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Eyepiece Review Pitfalls

Version 2 with Revisions and Additions, June 2001


If you have been reading eyepiece reviews you may have noticed that different reviewers give different results for the same eyepiece. Reviewers contradict their own results. You may have purchased a popular eyepiece that has rave reviews and find that it doesn’t give you the same results. How can this be? Are some people wrong and others right; are they all wrong or all right? Read on.

There are only a few basic eyepiece designs but there are a bewildering array of unique names and models for eyepieces, especially premium types. The manufacturers have done us no favors here. But you can often figure out which design underlies the unique name and the information presented here may help you.

In this second edition I have added notes and conclusions that I have made after reviewing about 100 eyepieces, new and old. I have expanded information on sky conditions, old vs. new eyepieces and more on differences in designs.

The "Prime Result"

An eyepiece has one main reason for existing: to gather the light from the objective and to focus it into a sharp point of light. I call this the "prime result". Different telescopes deliver the light to the eyepiece in different ways, however, and these differences affect how well an eyepiece does its job accomplishing the "prime result".

There are other desired results, of course, a wide view, successful eyeglass use, comfort and a sort of "can I see the thing I am expecting to see" result. Sometimes these individual results are obtained at the sacrifice of one another and sometimes even the prime result.

Some negative results include light loss, spatial distortion (things are not where they should be), spurious light reflections which may reduce contrast and false coloration of objects.

The Perfect Eyepiece

There is a perfect eyepiece design, it's called an Orthoscopic, and it's called perfect because it does the best job of delivering the prime result without adding any distortion of it's own. Sometime you will hear that a certain design of eyepiece is "orthoscopic" meaning it has no optical distortion like the Orthoscopic design.

The "Sweet Spot"

Without getting technical, there is for every telescope one or two sizes of eyepieces that work better than eyepieces of a different size. With those eyepieces the telescope, eyepiece and human eye combine to provide maximum acuity. This is strictly a product of the f/ratio, eyepiece focal length and your eye. A particular eyepiece may fall in the sweet spot in one telescope and be way off the mark in another. To find your sweet spot, just multiply your focal ratio by 2. For example, a f/8 scope will have maximum acuity with a 16mm eyepiece.

Differences in Telescopes affect Eyepiece Results

How an eyepiece performs is very much influenced by the telescope you put it in. An eyepiece will perform flawlessly in one telescope but poorly in another. Why?

Focal Lengths

Both the focal length of the objective and the focal length of the eyepiece determine the amount of magnification of a telescope. Fast scopes tend to have short focal lengths and give lower magnification for a particular focal length eyepiece. An eyepiece in one scope will magnify more than in another and so you can’t compare the results. Here again, you see eyepieces reviewed without stating the focal length or magnification of the scope used. It’s up to you to figure this out from the information given if you can.

For example, you read a rave review of a 7.5mm premium eyepiece, go out and buy it only to find that it makes fuzzy images in your scope. Then you find out that the reviewer used a 1000mm scope in which the eyepiece made 133x while in your 2350mm scope it yielded over 300x.

Focal Ratios

Each telescope will have a certain focal ratio. The lower the number the "faster" the scope. Faster scopes are shorter in physical length than "slower" scopes and bend light more. They will have a wider view. The light delivered to the eyepiece is coming in at a steeper angle. Yet you often see where an eyepiece is reviewed but the f/ratio of the scope used is never mentioned. A f/ratio of about 5.5 or greater (f/6, f/8, f/10…) can be considered as "slow" today, F/5 and f/4 scopes are "fast". Scopes have become faster over the years for a number of reasons and eyepieces try to keep up with them, newer eyepieces work better with faster scopes because they are designed to. More importantly, older designs work just fine with older scopes and slower scopes. Even more important to know is that sometimes a newer, premium eyepiece sometimes "overcorrects" and works poorly in a slower scope.

Sometimes you will hear reviewer say this or that eyepiece "snaps" into focus. This is a result of the f/ratio of the scope and says little about the eyepiece except that it will come to sharp focus.


There are two main types of telescopes, refractors and reflectors. Eyepieces don’t always perform the same in the two types, but more important, evaluation of the eyepiece may be different for the two.

