photo finish

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A race in which the leading contestants cross the finish line so close together that the winner must be determined by a photograph taken at the moment of crossing.
• noun Informal An extremely close competition.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The finish of a race in which the leaders are so close together that the winner cannot be identified using the naked eye but is determined by means of a photograph
• noun by extension Any competition with a very close outcome

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun in general, any very close finish; in particular, a finish of a race in which the contestants are so close together that the winner must be determined from a photograph taken at the instant of finishing