Eyepiece Types

Some names, like Ultrawide, give a clue to an eyepiece's principal characteristics. Others have classic names like Kellner, Orthoscopic, Plössl, and Erfle, or more specific modern variants like Nagler and Lanthanum. Optical aberrations (deviations from the "ideal" form) are better corrected in some designs than others, but like everything in life, the higher the price, the better the quality.

Even so, eyepieces with desirable characteristics still can be obtained at reasonable cost once you know what to look for. At the budget end of the market, three-element (meaning they have three lenses inside) eyepieces labeled Kellner or "MA" (modified achromat) can turn in a good performance with scopes of long focal ratio, such as Schmidt-Cassegrains and traditional refractors. They do not work well with telescopes of short focal ratio.

For optically "fast" Newtonians and the like, better results will be obtained with four-element designs, such as the slightly out-of-fashion but otherwise desirable Orthoscopic, or the ubiquitous Plössl. The latter is a good all-rounder, particularly when antireflection multicoated. A Plössl can deliver well-corrected, wide fields of view with good eye relief — meaning the eye can be positioned at a comfortable distance behind the rear lens and still see the whole field of view. It is worth pointing out that observers who wear glasses to correct for simple long- or short-sightedness (no astigmatism) don't need to use them at the telescope; a twist of the focuser will remedy that.

High-power lunar and planetary viewing entails the use of short-focal-length eyepieces, which can be a problem owing to the small eye relief of conventional designs. This has spawned the development of six- to eight-element designs that combine comfortable eye relief and wide apparent fields across the focal-length range. They may feature exotic rare-earth glass elements to reduce optical aberrations still further. Many regard these Lanthanum and Ultrawide designs as the pinnacle of eyepiece evolution (and rightly so), but for many people the price will be a barrier — and their physical size and weight may be an issue for delicately balanced small scopes.