Refractors have a defect called "color" or "false color" which shows up as a purple fringe around bright objects. Some eyepieces can help eliminate this color while others may make it worse by having their own "color". When an eyepiece shows color in a refractor it may be difficult to determine whether the problem is with the eyepiece or the scope itself.

Reflectors may have field curvature or coma. Sometimes field curvature or coma may be blamed on an eyepiece "edge defects" when the eyepiece is performing fine.

Some Newtonian reflectors have a small secondary mirror that affects the performance of ultra wide eyepieces by clipping the light path. A dim image may be the fault of the scope.

Reflectors all have a minimum magnification number where the secondary becomes visible and this sets the longest eyepiece focal length that can be used.

Differences in Targets mean Differences in Desired Eyepiece Results

Make sure that your candidate eyepiece is being reviewed on the targets you want to view. If you see reviewers rate an eyepiece as a "great planetary", it’s probably lousy for planetary nebulas.

Stars. Individual and double stars need the prime result only. In fact, to be able to see or "split" double stars you need very sharp points of light. You don’t need a wide field except perhaps to find the stars.

Clusters. If you are looking at more than one star then a wide field may become important. There are two kinds of clusters however, "open" and "globular". For open clusters, wide field and sharp focus are important, but many open clusters will not fit entirely in any eyepiece view while globulars always do. Globulars need high brightness (no light loss) and high contrast, a "prime result".

Planets. Planets and to some extent the moon are special cases. In fact, there are special types of eyepieces just for planets and some are best for the moon. For viewing planets wide fields are not required, no planet is going to fill your eyepiece. The Moon however is big. Here you want a wide field with lots of magnification. Contrast and brightness are not as important except for "local contrast" or contrast between similar shades or colors on the object surface.

Faint Fuzzies. Galaxies and nebulae are different. They are very faint. They are also extended objects, which means that a telescope can make them bigger but not brighter. Sometimes a wide field is important as is magnification but of most importance, more so than any other type of object, is contrast. The eye sees better if there is a large difference in contrast between the object and the sky around it. Contrast may be more important in medium sized scopes than in large or small ones. Large scopes tend to automatically have more contrast and you can’t see faint fuzzies in a small scope anyway. An eyepiece used for faint objects can be forgiven many defects if it loses little light and has high contrast.

Differences in Sites can affect Eyepiece Results

The place where an eyepiece is reviewed is important. This is because light pollution affects contrast. The greater the contrast is to start with, the easier it is for the eyepiece to do this job so the reviewer should indicate the sky conditions during the evaluation and the reader should consider same.

Different Nights

Turbulence is when the air between your telescope and the target is moving. This affects the image differently depending on the size of your telescope. Larger scopes will show a star surrounded by spikes or rays. It’s possible for a better quality eyepiece to actually accentuate the spikes by showing them better and so make the view seem "worse". So evaluating a sharp eyepiece on a night with bad turbulence and in a large scope may give a bad impression.

Give and Take

Say we start with a nearly perfect eyepiece. It provides good clear, sharp, high contrast images but it has a narrow view. The eyepiece maker may make the field wider but some sharpness or contrast or object brightness is lost. Say the loss is brightness and you are wanting to use the eyepiece on the moon. No problem, the moon is plenty bright. Don’t get me wrong, it may be possible to build the eyepiece to give both results, but there is always a price, either in dollars or results and sometimes both.

To increase the FOV light must simply be bent more. The more light is bent the more difficult it is to get the prime result. Remember that a fast scope is already bending the light quite a bit and a wide FOV eyepiece bends it more before focusing it to a point. Attempts to increase the FOV usually cause a loss somewhere else.

Eye relief is important if you wear eyeglasses because you must have enough eye relief to accommodate the space taken up by the eyeglass. Often overlooked is the fact that you can’t really "see" the entire view of some wide eyepieces because you cannot get your eye close enough to see the edges! Eye relief is a triple whammy as longer eye relief is also more "comfortable" up to a certain limit… about 20mm for most folks. So the amount of eye relief is more important for eyeglass wearers, wide views and comfortable viewing. Some eyepiece designs inherently have more eye relief.

Physical size can be a huge factor. Long or fat eyepieces may not work on your telescope for purely physical reasons.

