The incident took place in Edmonton, Canada, where Pitt is filming The Assassination of Jesse James by the Coward Robert Ford.
David Buston was first spotted by security guards. He was a free-lance photographer who worked with People magazine, but mostly photographed sporting events.
Edmonton has been filled with Canadians since Pitt and his girlfriend, Angelina Jolie, arrived last month and visited a museum.
According to sources, a photograph of the couple could be worth between $250,000 to $500,000.