Different Eyepiece Designs
Huygens. This eyepiece design was invented by Christiaan Huygens in the 17th century. This two-element design is now considered outdated, but sometimes such eyepieces (marked with capital ‘H’) are supplied together with cheap telescope models. Eye relief and field of view are quite small. Ramsden eyepiece design, which is a modified version of the Huygens eyepiece design, is much more efficient but is also outmoded (although still used in some microscopes).
Kellner. Three-element Kellner (and its close modifications – Achromatic Ramsden “AR” and Modified Achromatic “MA”) are considered to be the least expensive eyepiece for serious astronomy. Such eyepieces provide bright and clear images at small and medium magnifications. Kellner eyepieces work perfectly with small and medium-sized telescopes. These eyepieces have about 40° apparent fields of view and reasonable eye relief, though quite short for high magnifications.
Orthoscopic. The four-element orthoscopic eyepieces were once considered the best eyepieces for universal use. But now such optical design clearly loses the competition to more recent designs because of its quite narrow field of view. Orthos provide excellent clarity, color correction, contrast, and larger eye relief than Kellner eyepieces. They are especially good for lunar and planetary observations.
Plössl. The four-element Plössl design is the most popular eyepiece optical design that gives you excellent image quality, good eye relief, and a 50° apparent field of view. High-quality Plössl eyepieces provide high contrast and sharpness across the entire field of view. They are suited for any king of observation.
Erfle. The 5- or 6-element Erfle eyepieces are optimized for a wide apparent field of 60° to 70°. At low magnification these eyepieces produce spectacular views of stellar fields. At high magnifications the edge-to-edge clarity suffers a little.
Ultra Wide Angle Eyepieces. This group includes improved optical designs consisting of 6-8 elements and featuring wide fields of view up to 85°. It is so wide, you have to rotate your eye to see the whole panorama (by the way, not everyone loves it). Additional elements slightly increase light loss inside the eyepiece, but in general the resulting image quality is very high. So is the price.