Weight wide view eyepieces are often very heavy and you might not be able to balance your telescope. Some of the best wide view eyepieces are very heavy.

What makes a premium eyepiece premium?

Premium eyepieces attempt to provide enhanced results in one area while not affecting others excessively. For example, Nagler eyepieces achieve very wide views while keeping the image sharp and contrasty but with some spatial (pincushion) distortion, large size, weight and, in my opinion, light loss.

Do you really want to pay extra for that? You have to decide if you want to pay for the premium features that the more expensive eyepiece provides, but don’t assume the high price gets you better results in areas not mentioned by the manufacturer, because it likely don’t!

Sets and Sizes. Sometimes eyepieces come in a range of focal lengths. Don’t assume that you need every size, in fact some sizes are almost surely too long or too short for your particular scope. Check your sweet spot and stay between 16x and 300x or 32x and 300x for reflectors.

The 2" wide pitfall. I fell into this one big time. 2" eyepieces can give you a wide sweeping view, but they may not, depending on your scope. Having 2" eyepieces means having to come up with more eyepiece storage, it becomes difficult to keep all your eyepieces within reach, you are always swapping eyepiece adapters and 2" eyepieces cost more and are not available in all sizes. In my case it turns out that the type of telescopes I enjoy most are compound scopes with narrow views. There is a small gain by going to 2" with my scopes and it turned out not to be worth it. There is another factor here as well, there are not all that many objects up there that welcome a wide sweeping view and many of those look much better in binoculars.

Barlows can help an eyepiece do its job by reducing the angle of the light coming into the eyepiece. If you have a slow scope the angle is already low, or if your eyepiece already handles higher angles (or sometimes prefers them as in many "new" designs) the Barlow can only make the image worse. Naturally there will be combinations of telescope, Barlow and eyepiece that will work better than other combinations.

You may hear that a Barlow improves the eye relief of an eyepiece, it doesn’t. It does let you get the magnification of a shorter eyepiece while preserving the greater relief of the longer one that you are using. Premium eyepieces often give better eye relief or a constant eye relief for a set by building Barlows into the eyepiece and making them a longer focal length than marked. I prefer to supply my own Barlow when needed and save my money.

Some premium Orthoscopic eyepieces use premium glass in all lenses.

Can you see through this? The amount of glass affects eyepiece results.

Less glass is better right? Maybe, maybe not!

Glass is not really transparent; a thick enough piece of glass would be as transparent as a brick wall. This is true of all types of glass. So the more glass in an eyepiece the less light gets through.

Coatings help to prevent reflection and to make up for problems caused by a lens that has many air to glass surfaces. If each surface of each lens element is coated (called "fully coated") it can make a dramatic improvement in contrast and some improvement in brightness. Some eyepieces are multi-coated and you might think this is better but sometimes it is not. For one thing, the single coated lens is more likely to be fully coated than a multi-coated lens. Another factor is that when a lens is multi-coated, different combinations of metals are used and some are very soft so a multi-coated lens can be easier to scratch. While the difference between no coating and a single coating is quite substantial, over 10 percent or more, the difference between single coating and multi coating is more like 2 or 3 percent or less.

Personally, I would go for a well singly but fully coated eyepiece over one that is multicoated every time unless I know the eyepiece is fully multi coated and I can afford the price difference.

Surface polish is often stated as a feature of a premium eyepiece. I don’t think it’s a big factor in modern eyepieces though. Nearly all eyepiece lenses are finely polished with modern equipment.

Edge Blackening is especially important for eyepieces with thick glass elements. They should be blackened. Barrel blackening and baffling may be more important. All premium eyepieces should have these characteristics. All effect contrast but not necessarily local contrast however.

Number of Elements is very important in some situations. There is the overall amount of glass to consider and each air to glass surface in the eyepiece loses and diffuses some light. Many classic eyepiece designs have only 4 such surfaces. The minimum you can have is 2 and there is one eyepiece, the Monocentric that has just that. Many people find the Monocentric to have the brightest, sharpest and highest contrast image of any design (our prime result) but guess what? Its view is very narrow. That is the give and take that I stress.

