Wonders: Time Exposures; July 2000; Scientific American Magazine; by Morrison, Morrison; 2 Page(s)

Alittle group of science teachers, none of them with more than point-and-shoot photographic experience, had built a pinhole camera and were impatiently awaiting their first picture through the long minutes of time exposure. But when they saw the faithful image of the scene, it came as a surprise to all the experimenters. Although one person had reached into the middle of the view to make an adjustment there during the exposure (and all agreed they had seen it happen), there was no sign of it in the final composition. Somehow the camera had edited out the intruder, without losing any of what must have been hidden! For a while it seemed magical.

The pinhole camera required minutes of exposure to make its image, but taking a picture still seemed to these experimenters a single act, one not to be divided into parts. If you were in the picture, there you must appear. The real physical process had been subsumed into a single moment. Of course, the few seconds of intrusion meant that only a small fraction of the light energy that had formed the rest of the image was there to capture the vagrant arm, not enough for a noticeable record. No image is an instantaneous act; time exposures are all we ever see, short or long, and none will visibly report too brief a presence. Only the in-built photochemistry of suntanned skin provides a familiar long exposure: no snapshot of a passerby was ever caught via a suntan!