Binocular eyepieces usually consist of three or more lenses in two or more groups. The most common is the venerable Kellner configuration, a design dating from 1849 that consists of a singlet field lens and a doublet eye lens. Increasingly common are reversed Kellners, a design that was introduced in 1975 by David Rank of the Edmund Scientific Company and used in its RKE eyepieces. The field lens is the doublet and the eye lens is a singlet. The reversed Kellner has the advantages of a slightly wider field (50 degrees as opposed to the 45 degrees of a Kellner), over 50 percent more eye relief, and of working better with the short focal ratios that typify binocular objectives.Wide field binoculars usually use modifications of Erfle eyepieces. These consist of five or six elements in three groups. They can have a field of up to about 70 degrees, but eye relief tends to suffer when the field exceeds about 65 degrees (Figure 2.2).
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