A type of eyepiece consisting of two planoconvex lenses of the same focal length with their flat sides facing outwards. Its main advantage is its simplicity and low cost. It has poor eye relief, and suffers from chromatic aberration and spherical aberration, although not coma (see coma, optical). It was invented in 1782 by the English instrument-maker Jesse Ramsden (1735–1800). See also kellner eyepiece.