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Introduction to Telescope Eyepieces

To understand which eyepiece is best for you, we first need to understand magnification. Magnification is measured by how many more times larger an image appears than the naked eye. For example, a magnification of 100x will make the object 100 times larger than what our eyes see. To calculate magnification, we can use this simple formula:

Telescope Focal Length ÷ Eyepiece Focal Length = Magnification

For example, if you have a telescope of 1000mm in focal length and your eyepiece is 25mm, then you have a magnification of 40x. When we talk about a low magnification like this, it is known as low power. If you swapped out the 25mm eyepiece for one with a 5mm focal length, the magnification jumps to 200x, known as high power.

So high power eyepieces are better, right? Not always! Using a low or medium power eyepiece has its benefits. For one, the sky conditions and how shaky the atmosphere is can greatly impact the view through an eyepiece. This turbulence in the atmosphere, known as seeing, can change from night to night, hour to hour, minute to minute, and even second to second. When seeing is poor and the view is slightly blurry, a high power eyepiece is only going to magnify that blur.

Another reason you may want to avoid high power is because of limitations of your telescope. With telescopes, the larger the aperture, the more detail it can resolve. In contrast, the smaller the aperture, the less detail it can resolve. An easy rule of thumb is to take your telescope's aperture in millimeters and multiply it by two to get your maximum useful magnification:

Telescope Aperture (in mm) x 2 = Maximum Useful Magnification

For example, if your telescope has an aperture of 100mm, you would not want to use an eyepiece that exceeds 200x magnification. In truth, the maximum useful magnification should only be used when atmospheric seeing conditions are near perfect. For this reason, you'll want some eyepieces that fall well below the maximum useful magnification, such as 1/2 or 1/4 that figure.

If you want to learn even more about eyepiece fundamentals, head to our blog post titled A Guide To Choosing The Best Telescope Eyepieces. Now let's dive in to the types of eyepieces out there.

Long Focal Length Eyepieces

Best for observing large-sized objects like wide galaxies

Although it depends on the telescope, long focal length eyepieces are usually ideal for viewing large objects like the Andromeda Galaxy, the Orion Nebula, and the full disc of the moon. Long focal length eyepieces are generally considered to begin around 20mm and higher and give low power views. One of the greatest benefits of long focal length eyepieces is that they inherently have a long eye relief, which translates to comfortable viewing even with entry-level eyepieces. Another benefit is that no matter what the sky conditions are, a long focal length eyepiece still usually gives a great view because they're not as affected by atmospheric distortions. Long focal length eyepieces are not usually a good match for close-up views like planets or other small objects, but we recommend everyone have one in their eyepiece arsenal.

Explore Long Focal Length Eyepieces

Medium Focal Length Eyepieces

Best for "just right" views between short and long focal lengths

Medium focal length eyepieces are a nice middle-ground between short and long focal length eyepieces and can provide the missing link between the two. Medium focal length eyepieces generally fall between 10 to 20mm. When a long focal length eyepiece is too wide and short focal length is too narrow, a medium focal length eyepiece can often give that "just right" view. That being said, a medium focal length eyepiece is usually a great addition to any visual observer's collection. The biggest benefit of medium focal length eyepieces is that when sky conditions are poor and a high-power eyepiece is out of the question, a medium focal length eyepiece can still provide relatively close-up views without seeing too much blurring or shakiness caused by a turbulent atmosphere.

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Short Focal Length Eyepieces

Best for observing small-sized objects like planets

Short focal length eyepieces, when the sky conditions are right, can give jaw-dropping views of objects like the planets and extreme close-ups of the craters on the moon, among other tiny objects in the night sky. Generally categorized as below 10mm in focal length, these eyepieces will yield a higher power magnification than medium or long focal length eyepieces. Observing at high power isn't always possible due to distortions in the atmosphere. At very high power magnifications, a short focal length eyepiece may only appear to make the object blurrier. For this reason, you should reserve using your short focal length eyepieces for when the atmosphere is the most still. When the atmosphere cooperates, though, short focal length eyepieces can give views that you'll remember for a lifetime.

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Zoom Eyepieces

Best for the traveling observer or the beginner who wants varying focal lengths

By being able to zoom in and out between multiple focal lengths, zoom eyepieces can provide unmatched versatility. Some zoom eyepieces can combine low, medium, and high power magnifications all in one eyepiece. For that reason, if you only are able to bring one eyepiece with you when heading out to observe or want the convenience of not having to swap out eyepieces, a zoom can be a great choice. Some higher-quality zoom eyepieces can be a better overall choice than entry-level eyepiece kits. In terms of performance, fixed focal length eyepieces usually perform better optically than zooms, but they can be more costly for a full set. Because zoom eyepieces need the ability to move in and out, their field of view is usually smaller and more limited than fixed focal length eyepieces. All in all, though, a zoom eyepiece can be a great affordable option for beginner and intermediate observers.

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FAQs

Still have questions? We have answers.

Which eyepiece should I buy?

For any budding visual observer, it is recommended to start off with three eyepieces to start: one low-power, one medium-power, and one high-power. You can, of course, start off with just two or even one. Since many objects in the night sky vary in size and the conditions of the atmosphere can change, it's best to have a few different focal length eyepieces to choose from.

Make sure to also buy the correct sized eyepieces for your telescope. Many beginner telescopes can only accept 1.25" eyepieces, so a 2" eyepiece won't fit.

How do I tell what's a good quality eyepiece?

