photographic process. Fox Talbot's calotype was discovered earlier but did not enjoy commercial success. Petzval learned of the invention from his friend, Viennese professor Andreas von Ettingshausen. The daguerreotype was problematic in that it required exposure times as long as 30 minutes to create a portrait. With Ettingshausen's urging, Petzval set up a workshop and laboratory at Kahlenberg in Vienna and, after six months of complex computations, produced designs for improved objective lenses for both portraiture and landscape photography. Because the artillery was one of the few occupations that used advanced mathematical computations at the time, Archduke Ludwig lent eight artillery cannoneers and three corporals to the computational efforts. The calculations these men carried out in tandem with each other have been regarded as an early (albeit human) example of a parallel computer. Petzval's portrait objective lens (Petzval Porträtobjektiv) was an almost distortionless Anachromatischer vierlinser (double achromatic objective lens, with four lenses in three groups). The luminous intensity of this flat "portrait lens" was substantially higher than the daguerre standard of 1839, the Wollaston Chevalier lens (). The screen with a focal length of 160 mm made crucially shorter exposure times possible — using exposures of only about 15 to 30 seconds compared to the 10 minutes previously. Thus, snapshots became possible for the first time. The portrait objective lens consisted of a cemented double lens in front () and a double lens with a gap in the back. The rear double lens was necessary for the correction of spherical and coma errors. The Chevalier lens used two cemented double lenses, but was immediately replaced by the Petzval lens, so that the Petzval Porträtlinse was the first cemented lens in widespread use. The first portrait objective lenses were rather small and had a diameter of 2.6 cm. The 1856 Petzval lenses produced by Dietzler had a diameter of 15 cm and a weight of 15 kg, with which one could make portraits measuring 33 by 42 cm. In 1840, Petzval allowed the Viennese entrepreneur Peter Wilhelm Friedrich von Voigtländer to produce the lens for a one-time payment of 2,000 guldens, without a patent or a contract, which led later to a lasting controversy between Petzval and Voigtländer. Voigtländer, who had confirmed the process through his own calculations, produced a prototype in May 1840 and began production of the lens for the daguerrotype cameras in 1841, making a fortune in the