Movies You Haven't Seen But Should: The Asphyx

From Wikipedia:

The Asphyx is a 1973 British horror film directed by Peter Newbrook. It was also known as Spirit of the Dead and The Horror of Death.

It tells the exploits of rich Scientist-Philanthropist Sir Cunningham, who discovers an unimaginable blur being recorded while he searches the negatives of dying men whom he had recorded. Later on, at a party- he is making a motion picture of his wife and son, who are playing in a pool and ends up filming their accidental deaths as well. At first, Cunningham doesn't realize that he's captured that same misshapen blur on the camera, but once he does, he comes to a conclusion: that the blur is a person's soul leaving the body. It isn't until he re-examines the pictures and film however, that he makes the startling discovery that the blur was not moving away from the bodies of those who were dying, it was moving TOWARDS them. After doing some research, the good Doctor finds that he has found a ticket to immortality through the blur, an ancient Greek spirit known as an "Asphyx". According to legends, the spirit will appear only at the moment of a person's death to take them away to the Underworld. Cunningham, in his brilliant madness soon deduces that if he can only stop this process from happening at all, it would make any person immortal and unable to die- unless their particular Asphyx is released.

The Doctor soon goes to his task and makes a deal with his adopted son for help in perfecting and utilizing the procedures, and from there- his laboratory becomes filled with torturous devices and instruments of death. He soon comes to understand however, that immortality comes at a high price, not easily fulfilled by mortal man.

And here it is on the IMDB