Planar (Double-Gauss) Lens

Carl Friedrich Gauss had designed a telescope objective of two air-spaced lenses - a crown meniscus and a flint lens with differently curved facing surfaces - that corrected spherical aberration for two colours at once. Paul Rudolph at Zeiss turned it into a camera lens: two such groups placed symmetrically either side of the stop, each group achromatised with a cemented pair of glasses of nearly the same refractive index but different dispersion. **US 583,336**, "Object-glass", Rudolph, assigned to Carl Zeiss, filed 1. February 1897, granted 25. May 1897. Descendants of this double-Gauss layout became the standard fast normal lens for most of the twentieth century. **You test what the symmetry buys**: a grid imaged through a single lens with the stop in front, with it behind, and through a symmetric pair with the stop in the middle.