ABSTRACT
A lens designed and corrected for the Seidel aberrations may form a spatially correct image in a focal plane of point objects at all field angles. This is possible only in monochromatic light, as the refractive indices of the optical materials are a function of wavelength. Focal length therefore is inversely proportional to wavelength, and as refractive index decreases with wavelength then focal length increases with wavelength. Chromatic aberration may also be present in the field of a lens, as distinct from longitudinal chromatic aberration, which is a variation of image position with wavelength. The Gauss lens is an air-spaced doublet with a steeply curved meniscus, the additional degrees of freedom allowing even better correction. This design has found important use in symmetrically spaced pairs to give the double Gauss design for anastigmatic lenses. The non-linear spectra of the two glasses cause a residual, if much reduced, chromatic error called the secondary spectrum.
