Telescope 10mm vs 20mm: Know The Key Differences Between?

There are a variety of magnification levels, shapes, and sizes. The eyepiece is an essential part of the telescope as it magnifies objects and images so users can see them better.

The eyepiece of 10mm focal length is suitable for close-up observations, whereas one with a focal length of 20mm is ideally suited to viewing objects far away.

In this article, we will explain how Eyepiece design comes with different focal lengths such as 10mm vs. 20mm to help you choose which one you should use based on what you wish to achieve.

Differences of Telescope 10mm vs 20mm

Differences of Telescope 10mm and Telescope 20mm

Dimensions of the Lens and Magnification

Dimensions of the lens and magnification also determine the eyepiece and eyepiece magnification. 20mm eyepieces have a larger diameter compared to 10mm eyepieces, which means that objects will appear bigger when looking through them. The focal length of eyepieces is also measured in millimeters, and the higher the focal length, the greater the magnification you will get from your telescope.

10mm eyepieces yield a medium-low magnification, while telescopes with 20mm eyepieces magnify more than 50% stronger. That's because eyepiece diameter and field of view (FOV) go hand in hand: eyepieces with larger diameters provide a wider FOV, which means that you can see objects in more detail when looking through eyepieces with greater magnification.

The 20mm eyepiece, in particular, offers a FOV of 56°, while the 10mm eyepiece gives you a wider 41° field of view. With telescopes that have eyepieces with focal lengths lower than 20 mm, when looking at star clusters or galaxy groups with two to three bright stars close together, there are often gaps between the eyepiece's FOV and individual stars in these objects. This is due to the fact that you can't fit all the stars in one eyepiece frame because these objects are too far apart from each other.

Observer's Point of View

Observer's Point of View

When you look through eyepieces with focal lengths lower than 20mm, the eyepiece's FOV is too large for many deep-space objects. In some instances, it may cover several stars, which means that you can't enjoy them individually. With an eyepiece of 50 mm or higher on a telescope with a low magnification of about 40x - 50x per inch aperture, observing star clusters and galaxy groups is almost impossible because their observation area is on a different level from your eyes unless you position yourself on your knees to view up-side-down objects.

With eyepieces of 20mm or higher, however, the FOV is not as large but enough to give you a perfect view of both upside-down and right-side-up deep space objects. So, you get the best of both eyepieces with eyepieces of focal lengths higher than 20mm, which is why most eyepiece magnification recommendations for focal lengths between 10 - 50 mm are at least 30x to 40x per inch aperture.

Eliminating eyepiece view obstructed by a diagonal mirror When using a telescope's eyepiece through a 45° erecting prism or star diagonal, the eyepiece will be positioned above your line of sight. In doing so, it makes more room for other accessories on your telescope such as finder scopes and filters. It also eliminates eyepiece view obstruction from coming from a diagonal's mirror beneath the eyepiece.

Corrective Lenses for Eye Relief

When eyeglass wearers look through eyepieces, it's important to have long eye relief for eyeglasses, which is the distance between your eyeglasses frame and eyepiece lens. Eyepieces with short eye relief are not suitable for eyeglass users because they won't the essentialsee the entire field of view.

With eyeglasses on, you should be able to see all of the eyepiece's field of view without having to move your eyeglasses or head position forward or backward.

Because every type of telescope has different eyepiece dimensions, but almost all are measured in millimeters (mm), you can adapt eyeglass wearers by providing corrective lenses that fit into standard 1.25" eyepiece filter threading with the eyeglasses eyepiece lens closer to your eyeglass frame.

Eyepieces that are too large or have an eye relief that's too short for eyeglass wearers will force you to position your head at awkward angles where it may be impossible to center an object through your eyepiece viewfinder.

If this is the case, always lower the magnification of your telescope eyepiece down to about 10x per inch aperture or less until you can position yourself in a more comfortable viewing environment.

Size of the Barrel

Size of the Barrel

The eyepiece barrel size is also another factor in eyeglass-friendly eyepieces. Because eyeglass wearers are always concerned with the eyepiece's weight, when choosing eyepieces, never go for eyepieces with a barrel smaller than 1 inch.