Light loss may not be as serious as it sounds. An eyepiece with excellent contrast can be forgiven for being a little dimmer. Sometimes, you need the brightness and sometimes you don’t.

Eyepiece Designs

Over the years a number of different eyepieces have been designed. They are usually named after the designer, which is why you always see the name capitalized. [FOV]

Kellner (Ke or K) [40] Designed when f/6 was a "fast" scope and is better with f/6 or slower scopes. I accidentally put a Kellner in my f/10 telescope one night when I was viewing a star cluster. To my surprise I could see more stars and the whole scene was brighter. Now I use modern Kellners whenever I am viewing faint objects. Don’t bother reviewing a Kellner in a f4 scope though.

The Rank Kellner (RK) [45] was designed a bit later when scopes were a bit faster. The RKE is a later RK and uses a special glass element to provide a better wider view with even faster scopes (but not much). For some telescopes on many objects on most nights the RKE provides as good a view as many folks want.

I have found great variation in the quality of Kellners. In general newer, fully coated, K, RK and RKE are the best. Older Kellners, especially Orion brand are of less quality.

Orthographic (Orth) [45] the perfect eyepiece but a bit narrow on FOV and perhaps short on eye relief for some people. Four elements, four air to glass surfaces. This is the design that all the other modern designs try to improve on while keeping as good a prime result. Few succeed. Orthos made today are excellent while those made in the 1980s are probably the best eyepieces available as far as image quality.

Plössl [50] eyepieces give a wider view than an Ortho at the expense of eye relief only. While an Ortho will have eye relief the same as its focal length or a bit more, a Plössl gives only 80 percent of its. Therefore some folks use Orthos below about 15mm and Plössls above. I have found this to be a good idea. A Plössl is orthoscopic, having no optical deficiencies. Plössls are easier to make so they have a price advantage; very good Plössls can be had at very low prices. Most Plössls are actually symmetricals, having two identical doublets back to back. You can check this by using the eyepiece as a magnifier; it will magnify the same amount when you reverse it.

Older Plössls made in Japan are excellent, especially the 5 element Meade Series 4000 and Celestron Ultima Super Plössls. They can be bought today for around $50-55 and are hard to beat.

Several premium eyepieces are based on the Plössl.

The Erfle [65] is a newer design that attempts to get a wide FOV while keeping a sharp image. They were designed again when f/6 was fast and so tend to work better on slower scopes. On fast scopes they tend to distort the image near the edge. Some modern Erfles are said to be sharper and have higher contrast than any other type. They have a lot of glass in them, however, and could never be as bright as a modern Kellner. The eyepiece FOV is 60 to 70 degrees.

Erfles have large eye lenses; good eye relief and can be very comfortable to use. Although the Erfle design has a bit of a bad reputation for edge distortion they work very well in slower scopes and modern makes do not have this problem. In fact, several premium eyepieces are Erfles with a Barlow added!

Königs [65] get results very similar to an Erfle but has fewer elements. It may not be as sharp or have as much contrast but should be brighter. Some Königs have only 3 elements. Königs sometimes are stated to give less edge distortion than Erfles, the truth is that both may have problems in an f/4 scope and modern Erfles are fine in f/6 and slower scopes.

Nagler is the newest design. They offer very wide fields of view (80+) degrees and have many elements. The view through a Nagler can be spectacular on the right telescope; it is highly corrected and works well in fast and very fast telescopes. I found my Nagler to be a bit dim in my 6-inch telescope and only slightly wider than my Gary Russell UWA. Later I realized that the Nagler was actually quite a bit wider but this was a case where I could not get my eye close enough to see the full view. I kept the GRO UWA and sold the Nagler. Some reviewers say that the Nagler provides somewhat soft (less sharp) images. I did not find this to be the case.


Figure out what you want to observe and can observe with your size scope. A smaller scope, say six inches or less, will never be great for faint fuzzies. Scopes, having f/ratios of f/6 or above work fine with Kellners, Orthos and Plössls and modern Erfles and Königs will give fine wide field views.

If you must wear glasses at the eyepiece, avoid Plössls. If you don’t wear eyeglasses you probably don’t need premium eyepieces.

If you use compound telescopes (SCT, MCT, Klevzov Cassegrain) or very slow scope (f/10 or more), think hard before moving into 2" eyepieces.

