The Photo Finish Explained

Ever wondered why the spokes appear to bend in a sprint finish when you see the photo finish image from a race? Are the riders going so fast that they flex the spokes? Or is something else at work?

Wednesday’s stage in the Tour of Turkey saw Mark Renshaw of Rabobank beat Matthew Goss by 0.008 seconds, a winning margin that was hard to see even with the photo finish image.

All top level races have to have a photo finish camera and rightly so as often the result is too close for the human eye to detect. It’s worth defining the finish line first. Here’s the UCI rulebook:

1.2.099 The finish line shall comprise a line of 4 cm in width, painted in black on a white strip 72 cm wide thus leaving 34 cm of white on each side of the black line. 1.2.100 The finish occurs at the instant that the tyre of the front wheel meets the vertical plane rising from the starting edge of the finishing line. To this end, the verdict of the photo-finish shall be final.

In other words the finish line is the start of the black line on the white band. A rider wins when the front of their wheel crosses the line.

The optical effect

Now about those bending spokes. There is no bending. Look again at the images above and note that if the spokes appear to bend, the gaps between the drilled holes on the rim also appear to move and so does the “o” in the Campagnolo decal. Perhaps you could imagine a spoke bending or a different decal, but you cannot drill and lace a wheel for the camera. The second clue to the image being different is the road surface. Look at the horizontal stripes on the ground, this is not normal either.

It’s all down to the rolling shutter effect. The photo finish camera is not taking a series of photos, clicking away like a fashion photographer in a photoshoot. Instead a digital eye is scanning the finish line, taking a series of photos across the line in one go. Imagine a document scanner, only this one works much faster to scan the finish line. The camera on the Apple iPhone works this way too and people have snapped fast moving objects only to get weird images. Here are the propeller blades of an aircraft:

Now you might think spokes bend but propellor blades are not disintegrating in the shot above. Instead the camera is scanning the field only the blades are moving so fast we get an odd image. This Youtube clip shows what is happening, watch the yellow line going down the screen.

The yellow line represents the scan and the resulting image is generated by everything captured by the yellow line as it passes down the field. Note it doesn’t “snap” the red propellor, but instead it scans and picks up the rotation in segments.

Normally the scan is so fast but when an even faster object, like a propeller blade or a bicycle wheel, enters the field then this is distorted. Note the distortion is only on the fastest moving parts of the bike, namely the outside edges of the rim. Above all there is not distortion to the exact point on the finish line, the front edge of the tyre on the vertical plane is perfect.

Their use in bike races

Races often use timing chips where each bike is fitted with a transponder and a radio loop at the finish picks up the finishing position. But these are indicative for the time and position, after all a rider can change bikes during the race. The final word goes to the finish line camera, as the UCI rule quoted above insists.

Many events, including the Tour de France use the Lynx model of camera, capable of 10,000 frames per second. The Tour uses one for each intermediate sprint where a race official and staff are on stand-by to judge the result. At the finish two cameras are deployed, one as back-up just in case of a malfunction. They are placed inside a special box that resembles a large wardrobe that sits on the line.

Of course sometimes even the camera can’t split the difference. In 2010 Michael Matthews of Australia won the U-23 World Championships and John Degenkolb took the silver medal. But the photo finish could not separate Guillaume Boivin and Taylor Phinney and four riders stood on the podium with Boivin and Phinney being tied for third place and sharing the bronze medal.