“The image was a publicity effort by the Rockefeller Centre,” Corbis’s chief historian Ken Johnston told the Independent. “It seems pretty clear they were real workers, but the event was organised with a number of photographers.”
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I guess what makes this photo so powerful is the contrast of the nonchalance of the workmen and the perceived danger of falling to one's death. It's an almost automatic anxiety/fear response, like a blink, that works everytime I look at this photo. If you want the full version of skyscraper anxiety I suggest watching the old Harold Lloyd silent film SAFETY LAST.