Musings on optics, physics, astronomy, technology and life

At many times in my past, I’ve felt a bit whipsawed as I careen from one writing and editing assignment to another: from elder affairs to school board meetings, from dying kids to budgetary minutiae, from higher mathematics to apartment fires.

In recent months, I went straight from writing about photography in the American Civil War — the pre-light-bulb era — to putting together a feature on modern-day light pollution — which has a lot to do with poorly placed electrical lighting fixtures. So what could these topics possibly have in common?

Blank skies.

The blankness is obvious in this famous Crimean War photograph by Roger Fenton (which actually predates the American Civil War) and this image of a dead Confederate soldier at Gettysburg by Alexander Gardner. Even when there’s a sharply defined shadow, as in this Mathew Brady photo, you can’t distinguish between blue sky and white clouds. (The line at the top of that photo looks like an issue with the collodion plate coating or other chemicals.)

Yet when I got my first 35-mm SLR camera as a high school graduation present and started learning about photography, I read that, in order to increase the contrast between puffy white clouds and beautiful blue skies, one should use a red filter in black-and-white photography and a polarizing filter in color photography. So, why the blank skies in the Civil War?

The answer: prior to 1873, photographic chemicals were sensitive only to blue light. So the wet photographic plates picked up the blue light coming from the sky (Rayleigh scattering and all that) and the blue component of the light reflected off the clouds and recorded them all the same. Photography studios of the era often had walls painted in a sort of robin’s-egg blue, yet they looked lighter on the resulting ambrotype or tintype than they would on modern panchromatic black-and-white film (or digital images turned monochromatic in an image-editing program).

More blank skies are readily visible in this amazing set of Western American images by Timothy O’Sullivan, one of the Civil War photographers who kept on plying his trade after the fighting ended. It makes you wonder about some of the gorgeously detailed skies in the works of some 19th-century Romantic painters — were they trying to one-up the photographers?

Meanwhile, light pollution makes today’s urban night skies look just as blank to the naked eye as the daytime Civil War skies looked to the wet-collodion plates (see top and bottom images here). Different effect, same blah outcome.

My feature article on photography in the American Civil War just came out in the June 2012 issue of Optics & Photonics News. My story on light-pollution will appear in that magazine later this year. If you are an OSA member or can get to a library that subscribes to OPN, please read my article and let me know what you think.

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