Cortázar's Las babas del diablo Blow-Up (1966)
The source story is much weirder than the film
Antonioni's screenplay credits the film as "inspired by" Julio Cortázar's short story "Las babas del diablo" ("The Devil's Drool" or "Devil-Saliva"), published in 1959 in Cortázar's collection Las armas secretas. The story differs significantly from the film, and the differences are most of what is interesting about the adaptation.
The story's plot
A Franco-Chilean translator and amateur photographer named Roberto Michel is walking on the Île Saint-Louis in Paris. He sees a woman in her thirties seducing — possibly procuring — a teenage boy of fifteen on a quay near the Pont du Louis-Philippe. The translator photographs the encounter, partly out of voyeuristic interest and partly out of the suspicion that the boy is in danger. The woman runs over and demands the photograph; the boy uses the disturbance to flee.
Michel goes home, develops the photograph, and stares at it on his wall. As he writes about the photograph in the first-person narrative — switching erratically between first person, third person, and second person — the photograph appears to come back to life. He notices a man in a parked car at the edge of the original scene, a man whose presence he did not consciously register at the time. The narrator concludes that the woman was procuring the boy for the man in the car, and that his photograph saved the boy at the cost of redirecting the man's attention to himself. The narrator then experiences the photograph as a film, watches the woman walk toward the boy, and the story ends in a mix of grammatical persons and a description of clouds passing across the photograph. (wikipedia)
What Antonioni kept and what he changed
Kept: the photographer who interrupts a possible crime by photographing it, the woman who pursues him for the photograph, the enlargement that reveals what the photographer did not see at the time, the suggestion that another party in the photograph is the real threat, and the philosophical question of whether the photograph saves anyone.
Changed: the location (Paris to London), the photographer (amateur to fashion-industry professional), the crime (procuring a teenager to murdering an older man), the narrative voice (Cortázar's metaphysical fluctuation to Antonioni's external camera), and the ending (Cortázar's photograph-comes-alive to Antonioni's mime tennis match and pull-back).
"Cortázar's story is about a photograph that becomes a film. Antonioni's film is about a photograph that becomes useless. They are mirror images of the same idea." — Senses of Cinema, Blow-Up review (2017)
Cortázar on the adaptation
Cortázar saw the film and wrote about it in 1968. He was generous about Antonioni's choices and clear that the film was its own work, not an attempt to render the story.
"The film is not the story. The film is a film made by a man who read the story and went on from there. I have no objection. The story is still there. The film is a different thing." — Julio Cortázar, interview reprinted in La vuelta al día en ochenta mundos (1968) (book)
The two men reportedly met once, briefly, in Paris in the early 1970s.
Why the changes work
The film's most-discussed structural choice is the relocation from Paris to London — not because Paris and London are different but because Paris in 1959 is not Swinging London in 1966. The shift moves the story from Cortázar's almost timeless meditation on photography into a specific cultural moment in which the photographer is a public figure, fashion is the dominant medium, and the city itself is being photographed constantly. The change of crime — from a procuring scheme to a murder — gives the photograph stakes the source story does not have. The ending is entirely Antonioni; Cortázar's metaphysical close becomes the film's social close (the world that does not file the report).
"Antonioni took Cortázar's metaphysical question and put it inside a working profession. The question is the same. The world around the question is entirely different." — Roger Ebert, RogerEbert.com (1998)
Cortázar's career after the adaptation
Cortázar (1914–1984) was a major Argentinian writer of the Latin American Boom, best known for Hopscotch (Rayuela, 1963), Bestiario (1951), and All Fires the Fire (Todos los fuegos el fuego, 1966). His association with Blow-Up brought him a wider European and American readership. He continued publishing fiction and essays through the 1970s and lived in Paris from the 1950s until his death.
"Cortázar wrote one of the strangest stories in twentieth-century Spanish-language fiction, and Antonioni made one of the strangest films of the European 1960s out of it. Both works are still in print, still discussed, still teaching." — The Paris Review, Cortázar interview archive (1984)