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 finish line is that entire white or grey smear from one edge of the photo to the other. That red line is moved past the riders in the image to check the relative positions of the third- and fourth-placed riders. But third and fourth can’t have yet reached the line, since they just showed the red line lined up with the tire of the first-place rider, right? If it was a normal photo, you might think the fourth-place rider could pass third place by the time they reach the line. But the entire image was taken at the line; fourth is fourth and third is third.
Officials move that red line around to look at the positions of riders further back relative to each other, because they are all photographed as they crossed the finish line. Since the camera aperture is a vertical slit lined up with the finish, every one of those images of riders does indeed show them as they crossed the finish line. They look to be behind the riders ahead of them in space, but you are actually looking at the moment they crossed the line, behind the riders ahead. And if you know exactly how fast the film was moving when it was taken and the scale of the photo, you can actually measure the time gaps between riders with a ruler.