The Petzval portrait lens

How it began

Daguerre’s cameras used a slow landscape lens, so a portrait meant sitting still in bright sun for minutes. In 1840 Joseph Petzval, a mathematics professor in Vienna, took up the problem. With a team of army gunners as human calculators, he computed a lens of four elements in two groups instead of finding one by trial and error. The Voigtländer company made and sold it from 1841.

How it works

The amount of light a lens passes depends on its f-number, the focal length divided by the width of the opening. Petzval’s design worked at about f/3.6, roughly sixteen to twenty times brighter than the f/15 or so of the 1839 lens. It is sharp in the centre and softer at the edges, which suits a portrait. See why a lens gathers more light than a pinhole.

Why it mattered

Together with faster chemistry it cut daguerreotype portraits to tens of seconds or less, which made the portrait studio a business. It was also one of the first lenses designed by calculation, a method that became the rule.

Limits and what replaced it

It has a narrow field and a curved field of focus, so it is poor for landscapes or architecture. Anastigmats of the 1890s, such as the Cooke triplet and Zeiss Tessar, corrected these faults.

Today

Modern remakes of the Petzval design are sold for the swirling background blur it gives.

Sources

• Wikipedia: Petzval lens ↗
• Eder, ch. XXXIV (p. 289)