American football vs football field

American football

noun
  • A game similar to rugby football played on a field of 100 yards long and 53 1/3 yards wide (with two 10 yard-long endzones) in which two teams of 11 players attempt to get an ovoid ball into each other's territory. 

football field

noun
  • A playing field on which the game of Association football is played. 

  • An informal unit of measure of area, equal to the area of an American football field (57,600 square feet or 1.32 acres). 

  • A playing field on which the game of American football is played. 

How often have the words American football and football field occurred in a corpus of books? (source: Google Ngram Viewer )