ˌphoto ˈfinish noun [countable] the end of a race in which the leading runners finish so close together that a photograph of it has to be examined to decide which is the winner
[TahlilGaran] Dictionary of Contemporary English ▲
photo finish n. phr. A close finish in a race of people or animals, where the camera must decide the actual result, sometimes by millimeters.
The black horse was declared the winner in a photo finish. [TahlilGaran] English Idioms Dictionary ▲