How does Susan Sontag use Plato's Cave?

Sontag uses the chapter title as an allusion to an aspect of the philosophy of Plato, which is not discussed in the text. Briefly, in The Republic of Plato used an allegory of a cave in which prisoners are chained facing a wall whereupon shadows of real objects are cast. The prisoners see only the shadows and believe them to be the totality of reality. Sontag playfully suggests that photography has made humanity unable to perceive the real world beyond the photograph.