Everyone should have a set of modern Kellners. They are cheap, and a set can be had for well under $100 used. At times it can be like adding an extra inch or two to your scope.

The human eye has about a 50-degree FOV. Anything greater than this will be perceived as a wide view. Give 65-degree FOV eyepieces a chance before jumping into the 70-degree plus eyepieces.

Eyepieces made from surplus elements can have some big advantages. Gary Russell, for example, machines his eyepieces from Delrin, a metal like plastic that is very strong and light weight. When it’s 20 degrees f at the scope, these eyepieces are a pleasure to handle while the big metal ones are brutal to the fingers. There is another advantage. Most surplus glass is from the military and uses only the hardest types of coatings. Paul Rini and a gentleman named Siebert make eyepieces from surplus lens elements but I do not have experience with either.

Time and Trials. Give any eyepiece time and be sure to try it on many nights before you decide that it’s not for you. I had an eyepiece that I honestly thought was useless until I tried it one night on the Trapezium in my f/8 Newtonian. In that scope, on that target, that eyepiece is spectacular!

Focusing: The better the conditions the harder it is to focus. I have found that the nights with the best sky conditions are when Very careful focusing pays off. This is especially true when viewing clusters.

The best sky conditions are needed when evaluating the best eyepieces. I have found that the difference between a good and an excellent eyepiece don’t show up unless the seeing is very good. Very careful focus will be needed as well. You may find that you have to change your focus technique on the best nights, I did. Spend more time after each tweak of the focus knob evaluating the image.

Don’t assume that a particular premium eyepiece design or brand is better in all respects than one of a standard, named design. Read the manufacturers literature carefully, remembering that if one result is improved another may suffer. You might notice that an eyepiece is claimed to have a superior, even the best, FOV while little or no mention is made of sharpness or sharpness is simply "good". Be sure before you buy the wide view make sure that is what you want and are willing to pay for and you are not expecting increased sharpness too.

I prefer to know the actual type of design of my eyepieces, whether Kellner, Plössl, Erfle and so forth. Most eyepieces are based on one of named designs and I try to find out what that is so that I know if some feature is just an attempt to compensate for some defect inherent in that design. For example, I wouldn’t want to buy a premium eyepiece whose major feature is compensating for the short eye relief of a underlying Plössl design. I especially wouldn’t want to pay for this feature in say, a 25mm eyepiece since a 25mm Plössl has enough eye relief to start with. But such eyepieces are made and sold at a premium price anyway.

When you read an eyepiece review make sure the reviewer is using a telescope similar to yours, looking at similar objects under similar skies. At least make sure the scope is about the same f/ratio as your scope(s). I hope I have given you an understanding of what to look for and why and which things are important.

Conclusions and Generalities

I have sprinkled some conclusions through out this document. There are some things I have discovered that don’t fit elsewhere which I include here.

Eyepieces made in the 1980s and early 1990s generally outperform those made today, especially premium eyepieces. This is especially true when a design was changed to increase FOV or elements were dropped (from 5 to 4 elements). Don’t let the price fool you.

All of the Russian eyepieces I have tested are excellent although their FOV tends to be less than stated and the design stated is often incorrect. I have found an Erfle to narrow to be an Erfle and Plössls and Kellners too wide for those designs. Beware that there are "Russian" eyepieces made in China and Taiwan. The Russian eyepieces I am talking about can be identified by three characteristics: 1. The lettering is engraved and filled 2. The knurling is very fine and 3. The letters "S" will be symmetrical without the bottom curl of the letter being squashed.

BTW engraved and filled lettering is a mark of a fine eyepiece of any manufacture.

Hot Buys: Right now there are some very, very good generic Orthos floating around for about $30. They are sometimes completely unmarked and are lightweight, with strange flat rubber guards around the eye lens. Sizes include 7mm and 16.8mm. They may have been made for Bushnell for either telescopic or binocular use.

Almost any true Russian eyepiece is a good buy.

The Super Easy View eyepieces are an odd design that provides premium performance at a sub premium price.

Grab any Meade Series 4000 or Celestron Ultimas made in Japan that you see for $55 or less. Made in Japan in this case actually indicates the design. They are 5 element Super Plössls and are truly premium eyepieces.