This is a subjective question, but some features you'll want to look for in a good quality eyepiece are:

  • Long eye relief — especially important for eyeglass wearers.
    OK: 10mm / Good: 15mm / Great: 20mm & Longer
  • Large field of view — especially important for deep sky observing.
    OK: Under 60º | Good: 60º to 80º | Great: 80º to 100º | Excellent: Over 100º
  • Price — if the price seems too good to be true, it probably is.
    OK: Under $50 | Good: $100 | Great: $200 | Excellent: $200 & Up

Are eyepieces universal?

Yes, eyepieces are universal in that you can use any 2" eyepiece in a 2" telescope focuser. You can also size up an eyepiece — like use a 1.25" eyepiece in a 2" telescope focuser — as long as you have the correct adapter. You cannot, however, use a larger eyepiece with a 2" barrel in a smaller 1.25" telescope focuser. Some eyepieces do offer the ability to be used as both 2" or 1.25", so be sure to look out for those if you have two or more telescopes with different sized focusers.

What's the difference between 1.25", 2", 0.965", and 3" eyepieces?

This number refers to the diameter of the eyepiece where it attaches to the telescope's star diagonal, focuser, or visual back. Almost all entry-level telescopes can accept 1.25" barrel size eyepieces. Intermediate and advanced level telescopes usually accept 2" eyepieces. These larger eyepieces allow for a wider field of view.

0.965" and 3" eyepieces are much less common. 0.965" was an older standard eyepiece size that has since been replaced by the more common 1.25" size. 3" eyepieces are even more rare, and are only useful on very large telescopes.

Which eyepiece is best for observing planets?

The planets are small — and we mean really small — when viewed from Earth, much smaller than most deep sky objects! For that reason, you need a high-power eyepiece, good sky conditions, and ideally a large aperture telescope to see the planets well. For most telescopes, high-power eyepieces begin under 10mm in focal length, but this depends on which telescope is used, too. You can also use a Barlow lens which can magnify the view by 2x or more, but just be sure it's good quality otherwise it may worsen the view. 

Which eyepiece is best for observing deep sky objects?

Contrary to planets which require a high-power eyepiece, most deep sky objects are best observed with a low or medium power eyepiece. For most telescopes, that usually means an eyepiece with focal length above 10mm. Since deep sky objects vary so much in size, it's best to have at least 2-3 different focal lengths to choose from. Large aperture telescopes will also be able to resolve fainter deep sky objects with more detail.

Are the eyepieces that come with my telescope any good?

Many entry-level telescopes come with 10mm (high power) and 25mm (low power) Plossl eyepieces. While these eyepieces can be fine for kids and adults who are just getting started, you will see a drastic improvement in the view if you invest in some quality eyepieces. For this reason, if you have enough budget to shop with, we recommend reserving a portion of it for a nice quality eyepiece or two.

How do I attach an eyepiece to my telescope?

This depends on the telescope, but almost all telescopes (with the exception of some scopes designed specifically for astrophotography) come with a star diagonal, focuser, or visual back that can accept eyepieces. Eyepieces slide in to this opening and are held in place by set screws. For refractor and Schmidt-Cassegrain telescopes, a star diagonal makes for comfortable viewing if your telescope did not come with one.

Glossary

Focal Length
Focal length is an eyepiece’s most important feature. Eyepiece focal lengths usually range from around 2mm to 60mm. Contrary to telescopes, a longer focal length eyepiece actually gives a wider (low power) view, and a shorter focal length eyepiece gives a narrower (high power) view.

What to look for: Low power eyepieces, such as a 30mm eyepiece, are best for viewing faint deep sky objects and large objects like the full disc of the moon. High power eyepieces are best for viewing planets and other objects up close.

Apparent Field of View (AFOV)
An eyepiece’s apparent field of view is how wide of a view, measured in degrees, that an eyepiece gives when looking through it. The human eye’s full field of vision is around 120º. The larger the apparent field of view that an eyepiece has, the more it feels like you’re immersing yourself into space when looking through it. This is known as the “spacewalk” effect. Wide AFOV eyepieces also allow for easier use of averted vision observing on deep sky objects, which is the practice of looking slightly away from an object to see it in more detail.

What to look for: Eyepieces with a wide apparent field of view, such as 82º, 100º, or even higher, can give an incredible view that feels like you’re immersed in space. These ultra-wide AFOV eyepieces can get expensive, though, and if you’re just observing planets, a wide AFOV eyepiece is not necessarily needed.

Eye Relief
Eye relief is how far your eye needs to be from the eyepiece for the image to appear in focus. Eyepieces with a long eye relief (like 20mm) are more comfortable than short eye relief (like 10mm). When trying to hold still like when looking through an eyepiece, our heads naturally move slightly. The closer your eye is to the eyepiece, the more exaggerated the movements of your head will seem, which may even cause the image to drift out of view. For this reason, eyepieces with a longer eye relief is preferred. Those who wear glasses will need a long eye relief eyepiece.

What to look for: Eyepieces with a longer eye relief, ideally 15mm or more, are almost always preferred as they’ll make it easier to look through. If you wear eyeglasses, having an eye relief of 15mm or more should be considered essential, and ideally you’ll want to find an eyepiece with closer to 20mm of eye relief.

Exit Pupil
Exit pupil is the diameter (in mm) of the light path or beam exiting the eyepiece. The human iris in our eye, when fully dilated, can be up to 7mm wide in younger people and gets smaller with age to around 5mm wide in older people. We can calculate exit pupil for any eyepiece and telescope combination by using the following formula: Eyepiece focal length ÷ Telescope f/# = Exit Pupil (mm)

What to look for: When observing faint deep sky objects, we want as close to a 5-7mm exit pupil as we can get because it means we're letting our eye take full advantage of the telescope's light-gathering ability. For planetary observing, the objects are so bright that a smaller exit pupil can be used, such as 1-2mm.