A design of wide-field eyepiece consisting of three lenses, two or three of which may be doublets, that gives a field of view of 65–70°. Although the field of view is slightly curved and the images at the edge of the field suffer from astigmatism and chromatic aberration, the Erfle is cheaper to manufacture than many other wide-field designs. It was the first wide-field eyepiece, and was designed in 1917 by the German optician Heinrich Valentin Erfle (1884–1923). The König eyepiece is an adaptation of the Erfle, with shorter focal length, named after its inventor, the German optician Albert König (1871–1946).
Subjects: Astronomy and Astrophysics.