Petzval, Josef (1807-91), Hungarian mathematician who in 1840 became the first person to compute a lens design mathematically. His lens, designed specifically for cameras, used two achromatic pairs of elements, and with its then enormous aperture of f/3.6 reduced exposure times from minutes to seconds. Petzval's configuration is still the basis of some long-focus and projection lenses. He also worked out a formula (the Petzval sum) for the curvature of field of a lens in terms of the radii of curvature of the surfaces of its elements, thus enabling later lens designers to compute lenses with a completely flat field.
— Graham Saxby
