm Eyepiece Good?

In almost any telescope, a 20 mm is helpful, just like a 13 mm is. But all-purpose might not be the best descriptor. The focal length of telescopes ranges from less than 400 mm to over 2800 mm. A 20mm eyepiece is a low to mid-power deep sky eyepiece. It’s not a planetary in those telescopes.

Closing Note

That should clear up the confusion between a 4mm eyepiece vs 20mm eyepiece. I gave everything you needed to know as a beginner astrophotographer in this article.

Now it’s up to you to decide which eyepiece will cater to your needs better. 

Wishing you a great seeing condition!

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