A photo finish occurs in a sporting race, when two (or more) competitors cross the finishing line at near the same time. As the naked eye may not be able to discriminate between which of the competitors crossed the line first, a strip photo, a series of rapidly triggered photographs, or a video taken at the finish line may be used for a more accurate check. Nowadays, the photographs may be digital. A digital photo finish camera uses a 1-D array sensor to take sequential images of the finish line. Since only a single line of the CCD is read out at a time, the frame rates can be very high (up to 10 000 frames per second). Unlike a film based photo finish, there is no delay from developing the film, and the photo finish is available immediately. They may be triggered by a laser or photovoltaic means.
Historically, a hand cranked strip photograph was taken at the finishing line. Today, finish-line photos are still used in nearly every modern racing sport. Although some sports use electronic equipment to track the racers during a race, a photo is considered the most important evidence in selecting the winner. However, they may be examined only when a race is close or when a record has been broken, however are often used to give official times for events. An alternative is the use of manual touch pads to register a time by the athletes themselves, such as in competitive swimming.
In athletics, races have to be timed accurately to hundredths (or even thousandths) of a second. A battery of electronic devices are installed in high-profile events (such as the Olympic Games) to ensure that accurate timings are given swiftly both to the spectators and to the officials.
At the finish line, photocells and digital cameras are used to establish the placings. Sometimes, in a race as fast as the 100 m sprint, all eight athletes can be separated by less than half a second. It is not uncommon for two athletes to have the exact same time recorded without there being a deadheat.
The 2008 Summer Olympics saw the introduction of some of the fastest timekeeping equipment yet, with cameras that take photographs 3000 times a second (compared to 1996, which were 1000 times per second).
In horse racing, a factor known as a dead heat can occur, when two - or possibly more - horses cross the finish line at the same time. Photo finishes determine accurately where the horses were at the time of finish. Stewards at the racetrack usually put up PHOTO status on the races during these photo finishes; the status of objection or inquiry can also trigger if other horses or jockeys somehow interfered in the horse rankings and can factor in Dead Heats. The most notable dead heat was in 1989s Hambletonian, with both Park Avenue Joe and Probe finishing in a dead heat. A photo finish decided the winner of the 2005 edition of the Japan Cup, which was given to Alkaseed and narrowly beating Heart's Cry.
Types of photographs
There are two methods for creating a photo finish. The most common method uses a special slit camera, which produces a panoramic film strip. This camera uses a single vertical slit instead of a shutter, and the film is advanced continuously at a similar speed to the racers' images. This creates a 'virtual view' of the positions of each racer as they crossed the finish line, from the side without motion blur. Racers may appear compressed in this view based on the difference in the speed that the film is turning relative to their movement speed. Still objects at the finish line are imaged as streaks. Slit-scan photography is similar, however the camera moves rather than the subject.
The second method for creating this strip involves combining individual photographs. A high speed camera or a movie camera is used to take a continuous series of partial frame photos at a fast rate, while leaving no blank space between the cells.
With all methods, time markings along the bottom of the photo can be used to find the exact crossing time of any racer, or simply used to compare their finishing positions along the strip.
- In the TV series Futurama, the episode "The Luck of the Fryrish" starts with several horse races, one of which the finish is measured by an electron microscope, and the difference between the two lead horses is apparently measured in quarks. Professor Farnsworth angrily tears up his ticket after protesting "You changed the outcome by measuring it!," a reference to the observer effect.
- fully automatic timing