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Technical FAQ: How a photo-finish camera works
- Updated: July 19, 2016
Home » Bikes and Tech » Technical FAQ: How a photo-finish camera works
Dear Lennard, Thank you for your article on Coke can shims for seat post slippage. Some of your readers must love the idea of the Coke shim, like I do, while other do not like it at all!
Here’s my question: Why are the wheel spokes on a photo finish curved; see attached from stage 3, Tour de France 2016?—Pierre
Dear Pierre, The curving of the spokes in a photo-finish image is due to the fact that the camera is catching a single image, not a bunch of images taken at high shutter speed, when each rider crosses the line. There is no need for high-speed data storage or a really fast lens to capture the entire finish. That’s why Peter Sagan could get the image so quickly on his smartphone on Monday (stage 16) and see that he’d beaten Alexander Kristoff in Bern.
With film cameras for photo finishes, the film is moving at the approximate speed of the riders, and the camera aperture is a vertical slit lined up with the finish line, rather than being a round aperture closing and opening like the iris of an eye. As the moving film goes past the opening, the incoming light exposes the film. Nowadays, they use a digital equivalent, but I find it easier to explain if I continue to describe it as moving “film.”
The assumption people make when looking at a photo finish is that it is a normal photo, which has distorted images. After all, the bikes themselves, other than the spokes, look pretty normal — but the riders indeed look weird! But if it were a normal photo, there would be a background, and there is not, because the film is moving with the riders, whereas the background is stationary. This means there is no actual finish line to be seen in the photo, because the finish line is stationary — the entire photo is the finish line.
Do you ever wonder why the finish line in photo finishes is often shown as a thin red line, whereas the actual finish was two wide, white stripes with a narrow, black stripe in between? Maybe you thought the red line was just for emphasis because the finish line was blurry? No, the red line is a tool that can be moved by the computer across the screen, and the …
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