Wednesday, July 30, 2014

Depth of Field: Lens Technology in the Mid-19th Century

Wet-collodion processing led to faster shooting and much more reproducible images, but another important component of the evolution of photography was the camera itself. Modern photographers have a huge range of lenses to choose from, with a diverse assortment of capabilities. There is a deliberate choice about which lens to use for a particular shot to get a particular image; the reflection of those choices are in some ways a reflection of the photographer--what they wanted to show, their aesthetic preferences, their point of view. In the 19th century, lenses weren't interchangeable in the same way, and photographic techniques developed in different ways as lens technology changed.

Although photographic processes existed fairly early in the 19th century, the first convertible lens was not produced until about 1840. This lens, commissions via a design contest by the French society for the advancement of the industry (Société d'Encouragements pour l'Industrie Nationale), was designed by Charles Chevalier and called the "l'Objectif double à verres combinés." The lens was comprised of two curved glass disks inside a metal barrel, and by changing part of the barrel (and therefore swapping out one of the glass disks), the photographer could alter the focal length to change between a lens best for wide shots (landscape) and singular focus (portrait). 

an add for Chevalier's lens (shown in both focal lengths)

View of the Seine, Paris, 1843, Charles Chevalier (and presumable shot with his lens)
While the lens was revolutionary, its landscape configuration was by far stronger; the portrait length suffered from an overall lack of sharpness, leaving room for improvement.

Although submitted too late for consideration in the Societe's contest, another lens designed at the same time provided a much clearer portrait image. Designed by Joseph Petzval, a mathematics professor at the University of Vienna, the Petzval lens used complex mathematical equations applied to the spacing and curvature of the glass disks within the barrel to produce the fasts reflection of a flat field possible. The lens was quite sharp in the central focal point of the image and provided a much better resolution than Chevalier's lens.

diagram of Petzval's design
The fault of the large curvature of the glass was that Petzval's lens had a narrow area of focus, and the rest of the image featured blurring/vignetting. While that made it less ideal for landscapes, the lens was well liked for portraits (its intended purpose), and the general features of the design quickly became popular, resulting in many many copies. Thousands of these lenses were produced and sold, which helps to explain why the lens's distinct image features are so prevalent in 19th century photographs. The narrow depth of field and small focal area also change the process of setting up a shot--rather than thinking about a scene or composition of multiple components, photographers were isolating one moment or face or object of particular interest and loosing all the surrounding context. While a different type of deliberate image creation than choosing the lens, I think it is very much a window into how photographers captured to world.

Soldier, Crimea, 1850s
Soldier, crimea, c.1855
soldiers at camp, c.1861-65 (note how only one face is in focus)
Norwegian women, date unknown

In previous posts in this series, I've discussed how images contribute to our understanding of the past, and the way evolving image-processing technology allowed photographers to leave the studio and bring the world home. If this was interesting, check them out!


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