What is the "decisive moment" in photography, and why did a small camera make it possible?

Henri Cartier-Bresson believed that the best photographs come from a single instant, when everything in front of you falls into order. He called it the decisive moment. He defined photography as the simultaneous recognition, in a fraction of a second, of the significance of an event and of a precise organization of forms which give that event its proper expression. As he told The Washington Post in 1957, once you miss the moment, it is gone forever.

What made this possible was the size of the camera. Cartier-Bresson almost always used a Leica with a 50 mm lens. The Leica, first sold in 1925, was the first commercially successful camera to use 35 mm film, and its compactness and bright viewfinder let photographers move through busy streets and capture fleeting moments. He called such cameras "the velvet hand...the hawk's eye". The anonymity it gave him in a crowd was essential for overcoming the unnatural behaviour of people who knew they were being photographed, and he painted its shiny parts black to make it even less visible.

He was strict about the result. He composed in the viewfinder, not in the darkroom, and had almost all his prints made full-frame and uncropped.

The name itself has a twist. His 1952 book was titled Images à la sauvette in French, which means "images on the sly" or "hastily taken images". The English title, The Decisive Moment, came from his American publisher Dick Simon of Simon & Schuster.