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A Guide on Choosing the Best Telescope Eyepieces

A Guide on Choosing the Right Telescope Eyepieces

The optical elements of eyepieces allow you to focus light collected by a telescope, so you can observe a sharp view of the object or area where the telescope is pointing. It may seem like a small link in the chain, but it has a large effect on your telescope's optical system, and finding suitable eyepieces will greatly enhance its potential. 

With so many options to choose from, selecting the right set of eyepieces for you and your telescope can seem a little tricky. This guide offers some insight and explanations on different eyepiece types, specifications, and how it all ties together to optimize your astronomy and astrophotography sessions!

Focal Length and Magnification

The Focal Length is an important specification to consider when determining the magnification, also known as power, of an eyepiece and the telescope it's being used with. The following formula will help you determine the magnification based on your eyepiece and telescope's specifications:

Magnification  = Telescope Focal Length (mm) / Eyepiece Focal Length (mm)
 
For example:
  • A 20 mm eyepiece on a 2000 mm telescope (2000/20) gives you 100 power (100x), this makes objects appear 100 times closer to you through the telescope than they appear to your unaided eye. 
Note: When using your telescope at different powers, you generally have a choice of a small, sharp, and bright image at lower magnification; or a larger, yet blurred and dim image at higher magnification. The reason being, that the telescope gathers a fixed amount of light, and at higher magnifications, the same amount of light is being spread over a larger area, resulting in a dimmer image.

    Field of View: Apparent and True

    An eyepiece's Apparent Field of View (AFOV) is expressed in degrees (°). It is how much of the sky is seen edge-to-edge through the eyepiece alone. AFOV's range from narrow (25° - 30°) to an extra-wide angle (80° or more).

    An eyepiece's true field of view is the angle of sky seen through the eyepiece when it's attached to the telescope. The true field can be calculated using the following formula:

    True Field = Apparent Field / Magnification

    For example, suppose you have an 8-inch Schmidt-Cassegrain telescope with a 2000 mm focal length and a 20 mm eyepiece with a 50° apparent field. The magnification would be 2000 mm / 20 mm = 100x. The true field would be 50\100, or 0.5° - about the same apparent diameter as the full moon.



     
    Eye Relief and Corrective Lenses

    Eye Relief refers to the distance between your eye and the eyepiece lens when the image is in focus. Eye relief is traditionally in proportion with focal length: The shorter the focal length, the shorter the eye relief. However, some of the more modern eyepiece designs provide long-eye relief regardless of focal length, which is especially beneficial to those who wear glasses. If you like to keep your glasses on while using a telescope, the eye relief of an eyepiece is an important specification to consider (we recommend looking at long-eye relief eyepieces). 

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    2 mm - 4.9 mm Eyepieces

    These provide very high magnifications and work best on long focal length refractorsand Schmidt-Cassegrains. Unless you have very steady seeing conditions, this range more than likely will produce too much magnification for other telescope types.

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    5 mm - 6.9 mm Eyepieces

    These make good planetary detail and double star eyepieces for long focal length telescopes and will work satisfactorily in shorter focal length telescopes with steady seeing conditions.

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    7 mm - 9.9 mm Eyepieces

    Ideal high magnification eyepieces for shorter focal length telescopes and serve as good planetary, double star, and lunar detail units.

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    10 mm - 13.9 mm Eyepieces

    Good to use across all focal lengths and offer great background darkening capabilities for studying planetary nebula, small galaxies, planetary details, and lunar details.

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    14 mm - 17.9 mm Eyepieces

    A great mid-range magnification for all focal lengths and helps resolve globular clusters, galaxy details, and spot planetary nebulae.

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    18 mm - 24.9 mm Eyepieces

    Works nicely on long focal length telescopes to show wide field and extended objects. Shorter focal length telescopes will enjoy great mid-range magnification of galaxy clusters and large open clusters.

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    25 mm - 30.9  mm Eyepieces

    Longer focal lengths are good for large nebula and open clusters. Shorter focal lengths are great for large objects such as the Orion nebula, views of the full lunar disc, large open clusters, and more. It also makes for good "locator" eyepieces in all focal lengths.

