The photographer's 'decisive moment', as Henri Cartier-Bresson tagged it, often comes in the dark room as much as in the initial flurry of action or on the surreptitious stake out. It is the mark of chinagraph on contact sheet that can make history.

'Magnum Contact Sheets', a new exhibition at Amsterdam's Foam photography museum, examines that critical winnowing. The show includes 60 contact sheets, and a print of the single iconic image they contain, from photographers such as Robert Capa, Cartier-Bresson, Eve Arnold, Martin Parr, Jim Goldberg and Elliott Erwitt (Magnum members all, though some of the images pre-date the agency's founding in 1947). 

The show includes iconic images of the Second World War and the Prague Spring; of Che Guevara, Malcolm X, Martin Luther King and Margaret Thatcher; conflicts in the Balkans and the Middle East; and Brooklynites taking it easy with the Twin Towers disappearing into smoke across the East River, as well as Martin Parr's pasty British seasiders.

The joy, of course, is in tracking the moments before and after the decisive one. The exhibition includes the photographers' notes, explaining their choice (and, of course, inviting you to disagree and to pick another point frozen in time). It is also inevitably a lament for a lost process; the contact sheet has been replaced by Capture One Pro on a pimped-up Mac Book Pro, where the unchosen image leaves no physical trace.