The Leica, on the market from 1925, changed photography's physics: a camera using cinema film, fitting in the palm, needing no tripod and running almost silently. Working unnoticed in the street, outside the posed world of the portrait studio, became possible.

Henri Cartier-Bresson turned that possibility into an aesthetic. His book of 1952 was titled in French *Images à la sauvette* — images on the sly; the English edition named the concept: The Decisive Moment. The method is this: the photographer does not arrange the scene but composes the frame in the mind while waiting, and releases the shutter in the second when event and geometry coincide. The frame is not corrected afterwards — Cartier-Bresson printed to the very edge of the negative and refused to crop. That is the proof that the decision was made at the moment of exposure.

Magnum Photos, which he founded in 1947 with Robert Capa, David Seymour and George Rodger, was a cooperative that let photographers keep the copyright in their own work; the principle that the photographer, not the publication, decides the story established the professional ground of documentary photography. His influence continues, and so does the argument: because the idea of the decisive moment excludes staged, constructed and serially produced photography, later generations — from Sherman onward — opposed it directly. The language of street photography nevertheless still comes largely from this method: the geometry of the frame, the waiting, the refusal to intervene.