how to choose eyepiece for your telescope

How to Choose Eyepiece for Telescope – The Buyers Guide


For those who are just getting into learning about telescopes, you may feel a little overwhelmed by all of the choices you have available concerning telescope eyepieces.

With a little background knowledge and deciding what exactly you are interested in using your telescope for, you can confidently choose a telescope eyepiece to fit all of your scoping needs. So let’s dig a little deeper.

First, how does a telescope eyepiece work?

It is important to understand how telescope eyepieces actually work so you can be sure to select ones appropriate to the activity you will be using your telescope for.

Telescope Eyepiece

Every telescope utilizes a lens or a mirror and some even use both. They collect incoming light and in turn outline a figure from that light.

A telescope eyepiece takes the figure formed by this mirror or lens (or both) and enlarges it so you can see more details in said figure.

What are those numbers on your eyepiece?

When shopping for telescope eyepieces, you will notice in most cases that there are particular markings on each lens.

Generally, these markings consist of numbers and some even incorporate letters as well.

These serve the purpose of identifying what is known as the focal length.

For instance, an eyepiece marked “25mm” tells you that it carries a focal length of 25 millimeters and in turn allows you to calculate the amount of magnification that particular eyepiece will supply.

Some eyepieces have other markings on them like “H”, “PI” or “SR” (no need to learn about it right now). These identify the kind of eyepiece it is. This refers to the optical design of the lens and usually classifies how much glass is used in each piece.

By knowing what the markings on telescope eyepieces mean, you will able to choose lenses that meet not only your telescope’s design but your particular viewing aims as well.

Magnification of the eyepiece

the telescope magnificationNo eyepiece has a fixed magnification and it varies from with different telescopes. And you will get a clue from the formula below that is used to calculate it.

Mag. of eyepiece = F.L. of telescope / F.L. of the eyepiece

So if you have an eyepiece with larger focal length (FL), the magnification will be lower and vice-versa. And on the other hand, if you have a telescope with a larger FL, the magnification will be higher. 

[And you know how to find the FL of eyepiece from above, for FL of your telescope, look for specifications or tell me your telescope model in the comment below so I can help you find it]

Eye relief of the eyepiece

So listen, this one is an important thing here. Especially if you wear eyeglass! 

Eye relief of a telescope eyepiece is the distance between the eyepiece and your eyeball, from which you can see the complete image. So better go with an eyepiece with larger eye relief. 

What eyepiece is best for planets?

best eyepiece for planets

Our dearly planets are at a good enough distance. To see them with sensible details, you would want to have an eyepiece that gives you at least 100x of magnification. You should be able to catch the Saturns ring at 150-200x. 

Heres an example to explain you find that eyepiece for you. 

Say you have a telescope with 650mm focal length (like astromaster 130 does) then do this simple math;

Eyepiece F.L.   =  F.L. of telescopeReq. Mag.

Eg. 650/100= 65mm

That means for a telescope with 650mm of focal length, the eyepiece with a focal length of at least of about 65mm is must to observe planets.

Moreover, the picture quality produced also depends on the telescope so make sure you have a larger one. Here are some of our favorite telescope to see planets.

What more you can do is to add a Barlow lens in your arsenal.

What is a Barlow lens?

A Barlow lens is, you can say, an optical multiplier. It seats in the middle of your eyepiece adopter and eyepiece itself. And what role it plays is to multiply the magnifying ability of your eyepiece. 

Let’s say, your eyepiece is giving you 50x of magnification. When you put the Barlow lens in the middle, it doubles the power to 100x.

Isn’t that just great? Yeah, because these are usually cheaper as well. There are a variety of Barlows lens like 2x, 3x, 5x etc. A 3x will triple your eyepiece to 150x magnification.

Here’s the complementary content for you:


Bonus tips for you

When looking for eyepieces for your first telescope, these are the tips you can follow that may help you invest in the best bet.

You want to be sure your telescope accepts 1.25″ eyepieces. Telescopes that accept two-inch lenses are fine as you can find adapters relatively easily that will take the 1.25″ size.

Another pro tip is that if your scope comes with two different lenses, you are looking for one of them to generate a good low power magnification while the other generates a higher level. Generally, the best entry level eyepieces will include a 25mm focal length and a lens within the 10m to 7mm array.


Today’s technology can be very confusing, creating some challenges for consumers trying to find the best technology at affordable prices. 

But you don’t worry, I got your back. If you need even a little bit of help at this point and have whatever question, just go ahead and ask me, right in the comment below! 🙂

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