The Asphyx (1972) on Netflix

Plot summary

Sir Hugo Cunningham, a philanthropic scientist in Victorian England, belongs to a parapsychological society that studies psychic phenomena. Its members photograph people at the moment of death, and their photographs show a smudge that hovers around the body; the society takes it for evidence of the soul leaving, but Hugo is skeptical. While filming a riverside party, he records a boating accident, and in the film the blur moves toward his son rather than away from him. He concludes it is an asphyx, a spirit of death from Greek myth that comes for each person as they die. Filming a public hanging with a phosphorus spotlight of his own making, he watches an asphyx hang suspended in the beam, and theorizes that a trapped asphyx would leave its owner immortal.