Unlike conventional cameras the circular flow camera used a single vertical slit instead of a shutter; a strip of film moved horizontally across the fine vertical opening located in the focal plane. This limited the field of vision to no more than a few inches, the restricted field being aligned with the vertical line on the winning post on which the lens was focused. The strip film moved across the slit in the opposite direction to the race and at substantially the same speed as the rate of movement of the image of the horses as each passed the finishing line. This kept the image of the horses more or less stationary with respect to the film. As soon as the first horse started to pass over the line, the camera began to record its image on the moving film from the nose backwards, progressively along the length of the body, with the arrival of every horse at the finishing post in succession. This produced a strip photographic record of the horses as they passed the vertical plane (winning post). Film was advanced continuously at a pace equivalent to the average speed of a racing horse, resulting in distortions of length (slower-moving horses appear to be stretched), but still preserving the order of finishers.