It is generally recognized that an anastigmatic lens system has a relatively fixed amount of field curvature associated with its focal length. The present paper investigates the manner in which indices of refraction, thicknesses, and separations may be used to gain some control over Petzval curvature independently of back focal length.
W. WALLIN, "The Control of Petzval Curvature," J. Opt. Soc. Am. 41, 1029-1030 (1951)