Plossl and Kellner eyepieces are both popular choices for amateur astronomers, and they have some similarities and differences.
The Kellner eyepiece design, invented in 1849, uses three lenses: a cemented convex doublet lens acts as the “field lens” where light enters, and the final image is formed by a single convex eye lens. The Kellner is an evolution of the earlier Ramsden design, which uses a single lens in place of the cemented doublet.
The Plossl design uses a second cemented doublet lens as the eye lens in place of the Kellner’s single lens and thus represents an evolution of the Kellner. Many Plossls are of the “symmetrical” type, which use identical cemented doublets for both the field and eye lens, simplifying manufacturing.
Kellner eyepieces were a mainstay of astronomy from the 1940s through the 1990s, offering vast improvements over Huygens and Ramsden eyepieces and being ideal for telescopes with focal ratios of f/8 or slower. Kellner eyepieces taken out of surplus binoculars flooded the market after World War II, and higher-quality ones were produced by manufacturers in Japan soon after. The Kellner was killed as much by the demanding nature of faster Dobsonian telescopes as by economics: it became cheaper to use the same doublet lenses for the Plossl than whatever was saved by not using another piece of glass.
Plossl eyepieces have been around since 1860, but were few and far between until the 1980s, when computerized ray tracing software made it possible for them to be designed for efficient manufacture. Today, they dominate the kits included with beginner telescopes and are plentiful when sold à la carte, while it can be genuinely hard to even find a Kellner sold on its own.
Performance of Plossl vs. Kellner Eyepieces – Which is Right for Me?
There is pretty much no reason to go with a Kellner over a Plossl eyepiece based on performance; however, if your telescope simply came with Kellners, it may or may not be worth upgrading, and if you are using fairly nice Kellners, a cheap Plossl ocular may actually be a step down in viewing quality. Despite what some may say, the difference only matters if your telescope is below around f/6, and if your telescope is below f/6, you can and should go with something a lot better than either optical design anyway if you’re buying outright new eyepieces. Ultimately, the best eyepiece for you will depend on your specific needs and preferences. It’s generally a good idea to try out a few different eyepieces to see which one works best for you.
- Image Quality
Kellner eyepieces are a little more likely to have internal reflections than Plossls, but it’s hard to tell. The Kellner design has a little more chromatic aberration and problems with astigmatism at the edges of the field of view than a Plossl, so Kellners are generally only good at focal ratios of f/8 or above and at least work down to f/6. Plossls can be used with telescopes as fast as f/4.5, but they work best at f/6 or above. Kellners are often stopped down to a slightly narrower apparent field of view than an equivalent Plossl—perhaps 45–50 degrees instead of 50–55 degrees. However, Kellners with apparent fields as wide as 55 degrees exist, and Plossls are similarly limited to this field angle before they start to suffer from unacceptable aberrations.
- Eye Relief
Both Plossl and Kellner eyepieces are very short on eye relief; the eye lenses get smaller at short focal lengths, and you have to jam your eyeball into the lens to see anything at all. A Kellner or Plossl below around 11mm focal length is usually quite uncomfortable to look through, and below 8mm in focal length, your eyelashes will get in the way, and observing through one is almost impossible.
Distinguishing Kellner & Plossl Eyepieces from Other Inexpensive Designs
Manufacturers’ labeling is inconsistent at best when it comes to labeling inexpensive eyepiece designs. Kellners are usually labeled as “K’, “SMA” for “Super Modified Achromat”, or occasionally “A.R.” for “Achromatic Ramsden”. Plossls are usually “PL” or “P”, though the term “Super Plossl” is popular. Celestron’s E-Lux 2” eyepieces are Kellners, but the 1.25” ones are Plossls, which adds to the confusion.
In the absence of the word “Plossl,” it can be generally assumed that a “Super” eyepiece is not a Plossl. “Wide angle”, “SWA”, “Super Planetary” and other such eyepieces are usually of the Erfle design, which uses 5 lenses, often with another lens or two to improve the image.
“Super” eyepieces with no additional verbiage are often Königs, a 3-element design with a 50–60 degree apparent field of view and a design similar to a reversed Kellner with a singlet field lens and doublet eye lens with little space between all three elements. “RK” and “RKE” eyepieces are of the Reversed or Rank Kellner design, which has a doublet eye lens like the König but with wider spacing and easier manufacturing tolerances. König and RKE eyepieces are a lot easier to look through and often sharper than Kellner or Plossl eyepieces
If you’re looking at older eyepieces to round out a collection, other simple and inexpensive designs such as the Bertele and Orthoscopic are great performers; the Orthoscopic bests even the highest-quality Plossls and Kellners in sharpness and edge-of-field correction but is limited in eye relief or field of view, while the Bertele is a wide-angle design like the Erfle with severe sharpness limitations in fast telescopes.
How to Get a Great Plossl/Kellner
Good Plossl and Kellner eyepieces have fully multi-coated optics for minimal glare/reflections and good light transmission. Cheaper units with plastic parts are probably going to have accordingly sloppy optical quality control. The majority of good Plossl and Kellner eyepieces are high-end older oculars manufactured before the year 2000, though premium Plossls from companies like Tele-Vue and Masuyama still exist. However, regardless of what you end up choosing, remember that Plossl and Kellner eyepieces are very uncomfortable to use with focal lengths of below 11mm and perform poorly in faster telescopes.
Our Best Plossl Eyepiece Guide goes into detail on some of our favorite Plossl eyepieces available today. If you’re shopping used, the Celestron Ultima/Orion Ultrascopic and similar “Masuyama-type” Plossls, along with anything made by Clave, Tele-Vue, or Vixen, are likely to be good choices. For Kellners, few high-quality options are sold independently today, but most older units will do just fine; many Japan-made Kellners were imported by companies like Celestron and compare favorably to many of the cheaper and lower-tier Plossl oculars sold today.