Joseph Petzval's 1840 Portrait Lens Revolutionized
In 1840, mathematician Joseph Petzval designed the first computed photographic lens, gathering up to 20 times more light than previous optics and enabling
In May 1840, mathematics professor Joseph Petzval completed the design for a revolutionary portrait lens before any glass was even ground. His computed lens gathered roughly 20 times more light than the standard daguerreotype camera lenses of the era, a breakthrough reported by Fstoppers.
The Computed Design
Before 1840, camera lenses were crafted through trial and error, like furniture. Petzval changed that by mathematically tracing light rays through a proposed four-element design. He adjusted curves and glass types on paper until the numbers showed minimal aberration. This method, not only the resulting speed, made the Petzval objective the first photographic lens designed by computation.
A team of soldiers from the imperial artillery, skilled in ballistics calculations, performed the months of hand arithmetic under Petzval's direction. While specific roster details vary in historical accounts, the core story of military personnel acting as human computers for the project remains consistent.
A Dramatic Gain in Speed
The Petzval portrait objective operated at approximately f/3.6. Compared to the standard Chevalier landscape lens of 1839, which worked around f/15 to f/16, this represented a massive increase in light gathering. The gain is measured by squaring the ratio of the f-numbers.
| Reference Lens (f-number) | Petzval Lens (f-number) | Light Gathering Ratio |
|---|---|---|
| f/15 | f/3.7 | ~16x |
| f/16 | f/3.6 | ~20x |
| f/17 | f/3.6 | ~22x |
This spread explains why historical accounts cite different multipliers. In practical terms, 20 times more light equals a gain of over four stops. One lens swap provided more speed than jumping from ISO 100 to ISO 1600.
Transforming the Portrait Session
Early daguerreotype exposures lasted from three to fifteen minutes, making portraiture an ordeal. Petzval's faster lens, combined with concurrent improvements in plate chemistry using bromine and chlorine, slashed exposure times. Austrian accounts describe a drop from roughly 15 minutes to around 45 seconds.
The lens alone did not achieve one-second exposures. It worked in tandem with chemical accelerators. Studios used blinding direct sun or skylights, and sitters were braced against headrests to remain still for the duration. The stiff expressions in surviving portraits from the era testify to the discomfort of holding a pose under bright light for tens of seconds.
A Design Without Protection
Petzval handed his design to Viennese optician Peter Wilhelm Friedrich von Voigtländer, who built it into an all-metal camera for series production in 1841. Petzval received a single payment of about 2,000 gulden with no written contract or patent filed beforehand. As the MacTutor biography states, "Voigtländer made a fortune, Petzval had not taken out patents, and he received no royalties."
The unprotected design was copied widely. Voigtländer's firm produced the lens by the thousands, finishing its 10,000th objective by the end of 1861 and about 18,000 by 1865. A later patent for a second Petzval design, a landscape lens filed by optician Carl Dietzler in 1857, led to litigation with Voigtländer but did not stop production.
Petzval eventually walked away from optics. After a public dispute with physicist Christian Doppler, he spent his later working years on acoustics. His pioneering computed design, however, remained the industry standard for portrait photography for the next four decades.
