When a beginner is choosing eyepieces on a budget, there are a number of possible designs they may choose between. While eyepieces with relatively complex, proprietary, or bespoke designs are becoming more commonly available in the beginner price bracket, there is often good reason to choose a much simpler design.
Plossl and Kellner eyepieces are two very common examples of a simple eyepiece design, and they are probably the most common type of eyepiece you are likely to come across when eyepieces have been bundled with a telescope; they are also a very common aftermarket accessory.
Plossls and Kellners, like other simple eyepiece designs, have moderate fields of view and tend to have short eye relief as you scale them down to shorter focal lengths.
History of Plossl and Kellner
In the early days of telescopic astronomy, refractor telescopes were designed with a single lens at the front and a single lens at the back. Centuries of improvements led to achromatic refractors, where the primary lens is made up of two lenses of different types of glass, which reduces false color fringing and allows the telescope to be made considerably more compact. In addition, the eyepiece would be a Huygens or Ramsden design, where two lenses made of the same type of glass could be combined to yield minimal false color fringing, as long as the focal length of the telescope remained fairly long.
In 1849, Carl Kellner invented the Kellner eyepiece, which is essentially a Ramsden eyepiece where one lens has been replaced with an achromatic doublet made of two different types of glass. The three-lens eyepiece that came out of this was much better at controlling false color fringing and other distortions, even in telescopes with short focal lengths. By correcting for aberrations, the field of view of these eyepieces could be much larger than the “soda-straw” field of view in a Ramsden or Huygens, around 40 or perhaps up to 50 degrees. The tradeoff was that each lens surface added another internal reflection, reducing contrast and adding glare.
In 1860, Georg Simon Plossl invented the Plossl eyepiece. A four-element eyepiece was made by putting together two achromatic doublets with different focal lengths. This eyepiece was even better at correcting false color fringing, but because of internal reflections, the glare was even worse.
By the 1980s, amateur astronomy equipment suppliers were still selling new telescopes with Huygens and Ramsden eyepieces, not merely because they were cheap but because they lacked internal reflections. You might be able to find a Kellner, or perhaps a high-quality Orthoscopic. As new anti-reflection coatings were developed, it became possible to actually make use of complex lens arrangements (not just in relatively simple 4-element systems but also in convoluted, extreme wide-field eyepieces such as the TeleVue Nagler).
Optics manufacturers began selling and popularizing Plossl eyepieces, albeit with a generally simplified design where each achromatic doublet had the same focal length. These eyepieces are technically called Symmetrics, since Plossls can have different focal lengths for each doublet, but they are always marketed as Plossl.
Today, most beginner telescopes of any quality are sold with either Kellner or Plossl eyepieces. Those that still come with Huygens and Ramsdens are generally cheap and low quality, since practically the only reason to use those designs in modern times is that they are extremely cheap to manufacture. Very often, those eyepieces will be in the outdated 0.965” format rather than the modern 1.25” or 2” diameter barrel format.
Plossls vs Kellner
Despite one being fundamentally superior to the other in terms of image correction, Kellners and Plossls are generally quite comparable in terms of the image you actually see.
Kellners tend to have narrower apparent fields of view (40 degrees rather than 50 or 52 degrees in a Plossl), and in short focal length telescopes (focal ratio f/5 or shorter) they can have more false color fringing, but for the most part it is hard to notice the difference between a well-made Kellner and a well-made Plossl in terms of image quality. Many Kellners are made with field stops opened up to 50 degrees, in which case it can truly compare to a Plossl in any telescope with a focal ratio of f/6 or slower.
(Focal ratio is a term that refers to the telescope’s focal length divided by its aperture. Higher values are slower and lower values are faster–this is a holdover from camera lens terminology and has to do with exposure times.)
With modern multi-coated optics that thwart internal reflections, Plossls and Kellners have little difference in glare or transmission/opacity.
