The Australian filmmaker was working on a documentary about a child murderer in Oklahoma City, in the United States, and was casting about for characters outside a 7-11 store in the early hours of the morning.

There, he met Charles, a homeless man who was estranged from his family because of his schizophrenia, and was living by a pond behind the 7-11.

"He was actually guarding over our producer who was sleeping in our car, and he was weeping because he couldn't believe how beautiful this young woman who was sleeping was," Courtin-Wilson said.

"I just thought, my god, this is amazing, so I just asked him if he could stand underneath this streetlight as we shot his portrait. It was totally spontaneous."

The moving portrait, titled simply Charles, has just been announced as the winner of the 2016 Digital Portraiture Award at the National Portrait Gallery.

The $10,000 prize, along with a residency at The Edge at the State Library of Queensland, is awarded each year to a work that "enables the gallery to actively challenge conservative notions of portraiture".

The gallery's digital manager Gillian Raymond said the exhibition of seven finalists was "as much an exhibition about the genre of portraiture as it is about digital art".

"This year's finalists explore the boundaries between performance and portraiture, biology and portraiture, advertising and portraiture," she said.

"They riff on themes of artistic disillusionment, self-discovery and the development of identity. In the end, the judges were won over by the technical mastery of the winning work combined with the empathetic and poignant depiction of its subject."

The six-minute footage shows Charles standing with his head tilted back and eyes closed, as the camera moves in and out of focus.

Courtin-Wilson said it was remarkable to witness the man's transformation once the camera was focused on him.

"This guy is addicted to drugs, he's schizophrenic, and he was talking about how he would talk to the pond and the trees before he'd go to sleep, and that would help him go to sleep," he said.

"But when we gave him that moment of attention...you could feel his whole posture change because someone was showing him some attention and some love. It was really, heart-wrenchingly beautiful."

He said he had since returned to Oklahoma City three times to try and find him again, but to no avail.

"I'm going to keep trying," he said.

"He could be in Memphis - he said he wanted to catch a Greyhound bus to Memphis to find his daughter. I trust that maybe he's still alive. I'd like to give him some money as well."

The Digital Portraiture Award 2016 exhibition is on show at the National Portrait Gallery until April 9.