Thursday, July 23, 2015

The First Portrait Lens

Petzval Lens
Petzval Lens
In the earliest days of photography, sitting for a portrait was a long and uncomfortable process. The low sensitivity of the glass plates combined with the small aperture lenses to require an exposure time of 15 to 30 minutes.  Any movement during the exposure resulted in a blurred image. It is little wonder that everyone looked unhappy in those early portraits.

The situation was improved dramatically by the development of the Petsval lens by Joseph Petzval in 1840. With an aperture of f/3.7, the exposure times were able to drop to 15 to 30 seconds.  The 160 mm focal length made it a perfect portrait lens for the large glass negatives sizes of the period.

The Petzval lens consisted of two doublet lenses with an aperture stop mounted in the middle. The front lens is designed to correct for spherical abberration, but it introduces coma.  The second doublet corrects the coma and the aperture stop corrects most of the astigmatism. The lens suffers from field curvature and vignetting which limits its field of view to about 30 degrees.

The Petzval lens was one of many photographic innovations that would simplify photography and eventually lead the the smartphone cameras we use every now.

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