The Crib Sheet

Below are some of the numbers that you can use as a guideline to figure out which eyepiece is which. Pay special attention to the FOV. In fact, the basic design incorporated in a premium eyepiece is often revealed by the FOV. Remember that all focal lengths in a "set" having the same eye relief indicates that Barlows are in use, usually on a Plössl or Erfle design. Be sure to read the references at the end of this article too.


Field of View

Number of Elements

Underlying Design

May include





Special Glass

Super Plössl




Barlow, Special Glass





Special Glass, Barlow









Special Glass, Barlow





Special Glass, Barlow





Special Glass

SWA König




Special Glass







Special Glass, Barlow





Special Glass





Special Glass, Barlow


All About Eyepiece Designs by Dr. Marcus E. Hennecke http://www.atmpage.com/ep.html
http://www.weatherman.com/user.htm - especially the reviews by Darwin Bagley

Eyepiece Table

Following are the eyepieces I have used/tested

2" Eyepieces





52mm Omcon Erfle 3.5*

Heavy — blackout
40mm University Optics (UO) MK70 5*

32mm UO König II 4*
25mm UO MK 70 5*

22mm TeleVue Nagler Type 4 3*

Heavy, could not use all of the FOV
20mm Gary Russell (GRO) UWA "Baby Nagler" 5*

Great Eyepiece!
19mm GRO SWA 4*
13mm GRO SWA 5*
11mm GRO SWA 4*







42mm Celestron Ultima 4.9*

Like 40mm Meade below better
40mm Meade Series 4000 Super Plössl — Japan 5*

5 Element
40mm Celestron Plössl 2*

32mm Sovietski Plössl 3*
30mm Intes Micro Kellner 4*
32mm Pocono Mt. Optics (PMO) Super Plössl 4*
28mm Edmund’s RKE 5*

26mm Celestron Plössl 3*°
26mm PMO Super Plössl 4*
25mm TAL Plössl 5*
25mm TAL Plössl w/reticle 4*
25mm JSO Rank Kellner 5*
22mm UO Kellner 4*

20mm GRO SWA König 5*
20mm Meade Series 4000 Super Plössl — Japan 5*
20mm ? Kellner 4*
18mm Intes Erfle 4.5*

AFOV less than advertised, excellent otherwise
18mm GRO/Nikon SWA 4.5*
17mm Sovietski Plössl 5*
17mm GRO SWA 5*

16.8mm Telescope Warehouse Ortho 5*

15mm Meade Series 4000 Super Plössl — Japan 5*

5 element
15mm TAL Kellner 5*

Excellent on DSOs, esp. Globular Clusters
14mm Russell/LOMO König 5*
12.4mm Meade Series 4000 Super Plössl — Japan 5*

5 element?
12mm Brandon Ortho 5*

1980’s vintage
12mm Meade Kellner — Guiding 4*
12mm Meade Kellner — Astrometric 5*
11mm TeleVue Plössl 4.5

Premium Price
10mm Sovietski Plössl 4*
9.7mm Meade Series 4000 Super Plössl — Japan 5*
9.5mm Telescope Warehouse Super Easy View 5*

Premium performance
9mm Meade Modified Achromatic 2*

Tiny FOV
8mm Edmund’s RKE 5*

8mm TeleVue Plössl 3.5*

Lacks Eye Relief Premium Price
7.5mm Sovietski Plössl 4*

Good on M42 in TAL2
7.5mm Celestron Ultima Plössl— Japan 5*

5 element FMC
7mm Telescope Warehouse Ortho 5*

6.7mm Meade Series UWA 5*
6mm PMO (UO) Ortho 5*
6mm Intes Micro Monocentric 5*

Specialized eyepiece, narrow FOV
5mm Vixen Ortho 5*
5mm Orion Ultrascopic 5*
4mm GTO Plössl 3*

Lacks eye relief
4mm Celestron/Vixen Ortho 5*

Needs perfect conditions to excel

Note: My ratings (0-5 "*") are based on the eyepiece satisfaction as they performed in my scopes, according to my use and price is taken into consideration but has the lowest weight.

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