I have seen some gatekeeping in this hobby about Kellners, and I’m not sure I understand it. People may have heard, and rightly so, that Huygens or Ramsdens are crummy eyepieces and that they should get a Plossl instead, and they may believe the same applies to Kellners, but the truth is that Kellners are often good eyepieces. Yes, the wider field of view of a Plossl is very helpful, but 40 degrees of Kellners isn’t a soda straw (unless you are used to looking through very wide field eyepiece. Oh the pain of… uh… having access to really cool eyepieces.)
When I was getting my start in the hobby, I bought an ultra-cheap ($15?) 6mm Kellner eyepiece that I enjoyed a lot. Despite the narrow field of view and short eye relief, it showed shockingly good views of Saturn, Jupiter, and even the Moon. Even when I upgraded to a 6mm Goldline, I kept the Kellner around because of pretty annoying kidney-bean blackout issues on the Moon with that eyepiece. I foolishly destroyed it while experimenting with solar projection, because I hadn’t realized the metal-and-glass construction was aided by a plastic retaining ring on the field stop, and I hadn’t realized you do not need to change the eyepiece focal length when doing solar projection. Generally, don’t mess around with solar projection unless you’re okay with damaging the instrument. And of course, NEVER look through a telescope pointed at the sun that does not have a safe solar filter covering the front.
Field of View Comparison
Plossl eyepieces have wider fields of view than Kellners. By default, expect a 40-degree field of view in a Kellner and a 50-52 degree field of view in a Plossl. I have seen wider-field Plossls. There was an old Meade “Super-Plossl” design that used an additional lens element and could reach a wider field of view. I have a custom-built 32mm Plossl that has been opened up as wide as possible to 60 or 70 degrees, for use as a 2” eyepiece, although in my 10” Dobsonian at f/5 the edge distortions prevent it from seeing much use. There’s a reason Plossls are usually kept at 52 degrees.
The field of view of the eyepiece has a few effects when it comes to actually using it in the telescope. For one thing, a wider field of view is almost always more pleasant. You see the object in a wider expanse of space with more stars. In some cases, an object is so large (i.e., the Moon, the Pleiades, the Orion Nebula, the Andromeda Galaxy) that a narrow field of view can only show you part of the object, whereas a wider field of view could show the whole object at the same magnification.
Generally, to maximize the number of stars you see in an eyepiece with a given telescope, both the magnification and field of view should be maximized. Between Kellners and Plossls, this means a 32mm Plossl, which maximizes the field of view in the 1.25” barrel diameter format, will show more stars than either a 32mm Kellner with the same magnification but a smaller true field of view, or a 40mm Kellner (or 40mm 1.25” Plossl, which has a 40 degree apparent field of view) with a lower magnification and the same true field of view. Through the 32mm eyepiece, objects will look larger, and though they may have a lower surface brightness, the surface brightness of the background sky will also be lower.
Another useful aspect of a wider field of view is that, assuming the entire eyepiece is sharp enough to show good images right to the edge, you don’t have to nudge the telescope to track the rotation of the Earth as often with a Plossl (50 degrees) compared to a Kellner (40 degrees). The difference isn’t that severe, and I often track more often than is strictly necessary to keep the object centered anyway.
Finally, a wider field of view just makes it easier to find stuff. This is why we recommend you try to locate the object first at low power, then use a higher magnification if you like—the wider field of view on the sky provides more context in terms of asterisms and reference stars and makes it more likely that you’ll catch the object you are looking for.
Plossls are the clear winner here in most cases, unless you have one of the wider-field 50-degree Kellners.
Eye Relief Comparison
There is one aspect in which a Kellner can actually outperform a Plossl: eye relief.
Eye relief is the distance from the eye lens to your eyeball, and the higher the better (within reason). Eye relief of less than 10mm gets really uncomfortable—you tend to rub your eyepieces against the eyepiece or lens, and you have to hold your eye at just the right distance to keep from touching the lens. A long eye relief is also important for those who wear eyeglasses at the eyepiece, whether that’s a member of the general public at a star party or an astronomer with astigmatism, which, unlike near- or far-sightedness, cannot be corrected with the telescope’s own focuser.
