My nomination:
Blowup (1966)
"A successful mod photographer in London whose world is bounded by fashion, pop music, marijuana, and easy sex, feels his life is boring and despairing. But in the course of a single day he accidentally captures on film the commission of a murder. The fact that he has photographed a murder does not occur to him until he studies and then blows up his negatives, uncovering details, blowing up smaller and smaller elements, and finally putting the puzzle together."