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    31 mm - 39.9 mm Eyepieces

    These are well suited for shorter focal length telescopes for extended views and large, starry fields.

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    40 mm Eyepieces

    These are exclusively the domain of shorter focal length telescopes. This magnification range is superb for showing large, starry vistas as well as extended nebulae with star fields, etc.

    How Exit Pupil Relates to Power

    Exit pupil refers to the size of the bundle of light rays coming out of the eyepiece. Exit pupil size (in inches) can be calculated by:

    Exit pupil size (mm) = Telescope aperture (mm) / Telescope magnification
    or
    Exit pupil size (mm) = Eyepiece focal length (mm) / Telescope f-ratio

     

    In order for all the light rays to enter your pupil, the exit pupil must be smaller than the pupil of your eye. A young person's fully dark-adapted eyes may have 7 mm-wide pupils. As you age, the maximum pupil diameter decreases. For middle-aged adults, the practical maximum is closer to 5 mm.
    At the other end of the scale, magnifications that yield an exit pupil in the range of 0.5 mm to 1.0 mm, empty magnification begins to set in, depending on the quality of your telescope and your eyes. In other words, this much magnification starts to degrade the image you see.

    How Many Eyepieces Do I Really Need

    Although there is no specific number of eyepieces you should own, with a few different telescope eyepieces, you have a better chance of hitting the optimal power for the particular object you are observing, given the sky conditions at the time. Usually, you'll want to start with low power (i.e., long eyepiece focal length, such as 25 mm or 30 mm) to get the object in the field of view of the telescope. Then you might want to try a slightly higher-power (shorter focal length, maybe 18 mm or 15 mm) eyepiece and see if the view looks any better. If it does, swap in an even higher-power eyepiece, etc., until you hit that "sweet spot" where image brightness, image scale, and the amount of visible detail combine to form the most pleasing view. 

    What about Barlow Lenses?

    You can also choose a long focal length eyepiece with comfortable eye relief and use image amplifiers to increase power, such as a Barlow lens. A Barlow increases the effective focal length of an objective lens, increasing the magnification. The idea is that two eyepieces and a Barlow will give you the flexibility of magnification of four eyepieces, and will give higher magnifications with less powerful eyepieces.

    Using different eyepieces can profoundly increase the versatility and functionality of any telescope. 

    Basic Tips to Follow When Shopping for Eyepieces

    • Consider the focal length of your telescope, or telescopes, to make sure the eyepiece will provide an appropriate magnification to suit your needs.

    • If you wear eyeglasses while using a telescope, pay attention to the eye relief specification of different eyepieces, as ample eye relief can improve comfort and ease-of-use while wearing corrective lenses.

    • Depending on your observing goals, consider the apparent field of view of your eyepiece choices.

    • If versatility is paramount, consider a zoom eyepiece or Barlow lens to increase the number of possible magnifications to use.


    24 Responses

    OPT

    November 17, 2020

    Hey Richard, That’s a great telescope! You should be able to get some fantastic views with that and some nice eyepieces, especially for wide angle views. For planetary with that telescope’s large aperture, you can surely push the magnification some more. We recommend a nice quality eyepiece around 6mm in focal length for nice high-power views, like the Tele Vue Delos 6mm or the Explore Scientific 6.7mm. You can then pair that with a 2x Powermate for even more fantastic views when the atmosphere is very still. If you want more help, our Sales team is always around to recommend the right gear for you. Shoot us an email at InternetSales@optcorp.com

    OPT

    November 17, 2020

    Hi John, We’re glad you enjoyed the article! It sounds like what you’re experiencing looking at the planets is some poor seeing conditions. On many nights, the atmosphere is turbulent enough to distort and blur the view of the planets. We recommend letting your telescope adjust to the ambient air temperature for a little while before observing, especially on colder nights. You could also try a higher power eyepiece — on nights with good seeing, an eyepiece with 5mm focal length will provide twice the magnification that the 10mm does. We recommend a higher quality 5mm eyepiece instead of a 2x Barlow. Andromeda often is a fuzzy blur in most telescopes around that size, but just remember, the light that you’re seeing has traveled 2.5 million years to get here! We think it’s pretty remarkable you can see that at all 🙂 Hope this helps!

