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“The technical problems were formidable. To stop the motion of the subjects at such close range required a much faster shutter speed than the film of the day allowed in the shadowed city street. Furthermore, the problem of focusing accurately on a close-up moving subject would be almost impossibly difficult. To further complicate the issue, Callahan would also be a pedestrian, photographing as he walked, in order to avoid drawing attention to himself.
Callahan approached the problem by defining a procedure that would give the greatest effective film speed and eliminate the need for focusing, and then worked within the limits established by this procedure. He exposed his film as though it were eight times faster than the manufacturer and severely overdeveloped it to achieve maximum effective density, which produced a negative with little detail in the shadows and a rough abbreviated tonal scale. This required that the picture succeed in graphic terms, not through the illusion of continuous plastic relief that identifies classical photographic technique. To finesse the problem of selective focusing, he pre-focused his camera for the distance at which an average head, from chin to hairline, would fill the horizontal negative. Then he walked the streets, camera to his eye, firing on instinct when the right head filled the frame in the right way.” - John Szarkowski (http://silentbeings.tumblr.com/page/27)
Harry Morey Callahan (Oct. 22, 1912 – March 15, 1999); influential 20th century American photographer.
...Callahan left almost no written records—no diaries, letters, scrapbooks or teaching notes. His technical photographic method was to go out almost every morning, walk through the city he lived in and take numerous pictures. He then spent almost every afternoon making proof prints of that day's best negatives. Yet, for all his photographic activity, Callahan, at his own estimation, produced no more... (https://en.wikipedia.org/wiki/Harry_Callahan_(photographer))