Susan Sontag wrote Regarding the Pain of Others as a means of exploring peoples' reactions to photography, particularly war photography, and what can and cannot be gained from war photography.
One essential and fascinating argument in her book is that photography is biased. People like to think of photographs as instantaneous captures the whole truth; however, Sontag points out that all photographs must be taken from certain various angles, and as the editors of Literary Masterpieces, Volume 20, state in their paraphrase of Sontag's argument, "What the angle frames is also a testament to what is left out of the picture" (eNotes, "Summary"). In other words, the angle is chosen by the photographer out of his/her own biased opinion, and he/she chooses to leave details out based on his/her biased opinion. Hence, a photograph can never capture the whole truth; it can only capture the truth as the photographer sees it.
A second fascinating and essential point she argues in her book is that images create inescapable emotional distance because the act of viewing itself creates distance. While photographs can help generate feelings of sympathy, all they really do is show the distance between the viewer and the pain experienced by the subject in the photograph simply because the viewer is not experiencing that pain himself/herself. But the same holds true of spectators watching a painful event on the sidelines. Hence, she argues that it is not photographs themselves that create emotional distance but rather the act of viewing. What's more, it's very true that viewers of war photographs will never know what it's like to truly experience war.