A photograph that captures a 'decisive moment' does much more than merely pause a moment in time. It imbues that moment with a significance that gives it not only a past, a context, but also suggests the immediate future and, in the case of some of Henri Cartier-Bresson's great photographs, gives the image a deep historical resonance.

Take, for example, his photograph taken when the concentration camps were being liberated. It is usually entitled ''A gestapo informer impersonating a refugee is 'exposed' in Dassau''. Our initial focus is on the expressions and gestures of the three main protagonists - the female gestapo informer, the woman who has 'unmasked' her, and the man who runs the refugee camp.  Now study the expressions of the onlookers (one of whom is in the striped 'pyjamas' worn by concentration camp inmates) and you see so many feelings, from anger to disbelief, from curiosity to hatred. They seem to represent the myriad conflicting emotions and attitudes that must have dominated a devastated Europe at the end of WW2.  And they are all there in one extraordinary image.

A great 'decisive moment' photograph captures an incident in time that is decisive because in that flicker of a second the past and the future collide to create a present that is pregnant with social significance.