A lens or combination of lenses used to magnify the image formed by a telescope; also known as an ocular. The simplest form of eyepiece is a single converging lens of short focal length, but this has severe aberrations except at the very centre of its field of view. In practice, therefore, eyepieces usually have at least two elements. A field lens, facing the objective or mirror, gathers light over a wider field than a single lens, while the eye lens, through which the observer looks, provides the magnification. This combination gives a good field of view while keeping aberrations under control. A field stop (a diaphragm located so that it is in focus as seen through the eye lens) provides a hard edge to the field of view. Many additional elements can be included to improve performance. Popular designs include the Erfle, Huygenian, Kellner, monocentric, Nagler, orthoscopic, Plössl, and Ramsden eyepieces.