In order to avoid possible wrong decisions over the order in which racehorses, athletes, racing cars, etc. pass the finishing post when the result is very close, the finish of a race can be recorded by a special camera called a photo-finish camera. Until recently this has been a specially constructed or modified film camera, taking 70 mm or larger roll-film. Instead of a normal focal-plane shutter this camera has only a capping shutter, and in the focal plane a fixed vertical slit centred on the optic axis of the lens. The camera is set up so that the image of the finishing post falls on the slit, with a high viewpoint so that the competitors do not mask one another. To record the finish of the race the lens is uncapped and the film is set in motion at a speed matched to the movement of the image of the participants. This is calculated according to the formula
In horse races it is usually the relative positions of the horses' noses as they pass the finishing post that decides the outcome of the race; in athletics it is the heads of the competitors (hence the practice of ducking forward at the finishing line); with cars, cycles, etc. it is the front of the machine. In each case this will be the part that is moving at more or less the average speed, and this part of the image will appear sharp and undistorted, whereas other parts such as legs will inevitably show some distortion. In particular, a hoof or foot that lands precisely on the finishing line will be imaged as a smear with a length corresponding to the time it was stationary. If the film movement has been accurately judged the spacing between the noses etc. will give the precise time margins between the competitors. The winning post may also bear a moving image with timing marks, date, and race number. This will record as a full image on the film.
Owing to the delay in processing, film cameras are rapidly becoming obsolete, and are being replaced by digital recording cameras. These do not require a full frame of pixels, as a single vertical row of these suffices. There is no need for a slit, and the result can be displayed on a screen in near-real time, and printed out immediately in full colour.
— Graham Saxby
Bibliography
- Ray, S. (ed.), High Speed Photography and Photonics (1997)
