Anastigmat

An anastigmat or anastigmatic lens is a photographic lens completely corrected for the three main optical aberrations: spherical aberration, coma, and astigmatism. Early lenses often included the word Anastigmat in their name to advertise this new feature (Doppel-Anastigmat, Voigtländer Anastigmat Skopar, etc.). The first Anastigmat was designed by Paul Rudolph for the German firm Carl Zeiss AG in 1890.

Example of anastigmat lens designs: (1) Six elements in two groups, (2) six elements in four groups. The controllable aperture stop is typically placed in the middle of the composite lens (between the so-called eye- and field-lenses), and its maximum diameter (the one shown) dictates the minimum F-number for each lens.

All modern photographic lenses are close to being anastigmatic, meaning that they can create extremely sharp images for all objects across their field of view; the underlying limitation is that the lens can deliver the anastigmatic performance only up to a maximum aperture (i.e., it has a minimum F-number) and only within a given working distance (focusing range). Note that all optical aberrations (except spherical aberration) become more pronounced towards the edges of the field of view, even with high-grade anastigmatic lenses.

Anastigmatic performance is accomplished by a proper combination of multiple lenses (optical surfaces), usually three or more. Aspheric lenses can minimize the number of surfaces required and thus the bulk and weight of the composite lens; however, aspheric surfaces are more costly to manufacture than spherical and other conic section (hyperbolic, parabolic) ones. Many high-end catoptric telescopes are three-mirror anastigmat, while the corresponding catadioptric telescopes use two mirrors (reflector) and one lens (refractor) to accomplish the same result.