You will sometimes hear photographers talk about getting “the shot” — they captured what they wanted to get. If you were to compare photography to the way we commonly think about education, this picture is the equivalent of the diploma. It’s the reward. The achievement is complete. We’re done here.
This concept has some precedence in art history. Henri Cartier-Bresson was a French photographer of candid scenes and he was known for capturing “the decisive moment.” The elements in his photographs were so perfectly arranged that a picture taken even a split second earlier or later would appear obviously inferior.