Francis Bacon was obsessed with "doing one perfect image", and Henri Cartier Bresson searches constantly for "the decisive moment". Two of the most influential artists of the century, they both have current exhibitions at the Hayward Gallery. Bresson - who is 90 this year - explores Europe through photographs of people, and Bacon, in the first major showing of his work for a decade, probes the body itself in nudes and portraits. Producer Abigail Appleton