Eyepieces larger than 1-inch can be heavy, so always remember to keep it light by looking for eyepieces that have aluminum barrels or dielectric coatings which help make them lighter.

Aspects of Optical Correction

Eyepieces with better eyepiece correction will give you better views of objects in the night sky. This is why eyepiece manufacturers always list eyepiece optical specifications (type of glass used, eyepiece element count, the field of view, etc.) on their eyepieces barrels.

The most common eyepiece types that are eyeglass friendly are either Kellner or Orthoscopic eyepieces because their lenses use only two to four lens elements which significantly reduces aberrations, specifically chromatic aberration, and spherical aberration.

As an eyeglass wearer myself, I would recommend anything over 30mm for comfort but lower than 50mm so they can act as a decent grab-n-go eyepiece.

When to Zoom and When Not to Zoom

An eyepiece zoom eyepiece is a great eyepiece to use when you want to switch eyepieces without having to swap out eyepieces and filers. Not only will they save you time but also because there's less switching and more observing.

So, what should an eyeglass wearer look for in eyepiece zooms? When choosing an eyepiece zoom, pick one that has a low magnification range, such as 2x-3x or 4x-6x, because anything above that can be heavy on your neck or challenging to handle depending on the focal length of your telescope.

The Barlow Lens with Eyepieces

The Barlow Lens with Eyepieces

Some eyepiece manufacturers make eyepiece 2-in-1s or eyepieces with Barlow lenses built into them. If you're an eyeglass wearer looking for a premium eyepiece, skip the Barlow lens eyepieces because they can be heavy, difficult to handle, and get extremely expensive.

When it comes to choosing eyeglass-friendly quality eyepieces, look for eyepieces that have a focal length of 25mm or more but less than 40mm. Never go for anything lower than 20mm because they can be too close to your face and may not allow you enough space to see the apparent field of view through your telescope ocular without moving your head position forward or backward.

When choosing eyepieces with focal lengths under 25mm, always go for eyepieces that have at least 20mm of eye relief. This will ensure you have enough space to look through your eyepiece without moving your eyeglasses or head position back and forth.

Meade has a line of eyepiece filters called the Meade Long-Eye Relief eyepieces, which are made specifically for eyeglass wearers who want to magnify their view when looking through long focal length telescopes. They're available in 4mm, 6mm, 9mm, 12.4mm, 15mm, and 18mm focal lengths in 1.25" barrel sizes only, and they all have eye relief between 18-18.5mm.

Value eyepieces an eyeglass wearer can look for are eyepieces that have a focal length of 25-30mm and 20-25% more eyepiece elements than basic eyepieces. This will ensure you get better eyepiece correction, edge sharpness, field flatness, color accuracy, etc.

Which is better: Telescope 10mm or 20mm

The eyepiece will depend on the eyeglasses you are using. If, for example, if you are using eyeglasses with thin eyeglass frames, I would recommend a thin eyepiece. However, eyepieces are always about personal preference, so it really depends on what eyepiece feels comfortable to you.

For eyepieces that have eye relief over 15mm, the magnification will be higher than one with shorter eye relief which means that they're better for viewing objects at great distances. However, if magnification is not very important to you then an eyepiece with lower magnification that focuses on eyeglass comfort is just as good.

One eyepiece that eyeglass wearers can look for is eyepieces that have a focal length of 25-30mm and 20-25% more eyepiece elements than basic eyepieces. This will ensure better eyepiece correction, edge sharpness, field flatness, color accuracy, etc.

For beginners looking for eyeglass-friendly optics, we would recommend any Meade series 5000 from 4mm to 32mm Plossl eyepiece.

They're all 1.25" barrel sizes, but Meade has them available in five different body colors: black, brown, blue, green, and orange. All eyepieces have fold-down eye guards, rubber eyecups with three snap positions, and most importantly, they all have between 16-19.7mm of eye relief.

What is the Eyepiece and What Does it Do?

What is the Eyepiece and What Does it Do

An eyepiece is a cylindrical lens positioned in the eyepiece holder of a telescope. It magnifies the image which comes from an object such as a star or planet and makes it bigger so that observers can see it more clearly.

