the.com/photo finish

means A race finish so close that officials must examine a high-speed photograph to determine the winner.

from Born from horse racing and track in the early 20th century, once cameras fast enough to freeze a dead-heat moment became practical — the 'photo' being literal: an actual photograph taken at the finish line to settle what eyes alone couldn't. The phrase later galloped off the track entirely, now used for any contest — elections, exams, arguments — decided by the slimmest possible margin.