“To me, photography is the simultaneous recognition, in a fraction of a second, of the significance of an event,” wrote Henri Cartier-Bresson in a 1952 monograph. Magnum Photos Agency—founded in 1947 by Cartier-Bresson and fellow photographers Robert Capa, George Rodger, and David Seymour—echoes this philosophy. The agency is underpinned by a belief in photography as a form of instantaneous storytelling—the click of a lens captures just one moment, but that moment is a chronicle. Celebrating the medium’s spontaneity is “The Unexpected,” a week-long sale of square prints using images culled from Magnum’s archive. There are familiar figures seen from an unusual vantage (Marilyn Monroe reading Ulysses; an astronaut standing on a Senegalese beach), weighty images documenting political revolutions, and touching portraits of everyday people. Uniting these works is the snap judgement of the photographer, his or her keen sense for the story in their midst. —C.J.F.