Rio 2016: Photographer Cameron Spencer tells how he got that smiling shot of Usain Bolt

The Aussie photographer who took the now viral image of Usain Bolt smiling during his 100m semi-final at the Olympic Games admits he "took a punt and it worked".

Getty Images photographer Cameron Spencer is the man behind the image, which many sport commentators have said was one of the greatest moments captured in Rio.

Spencer was shooting the high jump when he decided to run across the field and capture the Jamaican sprinter's semi-final.

"Fortunately Bolt was running in the second semi and I decided to shoot a slow pan shot of him running past," he told 612 ABC Brisbane's Terri Begley.

"I stopped on the 70 metre line and waited for him to come by; no-one expected him to slow down and give a big smile to my direction.

"I couldn't believe it. I wanted to get a nice shot of him in action and it wasn't until I looked at the back of my camera when I realised he was smiling.

"As a photographer you want to try and do something different and I took the punt and it worked out."

'I was lucky the result was what it was'

Spencer said he could tell Bolt stopped to enjoy the last 20 metres knowing he had the race won.

"I think he was smiling at the other guys in the race but it happened to be in my direction," he said.

"The cheeky grin is such a reflection of his personality as he's such a larger-than-life character and an entertainer who loves to show off to the crowd."

Getty Images had 11 photographers covering the race, which allowed Spencer to take a risk by shooting a slower image.

"The chief photo manager told me there was 600 photographers at the 100 metres that evening; there were many lenses pointing at him," he said.

"I was lucky the result was what it was as I had only had minutes to prepare."