Michelangelo Antonioni's film about the existential nature of reality through photography, that Brian De Palma remade as Blowout.

David Hemmings is a swinging photographer in London during the 60s.
He comes across two lovers in a park and photographs them. The woman (Vanessa Redgrave) pursues him back to his flat and begs him to give her the film. He gives her a fake roll of film and proceeds to develop the real film, which appears to reveal a murder scene. He "blows up" the photos but is never really sure of what he sees. He goes back and finds a body, but is without his camera. He tries to get a witness but when he goes back, the body is gone. Later, the film in his flat is stolen. The film ends with two mimes playing mime tennis. David Hemmings throws an invisible tennis ball to them, while the sound track features the sounds of a real tennis game.

Thanks Don P!