    OPT

    November 13, 2020

    Hey Lou, thank you so much for the feedback! Plossl eyepieces, which often come packaged with telescopes, are good enough to get started but many people choose to upgrade to higher quality eyepieces as they will provide sharper (and sometimes brighter) views that are more comfortable to observe through. We definitely recommend trying out some higher quality eyepieces and seeing the difference for yourself. The nice thing about eyepieces is that they usually last a lifetime if you take care of them, so it’s often worth it to get a few nice eyepieces to supplement your telescope.

    Richard

    October 29, 2020

    I have a Discovery 15" f/5 Truss design with a 75" focal length. I am self taught and have never had a great viewing experience. I am hoping if I buy some new eyepieces my viewing experience will greatly improve. I am not sure which ones to buy. I currently own 3 eye pieces Meade Super Plossl 56mm, Meade Ultra Wide Angle 8.8 mm, and a TeleVue 35mm Panoptic. I would like to view the planets with better clarity. I am interested in buying a few new eyepieces. Which ones can you recommend for my telescope? Thanks

    John

    October 27, 2020

    Hi all and thanks for this great article — very helpful. I want to echo the question Lou just asked (a few days ago). I had a telescope as a kid and during COVID decided to renew my interest in astronomy so a Celestron 114LZ (1000mm focal — f/9 aperture) is what I bought. I was really hoping to see the basic stuff — the great red spot, defined rings of Saturn and most of all — Andromeda. I live in Indio in the Coachella Valley and have very good viewing conditions most of the time.
    Well, Jupiter looks like an overexposed white disk (I can see 4 moons), I can barely make out the rings of Saturn (10mm eyepiece) and last night Andromeda was just a fuzzy blur (10mm eyepiece).
    To Lou’s question — should I be looking at a new 10mm eyepiece of higher quality and/or a new Barlow with higher than 2x magnification? Any suggestions as to what brands of eyepieces are best?
    Many thanks!

    John

    Lou

    October 19, 2020

    Great article! My question is are more expensive eyepieces necessarily better or give a sharper image? I use the inexpensive Plossl eyepieces but have no real complaints with them. However, I hear from other astronomers in blogs, etc. that the eyepieces they use, which are usually more expensive, give sharper views. Sometimes I feel I may be missing out on sharper views by using the inexpensive ones. Thx!

    WillIam Saldana

    October 08, 2020

    I just purchased Gsyker 70400 telescope and I am looking for better definition I really want to use this telescope and learn. I believe if I learn on this telescope I will buy a better one.

    OPT

    October 08, 2020

    Hey Gene, congratulations on your purchase on the Celestron 11 SCT AVX! It’s a powerful scope. A Barlow lens is a nice upgrade to eyepiece collections. For other eyepieces, any eyepieces between 18 mm and 31.9 mm will work. Clear skies!

    Gene

    October 05, 2020

    I just purchased a Celestron 11 SCT AVX. I have the basic eyepieces that came with my old Celestron SLT. Do you suggest upgrading eyepieces? If so, which would be most useful? Thanks!

    OPT

    September 30, 2020

    Hey Richard, it sounds like the issue is closely related to eye relief. Sometimes people think the best approach is to put your eye right up to the eyepiece, like directly on it. This causes problems because your pupil has to be precisely aligned with the eyepiece, and any slight movement of your head causes the view to disappear. Start by positioning your eye an inch or two away from the eyepiece, and then gradually move closer until the view is best. If the eyepiece has a rubber cup around the top, make sure that is up and extended. When your eye is at the right distance away (correct eye relief) from the eyepiece, it will allow you to look through it even with your head slightly moving. A Barlow can help too if you’re still having trouble, as it keeps the same eye relief. Let us know if that works out for you. Clear skies!

    richard brook

    September 25, 2020

    Beginner with Celestron 6Inch SE GoTo 1500 mm. Aside from other troubles (focusing), circular field of view of night sky will suddenly disappear and when I move my eye around it will come back, then disappear etc. Telescope shop said it’s my eye piece—25mm (1.25) inexpensive Plossl that came with the scope. He said a better eyepiece would clear up the problem. You agree? Thanks, Dick Brook

    OPT

    September 21, 2020

    Hey Brent, a Barlow can help improve your current eyepiece’s magnitude but if you want to maximize its potential, a 4 or 5 x Barlow or Powermate would be great. Powermates are better because they don’t change the backfocus. Let us know if you have anymore questions, we are happy to help. Clear skies!

