How is a photo finish decided in Olympics?

World champion Noah Lyles roared to victory in 9.79 seconds to claim gold in a dramatic men's Olympic 100m final in Paris on Sunday.

Lyles won in the closest Olympic 100m finish in modern history as just five thousandths of a second separated him from Jamaica's Kishane Thompson.

Both were given the rounded-up time of 9.79s but the American's name carried the all-important (.784) to Thompson's (.789.), reports agencies.

Spectators and the athletes themselves were stunned by the result, with Thompson having led the majority of the race. The photo finish image appears to show Thompson's leg ahead of the finish line, further than Lyles. So how did Lyles then take the gold?

Here's a breakdown of the simple rules of a photo finish in athletics.

How does World Athletics - the body governing international athletics - explain a photo finish?

As the runners approach the finish line, a "slit-video" system scans an ultra-thin segment of the track precisely aligned with the finish line - scans it 2,000 times per second, providing an unbroken image of each athlete crossing the line - and coordinates it with the athlete's time.

In addition to that, there's a back-up camera on the other side of the field in case a runner is hidden or blocked by another runner.

The flash timer that clocks the final times of athletes has a margin of error, which, at worst, can throw up an error within 0.01 or 0.02 of the official time.

Now the most important part of determining the official timing, which in turn decides the race, is the body part that has to cross the finish line.

In the 100m final at the Paris Olympics, Thompson's foot looks like it breached first. The athlete's torso is the body part that has to cross the finish line for official timing registration. The torso, or trunk, of a person includes the chest, abdomen, pelvis, and back.

What this finish says about Lyles

Lyles got off to a far-from-ideal start - the slowest of the blocks in the eight-man final. A hundred meters of desperation later, he finished 5/1000 of a second in front of Thompson.

The Jamaican started out in fourth place and led the race right until the last 10m before Lyles took over and nosed ahead for gold.

Lyles was in eighth place for the first half of the race and pulled off a mid-race acceleration for the ages to eventually lead the pack.

This was already the most competitive final at the Olympic level, with it being the first time that an athlete with a sub-10 mark failed to make the last run. All eight finalists clocked sub-10 timings in the final, and fine margins separated one from the other.

Earlier, the man Lyles most feared, Jamaica's Kishane Thompson, was just as sluggish, reacting in 0.0176. Yet Thompson is the fastest man this year for a reason. He is a 100m specialist with power and a long stride, and he emerged from his drive phase at 30m with the lead.

By contrast, and despite being the 100m world champion, Lyles is a 200m runner by trade.