Plossl eyepieces have an eye relief equal to about 80% of their focal length. This is perfect for a 25mm Plossl (an extremely common standard) and downright luxurious with a 32mm Plossl, but when you get below about 12.5mm or 10mm, it can begin to get difficult to use.
I have several 10mm eyepieces in my eyepiece box—almost all of them are eyepieces that came with some telescope I bought—and I will almost always choose to use one of the Kellners over one of the Plossls, because the Kellners have a longer eye relief, which is comparable to the focal length. When I had a 6mm Kellner, I found the eye relief annoyingly short, sure, but when compared to the 6mm Plossl that often comes with “eyepiece kits,” it was luxurious.
A common problem are “eyepiece kits,” which are big boxes that some telescope suppliers will sell you with half a dozen eyepieces, most of which include annoyingly short focal length Plossl eyepieces. A 6mm Plossl has a tiny eye lens. I once had to use a 6mm Plossl to attempt to perform a star-test collimation for a telescope at the observatory. It was grueling, because I couldn’t even get the entire field of view—I had to keep my eye hovering above the exit pupil.
I think some of these kits even include 4mm eyepieces, and that’s ridiculous! Even if you did need a 4mm focal length (for high powers in an f/4 telescope, of course), neither a Kellner nor a Plossl would have long enough eye relief for that to be comfortably usable! You’d have to look into more complex designs which include a barlow lens, or simply use a longer focal length eyepiece with a quality barlow.
Even though both eyepieces are very common in beginner telescopes, it has become harder to find Kellner eyepieces on their own in the last two years, whether they come from a brand name or the original equipment manufacturer or a reseller. Meanwhile, Plossl eyepieces are much more common and easy to find. As of the past couple years, it seems possible that the choice between Kellners and Plossls has been made for us—you either get what you get with your beginner telescope, or you will have to find a Plossl.
Price Differences Between Kellners and Plossls
Kellners are often sold with cheaper, even more entry-level telescopes than Plossls. Plossls tend to come with large, ground-standing Dobsonians, whereas you’re more likely to find a Kellner sold with a tabletop Dobsonian. Most of Celestron’s entry-level telescopes are sold with Kellners (and other cheap but reasonable-quality designs like RKEs or Konigs), whereas their more expensive SCTs tend to come with a single 25mm Plossl.
Kellners can be as little as half the price of a Plossl in the same focal length, but they are generally very comparable, especially in recent years as telescope prices have fluctuated in response to changing supply chains and demand. Comparing Orion Telescope & Binoculars’ options, it appears that a Kellner can be found for almost half the price of a Plossl of the same focal length, although there are also equally cheap Plossls available from various sellers on Amazon that are highly comparable. (Svbony’s Plossls, for example, are optically identical to Orion’s Sirius Plossls).
If you have Kellners already
You probably don’t need to worry about upgrading them as soon as you get your telescope, unless you can see an optical problem in the one you’re using or you find the short eye relief for the short-focus eyepiece uncomfortable. If you do upgrade, you’ll probably want to go for a different eyepiece design than a Plossl–something with a sharper image or a wider field of view or a longer eye relief (or all three.)
If you want to upgrade
At this point, a Plossl and a Kellner are so close in price and performance that you may as well get the Plossl. If you wish to find a short-focal length eyepiece with a longer, more comfortable eye relief, you could use a long-focus eyepiece with a separate, high-quality barlow lens (an achromatic or apochromatic fully multi-coated barlow would be necessary), or you could purchase one of the several designs incorporating a smyth/barlow lens into the eyepiece design. These are effectively simple designs like Plossls, Kellners, Konigs, and RKEs, with long focal lengths, that have a lens included to increase their focal length. Within the budget-friendly realm are the 6mm and 9mm Goldlines/Redlines (66-degree Ultra-Wide-Angle) for telescopes with a focal ratio of f/8 or longer, and the excellent 58-degree Planetary eyepieces, sold by a variety of sellers on Amazon and by Agena Astro, which come in a wide variety of focal lengths and are optically superb for their price.