    Brent

    September 17, 2020

    Hi, I have a telescope 114-900mm , I have seen Jupiter with a bit of detail with a 4mm eyepiece but I am wondering if any other eyepiece can make it even better? I saw last night Saturnus with the ring but as I am interessted in more details I just want to see if there is another eyepiece to make it better?

    Thanks :D

    Eric

    September 04, 2020

    Hi Barbara,
    You can already see Uranus with your naked eye and Neptune with binoculars. If you want to make them out as really, really tiny but beautifully colored discs I would recommend an eyepiece that will give you a x150 magnification. To get the magnification divide the focal length of your scope (should be listed where you put the eyepiece in) by the aperture of the eye piece. For example I have a scope that has a focal length of 400mm, if I use a 10mm eyepiece I get x100 magnification (still enough to barely make out the disc but not ideal).
    I imagine you will most likely need a diameter of x10 or less. But remember anything more than what gives you a solid brightish image is a waste because with an 80mm scope you will never be able to make out any detail in the disc. You will need at the very least 200mm and even then we are talking a faint smudge visible. Really even in big scopes it’s beautiful but not very exciting.
    But it is worth looking for because even though the disc is about the size of a period on this post the color really jumps out! Anything past Neptune (Pluto etc…) you will not be able to make out either with the small scopes we have either.
    Happy gazing!
    Eric

    Barbara

    July 26, 2020

    I have a 80mm telescope, with a 25mm eyepiece. What eyepiece would work to see planets past Jupiter and Saturn?

    Del Johnson

    June 10, 2019

    Eyepiece focal lengths are selected by telescope f/ratio, not by telescope focal length. For a given telescope aperture, 40mm eyepieces would be used by telescopes with longer focal lengths, not shorter. Your advice for ultra short forcal length eyepieces is also reversed, and their use is not tied to telescope design as you described. Ultra short focal length eyepieces are definitely not tied to the common f/10 SCT.

    Alan Sanchez (OPT)

    May 08, 2019

    Hello Jim,
    For your first question regarding the “Corrective Lens” section, that is referring to the eyepiece focal length.

    For the second question, eyepiece focal lengths and diagonals essentially only function as connecting pieces. Nearly all diagonals these days are of excellent quality and you can rest easy know they won’t negatively affect your viewing. And by upgrading to 2-inch you’ll open up access to much larger focal length eyepieces!

    As for choosing the right one for you, I highly recommend getting in contact with one of our sales experts at internetsales@optcorp.com or opening a chat box on the bottom right of your screen here. They will set you up for success, I promise you that!

    Thank you for your question!

    Jim

    May 08, 2019

    Another question, how do the eyepiece focal lengths and diagonals work with each other. I’m considering upgrading to a 2-inch diagonal…I wear glasses and want to get a couple nice eyepieces that won’t break the bank. Also don’t want to waste money on things that I’m not going to be happy with.

    Jim

    May 08, 2019

    Nice article. Under the “Corrective Lens” Section, are those lengths (accompanied by example photos) referring to the eye relief distance or the eyepiece focal length?

    Jason

    April 06, 2019

    Thank you for braking the different types of eyepieces down so someone new (like me) to telescopes know what to look for in eyepieces.

    Alan Sanchez (OPT)

    March 18, 2019

    Soon you’ll be a newbie no more, Leroy my friend. Happy to hear this helped you out!

    Leroy Major

    March 17, 2019

    Great article! As a newbie, It answered a lot of questions I had about eyepieces. Thank you!

    Alan Sanchez (OPT)

    February 19, 2019

    Glad you like the article Gerry! Stay tuned for more.

    Gerry

    February 11, 2019

    Great article on choosing eyepieces. Easy to understand, especially for us beginners. Thank you.

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