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1. IT'S THE TORSO THAT COUNTS. The winner in track events is the runner whose torso, the body between the shoulders and the hips, crosses the finish line first. But there is no automated way to determine the winner of a race. While video cameras can be helpful, runners are so fast that they can change positions in between frames.

2. THERE'S NO ROOM FOR ERROR. In the case of a close call, track judges use a photo-finish camera trained on a narrow strip of space over the finish line. The device can shoot between 1000 and 2000 images per second, which develop as thin vertical strips. These individual image strips are then combined to form a single photo with a time scale superimposed along the bottom that allows officials to judge results to 1/100th second.

3. IT LOOKS A LITTLE WEIRD. Because the camera records time on the horizontal axis instead of space like a normal shot, photo finish pictures often show limbs of runners either shrunken or bloated in width. But these are not distortions, just depictions of the limbs at specific points in time. An arm that is swinging forward just as it crosses the line, for example, is going faster that the average speed and appears in the slit of space for less time than average, thus making it look thinner in the photo.

4. DIGITAL IS THE WAY OF THE FUTURE. In 2000, officials swapped their regular film photo finish cameras for their fully digital counterparts. The new system, introduced by Omega, is called Scan-o-Vision, and will be used in cycling, rowing and canoeing in addition to track events in Beijing.

Alex Cheng is the author of Splitting the Second—My Wacky Business in Olympic and Sports Timing, which chronicles his eight years as president of Seagull, Inc., exclusive agents for Omega Electronics.