It does this by using the eyepiece lens to converge light to the focal ratio of the objective lens. The eyepiece may also be able to change magnification levels. It can also change eyepieces for various purposes, such as viewing objects at close ranges and viewing those that are far away.

Eyepieces Come in Different Focal Lengths

The eyepiece is located in the eyepiece holder of the scope. It magnifies images and objects so that observers can see them more clearly. Eyepieces come in different focal lengths, which are used for various purposes.

Typically, eyepieces with a focal length of 10mm are used for close-up observations, while eyepieces with a 20mm focal length are better suited for viewing distant objects.

One of the essential eyepiece characteristics is the eyepiece focal length. A focal length is one of the eyepiece adjustments that can be done to magnify and reduce the size of any object that is being looked at.

The magnification of the eyepiece can be adjusted to magnify or reduce the size of any object that is being examined. Shorter focal length eyepieces are best for planets and other astronomical objects.

Eyepieces with high focal lengths are good for looking at deep space objects like nebulae or galaxies. The eyepiece magnification determines what you will see on the telescope eyepiece.

In astronomy, the eyepiece focal length is an often misunderstood but essential parameter. To understand this concept, it's necessary to know the eyepiece magnification, which is frequently incorrect or misunderstood.

The eyepiece focal length can be found on eyepiece specification sheets and is the point where light rays converge at the sharpest focus within an eyepiece.

How to Pick Eyepieces Based on Your Needs?

How to Pick Eyepieces Based on Your Needs

Choosing eyepieces at random without knowing what you need will only waste your money. Different eyepiece focal lengths are used for various purposes, such as viewing objects in detail or from a great distance.

The focal length of an eyepiece is one of the most essential characteristics. It is possible to adjust the focal length of an eyepiece to magnify or reduce the size of any object being viewed.

Eyepieces with low focal lengths work best for viewing planets, while eyepieces with high focal lengths are good for looking at deep space objects like nebulae or galaxies.


1. What is eyepiece magnification?

Magnification is the eyepiece's focal length divided by the eyepiece's barrel size. The barrel size of an eyepiece is its outer diameter in millimeters which you can usually find printed on eyepieces next to their focal length.

2. What are high magnification eyepieces?

High magnification eyepieces have a high focal length compared to their barrel size, so they give you more power when looking through them for better image brightness, contrast, and sharpness.

The only problem with higher magnification eyepieces is that they don't allow you to see as much at once because objects appear smaller through them than low magnification eyepieces, but this depends on your preference.

Some people like to see as much as possible at once, and others want to concentrate more on eyepiece sharpness than seeing a small piece of the object.

3. How does eyepiece focal length affect eyeglass wearers?

Eyepiece focal length is how far the eyepiece projects its focus on eyeglass wearers. A lower eyepiece focal length means that the eyepiece is better suited for eyeglasses than a higher eyepiece focal length.

However, a Short Focal length Eyepiece tends to have more extensive views of objects. Hence, it's really up to individual preference and what kind of conditions you'll be using your Telescope most often.

4. Do all eyepieces have the same focal length and magnifying power?

Do All Eyepieces Have the Same Focal Length and Magnifying Power

Not all eyepieces have the same focal length or magnification because eyepiece eyeglass compatibility is based on their barrel size. The eyepiece's barrel size will be stamped into its housing next to its focal length or printed in a separate handbook.

The eyepiece's focal length divided by the eyepiece's barrel size is what eyeglass wearers should look for when choosing an eyepiece.

This calculation will give you the equivalent eyeglass wearers' viewing magnification. It gives them the halfway point between their naked eye and telescope eyepiece magnifications so they can view objects at half-fainting strength, similar to if they were wearing glasses at all.

Last Thoughts

Now that you know more about what to look for when choosing eyepiece magnification, we hope you can find the perfect one for your telescope.

So keep in mind that eyepiece focal length will affect your telescope's magnification which, in turn, will affect the maximum power of objects you can see at once through your telescope.

However, this rule does not apply to all telescopes because some cannot reach their true magnification potential due to physical limitations. This is why we recommend that you buy bigger and better telescopes if your goal is to try to see as far into space as possible without having objects fade out of view because the atmosphere absorbs light.